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4.E: How Cells Obtain Energy (Exercises) - Biology

4.E: How Cells Obtain Energy (Exercises) - Biology


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4.1: Energy and Metabolism

Cells perform the functions of life through various chemical reactions. In studying energy, the term system refers to the matter and environment involved in energy transfers.

Review Questions

Which of the following is not an example of an energy transformation?

A. Heating up dinner in a microwave
B. Solar panels at work
C. Formation of static electricity
D. None of the above

D

Which of the following is not true about enzymes?

A. They are consumed by the reactions they catalyze.
B. They are usually made of amino acids.
C. They lower the activation energy of chemical reactions.
D. Each one is specific to the particular substrate(s) to which it binds.

A

Free Response

Does physical exercise to increase muscle mass involve anabolic and/or catabolic processes? Give evidence for your answer.

Physical exercise involves both anabolic and catabolic processes. Body cells break down sugars to provide ATP to do the work necessary for exercise, such as muscle contractions. This is catabolism. Muscle cells also must repair muscle tissue damaged by exercise by building new muscle. This is anabolism.

Explain in your own terms the difference between a spontaneous reaction and one that occurs instantaneously, and what causes this difference.

A spontaneous reaction is one that has a negative ∆G and thus releases energy. However, a spontaneous reaction need not occur quickly or suddenly like an instantaneous reaction. It may occur over long periods of time due to a large energy of activation, which prevents the reaction from occurring quickly.

With regard to enzymes, why are vitamins and minerals necessary for good health? Give examples.

Most vitamins and minerals act as cofactors and coenzymes for enzyme action. Many enzymes require the binding of certain cofactors or coenzymes to be able to catalyze their reactions. Since enzymes catalyze many important reactions, it is critical to obtain sufficient vitamins and minerals from diet and supplements. Vitamin C (ascorbic acid) is a coenzyme necessary for the action of enzymes that build collagen.

4.2: Glycolysis

ATP functions as the energy currency for cells. It allows cells to store energy briefly and transport it within itself to support endergonic chemical reactions. The structure of ATP is that of an RNA nucleotide with three phosphate groups attached. As ATP is used for energy, a phosphate group is detached, and ADP is produced. Energy derived from glucose catabolism is used to recharge ADP into ATP. Glycolysis is the first pathway used in the breakdown of glucose to extract energy.

Multiple Choice

Energy is stored long-term in the bonds of _____ and used short-term to perform work from a(n) _____ molecule.

A. ATP : glucose
B. an anabolic molecule : catabolic molecule
C. glucose : ATP
D. a catabolic molecule : anabolic molecule

C

The energy currency used by cells is _____.

A. ATP
B. ADP
C. AMP
D. adenosine

A

The glucose that enters the glycolysis pathway is split into two molecules of _________.

A. phosphate
C. NADH
D. pyruvate

D

Free Response

Both prokaryotic and eukaryotic organisms carry out some form of glycolysis. How does that fact support or not support the assertion that glycolysis is one of the oldest metabolic pathways?

If glycolysis evolved relatively late, it likely would not be as universal in organisms as it is. It probably evolved in very primitive organisms and persisted, with the addition of other pathways of carbohydrate metabolism that evolved later.

4.3: Citric Acid Cycle and Oxidative Phosphorylation

The citric acid cycle is a series of chemical reactions that removes high-energy electrons and uses them in the electron transport chain to generate ATP. One molecule of ATP (or an equivalent) is produced per each turn of the cycle. The electron transport chain is the portion of aerobic respiration that uses free oxygen as the final electron acceptor for electrons removed from the intermediate compounds in glucose catabolism.

Multiple Choice

What do the electrons added to NAD+ do?

A. They become part of a fermentation pathway.
B. They go to another pathway for ATP production.
C. They energize the entry of the acetyl group into the citric acid cycle.
D. They are converted into NADP.

B

Chemiosmosis involves

A. the movement of electrons across the cell membrane
B. the movement of hydrogen atoms across a mitochondrial membrane
C. the movement of hydrogen ions across a mitochondrial membran
D. the movement of glucose through the cell membrane

C

Free Response

We inhale oxygen when we breathe and exhale carbon dioxide. What is the oxygen used for and where does the carbon dioxide come from?

The oxygen we inhale is the final electron acceptor in the electron transport chain and allows aerobic respiration to proceed, which is the most efficient pathway for harvesting energy in the form of ATP from food molecules. The carbon dioxide we breathe out is formed during the citric acid cycle when the bonds in carbon compounds are broken.

