Information

21.3: Lipids - Biology

21.3: Lipids - Biology


We are searching data for your request:

Forums and discussions:
Manuals and reference books:
Data from registers:
Wait the end of the search in all databases.
Upon completion, a link will appear to access the found materials.

skills to develop

  • Describe the chemical composition of lipids
  • Describe the unique characteristics and diverse structures of lipids
  • Compare and contrast triacylglycerides (triglycerides) and phospholipids.
  • Describe how phospholipids are used to construct biological membranes.

Although they are composed primarily of carbon and hydrogen, lipid molecules may also contain oxygen, nitrogen, sulfur, and phosphorous. Lipids serve numerous and diverse purposes in the structure and functions of organisms. They can be a source of nutrients, a storage form for carbon, energy-storage molecules, or structural components of membranes and hormones. Lipids comprise a broad class of many chemically distinct compounds, the most common of which are discussed in this section.

Fatty Acids and Triacylglycerides

The fatty acids are lipids that contain long-chain hydrocarbons terminated with a carboxylic acid functional group. Because the long hydrocarbon chain, fatty acids are hydrophobic (“water fearing”) or nonpolar. Fatty acids with hydrocarbon chains that contain only single bonds are called saturated fatty acids because they have the greatest number of hydrogen atoms possible and are, therefore, “saturated” with hydrogen. Fatty acids with hydrocarbon chains containing at least one double bond are called unsaturated fatty acids because they have fewer hydrogen atoms. Saturated fatty acids have a straight, flexible carbon backbone, whereas unsaturated fatty acids have “kinks” in their carbon skeleton because each double bond causes a rigid bend of the carbon skeleton. These differences in saturated versus unsaturated fatty acid structure result in different properties for the corresponding lipids in which the fatty acids are incorporated. For example, lipids containing saturated fatty acids are solids at room temperature, whereas lipids containing unsaturated fatty acids are liquids.

A triacylglycerol, or triglyceride, is formed when three fatty acids are chemically linked to a glycerol molecule (Figure (PageIndex{1})). Triglycerides are the primary components of adipose tissue (body fat), and are major constituents of sebum (skin oils). They play an important metabolic role, serving as efficient energy-storage molecules that can provide more than double the caloric content of both carbohydrates and proteins.

Figure (PageIndex{1}): Triglycerides are composed of a glycerol molecule attached to three fatty acids by a dehydration synthesis reaction.

Exercise (PageIndex{1})

Explain why fatty acids with hydrocarbon chains that contain only single bonds are called saturated fatty acids.

Phospholipids and Biological Membranes

Triglycerides are classified as simple lipids because they are formed from just two types of compounds: glycerol and fatty acids. In contrast, complex lipids contain at least one additional component, for example, a phosphate group (phospholipids) or a carbohydrate moiety (glycolipids). Figure (PageIndex{2}) depicts a typical phospholipid composed of two fatty acids linked to glycerol (a diglyceride). The two fatty acid carbon chains may be both saturated, both unsaturated, or one of each. Instead of another fatty acid molecule (as for triglycerides), the third binding position on the glycerol molecule is occupied by a modified phosphate group.

Figure (PageIndex{2}): This illustration shows a phospholipid with two different fatty acids, one saturated and one unsaturated, bonded to the glycerol molecule. The unsaturated fatty acid has a slight kink in its structure due to the double bond.

The molecular structure of lipids results in unique behavior in aqueous environments. Figure (PageIndex{1}) depicts the structure of a triglyceride. Because all three substituents on the glycerol backbone are long hydrocarbon chains, these compounds are nonpolar and not significantly attracted to polar water molecules—they are hydrophobic. Conversely, phospholipids such as the one shown in Figure (PageIndex{2}) have a negatively charged phosphate group. Because the phosphate is charged, it is capable of strong attraction to water molecules and thus is hydrophilic, or “water loving.” The hydrophilic portion of the phospholipid is often referred to as a polar “head,” and the long hydrocarbon chains as nonpolar “tails.” A molecule presenting a hydrophobic portion and a hydrophilic moiety is said to be amphipathic. Notice the “R” designation within the hydrophilic head depicted in Figure (PageIndex{2}), indicating that a polar head group can be more complex than a simple phosphate moiety. Glycolipids are examples in which carbohydrates are bonded to the lipids’ head groups.

The amphipathic nature of phospholipids enables them to form uniquely functional structures in aqueous environments. As mentioned, the polar heads of these molecules are strongly attracted to water molecules, and the nonpolar tails are not. Because of their considerable lengths, these tails are, in fact, strongly attracted to one another. As a result, energetically stable, large-scale assemblies of phospholipid molecules are formed in which the hydrophobic tails congregate within enclosed regions, shielded from contact with water by the polar heads (Figure (PageIndex{3})). The simplest of these structures are micelles, spherical assemblies containing a hydrophobic interior of phospholipid tails and an outer surface of polar head groups. Larger and more complex structures are created from lipid-bilayer sheets, or unit membranes, which are large, two-dimensional assemblies of phospholipids congregated tail to tail. The cell membranes of nearly all organisms are made from lipid-bilayer sheets, as are the membranes of many intracellular components. These sheets may also form lipid-bilayer spheres that are the structural basis of vesicles and liposomes, subcellular components that play a role in numerous physiological functions.

Figure (PageIndex{3}): Phospholipids tend to arrange themselves in aqueous solution forming liposomes, micelles, or lipid bilayer sheets. (credit: modification of work by Mariana Ruiz Villarreal)

Exercise(PageIndex{2})

How is the amphipathic nature of phospholipids significant?

Isoprenoids and Sterols

The isoprenoids are branched lipids, also referred to as terpenoids, that are formed by chemical modifications of the isoprene molecule (Figure (PageIndex{4})). These lipids play a wide variety of physiological roles in plants and animals, with many technological uses as pharmaceuticals (capsaicin), pigments (e.g., orange beta carotene, xanthophylls), and fragrances (e.g., menthol, camphor, limonene [lemon fragrance], and pinene [pine fragrance]). Long-chain isoprenoids are also found in hydrophobic oils and waxes. Waxes are typically water resistant and hard at room temperature, but they soften when heated and liquefy if warmed adequately. In humans, the main wax production occurs within the sebaceous glands of hair follicles in the skin, resulting in a secreted material called sebum, which consists mainly of triacylglycerol, wax esters, and the hydrocarbon squalene. There are many bacteria in the microbiota on the skin that feed on these lipids. One of the most prominent bacteria that feed on lipids is Propionibacterium acnes, which uses the skin’s lipids to generate short-chain fatty acids and is involved in the production of acne.

Figure (PageIndex{4}): Five-carbon isoprene molecules are chemically modified in various ways to yield isoprenoids.

Another type of lipids are steroids, complex, ringed structures that are found in cell membranes; some function as hormones. The most common types of steroids are sterols, which are steroids containing an OH group. These are mainly hydrophobic molecules, but also have hydrophilic hydroxyl groups. The most common sterol found in animal tissues is cholesterol. Its structure consists of four rings with a double bond in one of the rings, and a hydroxyl group at the sterol-defining position. The function of cholesterol is to strengthen cell membranes in eukaryotes and in bacteria without cell walls, such as Mycoplasma. Prokaryotes generally do not produce cholesterol, although bacteria produce similar compounds called hopanoids, which are also multiringed structures that strengthen bacterial membranes (Figure (PageIndex{5})). Fungi and some protozoa produce a similar compound called ergosterol, which strengthens the cell membranes of these organisms.

Figure (PageIndex{5}): Cholesterol and hopene (a hopanoid compound) are molecules that reinforce the structure of the cell membranes in eukaryotes and prokaryotes, respectively.

This video provides additional information about phospholipids and liposomes.

Exercise (PageIndex{3})

How are isoprenoids used in technology?

Key Concepts and Summary

  • Lipids are composed mainly of carbon and hydrogen, but they can also contain oxygen, nitrogen, sulfur, and phosphorous. They provide nutrients for organisms, store carbon and energy, play structural roles in membranes, and function as hormones, pharmaceuticals, fragrances, and pigments.
  • Fatty acids are long-chain hydrocarbons with a carboxylic acid functional group. Their relatively long nonpolar hydrocarbon chains make them hydrophobic. Fatty acids with no double bonds are saturated; those with double bonds are unsaturated.
  • Fatty acids chemically bond to glycerol to form structurally essential lipids such as triglycerides and phospholipids. Triglycerides comprise three fatty acids bonded to glycerol, yielding a hydrophobic molecule. Phospholipids contain both hydrophobic hydrocarbon chains and polar head groups, making them amphipathicand capable of forming uniquely functional large scale structures.
  • Biological membranes are large-scale structures based on phospholipid bilayers that provide hydrophilic exterior and interior surfaces suitable for aqueous environments, separated by an intervening hydrophobic layer. These bilayers are the structural basis for cell membranes in most organisms, as well as subcellular components such as vesicles.
  • Isoprenoids are lipids derived from isoprene molecules that have many physiological roles and a variety of commercial applications.
  • A wax is a long-chain isoprenoid that is typically water resistant; an example of a wax-containing substance is sebum, produced by sebaceous glands in the skin. Steroids are lipids with complex, ringed structures that function as structural components of cell membranes and as hormones. Sterols are a subclass of steroids containing a hydroxyl group at a specific location on one of the molecule’s rings; one example is cholesterol.
  • Bacteria produce hopanoids, structurally similar to cholesterol, to strengthen bacterial membranes. Fungi and protozoa produce a strengthening agent called ergosterol.

Multiple Choice

Which of the following describes lipids?

A. a source of nutrients for organisms
B. energy-storage molecules
C. molecules having structural role in membranes
D. molecules that are part of hormones and pigments
E. all of the above

E

Molecules bearing both polar and nonpolar groups are said to be which of the following?

A. hydrophilic
B. amphipathic
C. hydrophobic
D. polyfunctional

B

True/False

Lipids are a naturally occurring group of substances that are not soluble in water but are freely soluble in organic solvents.

False

Fatty acids having no double bonds are called “unsaturated.”

True

A triglyceride is formed by joining three glycerol molecules to a fatty acid backbone in a dehydration reaction.

False

Fill in the Blank

Waxes contain esters formed from long-chain __________ and saturated __________, and they may also contain substituted hydrocarbons.

alcohols; fatty acids

Cholesterol is the most common member of the __________ group, found in animal tissues; it has a tetracyclic carbon ring system with a __________ bond in one of the rings and one free __________group.

steroid; double; hydroxyl

Critical Thinking

Microorganisms can thrive under many different conditions, including high-temperature environments such as hot springs. To function properly, cell membranes have to be in a fluid state. How do you expect the fatty acid content (saturated versus unsaturated) of bacteria living in high-temperature environments might compare with that of bacteria living in more moderate temperatures?

Short Answer

Describe the structure of a typical phospholipid. Are these molecules polar or nonpolar?

