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5.3: Bacterial Cell Walls - Biology

5.3: Bacterial Cell Walls - Biology



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Function of bacterial cell walls= prevent osmotic lysis

  • What is osmotic lysis?
  • osmosis: the diffusion of water from an area of high water concentration to an area of low water concentration across a semi-permeable membrane
    • -Because solutes (ions, molecules) take up space, there is an inverse relationship between solute concentration and water concentration:
      • -If high solute concentration=low water concentration
      • -If low solute concentration=high water concentration

-Because we are interested in what happens to cells, we need to describe the environment in which the cell is living

Hypoosmotic /hypotonic environments: solute concentration of environment is less than within cell

Hyperosmotic /hypertonic environments: solute concentration of environment is higher than within cell

Isoosmotic /Isotonic environments: solute concentration of environment is equal to that within cell

-If a compartment (for example a cell) has a higher solute concentration than its environment (outside of cell), the water concentration inside the cell will be less than outside the cell. Consequently there will be net movement/osmosis of water into the cell from the environment. As water moves in, pressure builds up inside the cell and eventually the cytoplasmic membrane will break in a process called osmotic lysis (similar to explosion of a water balloon). Osmotic lysis always kills a cell.

Figure 1 : a.Cell in Hypoosmotic environment undergoing osmotic lysis. b. Bacterial cell wall peptidoglycan prevents osmotic lysis

Bacteria cell wall peptidoglycan prevents osmotic lysis: Most bacteria grow in hypo-osmotic environments. How do bacteria prevent osmotic lysis? Most bacteria synthesize a strong cell wall made of cross-linked peptidoglycan. The cell wall is outside the cytoplasmic membrane similar to a “boiler plate” or suit of armor. The cell wall peptidoglycan is similar to cross-linked wire. The peptidoglycan of the cell wall prevents osmotic lysis when water moves into the cell, but ONLY if the cell wall peptidoglycan is cross-linked. Anything which prevents the cross links from forming or which cuts the cross-links will weaken the peptidoglycan so that it no longer can prevent osmotic lysis.

Peptidoglycan

Peptidoglycan is made up of long chains of modified sugars, alternating N-acetylglucosamine (‘NAG”) and N-acetylmuramic acid (“NAM”). Strong covalent bonds link the sugars together in chains (polymers). The Nam subunits have short “tails” made of 4 amino acids. Special bacterial enzymes link these “peptide tails” to corss-link the peptidoglycan. Bacterial enzymes which help crosslink peptidoglycan are called “bacterial transpeptidases”. See diagram passed out in lecture

Peptidoglycan: NAM= N-acetylglucosamine NAM= N-acetlymuramic acid aa=amino acid

NAM---NAG-----NAM------NAG------NAM------NAG-----NAM

aa aa aa aa

aa aa aa aa

aa aa /-> aa aa

aa aa /-> aa -> aa/ aa aa aa aa

-> aa/ aa aa aa

aa aa aa aa

aa aa aa aa

NAM---NAG-----NAM------NAG------NAM------NAG-----NAM

  • /-> Indicates bonds formed by bacterial transpeptidase; these bonds are not formed in the presence of beta-lactam antibiotics

*Key Idea: Beta-lactam antibiotics (e.g. penicillin,ampicillin, amoxicillin, oxacillin methicillin) bind to and inhibit bacterial transpeptidase. Consequently peptidoglycan is not crosslinked, is weakened and cannot prevent osmotic lysis and the bacterium dies

Humans infected with bacterial pathogens can thus take beta-lactam antibiotics such as penicillin or ampicillin, triggering lysis and death of the bacteria without harming the human cells.

Vancomycin is another powerful antibiotic which also inhibits cross-linking of bacterial peptidoglycan causing osmotic lysis in growing bacteria


The bacterial cell envelope

The bacteria cell envelope is a complex multilayered structure that serves to protect these organisms from their unpredictable and often hostile environment. The cell envelopes of most bacteria fall into one of two major groups. Gram-negative bacteria are surrounded by a thin peptidoglycan cell wall, which itself is surrounded by an outer membrane containing lipopolysaccharide. Gram-positive bacteria lack an outer membrane but are surrounded by layers of peptidoglycan many times thicker than is found in the gram-negatives. Threading through these layers of peptidoglycan are long anionic polymers, called teichoic acids. The composition and organization of these envelope layers and recent insights into the mechanisms of cell envelope assembly are discussed.