4.4: Fermentation

If NADH cannot be metabolized through aerobic respiration, another electron acceptor is used. Most organisms will use some form of fermentation to accomplish the regeneration of NAD+, ensuring the continuation of glycolysis. The regeneration of NAD+ in fermentation is not accompanied by ATP production; therefore, the potential for NADH to produce ATP using an electron transport chain is not utilized.

Review Questions

Which of the following fermentation methods can occur in animal skeletal muscles?

A. lactic acid fermentation
B. alcohol fermentation
C. mixed acid fermentation
D. propionic fermentation

A

Free Response

When muscle cells run out of oxygen, what happens to the potential for energy extraction from sugars and what pathways do the cell use?

Without oxygen, oxidative phosphorylation and the citric acid cycle stop, so ATP is no longer generated through this mechanism, which extracts the greatest amount of energy from a sugar molecule. In addition, NADH accumulates, preventing glycolysis from going forward because of an absence of NAD+. Lactic acid fermentation uses the electrons in NADH to generate lactic acid from pyruvate, which allows glycolysis to continue and thus a smaller amount of ATP can be generated by the cell.

4.5: Connections to Other Metabolic Pathways

Metabolic pathways should be thought of as porous—that is, substances enter from other pathways, and other substances leave for other pathways. These pathways are not closed systems. Many of the products in a particular pathway are reactants in other pathways.

Multiple Choice

The cholesterol synthesized by cells uses which component of the glycolytic pathway as a starting point?

A. glucose
B. acetyl CoA
C. pyruvate
D. carbon dioxide

B

Beta oxidation is ________.

A. the breakdown of sugars
B. the assembly of sugars
C. the breakdown of fatty acids
D. the removal of amino groups from amino acids

C

Free Response

Would you describe metabolic pathways as inherently wasteful or inherently economical, and why?

They are very economical. The substrates, intermediates, and products move between pathways and do so in response to finely tuned feedback inhibition loops that keep metabolism overall on an even keel. Intermediates in one pathway may occur in another, and they can move from one pathway to another fluidly in response to the needs of the cell.


Chapter 4: Introduction to How Cells Obtain Energy

Figure 4.1 A hummingbird needs energy to maintain prolonged flight. The bird obtains its energy from taking in food and transforming the energy contained in food molecules into forms of energy to power its flight through a series of biochemical reactions. (credit: modification of work by Cory Zanker)

Virtually every task performed by living organisms requires energy. Energy is needed to perform heavy labor and exercise, but humans also use energy while thinking, and even during sleep. In fact, the living cells of every organism constantly use energy. Nutrients and other molecules are imported into the cell, metabolized (broken down) and possibly synthesized into new molecules, modified if needed, transported around the cell, and possibly distributed to the entire organism. For example, the large proteins that make up muscles are built from smaller molecules imported from dietary amino acids. Complex carbohydrates are broken down into simple sugars that the cell uses for energy. Just as energy is required to both build and demolish a building, energy is required for the synthesis and breakdown of molecules as well as the transport of molecules into and out of cells. In addition, processes such as ingesting and breaking down pathogenic bacteria and viruses, exporting wastes and toxins, and movement of the cell require energy. From where, and in what form, does this energy come? How do living cells obtain energy, and how do they use it? This chapter will discuss different forms of energy and the physical laws that govern energy transfer. This chapter will also describe how cells use energy and replenish it, and how chemical reactions in the cell are performed with great efficiency.


Virtually every task performed by living organisms requires energy. Energy is needed to perform heavy labor and exercise, but humans also use energy while thinking, and even during sleep. In fact, the living cells of every organism constantly use energy. Nutrients and other molecules are imported into the cell, metabolized (broken down) and possibly synthesized into new molecules, modified if needed, transported around the cell, and possibly distributed to the entire organism. For example, the large proteins that make up muscles are built from smaller molecules imported from dietary amino acids. Complex carbohydrates are broken down into simple sugars that the cell uses for energy. Just as energy is required to both build and demolish a building, energy is required for the synthesis and breakdown of molecules as well as the transport of molecules into and out of cells. In addition, processes such as ingesting and breaking down pathogenic bacteria and viruses, exporting wastes and toxins, and movement of the cell require energy. From where, and in what form, does this energy come? How do living cells obtain energy, and how do they use it? This chapter will discuss different forms of energy and the physical laws that govern energy transfer. This chapter will also describe how cells use energy and replenish it, and how chemical reactions in the cell are performed with great efficiency.