Contributor

  • Nina Parker, (Shenandoah University), Mark Schneegurt (Wichita State University), Anh-Hue Thi Tu (Georgia Southwestern State University), Philip Lister (Central New Mexico Community College), and Brian M. Forster (Saint Joseph’s University) with many contributing authors. Original content via Openstax (CC BY 4.0; Access for free at https://openstax.org/books/microbiology/pages/1-introduction)


21.3 Prevention and Treatment of Viral Infections

By the end of this section, you will be able to do the following:

  • Identify major viral illnesses that affect humans
  • Compare vaccinations and anti-viral drugs as medical approaches to viruses

Viruses cause a variety of diseases in animals, including humans, ranging from the common cold to potentially fatal illnesses like meningitis (Figure 21.13). These diseases can be treated by antiviral drugs or by vaccines however, some viruses, such as HIV, are capable both of avoiding the immune response and of mutating within the host organism to become resistant to antiviral drugs.

Vaccines for Prevention

The primary method of controlling viral disease is by vaccination , which is intended to prevent outbreaks by building immunity to a virus or virus family (Figure 21.14). Vaccines may be prepared using live viruses, killed viruses, or molecular subunits of the virus. Note that the killed viral vaccines and subunit viruses are both incapable of causing disease, nor is there any valid evidence that vaccinations contribute to autism.

Live viral vaccines are designed in the laboratory to cause few symptoms in recipients while giving them protective immunity against future infections. Polio was one disease that represented a milestone in the use of vaccines. Mass immunization campaigns in the 1950s (killed vaccine) and 1960s (live vaccine) significantly reduced the incidence of the disease, which caused muscle paralysis in children and generated a great amount of fear in the general population when regional epidemics occurred. The success of the polio vaccine paved the way for the routine dispensation of childhood vaccines against measles, mumps, rubella, chickenpox, and other diseases.

The issue with using live vaccines (which are usually more effective than killed vaccines), is the low but significant danger that these viruses will revert to their disease-causing form by back mutations . Live vaccines are usually made by attenuating (weakening) the “wild-type” (disease-causing) virus by growing it in the laboratory in tissues or at temperatures different from what the virus is accustomed to in the host. Adaptations to these new cells or temperatures induce mutations in the genomes of the virus, allowing it to grow better in the laboratory while inhibiting its ability to cause disease when reintroduced into conditions found in the host. These attenuated viruses thus still cause infection, but they do not grow very well, allowing the immune response to develop in time to prevent major disease. Back mutations occur when the vaccine undergoes mutations in the host such that it readapts to the host and can again cause disease, which can then be spread to other humans in an epidemic. This type of scenario happened as recently as 2007 in Nigeria where mutations in a polio vaccine led to an epidemic of polio in that country.

Some vaccines are in continuous development because certain viruses, such as influenza and HIV, have a high mutation rate compared to that of other viruses and normal host cells. With influenza, mutations in the surface molecules of the virus help the organism evade the protective immunity that may have been obtained in a previous influenza season, making it necessary for individuals to get vaccinated every year. Other viruses, such as those that cause the childhood diseases measles, mumps, and rubella, mutate so infrequently that the same vaccine is used year after year.

Link to Learning

Watch this NOVA video to learn how microbiologists are attempting to replicate the deadly 1918 Spanish influenza virus so they can understand more about virology.

Vaccines and Antiviral Drugs for Treatment

In some cases, vaccines can be used to treat an active viral infection. The concept behind this is that by giving the vaccine, immunity is boosted without adding more disease-causing virus. In the case of rabies, a fatal neurological disease transmitted via the saliva of rabies virus-infected animals, the progression of the disease from the time of the animal bite to the time it enters the central nervous system may be two weeks or longer. This is enough time to vaccinate individuals who suspect that they have been bitten by a rabid animal, and their boosted immune response is sufficient to prevent the virus from entering nervous tissue. Thus, the potentially fatal neurological consequences of the disease are averted, and the individual only has to recover from the infected bite. This approach is also being used for the treatment of Ebola, one of the fastest and most deadly viruses on Earth. Transmitted by bats and great apes, this disease can cause death in 70 to 90 percent of infected humans within two weeks. Using newly developed vaccines that boost the immune response in this way, there is hope that affected individuals will be better able to control the virus, potentially saving a greater percentage of infected persons from a rapid and very painful death.

Another way of treating viral infections is the use of antiviral drugs. Because viruses use the resources of the host cell for replication and the production of new virus proteins, it is difficult to block their activities without damaging the host. However, we do have some effective antiviral drugs, such as those used to treat HIV and influenza. Some antiviral drugs are specific for a particular virus and others have been used to control and reduce symptoms for a wide variety of viral diseases. For most viruses, these drugs can inhibit the virus by blocking the actions of one or more of its proteins. It is important to note that the targeted proteins be encoded by viral genes and that these molecules are not present in a healthy host cell. In this way, viral growth is inhibited without damaging the host.

Antivirals have been developed to treat genital herpes (herpes simplex II) and influenza. For genital herpes, drugs such as acyclovir can reduce the number and duration of episodes of active viral disease, during which patients develop viral lesions in their skin cells. As the virus remains latent in nervous tissue of the body for life, this drug is not curative but can make the symptoms of the disease more manageable. For influenza, drugs like Tamiflu (oseltamivir) (Figure 21.15) can reduce the duration of “flu” symptoms by one or two days, but the drug does not prevent symptoms entirely. Tamiflu works by inhibiting an enzyme (viral neuraminidase) that allows new virions to leave their infected cells. Thus, Tamiflu inhibits the spread of virus from infected to uninfected cells. Other antiviral drugs, such as Ribavirin, have been used to treat a variety of viral infections, although its mechanism of action against certain viruses remains unclear.

By far, the most successful use of antivirals has been in the treatment of the retrovirus HIV, which causes a disease that, if untreated, is usually fatal within 10 to 12 years after infection. Anti-HIV drugs have been able to control viral replication to the point that individuals receiving these drugs survive for a significantly longer time than the untreated.

Anti-HIV drugs inhibit viral replication at many different phases of the HIV replicative cycle (Figure 21.16). Drugs have been developed that inhibit the fusion of the HIV viral envelope with the plasma membrane of the host cell (fusion inhibitors), the conversion of its RNA genome into double-stranded DNA (reverse transcriptase inhibitors, like AZT), the integration of the viral DNA into the host genome (integrase inhibitors), and the processing of viral proteins (protease inhibitors).

Unfortunately, when any of these drugs are used individually, the high mutation rate of the virus allows it to easily and rapidly develop resistance to the drug, limiting the drug’s effectiveness. The breakthrough in the treatment of HIV was the development of HAART, highly active anti-retroviral therapy, which involves a mixture of different drugs, sometimes called a drug “cocktail.” By attacking the virus at different stages of its replicative cycle, it is much more difficult for the virus to develop resistance to multiple drugs at the same time. Still, even with the use of combination HAART therapy, there is concern that, over time, the virus will develop resistance to this therapy. Thus, new anti-HIV drugs are constantly being developed with the hope of continuing the battle against this highly fatal virus.

Everyday Connection

Applied Virology:

The study of viruses has led to the development of a variety of new ways to treat non-viral diseases. Viruses have been used in gene therapy . Gene therapy is used to treat genetic diseases such as severe combined immunodeficiency (SCID), a heritable, recessive disease in which children are born with severely compromised immune systems. One common type of SCID is due to the lack of an enzyme, adenosine deaminase (ADA), which breaks down purine bases. To treat this disease by gene therapy, bone marrow cells are taken from a SCID patient and the ADA gene is inserted. This is where viruses come in, and their use relies on their ability to penetrate living cells and bring genes in with them. Viruses such as adenovirus, an upper-respiratory human virus, are modified by the addition of the ADA gene, and the virus then transports this gene into the cell. The modified cells, now capable of making ADA, are then given back to the patients in the hope of curing them. Gene therapy using viruses as carriers of genes (viral vectors), although still experimental, holds promise for the treatment of many genetic diseases. Still, many technological problems need to be solved for this approach to be a viable method for treating genetic disease.

Another medical use for viruses relies on their specificity and ability to kill the cells they infect. Oncolytic viruses are engineered in the laboratory specifically to attack and kill cancer cells. A genetically modified adenovirus known as H101 has been used since 2005 in clinical trials in China to treat head and neck cancers. The results have been promising, with a greater short-term response rate to the combination of chemotherapy and viral therapy than to chemotherapy treatment alone. This ongoing research may herald the beginning of a new age of cancer therapy, where viruses are engineered to find and specifically kill cancer cells, regardless of where in the body they may have spread.


3.3 Lipids

In this section, you will explore the following questions:

  • What are the four major types of lipids?
  • What are functions of fats in living organisms?
  • What is the difference between saturated and unsaturated fatty acids?
  • What is the molecular structure of phospholipids, and what is the role of phospholipids in cells?
  • What is the basic structure of a steroid, and what are examples of their functions?
  • How does cholesterol help maintain the fluid nature of the plasma membrane of cells?

Connection for AP ® Courses

Lipids also are sources of energy that power cellular processes. Like carbohydrates, lipids are composed of carbon, hydrogen, and oxygen, but these atoms are arranged differently. Most lipids are nonpolar and hydrophobic. Major types include fats and oils, waxes, phospholipids, and steroids. A typical fat consists of three fatty acids bonded to one molecule of glycerol, forming triglycerides or triacylglycerols. The fatty acids may be saturated or unsaturated, depending on the presence or absence of double bonds in the hydrocarbon chain a saturated fatty acid has the maximum number of hydrogen atoms bonded to carbon and, thus, only single bonds. In general, fats that are liquid at room temperature (e.g., canola oil) tend to be more unsaturated than fats that are solid at room temperature. In the food industry, oils are artificially hydrogenated to make them chemically more appropriate for use in processed foods. During this hydrogenation process, double bonds in the cis- conformation in the hydrocarbon chain may be converted to double bonds in the trans- conformation unfortunately, trans fats have been shown to contribute to heart disease. Phospholipids are a special type of lipid associated with cell membranes and typically have a glycerol (or sphingosine) backbone to which two fatty acid chains and a phosphate-containing group are attached. As a result, phospholipids are considered amphipathic because they have both hydrophobic and hydrophilic components. (In Chapters 4 and 5 we will explore in more detail how the amphipathic nature of phospholipids in plasma cell membranes helps regulate the passage of substances into and out of the cell.) Although the molecular structures of steroids differ from that of triglycerides and phospholipids, steroids are classified as lipids based on their hydrophobic properties. Cholesterol is a type of steroid in animal cells’ plasma membrane. Cholesterol is also the precursor of steroid hormones such as testosterone.

Information presented and the examples highlighted in the section, support concepts outlined in Big Idea 4 of the AP ® Biology Curriculum Framework. The learning objectives listed in the Curriculum Framework provide a transparent foundation for the AP ® Biology course, an inquiry-based laboratory experience, instructional activities, and AP ® exam questions. A Learning Objective merges required content with one or more of the seven Science Practices.