Figures

Transenvelope machines in the Gram-negative…

Transenvelope machines in the Gram-negative cell envelope. The AcrA/B proteins together with TolC…

The cellular machineries required for…

The cellular machineries required for OM biogenesis. The Lpt pathway, together with MsbA,…

Depiction of Gram-positive and Gram-negative…

Depiction of Gram-positive and Gram-negative cell envelopes: CAP = covalently attached protein IMP,…


Visual Connection

Which of the following statements is true?

  1. Gram-positive bacteria have a single cell wall anchored to the cell membrane by lipoteichoic acid.
  2. Porins allow entry of substances into both Gram-positive and Gram-negative bacteria.
  3. The cell wall of Gram-negative bacteria is thick, and the cell wall of Gram-positive bacteria is thin.
  4. Gram-negative bacteria have a cell wall made of peptidoglycan, whereas Gram-positive bacteria have a cell wall made of lipoteichoic acid.

Archaean cell walls do not have peptidoglycan. There are four different types of archaean cell walls. One type is composed of pseudopeptidoglycan , which is similar to peptidoglycan in morphology but contains different sugars in the polysaccharide chain. The other three types of cell walls are composed of polysaccharides, glycoproteins, or pure protein. Other differences between Bacteria and Archaea are seen in (Figure). Note that features related to DNA replication, transcription and translation in Archaea are similar to those seen in eukaryotes.

Differences and Similarities between Bacteria and Archaea
Structural Characteristic Bacteria Archaea
Cell type Prokaryotic Prokaryotic
Cell morphology Variable Variable
Cell wall Contains peptidoglycan Does not contain peptidoglycan
Cell membrane type Lipid bilayer Lipid bilayer or lipid monolayer
Plasma membrane lipids Fatty acids-glycerol ester Phytanyl-glycerol ethers
Chromosome Typically circular Typically circular
Replication origins Single Multiple
RNA polymerase Single Multiple
Initiator tRNA Formyl-methionine Methionine
Streptomycin inhibition Sensitive Resistant
Calvin cycle Yes No


5.3 Reproduction and Meiosis

Cell division is how organisms grow and repair themselves. It is also how they produce offspring. Many single-celled organisms reproduce by binary fission. The parent cell simply divides to form two daughter cells that are identical to the parent. In many other organisms, two parents are involved, and the offspring are not identical to the parents. In fact, each offspring is unique. Look at the family in Figure below. The children resemble their parents, but they are not identical to them. Instead, each has a unique combination of characteristics inherited from both parents. In this lesson, you will learn how this happens.

Reproduction: Asexual vs. Sexual

Reproduction is the process by which organisms give rise to offspring. It is one of the defining characteristics of living things. There are two basic types of reproduction: asexual reproduction and sexual reproduction.

Asexual Reproduction

Asexual reproduction involves a single parent. It results in offspring that are genetically identical to each other and to the parent. All prokaryotes and some eukaryotes reproduce this way. There are several different methods of asexual reproduction. They include binary fission, fragmentation, and budding.

  • Binary fission occurs when a parent cell splits into two identical daughter cells of the same size. This process was described in detail in the lesson “Cell Division and the Cell Cycle.”
  • Fragmentation occurs when a parent organism breaks into fragments, or pieces, and each fragment develops into a new organism. Starfish, like the one in Figure below, reproduce this way. A new starfish can develop from a single ray, or arm.

  • Budding occurs when a parent cell forms a bubble-like bud. The bud stays attached to the parent cell while it grows and develops. When the bud is fully developed, it breaks away from the parent cell and forms a new organism. Budding in yeast is shown in Figure below.

Asexual reproduction can be very rapid. This is an advantage for many organisms. It allows them to crowd out other organisms that reproduce more slowly. Bacteria, for example, may divide several times per hour. Under ideal conditions, 100 bacteria can divide to produce millions of bacterial cells in just a few hours!

Watch this quick video that shows just how fast bacteria can reproduce:

However, most bacteria do not live under ideal conditions. If they did, the entire surface of the planet would soon be covered with them. Instead, their reproduction is kept in check by limited resources, predators, and their own wastes. This is true of most other organisms as well.