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

  • Explain what metabolic pathways are
  • State the first and second laws of thermodynamics
  • Explain the difference between kinetic and potential energy
  • Describe endergonic and exergonic reactions
  • Discuss how enzymes function as molecular catalysts

Scientists use the term bioenergetics to describe the concept of energy flow (Figure) through living systems, such as cells. Cellular processes such as the building and breaking down of complex molecules occur through stepwise chemical reactions. Some of these chemical reactions are spontaneous and release energy, whereas others require energy to proceed. Just as living things must continually consume food to replenish their energy supplies, cells must continually produce more energy to replenish that used by the many energy-requiring chemical reactions that constantly take place. Together, all of the chemical reactions that take place inside cells, including those that consume or generate energy, are referred to as the cell’s metabolism.

Figure 4.2 Ultimately, most life forms get their energy from the sun. Plants use photosynthesis to capture sunlight, and herbivores eat the plants to obtain energy. Carnivores eat the herbivores, and eventual decomposition of plant and animal material contributes to the nutrient pool.

Metabolic Pathways

Consider the metabolism of sugar. This is a classic example of one of the many cellular processes that use and produce energy. Living things consume sugars as a major energy source, because sugar molecules have a great deal of energy stored within their bonds. For the most part, photosynthesizing organisms like plants produce these sugars. During photosynthesis, plants use energy (originally from sunlight) to convert carbon dioxide gas (CO2) into sugar molecules (like glucose: C6H12O6). They consume carbon dioxide and produce oxygen as a waste product. This reaction is summarized as:

Because this process involves synthesizing an energy-storing molecule, it requires energy input to proceed. During the light reactions of photosynthesis, energy is provided by a molecule called adenosine triphosphate (ATP), which is the primary energy currency of all cells. Just as the dollar is used as currency to buy goods, cells use molecules of ATP as energy currency to perform immediate work. In contrast, energy-storage molecules such as glucose are consumed only to be broken down to use their energy. The reaction that harvests the energy of a sugar molecule in cells requiring oxygen to survive can be summarized by the reverse reaction to photosynthesis. In this reaction, oxygen is consumed and carbon dioxide is released as a waste product. The reaction is summarized as:

Both of these reactions involve many steps.

The processes of making and breaking down sugar molecules illustrate two examples of metabolic pathways. A metabolic pathway is a series of chemical reactions that takes a starting molecule and modifies it, step-by-step, through a series of metabolic intermediates, eventually yielding a final product. In the example of sugar metabolism, the first metabolic pathway synthesized sugar from smaller molecules, and the other pathway broke sugar down into smaller molecules. These two opposite processes—the first requiring energy and the second producing energy—are referred to as anabolic pathways (building polymers) and catabolic pathways (breaking down polymers into their monomers), respectively. Consequently, metabolism is composed of synthesis (anabolism) and degradation (catabolism) (Figure 4.3).

It is important to know that the chemical reactions of metabolic pathways do not take place on their own. Each reaction step is facilitated, or catalyzed, by a protein called an enzyme. Enzymes are important for catalyzing all types of biological reactions—those that require energy as well as those that release energy.

Figure 4.3 Catabolic pathways are those that generate energy by breaking down larger molecules. Anabolic pathways are those that require energy to synthesize larger molecules. Both types of pathways are required for maintaining the cell’s energy balance.

Energy

Thermodynamics refers to the study of energy and energy transfer involving physical matter. The matter relevant to a particular case of energy transfer is called a system, and everything outside of that matter is called the surroundings. For instance, when heating a pot of water on the stove, the system includes the stove, the pot, and the water. Energy is transferred within the system (between the stove, pot, and water). There are two types of systems: open and closed. In an open system, energy can be exchanged with its surroundings. The stovetop system is open because heat can be lost to the air. A closed system cannot exchange energy with its surroundings.

Biological organisms are open systems. Energy is exchanged between them and their surroundings as they use energy from the sun to perform photosynthesis or consume energy-storing molecules and release energy to the environment by doing work and releasing heat. Like all things in the physical world, energy is subject to physical laws. The laws of thermodynamics govern the transfer of energy in and among all systems in the universe.

In general, energy is defined as the ability to do work, or to create some kind of change. Energy exists in different forms. For example, electrical energy, light energy, and heat energy are all different types of energy. To appreciate the way energy flows into and out of biological systems, it is important to understand two of the physical laws that govern energy.