Big Idea 4 Biological systems interact, and these systems and their interactions possess complex properties.
Enduring Understanding 4.A Interactions within biological systems lead to complex properties.
Essential Knowledge 4.A.1 The subcomponents of biological molecules and their sequence determine the properties of that molecule.
Science Practice 7.1 The student can connect phenomena and models across spatial and temporal scales.
Learning Objective 4.1 The student is able to explain the connection between the sequence and the subcomponents of a biological polymer and its properties.
Essential Knowledge 4.A.1 The subcomponents of biological molecules and their sequence determine the properties of that molecule.
Science Practice 1.3 The student can refine representations and models of natural or man-made phenomena and systems in the domain.
Learning Objective 4.2 The student is able to refine representations and models to explain how the subcomponents of a biological polymer and their sequence determine the properties of that polymer.
Essential Knowledge 4.A.1 The subcomponents of biological molecules and their sequence determine the properties of that molecule.
Science Practice 6.1 The student can justify claims with evidence.
Science Practice 6.4 The student can make claims and predictions about natural phenomena based on scientific theories and models.
Learning Objective 4.3 The student is able to use models to predict and justify that changes in the subcomponents of a biological polymer affect the functionality of the molecules.

Teacher Support

An important misconception to overcome for students is that lipids are not bad for the body. They are absolutely essential to the body’s functions, including for growth and survival.

Another concept to discuss is the insolubility of lipids in water. It is obvious in salad dressing, but why does it occur? If other functional groups are attached to lipids, they may contain some charges and give a degree of solubility to the lipid, but most lipids do not have any charges on the surface of the molecules and are not soluble in water, therefore, lipids are usually described as being hydrophobic.

Insoluble lipids must be attached to proteins in the body to become soluble in body fluids. Have the class research the proteins that transport and carry lipids. Identify their contributions to health or sickness.

The Science Practice Challenge Questions contain additional test questions for this section that will help you prepare for the AP exam. These questions address the following standards:
[APLO 2.9] [APLO 2.10] [APLO 2.12] [APLO 2.13][APLO 2.14][APLO 4.14]

Fats and Oils

Lipids include a diverse group of compounds that are largely nonpolar in nature. This is because they are hydrocarbons that include mostly nonpolar carbon–carbon or carbon–hydrogen bonds. Non-polar molecules are hydrophobic (“water fearing”), or insoluble in water. Lipids perform many different functions in a cell. Cells store energy for long-term use in the form of fats. Lipids also provide insulation from the environment for plants and animals (Figure 3.13). For example, their water-repellant hydrophobic nature can help keep aquatic birds and mammals dry by forming a protective layer over fur or feathers. Lipids are also the building blocks of many hormones and an important constituent of all cellular membranes. Lipids include fats, waxes, phospholipids, and steroids.

Teacher Support

The difference between a fat and an oil is the state of the compound at room temperature (68°F). A fat is a solid or semisolid material and an oil is a liquid at this temperature. Both fats and oils are made up of glycerol and two or three fatty acid chains attached to its carbons by way of dehydration synthesis. A fatty acid is a chain of carbon atoms with hydrogen atoms attached at the open bonding sites. If the chain is fully saturated with hydrogen atoms, it is termed a saturated fat. This tends to give the compound a relatively stiff configuration and helps it to be a solid. If any of the hydrogen atoms are missing, it is called an unsaturated fat or oil. The absence of hydrogen atoms along the chain causes double bonds to form between adjacent carbon atoms, which results in a bend in the chain. This causes the molecules to push away other molecules near it, preventing the packing of fatty acid chains, and resulting in a liquid at room temperature. Fats tend to contain a high concentration of saturated fatty acids and oils tend to contain more unsaturated fatty acid chains. Both types have an effect on health a high amount of saturated fats is significantly less healthy than a higher amount of unsaturated lipids. An exception trans fat, an unsaturated fat found in processed foods. Trans fats behave like a saturated lipid.

Divide the class into three sections: section 1: dairy department section 2: salad dressings, and section 3: potato chips.. Each section will visit the supermarket and identify which fats or oils are in five items in their category. Then, each section will prepare a chart listing their findings and share it with the class.

A fat molecule consists of two main components—glycerol and fatty acids. Glycerol is an organic compound (alcohol) with three carbons, five hydrogens, and three hydroxyl (OH) groups. Fatty acids have a long chain of hydrocarbons to which a carboxyl group is attached, hence the name “fatty acid.” The number of carbons in the fatty acid may range from 4 to 36 most common are those containing 12–18 carbons. In a fat molecule, the fatty acids are attached to each of the three carbons of the glycerol molecule with an ester bond through an oxygen atom (Figure 3.14).

During this ester bond formation, three water molecules are released. The three fatty acids in the triacylglycerol may be similar or dissimilar. Fats are also called triacylglycerols or triglycerides because of their chemical structure. Some fatty acids have common names that specify their origin. For example, palmitic acid, a saturated fatty acid , is derived from the palm tree. Arachidic acid is derived from Arachis hypogea, the scientific name for groundnuts or peanuts.

Fatty acids may be saturated or unsaturated. In a fatty acid chain, if there are only single bonds between neighboring carbons in the hydrocarbon chain, the fatty acid is said to be saturated. Saturated fatty acids are saturated with hydrogen in other words, the number of hydrogen atoms attached to the carbon skeleton is maximized. Stearic acid is an example of a saturated fatty acid (Figure 3.15)

When the hydrocarbon chain contains a double bond, the fatty acid is said to be unsaturated . Oleic acid is an example of an unsaturated fatty acid (Figure 3.16).

Most unsaturated fats are liquid at room temperature and are called oils. If there is one double bond in the molecule, then it is known as a monounsaturated fat (e.g., olive oil), and if there is more than one double bond, then it is known as a polyunsaturated fat (e.g., canola oil).

When a fatty acid has no double bonds, it is known as a saturated fatty acid because no more hydrogen may be added to the carbon atoms of the chain. A fat may contain similar or different fatty acids attached to glycerol. Long straight fatty acids with single bonds tend to get packed tightly and are solid at room temperature. Animal fats with stearic acid and palmitic acid (common in meat) and the fat with butyric acid (common in butter) are examples of saturated fats. Mammals store fats in specialized cells called adipocytes, where globules of fat occupy most of the cell’s volume. In plants, fat or oil is stored in many seeds and is used as a source of energy during seedling development. Unsaturated fats or oils are usually of plant origin and contain cis unsaturated fatty acids. Cis and trans indicate the configuration of the molecule around the double bond. If hydrogens are present in the same plane, it is referred to as a cis fat if the hydrogen atoms are on two different planes, it is referred to as a trans fat . The cis double bond causes a bend or a “kink” that prevents the fatty acids from packing tightly, keeping them liquid at room temperature (Figure 3.17). Olive oil, corn oil, canola oil, and cod liver oil are examples of unsaturated fats. Unsaturated fats help to lower blood cholesterol levels whereas saturated fats contribute to plaque formation in the arteries.

Trans Fats

In the food industry, oils are artificially hydrogenated to make them semi-solid and of a consistency desirable for many processed food products. Simply speaking, hydrogen gas is bubbled through oils to solidify them. During this hydrogenation process, double bonds of the cis- conformation in the hydrocarbon chain may be converted to double bonds in the trans- conformation.

Margarine, some types of peanut butter, and shortening are examples of artificially hydrogenated trans fats. Recent studies have shown that an increase in trans fats in the human diet may lead to an increase in levels of low-density lipoproteins (LDL), or “bad” cholesterol, which in turn may lead to plaque deposition in the arteries, resulting in heart disease. Many fast food restaurants have recently banned the use of trans fats, and food labels are required to display the trans fat content.

Omega Fatty Acids

Essential fatty acids are fatty acids required but not synthesized by the human body. Consequently, they have to be supplemented through ingestion via the diet. Omega -3 fatty acids (like that shown in Figure 3.18) fall into this category and are one of only two known for humans (the other being omega-6 fatty acid). These are polyunsaturated fatty acids and are called omega-3 because the third carbon from the end of the hydrocarbon chain is connected to its neighboring carbon by a double bond.

The farthest carbon away from the carboxyl group is numbered as the omega (ω) carbon, and if the double bond is between the third and fourth carbon from that end, it is known as an omega-3 fatty acid. Nutritionally important because the body does not make them, omega-3 fatty acids include alpha-linoleic acid (ALA), eicosapentaenoic acid (EPA), and docosahexaenoic acid (DHA), all of which are polyunsaturated. Salmon, trout, and tuna are good sources of omega-3 fatty acids. Research indicates that omega-3 fatty acids reduce the risk of sudden death from heart attacks, reduce triglycerides in the blood, lower blood pressure, and prevent thrombosis by inhibiting blood clotting. They also reduce inflammation, and may help reduce the risk of some cancers in animals.

Like carbohydrates, fats have received a lot of bad publicity. It is true that eating an excess of fried foods and other “fatty” foods leads to weight gain. However, fats do have important functions. Many vitamins are fat soluble, and fats serve as a long-term storage form of fatty acids: a source of energy. They also provide insulation for the body. Therefore, “healthy” fats in moderate amounts should be consumed on a regular basis.

Teacher Support

This question is an application Learning Objective 4.3 and Science Practices 6.1 and 6.4 because students are predicting how a change in the subcomponents of a molecule can affect the properties of the molecule.

A phospholipid is made of a phosphate group bonded to a glycerol that is linked to two fatty acid chains. One of the fatty acid chains is saturated and the other unsaturated. The saturated one is straight, while the unsaturated chain contains a bend. Phospholipids make up lipid bilayers, the main component of most plasma membranes and give it a fluid like property, a result of the fatty acid tails creating space between phospholipid molecules.

The concept of a bent fatty acid tail contributing to the fluidity of a cell membrane can be difficult to visualize. Obtain some old fashioned, wooden clothes-pins. The knob at the top becomes a phosphate molecule. The two prongs of the pins become fatty acids. Both prongs are stiff, so they are saturated fatty acids. There are no unsaturated fatty acids in this demonstration. Hold a number of the pins tightly in your hand and ask a student to remove a pin in the center. They shouldn’t be able to, as you are pressing the prongs of all of the pins together. This would be in a cell membrane without any unsaturated fatty acids pushing adjacent chains away, creating spaces that allow the membrane to behave like a fluid.


3.3 Lipids

By the end of this section, you will be able to do the following:

  • Describe the four major types of lipids
  • Explain the role of fats in storing energy
  • Differentiate between saturated and unsaturated fatty acids
  • Describe phospholipids and their role in cells
  • Define the basic structure of a steroid and some steroid functions
  • Explain how cholesterol helps maintain the plasma membrane's fluid nature

Lipids include a diverse group of compounds that are largely nonpolar in nature. This is because they are hydrocarbons that include mostly nonpolar carbon–carbon or carbon–hydrogen bonds. Non-polar molecules are hydrophobic (“water fearing”), or insoluble in water. Lipids perform many different functions in a cell. Cells store energy for long-term use in the form of fats. Lipids also provide insulation from the environment for plants and animals (Figure 3.12). For example, they help keep aquatic birds and mammals dry when forming a protective layer over fur or feathers because of their water-repellant hydrophobic nature. Lipids are also the building blocks of many hormones and are an important constituent of all cellular membranes. Lipids include fats, oils, waxes, phospholipids, and steroids.