Sexual Reproduction

Sexual reproduction involves two parents. As you can see from Figure below (past the root word alert and the video), in sexual reproduction, parents produce reproductive cells—called gametes—that unite to form an offspring. Gametes are haploid cells. This means they contain only half the number of chromosomes found in other cells of the organism. Gametes are produced by a type of cell division called meiosis, which is described in detail below. The process in which two gametes unite is called fertilization.

The fertilized cell that results is referred to as a zygote. A zygote is diploid cell, which means that it has twice the number of chromosomes as a gamete.

Mitosis, Meiosis, and Sexual Reproduction is discussed at http://www.youtube.com/watch?v=kaSIjIzAtYA.

Meiosis

The process that produces haploid gametes is meiosis (see Figure above). Meiosis is a type of cell division in which the number of chromosomes is reduced by half. It occurs only in certain special cells of the organisms. During meiosis, homologous chromosomes separate, and haploid cells form that have only one chromosome from each pair. Two cell divisions occur during meiosis, and a total of four haploid cells are produced. The two cell divisions are called meiosis I and meiosis II. The overall process of meiosis is summarized in Figure below.

Ready to fill out a printable that will help you remember the content from the video above? Here ya go!

I know that filling out printables and worksheets can seem like it’s so boring, but it does really help you learn and remember material. You’ll thank me when you take your college biology class… or when you are at a social gathering someday and you sound really well-educated as you impress some non-homeschoolers with your profound knowledge of the difference between mitosis and meiosis. Yay homeschooling!

It is also described in detail below. You can watch an animation of meiosis at this link:

Phases of Meiosis

Meiosis I begins after DNA replicates during interphase. In both meiosis I and meiosis II, cells go through the same four phases as mitosis. However, there are important differences between meiosis I and mitosis. The flowchart in Figure below shows what happens in both meiosis I and II. You can follow the changes in the flowchart as you read about them below.

The phases of meiosis are discussed at http://www.youtube.com/watch?v=ijLc52LmFQg (27:23). OPTIONAL video alert!

Meiosis I
  1. Prophase I: The nuclear envelope begins to break down, and the chromosomes condense. Centrioles start moving to opposite poles of the cell, and a spindle begins to form. Importantly, homologous chromosomes pair up, which is unique to prophase I. In prophase of mitosis and meiosis II, homologous chromosomes do not form pairs in this way.
  2. Metaphase I: Spindle fibers attach to the paired homologous chromosomes. The paired chromosomes line up along the equator of the cell. This occurs only in metaphase I. In metaphase of mitosis and meiosis II, it is sister chromatids that line up along the equator of the cell.
  3. Anaphase I: Spindle fibers shorten, and the chromosomes of each homologous pair start to separate from each other. One chromosome of each pair moves toward one pole of the cell, and the other chromosome moves toward the opposite pole.
  4. Telophase I and Cytokinesis: The spindle breaks down, and new nuclear membranes form. The cytoplasm of the cell divides, and two haploid daughter cells result. The daughter cells each have a random assortment of chromosomes, with one from each homologous pair. Both daughter cells go on to meiosis II.
Meiosis II
  1. Prophase II: The nuclear envelope breaks down and the spindle begins to form in each haploid daughter cell from meiosis I. The centrioles also start to separate.
  2. Metaphase II: Spindle fibers line up the sister chromatids of each chromosome along the equator of the cell.
  3. Anaphase II: Sister chromatids separate and move to opposite poles.
  4. Telophase II and Cytokinesis: The spindle breaks down, and new nuclear membranes form. The cytoplasm of each cell divides, and four haploid cells result. Each cell has a unique combination of chromosomes.

Gametogenesis

At the end of meiosis, four haploid cells have been produced, but the cells are not yet gametes. The cells need to develop before they become mature gametes capable of fertilization. The development of haploid cells into gametes is called gametogenesis. Gametogenesis may differ between males and females. Male gametes are called sperm. Female gametes are called eggs. In human males, for example, the process that produces mature sperm cells is called spermatogenesis. During this process, sperm cells grow a tail and gain the ability to “swim,” like the human sperm cell shown in Figure below. In human females, the process that produces mature eggs is called oogenesis. Just one egg is produced from the four haploid cells that result from meiosis. The single egg is a very large cell, as you can see from the human egg in Figure below.