Thermodynamics

The first law of thermodynamics states that the total amount of energy in the universe is constant and conserved. In other words, there has always been, and always will be, exactly the same amount of energy in the universe. Energy exists in many different forms. According to the first law of thermodynamics, energy may be transferred from place to place or transformed into different forms, but it cannot be created or destroyed. The transfers and transformations of energy take place around us all the time. Light bulbs transform electrical energy into light and heat energy. Gas stoves transform chemical energy from natural gas into heat energy. Plants perform one of the most biologically useful energy transformations on earth: that of converting the energy of sunlight to chemical energy stored within organic molecules (Figure 4.2). Some examples of energy transformations are shown in Figure 4.4.

The challenge for all living organisms is to obtain energy from their surroundings in forms that they can transfer or transform into usable energy to do work. Living cells have evolved to meet this challenge. Chemical energy stored within organic molecules such as sugars and fats is transferred and transformed through a series of cellular chemical reactions into energy within molecules of ATP. Energy in ATP molecules is easily accessible to do work. Examples of the types of work that cells need to do include building complex molecules, transporting materials, powering the motion of cilia or flagella, and contracting muscle fibers to create movement.

Figure 4.4 Shown are some examples of energy transferred and transformed from one system to another and from one form to another. The food we consume provides our cells with the energy required to carry out bodily functions, just as light energy provides plants with the means to create the chemical energy they need. (credit "ice cream": modification of work by D. Sharon Pruitt credit "kids": modification of work by Max from Providence credit "leaf": modification of work by Cory Zanker)

A living cell’s primary tasks of obtaining, transforming, and using energy to do work may seem simple. However, the second law of thermodynamics explains why these tasks are harder than they appear. All energy transfers and transformations are never completely efficient. In every energy transfer, some amount of energy is lost in a form that is unusable. In most cases, this form is heat energy. Thermodynamically, heat energy is defined as the energy transferred from one system to another that is not work. For example, when a light bulb is turned on, some of the energy being converted from electrical energy into light energy is lost as heat energy. Likewise, some energy is lost as heat energy during cellular metabolic reactions.

An important concept in physical systems is that of order and disorder. The more energy that is lost by a system to its surroundings, the less ordered and more random the system is. Scientists refer to the measure of randomness or disorder within a system as entropy. High entropy means high disorder and low energy. Molecules and chemical reactions have varying entropy as well. For example, entropy increases as molecules at a high concentration in one place diffuse and spread out. The second law of thermodynamics says that energy will always be lost as heat in energy transfers or transformations.

Living things are highly ordered, requiring constant energy input to be maintained in a state of low entropy.

Potential and Kinetic Energy

When an object is in motion, there is energy associated with that object. Think of a wrecking ball. Even a slow-moving wrecking ball can do a great deal of damage to other objects. Energy associated with objects in motion is called kinetic energy (Figure 4.5). A speeding bullet, a walking person, and the rapid movement of molecules in the air (which produces heat) all have kinetic energy.

Now what if that same motionless wrecking ball is lifted two stories above ground with a crane? If the suspended wrecking ball is unmoving, is there energy associated with it? The answer is yes. The energy that was required to lift the wrecking ball did not disappear, but is now stored in the wrecking ball by virtue of its position and the force of gravity acting on it. This type of energy is called potential energy (Figure 4.5). If the ball were to fall, the potential energy would be transformed into kinetic energy until all of the potential energy was exhausted when the ball rested on the ground. Wrecking balls also swing like a pendulum through the swing, there is a constant change of potential energy (highest at the top of the swing) to kinetic energy (highest at the bottom of the swing). Other examples of potential energy include the energy of water held behind a dam or a person about to skydive out of an airplane.

Figure 4.5 Still water has potential energy moving water, such as in a waterfall or a rapidly flowing river, has kinetic energy. (credit "dam": modification of work by "Pascal"/Flickr credit "waterfall": modification of work by Frank Gualtieri)

Potential energy is not only associated with the location of matter, but also with the structure of matter. Even a spring on the ground has potential energy if it is compressed so does a rubber band that is pulled taut. On a molecular level, the bonds that hold the atoms of molecules together exist in a particular structure that has potential energy. Remember that anabolic cellular pathways require energy to synthesize complex molecules from simpler ones and catabolic pathways release energy when complex molecules are broken down. The fact that energy can be released by the breakdown of certain chemical bonds implies that those bonds have potential energy. In fact, there is potential energy stored within the bonds of all the food molecules we eat, which is eventually harnessed for use. This is because these bonds can release energy when broken. The type of potential energy that exists within chemical bonds, and is released when those bonds are broken, is called chemical energy. Chemical energy is responsible for providing living cells with energy from food. The release of energy occurs when the molecular bonds within food molecules are broken.