Fats and Oils

A fat molecule consists of two main components—glycerol and fatty acids. Glycerol is an organic compound (alcohol) with three carbons, five hydrogens, and three hydroxyl (OH) groups. Fatty acids have a long chain of hydrocarbons to which a carboxyl group is attached, hence the name “fatty acid.” The number of carbons in the fatty acid may range from 4 to 36. The most common are those containing 12–18 carbons. In a fat molecule, the fatty acids attach to each of the glycerol molecule's three carbons with an ester bond through an oxygen atom (Figure 3.13).

During this ester bond formation, three water molecules are released. The three fatty acids in the triacylglycerol may be similar or dissimilar. We also call fats triacylglycerols or triglycerides because of their chemical structure. Some fatty acids have common names that specify their origin. For example, palmitic acid, a saturated fatty acid , is derived from the palm tree. Arachidic acid is derived from Arachis hypogea, the scientific name for groundnuts or peanuts.

Fatty acids may be saturated or unsaturated. In a fatty acid chain, if there are only single bonds between neighboring carbons in the hydrocarbon chain, the fatty acid is saturated. Saturated fatty acids are saturated with hydrogen. In other words, the number of hydrogen atoms attached to the carbon skeleton is maximized. Stearic acid is an example of a saturated fatty acid (Figure 3.14).

When the hydrocarbon chain contains a double bond, the fatty acid is unsaturated . Oleic acid is an example of an unsaturated fatty acid (Figure 3.15).

Most unsaturated fats are liquid at room temperature. We call these oils. If there is one double bond in the molecule, then it is a monounsaturated fat (e.g., olive oil), and if there is more than one double bond, then it is a polyunsaturated fat (e.g., canola oil).

When a fatty acid has no double bonds, it is a saturated fatty acid because it is not possible to add more hydrogen to the chain's carbon atoms. A fat may contain similar or different fatty acids attached to glycerol. Long straight fatty acids with single bonds generally pack tightly and are solid at room temperature. Animal fats with stearic acid and palmitic acid (common in meat) and the fat with butyric acid (common in butter) are examples of saturated fats. Mammals store fats in specialized cells, or adipocytes, where fat globules occupy most of the cell’s volume. Plants store fat or oil in many seeds and use them as a source of energy during seedling development. Unsaturated fats or oils are usually of plant origin and contain cis unsaturated fatty acids. Cis and trans indicate the configuration of the molecule around the double bond. If hydrogens are present in the same plane, it is a cis fat. If the hydrogen atoms are on two different planes, it is a trans fat . The cis double bond causes a bend or a “kink” that prevents the fatty acids from packing tightly, keeping them liquid at room temperature (Figure 3.16). Olive oil, corn oil, canola oil, and cod liver oil are examples of unsaturated fats. Unsaturated fats help to lower blood cholesterol levels whereas, saturated fats contribute to plaque formation in the arteries.

Trans Fats

The food industry artificially hydrogenates oils to make them semi-solid and of a consistency desirable for many processed food products. Simply speaking, hydrogen gas is bubbled through oils to solidify them. During this hydrogenation process, double bonds of the cis- conformation in the hydrocarbon chain may convert to double bonds in the trans- conformation.

Margarine, some types of peanut butter, and shortening are examples of artificially hydrogenated trans fats. Recent studies have shown that an increase in trans fats in the human diet may lead to higher levels of low-density lipoproteins (LDL), or “bad” cholesterol, which in turn may lead to plaque deposition in the arteries, resulting in heart disease. Many fast food restaurants have recently banned using trans fats, and food labels are required to display the trans fat content.

Omega Fatty Acids

Essential fatty acids are those that the human body requires but does not synthesize. Consequently, they have to be supplemented through ingestion via the diet. Omega -3 fatty acids (like those in Figure 3.17) fall into this category and are one of only two known for humans (the other is omega-6 fatty acid). These are polyunsaturated fatty acids and are omega-3 because a double bond connects the third carbon from the hydrocarbon chain's end to its neighboring carbon.

The farthest carbon away from the carboxyl group is numbered as the omega (ω) carbon, and if the double bond is between the third and fourth carbon from that end, it is an omega-3 fatty acid. Nutritionally important because the body does not make them, omega-3 fatty acids include alpha-linoleic acid (ALA), eicosapentaenoic acid (EPA), and docosahexaenoic acid (DHA), all of which are polyunsaturated. Salmon, trout, and tuna are good sources of omega-3 fatty acids. Research indicates that omega-3 fatty acids reduce the risk of sudden death from heart attacks, lower triglycerides in the blood, decrease blood pressure, and prevent thrombosis by inhibiting blood clotting. They also reduce inflammation, and may help lower the risk of some cancers in animals.

Like carbohydrates, fats have received considerable bad publicity. It is true that eating an excess of fried foods and other “fatty” foods leads to weight gain. However, fats do have important functions. Many vitamins are fat soluble, and fats serve as a long-term storage form of fatty acids: a source of energy. They also provide insulation for the body. Therefore, we should consume “healthy” fats in moderate amounts on a regular basis.

Waxes

Wax covers some aquatic birds' feathers and some plants' leaf surfaces. Because of waxes' hydrophobic nature, they prevent water from sticking on the surface (Figure 3.18). Long fatty acid chains esterified to long-chain alcohols comprise waxes.

Phospholipids

Phospholipids are major plasma membrane constituents that comprise cells' outermost layer. Like fats, they are comprised of fatty acid chains attached to a glycerol or sphingosine backbone. However, instead of three fatty acids attached as in triglycerides, there are two fatty acids forming diacylglycerol, and a modified phosphate group occupies the glycerol backbone's third carbon (Figure 3.19). A phosphate group alone attached to a diacylglycerol does not qualify as a phospholipid. It is phosphatidate (diacylglycerol 3-phosphate), the precursor of phospholipids. An alcohol modifies the phosphate group. Phosphatidylcholine and phosphatidylserine are two important phospholipids that are in plasma membranes.

A phospholipid is an amphipathic molecule, meaning it has a hydrophobic and a hydrophilic part. The fatty acid chains are hydrophobic and cannot interact with water whereas, the phosphate-containing group is hydrophilic and interacts with water (Figure 3.20).

The head is the hydrophilic part, and the tail contains the hydrophobic fatty acids. In a membrane, a bilayer of phospholipids forms the structure's matrix, phospholipids' fatty acid tails face inside, away from water whereas, the phosphate group faces the outside, aqueous side (Figure 3.20).

Phospholipids are responsible for the plasma membrane's dynamic nature. If a drop of phospholipids is placed in water, it spontaneously forms a structure that scientists call a micelle, where the hydrophilic phosphate heads face the outside and the fatty acids face the structure's interior.

Steroids

Unlike the phospholipids and fats that we discussed earlier, steroids have a fused ring structure. Although they do not resemble the other lipids, scientists group them with them because they are also hydrophobic and insoluble in water. All steroids have four linked carbon rings and several of them, like cholesterol, have a short tail (Figure 3.21). Many steroids also have the –OH functional group, which puts them in the alcohol classification (sterols).

Cholesterol is the most common steroid. The liver synthesizes cholesterol and is the precursor to many steroid hormones such as testosterone and estradiol, which gonads and endocrine glands secrete. It is also the precursor to Vitamin D. Cholesterol is also the precursor of bile salts, which help emulsifying fats and their subsequent absorption by cells. Although lay people often speak negatively about cholesterol, it is necessary for the body's proper functioning. Sterols (cholesterol in animal cells, phytosterol in plants) are components of the plasma membrane of cells and are found within the phospholipid bilayer.

Link to Learning

For an additional perspective on lipids, explore the interactive animation “Biomolecules: The Lipids”.


Biochemical analyses of nematodes have revealed that neutral lipids (especially triglycerides) are the main source of energy reserves, which is depleted as the nematodes age. Several methodologies have been developed to visualise triglyceride-rich fat stores in plant-parasitic nematodes using non-fluorescent, lipophilic dyes, such as Oil Red O. Here, we propose a robust and reproducible fluorescence-based Nile Red staining method (followed by image analysis) for rapid detection of neutral lipid droplets in Meloidogyne incognita. This unique lipophilic dye selectively fluoresces in red and green spectra in a lipid-rich environment. The neutral lipid content of M. incognita juveniles gradually diminished during different periods of food deprivation, and this was significantly correlated with reduction in parasitic success of M. incognita in eggplant. Additionally, variation in fat reserves in different developmental stages of M. incognita infecting adzuki bean was also demonstrated. This investigation may aid future metabolic research, including functional analysis of lipid regulatory genes in plant-parasitic nematodes.

Sections
References

Abràmoff , M.D. , Magalhães , P.J. & Ram , S.J. ( 2004 ). Image processing with ImageJ . Biophotonics International 11 , 36 - 42 .

Abràmoff , M.D. , Magalhães , P.J. & Ram , S.J.

Andaló , V. , Moino Jr , A. , Maximiniano , C. , Campos , V.P. & Mendonça , L.A. ( 2011 ). Influence of temperature and duration of storage on the lipid reserves of entomopathogenic nematodes . Revista Colombiana de Entomología 37 , 203 - 209 .

Andaló , V. , Moino Jr , A. , Maximiniano , C. , Campos , V.P. & Mendonça , L.A.

Andrássy , I. ( 1956 ). Die Rauminhalts- und Gewichsbestmmung der Fadenwurmer (Nematoden) . Acta Zoologica Academiae Scientiarum Hungaricae 2 , 1 - 15 .

Badhwar , R. , Raheja , R.K. , Ahuju , S. & Ahuju , S.P. ( 1995 ). Lipid composition of the seed gall nematode, Anguina tritici . Nematologica 41 , 584 - 591 . DOI: 10.1163/003925995X00521

Badhwar , R. , Raheja , R.K. , Ahuju , S. & Ahuju , S.P.

Chitwood , D.J. ( 1998 ). Biosynthesis . In: Perry , R.N. & Wright , D.J. (Eds). The physiology and biochemistry of free-living and plant-parasitic nematodes . Wallingford, UK , CAB International , pp. 303 - 330 .

(Eds). The physiology and biochemistry of free-living and plant-parasitic nematodes.

Chitwood , D.J. ( 1999 ). Biochemistry and function of nematode steroids . Critical Reviews in Biochemistry and Molecular Biology 34 , 273 - 284 . DOI: 10.1080/10409239991209309

Christophers , A.E.P. , Patel , M.N. , Benson , J.A. , Saka , V.W. , Evans , A.A.F. & Wright , D.J. ( 1997 ). A rapid field-laboratory bioassay to assess the infectivity of Meloidogyne spp. second stage juveniles . Nematologica 43 , 117 - 120 . DOI: 10.1163/004725997X00089

Christophers , A.E.P. , Patel , M.N. , Benson , J.A. , Saka , V.W. , Evans , A.A.F. & Wright , D.J.

Cirulis , J.T. , Strasser , B.C. , Scott , J.A. & Ross , G.M. ( 2012 ). Optimization of staining conditions for microalgae with three lipophilic dyes to reduce precipitation and fluorescence variability . Cytometry A 81 , 618 - 626 . DOI: 10.1002/cyto.a.22066

Cirulis , J.T. , Strasser , B.C. , Scott , J.A. & Ross , G.M.