Sexual Reproduction and Genetic Variation

Sexual reproduction results in offspring that are genetically unique. They differ from both parents and also from each other. This occurs for a number of reasons.

  • When homologous chromosomes pair up during meiosis I, crossing-over can occur. Crossing-over is the exchange of genetic material between homologous chromosomes. It results in new combinations of genes on each chromosome.
  • When cells divide during meiosis, homologous chromosomes are randomly distributed to daughter cells, and different chromosomes segregate independently of each other. This called is called independent assortment. It results in gametes that have unique combinations of chromosomes.
  • In sexual reproduction, two gametes unite to produce an offspring. But which two of the millions of possible gametes will it be? This is likely to be a matter of chance. It is obviously another source of genetic variation in offspring.

All of these mechanisms working together result in an amazing amount of potential variation. Each human couple, for example, has the potential to produce more than 64 trillion genetically unique children. No wonder we are all different!

Sexual Reproduction and Life Cycles

Sexual reproduction occurs in a cycle. Diploid parents produce haploid gametes that unite and develop into diploid adults, which repeat the cycle. This series of life stages and events that a sexually reproducing organism goes through is called itslife cycle. Sexually reproducing organisms can have different types of life cycles. Three are described in the following sections.

Haploid Life Cycle

The haploid life cycle (Figure below) is the simplest life cycle. It is found in many single-celled organisms. Organisms with a haploid life cycle spend the majority of their lives as haploid gametes. When the haploid gametes fuse, they form a diploid zygote. It quickly undergoes meiosis to produce more haploid gametes that repeat the life cycle.

Diploid Life Cycle

Organisms with a diploid life cycle (Figure below) spend the majority of their lives as diploid adults. When they are ready to reproduce, they undergo meiosis and produce haploid gametes. Gametes then unite in fertilization and form a diploid zygote. The zygote develops into a diploid adult that repeats the life cycle. Can you think of an organism with a diploid life cycle? (Hint: What type of life cycle do humans have?)

Alternation of Generations

Organisms that have a life cycle with alternating generations (Figure below) switch back and forth between diploid and haploid stages. Organisms with this type of life cycle include plants, algae, and some protists. These life cycles may be quite complicated. You can read about them in later chapters.

Lesson Summary

  • Asexual reproduction involves one parent and produces offspring that are genetically identical to each other and to the parent. Sexual reproduction involves two parents and produces offspring that are genetically unique.
  • During sexual reproduction, two haploid gametes join in the process of fertilization to produce a diploid zygote. Meiosis is the type of cell division that produces gametes. It involves two cell divisions and produces four haploid cells.
  • Sexual reproduction has the potential to produce tremendous genetic variation in offspring. This variation is due to independent assortment and crossing-over during meiosis and random union of gametes during fertilization.
  • A life cycle is the sequence of stages an organisms goes through from one generation to the next. Organisms that reproduce sexually can have different types of life cycles, such as haploid or diploid life cycles.

Lesson Review Questions

Recall

1. What are three types of asexual reproduction?

2. Define gamete and zygote. What number of chromosomes does each have?

3. What happens during fertilization?

4. Outline the phases of meiosis.

6. What is gametogenesis, and when does it occur?

Apply Concepts

7. Create a diagram to show how crossing-over occurs and how it creates new gene combinations on each chromosome.

8. An adult organism produces gametes that quickly go through fertilization and form diploid zygotes. The zygotes mature into adults, which live for many years. Eventually the adults produce gametes and the cycle repeats. What type of life cycle does this organism have? Explain your answer.

Think Critically

9. Compare and contrast asexual and sexual reproduction.

10. Explain why sexual reproduction results in genetically unique offspring.

11. Explain how meiosis I differs from mitosis.

Points to Consider

In sexually reproducing organisms, parents pass a copy of each type of chromosome to their offspring by producing gametes. When gametes are fertilized and form offspring, each has a unique combination of chromosomes and genes from both parents. The inherited gene combination determines the characteristics of the offspring.


Watch the video: Bacterial Structure and Functions (August 2022).