CONCEPT IN ACTION

Visit the site (http://openstaxcollege.org/l/simple_pendulu2) and select “Pendulum” from the “Work and Energy” menu to see the shifting kinetic and potential energy of a pendulum in motion.

Free and Activation Energy

After learning that chemical reactions release energy when energy-storing bonds are broken, an important next question is the following: How is the energy associated with these chemical reactions quantified and expressed? How can the energy released from one reaction be compared to that of another reaction? A measurement of free energy is used to quantify these energy transfers. Recall that according to the second law of thermodynamics, all energy transfers involve the loss of some amount of energy in an unusable form such as heat. Free energy specifically refers to the energy associated with a chemical reaction that is available after the losses are accounted for. In other words, free energy is usable energy, or energy that is available to do work.

If energy is released during a chemical reaction, then the change in free energy, signified as ∆G (delta G) will be a negative number. A negative change in free energy also means that the products of the reaction have less free energy than the reactants, because they release some free energy during the reaction. Reactions that have a negative change in free energy and consequently release free energy are called exergonic reactions. Think: exergonic means energy is exiting the system. These reactions are also referred to as spontaneous reactions, and their products have less stored energy than the reactants. An important distinction must be drawn between the term spontaneous and the idea of a chemical reaction occurring immediately. Contrary to the everyday use of the term, a spontaneous reaction is not one that suddenly or quickly occurs. The rusting of iron is an example of a spontaneous reaction that occurs slowly, little by little, over time.

If a chemical reaction absorbs energy rather than releases energy on balance, then the ∆G for that reaction will be a positive value. In this case, the products have more free energy than the reactants. Thus, the products of these reactions can be thought of as energy-storing molecules. These chemical reactions are called endergonic reactions and they are non-spontaneous. An endergonic reaction will not take place on its own without the addition of free energy.

ART CONNECTION

Figure 4.6 Shown are some examples of endergonic processes (ones that require energy) and exergonic processes (ones that release energy). (credit a: modification of work by Natalie Maynor credit b: modification of work by USDA credit c: modification of work by Cory Zanker credit d: modification of work by Harry Malsch)

Look at each of the processes shown and decide if it is endergonic or exergonic.

There is another important concept that must be considered regarding endergonic and exergonic reactions. Exergonic reactions require a small amount of energy input to get going, before they can proceed with their energy-releasing steps. These reactions have a net release of energy, but still require some energy input in the beginning. This small amount of energy input necessary for all chemical reactions to occur is called the activation energy.

CONCEPT IN ACTION

Watch an animation (http://openstaxcollege.org/l/energy_reactio2) of the move from free energy to transition state of the reaction.

Enzymes

A substance that helps a chemical reaction to occur is called a catalyst, and the molecules that catalyze biochemical reactions are called enzymes. Most enzymes are proteins and perform the critical task of lowering the activation energies of chemical reactions inside the cell. Most of the reactions critical to a living cell happen too slowly at normal temperatures to be of any use to the cell. Without enzymes to speed up these reactions, life could not persist. Enzymes do this by binding to the reactant molecules and holding them in such a way as to make the chemical bond-breaking and -forming processes take place more easily. It is important to remember that enzymes do not change whether a reaction is exergonic (spontaneous) or endergonic. This is because they do not change the free energy of the reactants or products. They only reduce the activation energy required for the reaction to go forward (Figure 4.7). In addition, an enzyme itself is unchanged by the reaction it catalyzes. Once one reaction has been catalyzed, the enzyme is able to participate in other reactions.

Figure 4.7 Enzymes lower the activation energy of the reaction but do not change the free energy of the reaction.

The chemical reactants to which an enzyme binds are called the enzyme’s substrates. There may be one or more substrates, depending on the particular chemical reaction. In some reactions, a single reactant substrate is broken down into multiple products. In others, two substrates may come together to create one larger molecule. Two reactants might also enter a reaction and both become modified, but they leave the reaction as two products. The location within the enzyme where the substrate binds is called the enzyme’s active site. The active site is where the “action” happens. Since enzymes are proteins, there is a unique combination of amino acid side chains within the active site. Each side chain is characterized by different properties. They can be large or small, weakly acidic or basic, hydrophilic or hydrophobic, positively or negatively charged, or neutral. The unique combination of side chains creates a very specific chemical environment within the active site. This specific environment is suited to bind to one specific chemical substrate (or substrates).