Croll , N.A. ( 1972 ). Energy utilization of infective Ancylostoma tubaeforme larvae . Parasitology 64 , 355 - 365 . DOI: 10.1017/S0031182000045431

Dash , M. , Dutta , T.K. , Phani , V. , Papolu , P.K. , Shivakumara , T.N. & Rao , U. ( 2017 ). RNAi-mediated disruption of neuropeptide genes, nlp-3 and nlp-12 , cause multiple behavioral defects in Meloidogyne incognita . Biochemical and Biophysical Research Communications 490 , 933 - 940 . DOI: 10.1016/j.bbrc.2017.06.143

Dash , M. , Dutta , T.K. , Phani , V. , Papolu , P.K. , Shivakumara , T.N. & Rao , U.

de Almeida Barros , A.G. , Liu , J. , Lemieux , G.A. , Mullaney , B.C. & Ashrafi , K. ( 2012 ). Analyses of C. elegans fat metabolic pathways . Methods in Cell Biology 107 , 383 - 407 . DOI: 10.1016/B978-0-12-394620-1.00013-8

de Almeida Barros , A.G. , Liu , J. , Lemieux , G.A. , Mullaney , B.C. & Ashrafi , K.

Diaz , G. , Melis , M. , Batetta , B. , Angius , F. & Falchi , A.M. ( 2008 ). Hydrophobic characterization of intracellular lipids in situ by Nile Red red/yellow emission ratio . Micron 39 , 819 - 824 . DOI: 10.1016/j.micron.2008.01.001

Diaz , G. , Melis , M. , Batetta , B. , Angius , F. & Falchi , A.M.

Dutta , T.K. , Papolu , P.K. , Banakar , P. , Choudhary , D. , Sirohi , A. & Rao , U. ( 2015 ). Tomato transgenic plants expressing hairpin construct of a nematode protease gene conferred enhanced resistance to root-knot nematodes . Frontiers in Microbiology 6 , 260 . DOI: 10.3389/fmicb.2015.00260

Dutta , T.K. , Papolu , P.K. , Banakar , P. , Choudhary , D. , Sirohi , A. & Rao , U.

Fitters , P.F.L. & Griffin , C.T. ( 2004 ). Spontaneous and induced activity of Heterorhabditis megidis infective juveniles during storage . Nematology 6 , 911 - 917 . DOI: 10.1163/1568541044038597

Fitters , P.F.L. & Griffin , C.T.

Fitters , P.F.L. , Meijer , E.M.J. , Wright , D.J. & Griffin , C.T. ( 1997 ). Estimation of lipid reserves in unstained living and dead nematodes by image analysis . Journal of Nematology 29 , 160 - 167 .

Fitters , P.F.L. , Meijer , E.M.J. , Wright , D.J. & Griffin , C.T.

Grewal , P.G. ( 1995 ). Nematode quality . In: The second international symposium on entomopathogenic nematodes and their symbiotic bacteria . Honolulu, HI, USA , University of Hawaii , pp. 38 - 40 .

( 1995 ). Nematode quality . In: The second international symposium on entomopathogenic nematodes and their symbiotic bacteria.

Gutbrod , P. , Gutbrod , K. , Nauen , R. , Elashry , A. , Siddique , S. , Benting , J. , Dormann , P. & Grundler , F.M.W. ( 2018 ). Inhibition of acetyl-CoA carboxylase by spirotetramat causes lipid depletion and surface coat deficiency in nematodes. bioRxiv preprint (March 7, 2018). DOI: 10.1101/278093

Hooper , D.J. ( 1986 ). Extraction of free-living stages from soil . In: Southey , J.F. (Ed.). Laboratory methods for work with plant and soil nematodes . London, UK , Ministry of Agriculture, Fisheries and Food , pp. 5 - 30 .

( 1986 ). Extraction of free-living stages from soil . In:

(Ed.). Laboratory methods for work with plant and soil nematodes.

Ministry of Agriculture, Fisheries and Food

Jones , J.T. , Haegeman , A. , Danchin , E.G.J. , Gaur , H. , Helder , J. , Jones , M.G.K. , Kikuchi , T. , Manzanilla-López , R. , Palomares-Rius , J.E. , Wesemael , W.M.L. et al. ( 2013 ). Top 10 plant-parasitic nematodes in molecular plant pathology . Molecular Plant Pathology 14 , 946 - 961 . DOI: 10.1111/mpp.12057

Jones , J.T. , Haegeman , A. , Danchin , E.G.J. , Gaur , H. , Helder , J. , Jones , M.G.K. , Kikuchi , T. , Manzanilla-López , R. , Palomares-Rius , J.E. , Wesemael , W.M.L.

Kearn , J. , Lilley , C. , Urwin , P. , O’Connor , V. & Holden-Dye , L. ( 2017 ). Progressive metabolic impairment underlies the novel nematicidal action of fluensulfone on the potato cyst nematode Globodera pallida . Pesticide Biochemistry and Physiology 142 , 83 - 90 . DOI: 10.1016/j.pestbp.2017.01.009

Kearn , J. , Lilley , C. , Urwin , P. , O’Connor , V. & Holden-Dye , L.

Lee , D.L. & Atkinson , H.J. ( 1977 ). Physiology of nematodes . New York, NY, USA , Columbia University Press .

( 1977 ). Physiology of nematodes.

Columbia University Press

Marella , H.H. , Nielsen , E. , Schachtman , D.P. & Taylor , C.G. ( 2013 ). The amino acid permeases AAP3 and AAP6 are involved in root-knot nematode parasitism of Arabidopsis . Molecular Plant-Microbe Interactions 26 , 44 - 54 . DOI: 10.1094/MPMI-05-12-0123-FI

Marella , H.H. , Nielsen , E. , Schachtman , D.P. & Taylor , C.G.

O’Rourke , E.J. , Soukas , A.A. , Carr , C.E. & Ruvkun , G. ( 2009 ). C. elegans major fats are stored in vesicles distinct from lysosome-related organelles . Cell Metabolism 10 , 430 - 435 . DOI: 10.1016/j.cmet.2009.10.002

O’Rourke , E.J. , Soukas , A.A. , Carr , C.E. & Ruvkun , G.

Papolu , P.K. , Gantasala , N.P. , Kamaraju , D. , Banakar , P. , Sreevathsa , R. & Rao , U. ( 2013 ). Utility of host delivered RNAi of two FMRF amide like peptides, flp-14 and flp-18 , for the management of root knot nematode, Meloidogyne incognita . PloS ONE 8 , e80603 . DOI: 10.1371/journal.pone.0080603

Papolu , P.K. , Gantasala , N.P. , Kamaraju , D. , Banakar , P. , Sreevathsa , R. & Rao , U.

Patel , M.N. , Stolinski , M. & Wright , D.J. ( 1997 ). Neutral lipids and the assessment of infectivity in entomopathogenic nematodes: observations on four Steinernema species . Parasitology 114 , 489 - 496 .

Patel , M.N. , Stolinski , M. & Wright , D.J.

Pino , E.C. , Webster , C.M. , Carr , C.E. & Soukas , A.A. ( 2013 ). Biochemical and high throughput microscopic assessment of fat mass in Caenorhabditis elegans . Journal of Visualized Experiments 73 , e50180 . DOI: 10.3791/50180

Pino , E.C. , Webster , C.M. , Carr , C.E. & Soukas , A.A.

Qiu , L. & Bedding , R. ( 1999 ). A rapid method for the estimation of mean dry weight and lipid content of the infective juveniles of entomopathogenic nematodes using image analysis . Nematology 1 , 655 - 660 . DOI: 10.1163/156854199508612

Robinson , M.P. , Atkinson , H.J. & Perry , R.N. ( 1987 ). The influence of soil moisture and storage time on the motility, infectivity and lipid utilization of second stage juveniles of the potato cyst nematodes Globodera rostochiensis and G. pallida . Revue de Nématologie 10 , 343 - 348 .

Robinson , M.P. , Atkinson , H.J. & Perry , R.N.

Rocha , F.S. , Campos , V.P. & de Souza , J.T. ( 2010 ). Variation in lipid reserves of second-stage juveniles of Meloidogyne exigua in a coffee field and its relationship with infectivity . Nematology 12 , 365 - 371 . DOI: 10.1163/138855409X12548945788367

Rocha , F.S. , Campos , V.P. & de Souza , J.T.

Rocha , F.S. , Campos , V.P. , Catão , H.C.R.M. , Muniz , M.D.F.S. & Civil , N. ( 2015 ). Correlations among methods to estimate lipid reserves of second-stage juveniles and its relationships with infectivity and reproduction of Meloidogyne incognita . Nematology 17 , 345 - 352 . DOI: 10.1163/15685411-00002871

Rocha , F.S. , Campos , V.P. , Catão , H.C.R.M. , Muniz , M.D.F.S. & Civil , N.

Satouchi , K. , Hirano , K. , Sakaguchi , M. , Takehara , H. & Matsuura , F. ( 1993 ). Phospholipids from the free-living nematode Caenorhabditis elegans . Lipids 28 , 837 - 840 . DOI: 10.1007/BF02536239

Satouchi , K. , Hirano , K. , Sakaguchi , M. , Takehara , H. & Matsuura , F.


Access options

Get full journal access for 1 year

All prices are NET prices.
VAT will be added later in the checkout.
Tax calculation will be finalised during checkout.

Get time limited or full article access on ReadCube.

All prices are NET prices.


Contents

When phospholipids are exposed to water, they self-assemble into a two-layered sheet with the hydrophobic tails pointing toward the center of the sheet. This arrangement results in two “leaflets” that are each a single molecular layer. The center of this bilayer contains almost no water and excludes molecules like sugars or salts that dissolve in water. The assembly process is driven by interactions between hydrophobic molecules (also called the hydrophobic effect). An increase in interactions between hydrophobic molecules (causing clustering of hydrophobic regions) allows water molecules to bond more freely with each other, increasing the entropy of the system. This complex process includes non-covalent interactions such as van der Waals forces, electrostatic and hydrogen bonds.

Cross section analysis Edit

The lipid bilayer is very thin compared to its lateral dimensions. If a typical mammalian cell (diameter

10 micrometers) were magnified to the size of a watermelon (

1 ft/30 cm), the lipid bilayer making up the plasma membrane would be about as thick as a piece of office paper. Despite being only a few nanometers thick, the bilayer is composed of several distinct chemical regions across its cross-section. These regions and their interactions with the surrounding water have been characterized over the past several decades with x-ray reflectometry, [4] neutron scattering [5] and nuclear magnetic resonance techniques.

The first region on either side of the bilayer is the hydrophilic headgroup. This portion of the membrane is completely hydrated and is typically around 0.8-0.9 nm thick. In phospholipid bilayers the phosphate group is located within this hydrated region, approximately 0.5 nm outside the hydrophobic core. [6] In some cases, the hydrated region can extend much further, for instance in lipids with a large protein or long sugar chain grafted to the head. One common example of such a modification in nature is the lipopolysaccharide coat on a bacterial outer membrane, [7] which helps retain a water layer around the bacterium to prevent dehydration.

Next to the hydrated region is an intermediate region that is only partially hydrated. This boundary layer is approximately 0.3 nm thick. Within this short distance, the water concentration drops from 2M on the headgroup side to nearly zero on the tail (core) side. [8] [9] The hydrophobic core of the bilayer is typically 3-4 nm thick, but this value varies with chain length and chemistry. [4] [10] Core thickness also varies significantly with temperature, in particular near a phase transition. [11]

Asymmetry Edit

In many naturally occurring bilayers, the compositions of the inner and outer membrane leaflets are different. In human red blood cells, the inner (cytoplasmic) leaflet is composed mostly of phosphatidylethanolamine, phosphatidylserine and phosphatidylinositol and its phosphorylated derivatives. By contrast, the outer (extracellular) leaflet is based on phosphatidylcholine, sphingomyelin and a variety of glycolipids. [12] [13] In some cases, this asymmetry is based on where the lipids are made in the cell and reflects their initial orientation. [14] The biological functions of lipid asymmetry are imperfectly understood, although it is clear that it is used in several different situations. For example, when a cell undergoes apoptosis, the phosphatidylserine — normally localised to the cytoplasmic leaflet — is transferred to the outer surface: There, it is recognised by a macrophage that then actively scavenges the dying cell.

Lipid asymmetry arises, at least in part, from the fact that most phospholipids are synthesised and initially inserted into the inner monolayer: those that constitute the outer monolayer are then transported from the inner monolayer by a class of enzymes called flippases. [15] [16] Other lipids, such as sphingomyelin, appear to be synthesised at the external leaflet. Flippases are members of a larger family of lipid transport molecules that also includes floppases, which transfer lipids in the opposite direction, and scramblases, which randomize lipid distribution across lipid bilayers (as in apoptotic cells). In any case, once lipid asymmetry is established, it does not normally dissipate quickly because spontaneous flip-flop of lipids between leaflets is extremely slow. [17]

It is possible to mimic this asymmetry in the laboratory in model bilayer systems. Certain types of very small artificial vesicle will automatically make themselves slightly asymmetric, although the mechanism by which this asymmetry is generated is very different from that in cells. [18] By utilizing two different monolayers in Langmuir-Blodgett deposition [19] or a combination of Langmuir-Blodgett and vesicle rupture deposition [20] it is also possible to synthesize an asymmetric planar bilayer. This asymmetry may be lost over time as lipids in supported bilayers can be prone to flip-flop. [21]

Phases and phase transitions Edit

At a given temperature a lipid bilayer can exist in either a liquid or a gel (solid) phase. All lipids have a characteristic temperature at which they transition (melt) from the gel to liquid phase. In both phases the lipid molecules are prevented from flip-flopping across the bilayer, but in liquid phase bilayers a given lipid will exchange locations with its neighbor millions of times a second. This random walk exchange allows lipid to diffuse and thus wander across the surface of the membrane. [22] Unlike liquid phase bilayers, the lipids in a gel phase bilayer have less mobility.

The phase behavior of lipid bilayers is determined largely by the strength of the attractive Van der Waals interactions between adjacent lipid molecules. Longer-tailed lipids have more area over which to interact, increasing the strength of this interaction and, as a consequence, decreasing the lipid mobility. Thus, at a given temperature, a short-tailed lipid will be more fluid than an otherwise identical long-tailed lipid. [10] Transition temperature can also be affected by the degree of unsaturation of the lipid tails. An unsaturated double bond can produce a kink in the alkane chain, disrupting the lipid packing. This disruption creates extra free space within the bilayer that allows additional flexibility in the adjacent chains. [10] An example of this effect can be noted in everyday life as butter, which has a large percentage saturated fats, is solid at room temperature while vegetable oil, which is mostly unsaturated, is liquid.

Most natural membranes are a complex mixture of different lipid molecules. If some of the components are liquid at a given temperature while others are in the gel phase, the two phases can coexist in spatially separated regions, rather like an iceberg floating in the ocean. This phase separation plays a critical role in biochemical phenomena because membrane components such as proteins can partition into one or the other phase [23] and thus be locally concentrated or activated. One particularly important component of many mixed phase systems is cholesterol, which modulates bilayer permeability, mechanical strength, and biochemical interactions.

Surface chemistry Edit

While lipid tails primarily modulate bilayer phase behavior, it is the headgroup that determines the bilayer surface chemistry. Most natural bilayers are composed primarily of phospholipids, but sphingolipids and sterols such as cholesterol are also important components. [24] Of the phospholipids, the most common headgroup is phosphatidylcholine (PC), accounting for about half the phospholipids in most mammalian cells. [25] PC is a zwitterionic headgroup, as it has a negative charge on the phosphate group and a positive charge on the amine but, because these local charges balance, no net charge.

Other headgroups are also present to varying degrees and can include phosphatidylserine (PS) phosphatidylethanolamine (PE) and phosphatidylglycerol (PG). These alternate headgroups often confer specific biological functionality that is highly context-dependent. For instance, PS presence on the extracellular membrane face of erythrocytes is a marker of cell apoptosis, [26] whereas PS in growth plate vesicles is necessary for the nucleation of hydroxyapatite crystals and subsequent bone mineralization. [27] [28] Unlike PC, some of the other headgroups carry a net charge, which can alter the electrostatic interactions of small molecules with the bilayer. [29]

Containment and separation Edit

The primary role of the lipid bilayer in biology is to separate aqueous compartments from their surroundings. Without some form of barrier delineating “self” from “non-self”, it is difficult to even define the concept of an organism or of life. This barrier takes the form of a lipid bilayer in all known life forms except for a few species of archaea that utilize a specially adapted lipid monolayer. [7] It has even been proposed that the very first form of life may have been a simple lipid vesicle with virtually its sole biosynthetic capability being the production of more phospholipids. [30] The partitioning ability of the lipid bilayer is based on the fact that hydrophilic molecules cannot easily cross the hydrophobic bilayer core, as discussed in Transport across the bilayer below. The nucleus, mitochondria and chloroplasts have two lipid bilayers, while other sub-cellular structures are surrounded by a single lipid bilayer (such as the plasma membrane, endoplasmic reticula, Golgi apparatus and lysosomes). See Organelle. [31]

Prokaryotes have only one lipid bilayer - the cell membrane (also known as the plasma membrane). Many prokaryotes also have a cell wall, but the cell wall is composed of proteins or long chain carbohydrates, not lipids. In contrast, eukaryotes have a range of organelles including the nucleus, mitochondria, lysosomes and endoplasmic reticulum. All of these sub-cellular compartments are surrounded by one or more lipid bilayers and, together, typically comprise the majority of the bilayer area present in the cell. In liver hepatocytes for example, the plasma membrane accounts for only two percent of the total bilayer area of the cell, whereas the endoplasmic reticulum contains more than fifty percent and the mitochondria a further thirty percent. [32]

Signaling Edit

Probably the most familiar form of cellular signaling is synaptic transmission, whereby a nerve impulse that has reached the end of one neuron is conveyed to an adjacent neuron via the release of neurotransmitters. This transmission is made possible by the action of synaptic vesicles loaded with the neurotransmitters to be released. These vesicles fuse with the cell membrane at the pre-synaptic terminal and release its contents to the exterior of the cell. The contents then diffuse across the synapse to the post-synaptic terminal.

Lipid bilayers are also involved in signal transduction through their role as the home of integral membrane proteins. This is an extremely broad and important class of biomolecule. It is estimated that up to a third of the human proteome are membrane proteins. [33] Some of these proteins are linked to the exterior of the cell membrane. An example of this is the CD59 protein, which identifies cells as “self” and thus inhibits their destruction by the immune system. The HIV virus evades the immune system in part by grafting these proteins from the host membrane onto its own surface. [32] Alternatively, some membrane proteins penetrate all the way through the bilayer and serve to relay individual signal events from the outside to the inside of the cell. The most common class of this type of protein is the G protein-coupled receptor (GPCR). GPCRs are responsible for much of the cell's ability to sense its surroundings and, because of this important role, approximately 40% of all modern drugs are targeted at GPCRs. [34]

In addition to protein- and solution-mediated processes, it is also possible for lipid bilayers to participate directly in signaling. A classic example of this is phosphatidylserine-triggered phagocytosis. Normally, phosphatidylserine is asymmetrically distributed in the cell membrane and is present only on the interior side. During programmed cell death a protein called a scramblase equilibrates this distribution, displaying phosphatidylserine on the extracellular bilayer face. The presence of phosphatidylserine then triggers phagocytosis to remove the dead or dying cell.

The lipid bilayer is a very difficult structure to study because it is so thin and fragile. In spite of these limitations dozens of techniques have been developed over the last seventy years to allow investigations of its structure and function.

Electrical measurements Edit

Electrical measurements are a straightforward way to characterize an important function of a bilayer: its ability to segregate and prevent the flow of ions in solution. By applying a voltage across the bilayer and measuring the resulting current, the resistance of the bilayer is determined. This resistance is typically quite high (10 8 Ohm-cm 2 or more) [35] since the hydrophobic core is impermeable to charged species. The presence of even a few nanometer-scale holes results in a dramatic increase in current. [36] The sensitivity of this system is such that even the activity of single ion channels can be resolved. [37]

Fluorescence microscopy Edit

Electrical measurements do not provide an actual picture like imaging with a microscope can. Lipid bilayers cannot be seen in a traditional microscope because they are too thin. In order to see bilayers, researchers often use fluorescence microscopy. A sample is excited with one wavelength of light and observed in a different wavelength, so that only fluorescent molecules with a matching excitation and emission profile will be seen. Natural lipid bilayers are not fluorescent, so a dye is used that attaches to the desired molecules in the bilayer. Resolution is usually limited to a few hundred nanometers, much smaller than a typical cell but much larger than the thickness of a lipid bilayer.

Electron microscopy Edit

Electron microscopy offers a higher resolution image. In an electron microscope, a beam of focused electrons interacts with the sample rather than a beam of light as in traditional microscopy. In conjunction with rapid freezing techniques, electron microscopy has also been used to study the mechanisms of inter- and intracellular transport, for instance in demonstrating that exocytotic vesicles are the means of chemical release at synapses. [38]

Nuclear magnetic resonance spectroscopy Edit

31 P-NMR(nuclear magnetic resonance) spectroscopy is widely used for studies of phospholipid bilayers and biological membranes in native conditions. The analysis [39] of 31 P-NMR spectra of lipids could provide a wide range of information about lipid bilayer packing, phase transitions (gel phase, physiological liquid crystal phase, ripple phases, non bilayer phases), lipid head group orientation/dynamics, and elastic properties of pure lipid bilayer and as a result of binding of proteins and other biomolecules.

Atomic force microscopy Edit

A new method to study lipid bilayers is Atomic force microscopy (AFM). Rather than using a beam of light or particles, a very small sharpened tip scans the surface by making physical contact with the bilayer and moving across it, like a record player needle. AFM is a promising technique because it has the potential to image with nanometer resolution at room temperature and even under water or physiological buffer, conditions necessary for natural bilayer behavior. Utilizing this capability, AFM has been used to examine dynamic bilayer behavior including the formation of transmembrane pores (holes) [40] and phase transitions in supported bilayers. [41] Another advantage is that AFM does not require fluorescent or isotopic labeling of the lipids, since the probe tip interacts mechanically with the bilayer surface. Because of this, the same scan can image both lipids and associated proteins, sometimes even with single-molecule resolution. [40] [42] AFM can also probe the mechanical nature of lipid bilayers. [43]

Dual polarisation interferometry Edit

Lipid bilayers exhibit high levels of birefringence where the refractive index in the plane of the bilayer differs from that perpendicular by as much as 0.1 refractive index units. This has been used to characterise the degree of order and disruption in bilayers using dual polarisation interferometry to understand mechanisms of protein interaction.

Quantum chemical calculations Edit

Lipid bilayers are complicated molecular systems with many degrees of freedom. Thus, atomistic simulation of membrane and in particular ab initio calculations of its properties is difficult and computationally expensive. Quantum chemical calculations has recently been successfully performed to estimate dipole and quadrupole moments of lipid membranes. [44]

Passive diffusion Edit

Most polar molecules have low solubility in the hydrocarbon core of a lipid bilayer and, as a consequence, have low permeability coefficients across the bilayer. This effect is particularly pronounced for charged species, which have even lower permeability coefficients than neutral polar molecules. [45] Anions typically have a higher rate of diffusion through bilayers than cations. [46] [47] Compared to ions, water molecules actually have a relatively large permeability through the bilayer, as evidenced by osmotic swelling. When a cell or vesicle with a high interior salt concentration is placed in a solution with a low salt concentration it will swell and eventually burst. Such a result would not be observed unless water was able to pass through the bilayer with relative ease. The anomalously large permeability of water through bilayers is still not completely understood and continues to be the subject of active debate. [48] Small uncharged apolar molecules diffuse through lipid bilayers many orders of magnitude faster than ions or water. This applies both to fats and organic solvents like chloroform and ether. Regardless of their polar character larger molecules diffuse more slowly across lipid bilayers than small molecules. [49]

Ion pumps and channels Edit

Two special classes of protein deal with the ionic gradients found across cellular and sub-cellular membranes in nature- ion channels and ion pumps. Both pumps and channels are integral membrane proteins that pass through the bilayer, but their roles are quite different. Ion pumps are the proteins that build and maintain the chemical gradients by utilizing an external energy source to move ions against the concentration gradient to an area of higher chemical potential. The energy source can be ATP, as is the case for the Na + -K + ATPase. Alternatively, the energy source can be another chemical gradient already in place, as in the Ca 2+ /Na + antiporter. It is through the action of ion pumps that cells are able to regulate pH via the pumping of protons.

In contrast to ion pumps, ion channels do not build chemical gradients but rather dissipate them in order to perform work or send a signal. Probably the most familiar and best studied example is the voltage-gated Na + channel, which allows conduction of an action potential along neurons. All ion pumps have some sort of trigger or “gating” mechanism. In the previous example it was electrical bias, but other channels can be activated by binding a molecular agonist or through a conformational change in another nearby protein. [50]

Endocytosis and exocytosis Edit

Some molecules or particles are too large or too hydrophilic to pass through a lipid bilayer. Other molecules could pass through the bilayer but must be transported rapidly in such large numbers that channel-type transport is impractical. In both cases, these types of cargo can be moved across the cell membrane through fusion or budding of vesicles. When a vesicle is produced inside the cell and fuses with the plasma membrane to release its contents into the extracellular space, this process is known as exocytosis. In the reverse process, a region of the cell membrane will dimple inwards and eventually pinch off, enclosing a portion of the extracellular fluid to transport it into the cell. Endocytosis and exocytosis rely on very different molecular machinery to function, but the two processes are intimately linked and could not work without each other. The primary mechanism of this interdependence is the large amount of lipid material involved. [51] In a typical cell, an area of bilayer equivalent to the entire plasma membrane will travel through the endocytosis/exocytosis cycle in about half an hour. [52] If these two processes were not balancing each other, the cell would either balloon outward to an unmanageable size or completely deplete its plasma membrane within a short time.

Exocytosis in prokaryotes: Membrane vesicular exocytosis, popularly known as membrane vesicle trafficking, a Nobel prize-winning (year, 2013) process, is traditionally regarded as a prerogative of eukaryotic cells. [53] This myth was however broken with the revelation that nanovesicles, popularly known as bacterial outer membrane vesicles, released by gram-negative microbes, translocate bacterial signal molecules to host or target cells [54] to carry out multiple processes in favour of the secreting microbe e.g., in host cell invasion [55] and microbe-environment interactions, in general. [56]

Electroporation Edit

Electroporation is the rapid increase in bilayer permeability induced by the application of a large artificial electric field across the membrane. Experimentally, electroporation is used to introduce hydrophilic molecules into cells. It is a particularly useful technique for large highly charged molecules such as DNA, which would never passively diffuse across the hydrophobic bilayer core. [57] Because of this, electroporation is one of the key methods of transfection as well as bacterial transformation. It has even been proposed that electroporation resulting from lightning strikes could be a mechanism of natural horizontal gene transfer. [58]

This increase in permeability primarily affects transport of ions and other hydrated species, indicating that the mechanism is the creation of nm-scale water-filled holes in the membrane. Although electroporation and dielectric breakdown both result from application of an electric field, the mechanisms involved are fundamentally different. In dielectric breakdown the barrier material is ionized, creating a conductive pathway. The material alteration is thus chemical in nature. In contrast, during electroporation the lipid molecules are not chemically altered but simply shift position, opening up a pore that acts as the conductive pathway through the bilayer as it is filled with water.

Lipid bilayers are large enough structures to have some of the mechanical properties of liquids or solids. The area compression modulus Ka, bending modulus Kb, and edge energy Λ , can be used to describe them. Solid lipid bilayers also have a shear modulus, but like any liquid, the shear modulus is zero for fluid bilayers. These mechanical properties affect how the membrane functions. Ka and Kb affect the ability of proteins and small molecules to insert into the bilayer, [59] [60] and bilayer mechanical properties have been shown to alter the function of mechanically activated ion channels. [61] Bilayer mechanical properties also govern what types of stress a cell can withstand without tearing. Although lipid bilayers can easily bend, most cannot stretch more than a few percent before rupturing. [62]

As discussed in the Structure and organization section, the hydrophobic attraction of lipid tails in water is the primary force holding lipid bilayers together. Thus, the elastic modulus of the bilayer is primarily determined by how much extra area is exposed to water when the lipid molecules are stretched apart. [63] It is not surprising given this understanding of the forces involved that studies have shown that Ka varies strongly with osmotic pressure [64] but only weakly with tail length and unsaturation. [10] Because the forces involved are so small, it is difficult to experimentally determine Ka. Most techniques require sophisticated microscopy and very sensitive measurement equipment. [43] [65]

In contrast to Ka, which is a measure of how much energy is needed to stretch the bilayer, Kb is a measure of how much energy is needed to bend or flex the bilayer. Formally, bending modulus is defined as the energy required to deform a membrane from its intrinsic curvature to some other curvature. Intrinsic curvature is defined by the ratio of the diameter of the head group to that of the tail group. For two-tailed PC lipids, this ratio is nearly one so the intrinsic curvature is nearly zero. If a particular lipid has too large a deviation from zero intrinsic curvature it will not form a bilayer and will instead form other phases such as micelles or inverted micelles. Addition of small hydrophilic molecules like sucrose into mixed lipid lamellar liposomes made from galactolipid-rich thylakoid membranes destabilises bilayers into micellar phase. [66] Typically, Kb is not measured experimentally but rather is calculated from measurements of Ka and bilayer thickness, since the three parameters are related.

Fusion is the process by which two lipid bilayers merge, resulting in one connected structure. If this fusion proceeds completely through both leaflets of both bilayers, a water-filled bridge is formed and the solutions contained by the bilayers can mix. Alternatively, if only one leaflet from each bilayer is involved in the fusion process, the bilayers are said to be hemifused. Fusion is involved in many cellular processes, in particular in eukaryotes, since the eukaryotic cell is extensively sub-divided by lipid bilayer membranes. Exocytosis, fertilization of an egg by sperm activation, and transport of waste products to the lysozome are a few of the many eukaryotic processes that rely on some form of fusion. Even the entry of pathogens can be governed by fusion, as many bilayer-coated viruses have dedicated fusion proteins to gain entry into the host cell.

There are four fundamental steps in the fusion process. [25] First, the involved membranes must aggregate, approaching each other to within several nanometers. Second, the two bilayers must come into very close contact (within a few angstroms). To achieve this close contact, the two surfaces must become at least partially dehydrated, as the bound surface water normally present causes bilayers to strongly repel. The presence of ions, in particular divalent cations like magnesium and calcium, strongly affects this step. [69] [70] One of the critical roles of calcium in the body is regulating membrane fusion. Third, a destabilization must form at one point between the two bilayers, locally distorting their structures. The exact nature of this distortion is not known. One theory is that a highly curved "stalk" must form between the two bilayers. [71] Proponents of this theory believe that it explains why phosphatidylethanolamine, a highly curved lipid, promotes fusion. [72] Finally, in the last step of fusion, this point defect grows and the components of the two bilayers mix and diffuse away from the site of contact.

The situation is further complicated when considering fusion in vivo since biological fusion is almost always regulated by the action of membrane-associated proteins. The first of these proteins to be studied were the viral fusion proteins, which allow an enveloped virus to insert its genetic material into the host cell (enveloped viruses are those surrounded by a lipid bilayer some others have only a protein coat). Eukaryotic cells also use fusion proteins, the best-studied of which are the SNAREs. SNARE proteins are used to direct all vesicular intracellular trafficking. Despite years of study, much is still unknown about the function of this protein class. In fact, there is still an active debate regarding whether SNAREs are linked to early docking or participate later in the fusion process by facilitating hemifusion. [74]

In studies of molecular and cellular biology it is often desirable to artificially induce fusion. The addition of polyethylene glycol (PEG) causes fusion without significant aggregation or biochemical disruption. This procedure is now used extensively, for example by fusing B-cells with myeloma cells. [75] The resulting “hybridoma” from this combination expresses a desired antibody as determined by the B-cell involved, but is immortalized due to the melanoma component. Fusion can also be artificially induced through electroporation in a process known as electrofusion. It is believed that this phenomenon results from the energetically active edges formed during electroporation, which can act as the local defect point to nucleate stalk growth between two bilayers. [76]

Lipid bilayers can be created artificially in the lab to allow researchers to perform experiments that cannot be done with natural bilayers. They can also be used in the field of Synthetic Biology, to define the boundaries of artificial cells. These synthetic systems are called model lipid bilayers. There are many different types of model bilayers, each having experimental advantages and disadvantages. They can be made with either synthetic or natural lipids. Among the most common model systems are:

To date, the most successful commercial application of lipid bilayers has been the use of liposomes for drug delivery, especially for cancer treatment. (Note- the term “liposome” is in essence synonymous with “vesicle” except that vesicle is a general term for the structure whereas liposome refers to only artificial not natural vesicles) The basic idea of liposomal drug delivery is that the drug is encapsulated in solution inside the liposome then injected into the patient. These drug-loaded liposomes travel through the system until they bind at the target site and rupture, releasing the drug. In theory, liposomes should make an ideal drug delivery system since they can isolate nearly any hydrophilic drug, can be grafted with molecules to target specific tissues and can be relatively non-toxic since the body possesses biochemical pathways for degrading lipids. [77]

The first generation of drug delivery liposomes had a simple lipid composition and suffered from several limitations. Circulation in the bloodstream was extremely limited due to both renal clearing and phagocytosis. Refinement of the lipid composition to tune fluidity, surface charge density, and surface hydration resulted in vesicles that adsorb fewer proteins from serum and thus are less readily recognized by the immune system. [78] The most significant advance in this area was the grafting of polyethylene glycol (PEG) onto the liposome surface to produce “stealth” vesicles, which circulate over long times without immune or renal clearing. [79]

The first stealth liposomes were passively targeted at tumor tissues. Because tumors induce rapid and uncontrolled angiogenesis they are especially “leaky” and allow liposomes to exit the bloodstream at a much higher rate than normal tissue would. [80] More recently [ when? ] work has been undertaken to graft antibodies or other molecular markers onto the liposome surface in the hope of actively binding them to a specific cell or tissue type. [81] Some examples of this approach are already in clinical trials. [82]

Another potential application of lipid bilayers is the field of biosensors. Since the lipid bilayer is the barrier between the interior and exterior of the cell, it is also the site of extensive signal transduction. Researchers over the years have tried to harness this potential to develop a bilayer-based device for clinical diagnosis or bioterrorism detection. Progress has been slow in this area and, although a few companies have developed automated lipid-based detection systems, they are still targeted at the research community. These include Biacore (now GE Healthcare Life Sciences), which offers a disposable chip for utilizing lipid bilayers in studies of binding kinetics [83] and Nanion Inc., which has developed an automated patch clamping system. [84] Other, more exotic applications are also being pursued such as the use of lipid bilayer membrane pores for DNA sequencing by Oxford Nanolabs. To date, this technology has not proven commercially viable.

A supported lipid bilayer (SLB) as described above has achieved commercial success as a screening technique to measure the permeability of drugs. This parallel artificial membrane permeability assay PAMPA technique measures the permeability across specifically formulated lipid cocktail(s) found to be highly correlated with Caco-2 cultures, [85] [86] the gastrointestinal tract, [87] blood–brain barrier [88] and skin. [89]

By the early twentieth century scientists had come to believe that cells are surrounded by a thin oil-like barrier, [90] but the structural nature of this membrane was not known. Two experiments in 1925 laid the groundwork to fill in this gap. By measuring the capacitance of erythrocyte solutions, Hugo Fricke determined that the cell membrane was 3.3 nm thick. [91]

Although the results of this experiment were accurate, Fricke misinterpreted the data to mean that the cell membrane is a single molecular layer. Prof. Dr. Evert Gorter [92] (1881–1954) and F. Grendel of Leiden University approached the problem from a different perspective, spreading the erythrocyte lipids as a monolayer on a Langmuir-Blodgett trough. When they compared the area of the monolayer to the surface area of the cells, they found a ratio of two to one. [93] Later analyses showed several errors and incorrect assumptions with this experiment but, serendipitously, these errors canceled out and from this flawed data Gorter and Grendel drew the correct conclusion- that the cell membrane is a lipid bilayer. [25]

This theory was confirmed through the use of electron microscopy in the late 1950s. Although he did not publish the first electron microscopy study of lipid bilayers [94] J. David Robertson was the first to assert that the two dark electron-dense bands were the headgroups and associated proteins of two apposed lipid monolayers. [95] [96] In this body of work, Robertson put forward the concept of the “unit membrane.” This was the first time the bilayer structure had been universally assigned to all cell membranes as well as organelle membranes.

Around the same time, the development of model membranes confirmed that the lipid bilayer is a stable structure that can exist independent of proteins. By “painting” a solution of lipid in organic solvent across an aperture, Mueller and Rudin were able to create an artificial bilayer and determine that this exhibited lateral fluidity, high electrical resistance and self-healing in response to puncture, [97] all of which are properties of a natural cell membrane. A few years later, Alec Bangham showed that bilayers, in the form of lipid vesicles, could also be formed simply by exposing a dried lipid sample to water. [98] This was an important advance, since it demonstrated that lipid bilayers form spontaneously via self assembly and do not require a patterned support structure.

In 1977, a totally synthetic bilayer membrane was prepared by Kunitake and Okahata, from a single organic compound, didodecyldimethylammonium bromide. [99] It clearly shows that the bilayer membrane was assembled by the van der Waals interaction.


Informational injections

Another key to making a cell is getting the software right. Enabling a synthetic cell to follow scientists’ instructions and to replicate itself will require some way of storing and retrieving information. For living systems, this is done by genes — from hundreds for some microbes, to tens of thousands for humans.

How many genes a synthetic cell will need to run itself is a matter of healthy debate. Schwille and others would like to keep it in the neighbourhood of a few dozen. Others, such as Adamala, think that synthetic cells need 200–300 genes.

Some have chosen to start with something living. Synthetic biologist John Glass and his colleagues at the J. Craig Venter Institute (JCVI) in La Jolla, California, took one of the smallest-known microbial genomes on the planet, that of the bacterium Mycoplasma mycoides, and systematically disrupted its genes to identify the essential ones. Once they had that information, they chemically stitched together a minimal genome in the laboratory.

This synthesized genome contained 473 genes — about half of what was in the original organism — and it was transplanted into a related bacterial species, Mycoplasma capricolum 9 . In 2016, the team showed that this minimal synthetic genome could ‘boot up’ a free-living, although slow-growing organism 10 . Glass thinks that it will be hard to decrease that number much more: take any gene away, and it either kills the cells or slows their growth to near zero, he says.

He and his JCVI colleagues are compiling a list of ‘cellular tasks’ based on the latest version of their creation, JCVI-syn3.0a, which could act as a blueprint of a cell’s minimal to-do list. But for about 100 of these genes, they can’t identify what they do that makes them essential.

As a next step, and supported by an NSF grant of nearly $1 million, Glass and Adamala will attempt to install the JCVI-syn3.0a genome into a synthetic liposome containing the machinery needed to convert DNA into protein, to see whether it can survive. In that case, both the software and the hardware of the cell would be synthetic from the start.

If it could grow and divide, that would be a tremendous step. But many argue that to truly represent a living system, it would also have to evolve and adapt to its environment. This is the goal with the most unpredictable results and also the biggest challenges, says Schwille. “A thing that just makes itself all the time is not life — although I would be happy with that!” she says. “For a cell to be living, it needs to develop new functionality.”

Glass’s team at the JCVI has been doing adaptive laboratory evolution experiments with JCVI-syn3.0a, selecting for organisms that grow faster in a nutrient-rich broth. So far, after about 400 divisions, he and his team have obtained cells that grow about 15% faster than the original organism. And they have seen a handful of gene-sequence changes popping up. But there’s no evidence yet of the microbe developing new cellular functions or increasing its fitness by leaps and bounds.

Erb says that working out how to add evolution to synthetic cells is the only way to make them interesting. That little bit of messiness in biological systems is what allows them to improve their performance. “As engineers, we can’t build a perfect synthetic cell. We have to build a self-correcting system that becomes better as it goes,” he says.

Synthetic cells could lead to insights about how life might look on other planets. And synthetic bioreactors under a researcher’s complete control might offer new solutions to treating cancer, tackling antibiotic resistance or cleaning up toxic sites. Releasing such an organism into the human body or the environment would be risky, but a top-down engineered organism with unknown and unpredictable behaviours might be even riskier.

Dogterom says that synthetic living cells also bring other philosophical and ethical questions: “Will this be a life? Will it be autonomous? Will we control it?” These conversations should take place between scientists and the public, she says. As for concerns that synthetic cells will run amok, Dogterom is less worried. “I’m convinced our first synthetic cell will be a lousy mimic of what already exists.” And as the engineers of synthetic life, she and her colleagues can easily incorporate controls or a kill switch that renders the cells harmless.

She and other synthetic biologists will keep pushing ahead exploring the frontiers of life. “The timing is right,” says Dogterom. “We have the genomes, the parts list. The minimal cell needs only a few hundred genes to have something that looks sort of alive. Hundreds of parts is a tremendous challenge, but it’s not thousands — that’s very exciting.”

Nature 563, 172-175 (2018)


Abstract

Plant lipid transfer proteins (LTPs) are soluble proteins which are characterized by their in vitro ability to transfer phospholipids between two membranes. We have compared the functional properties of two LTPs purified from maize and wheat seeds knowing that, despite a high degree of sequence identity, the two proteins exhibit structural differences. It was found that wheat LTP had a lower transfer activity than the maize LTP, consistent with a lower kinetics of fatty acid binding. The lower affinity for the fatty acids of the wheat LTP could be explained by a narrowing occurring in the middle part of the binding site, as revealed by comparing the fluorescence spectra of various anthroyloxy-labeled fatty acids associated with the two LTPs. The affinity for some natural fatty acids was studied by competition with fluorescent fatty acids toward binding to the protein. Again, wheat LTP had a lower affinity for those molecules. All together, these observations reveal the complexity of the LTP family in plants, probably reflecting the multiple roles played by these proteins.

Work supported by grants from the Ministère de l'Education Nationale, de la Recherche et de la Technologie, the Centre National de la Recherche Scientifique, and the Université Pierre et Marie Curie (UMR 7632).

Corresponding author. Telephone: +33 (0)1 44 27 72 59. Fax: +33 (0)1 44 27 61 51. E-mail: [email protected]


6 Conclusions

When exposed to highly osmolar media, S. cerevisiae cells respond by rapid intracellular accumulation of glycerol to counteract their dehydration. Moreover, a finely tuned regulation of intracellular glycerol content seems to be present. It has been shown that the accumulation of glycerol is mainly the result of an enhanced rate of production and increased retention of glycerol by the plasma membrane. Enhanced uptake of glycerol from the medium during osmotic stress has not yet been demonstrated in S. cerevisiae. Most attention has been paid to the markedly enhanced production rate of glycerol during osmotic stress and its molecular basis. In fact, the key enzyme of glycerol formation, the cytosolic NAD dependent glycerol 3-phosphate dehydrogenase (ctGPD), is strongly induced at the transcriptional level during osmotic stress via a specific osmoresponsive signal transduction pathway (HOG pathway). Furthermore, there is evidence that an increase simply in the ctGPD level can provoke a marked increase in the production of glycerol, at least when cells are grown on glucose as a carbon source. Besides the induction of ctGPD, further activity alterations of enzymes involved in glycerol metabolism and carbon catabolism have been reported, which were, however, less dramatic. These may reflect both a general adjustment of yeast metabolism to increased glycerol production and a fine regulation of intracellular glycerol content during growth under hyperosmotic stress.


Watch the video: Lipid Metabolism Overview, Animation (July 2022).


Comments:

  1. Bodwyn

    I suggest seeing a site that has a lot of information on this topic.

  2. Aldrich

    Many thanks for the help in this matter.

  3. Nik

    Absolutely with you it agree. In it something is and it is excellent idea. I support you.

  4. Sinai

    In my opinion you are mistaken. I can prove it. Write to me in PM.



Write a message