Active sites are subject to influences of the local environment. Increasing the environmental temperature generally increases reaction rates, enzyme-catalyzed or otherwise. However, temperatures outside of an optimal range reduce the rate at which an enzyme catalyzes a reaction. Hot temperatures will eventually cause enzymes to denature, an irreversible change in the three-dimensional shape and therefore the function of the enzyme. Enzymes are also suited to function best within a certain pH and salt concentration range, and, as with temperature, extreme pH, and salt concentrations can cause enzymes to denature.

For many years, scientists thought that enzyme-substrate binding took place in a simple “lock and key” fashion. This model asserted that the enzyme and substrate fit together perfectly in one instantaneous step. However, current research supports a model called induced fit (Figure 4.8 ). The induced-fit model expands on the lock-and-key model by describing a more dynamic binding between enzyme and substrate. As the enzyme and substrate come together, their interaction causes a mild shift in the enzyme’s structure that forms an ideal binding arrangement between enzyme and substrate.

CONCEPT IN ACTION

When an enzyme binds its substrate, an enzyme-substrate complex is formed. This complex lowers the activation energy of the reaction and promotes its rapid progression in one of multiple possible ways. On a basic level, enzymes promote chemical reactions that involve more than one substrate by bringing the substrates together in an optimal orientation for reaction. Another way in which enzymes promote the reaction of their substrates is by creating an optimal environment within the active site for the reaction to occur. The chemical properties that emerge from the particular arrangement of amino acid R groups within an active site create the perfect environment for an enzyme’s specific substrates to react.

The enzyme-substrate complex can also lower activation energy by compromising the bond structure so that it is easier to break. Finally, enzymes can also lower activation energies by taking part in the chemical reaction itself. In these cases, it is important to remember that the enzyme will always return to its original state by the completion of the reaction. One of the hallmark properties of enzymes is that they remain ultimately unchanged by the reactions they catalyze. After an enzyme has catalyzed a reaction, it releases its product(s) and can catalyze a new reaction.

Figure 4.8 The induced-fit model is an adjustment to the lock-and-key model and explains how enzymes and substrates undergo dynamic modifications during the transition state to increase the affinity of the substrate for the active site.

It would seem ideal to have a scenario in which all of an organism's enzymes existed in abundant supply and functioned optimally under all cellular conditions, in all cells, at all times. However, a variety of mechanisms ensures that this does not happen. Cellular needs and conditions constantly vary from cell to cell, and change within individual cells over time. The required enzymes of stomach cells differ from those of fat storage cells, skin cells, blood cells, and nerve cells. Furthermore, a digestive organ cell works much harder to process and break down nutrients during the time that closely follows a meal compared with many hours after a meal. As these cellular demands and conditions vary, so must the amounts and functionality of different enzymes.

Since the rates of biochemical reactions are controlled by activation energy, and enzymes lower and determine activation energies for chemical reactions, the relative amounts and functioning of the variety of enzymes within a cell ultimately determine which reactions will proceed and at what rates. This determination is tightly controlled in cells. In certain cellular environments, enzyme activity is partly controlled by environmental factors like pH, temperature, salt concentration, and, in some cases, cofactors or coenzymes.

Enzymes can also be regulated in ways that either promote or reduce enzyme activity. There are many kinds of molecules that inhibit or promote enzyme function, and various mechanisms by which they do so. In some cases of enzyme inhibition, an inhibitor molecule is similar enough to a substrate that it can bind to the active site and simply block the substrate from binding. When this happens, the enzyme is inhibited through competitive inhibition, because an inhibitor molecule competes with the substrate for binding to the active site.

On the other hand, in noncompetitive inhibition, an inhibitor molecule binds to the enzyme in a location other than the active site, called an allosteric site, but still manages to block substrate binding to the active site. Some inhibitor molecules bind to enzymes in a location where their binding induces a conformational change that reduces the affinity of the enzyme for its substrate. This type of inhibition is called allosteric inhibition (Figure 4.9). Most allosterically regulated enzymes are made up of more than one polypeptide, meaning that they have more than one protein subunit. When an allosteric inhibitor binds to a region on an enzyme, all active sites on the protein subunits are changed slightly such that they bind their substrates with less efficiency. There are allosteric activators as well as inhibitors. Allosteric activators bind to locations on an enzyme away from the active site, inducing a conformational change that increases the affinity of the enzyme’s active site(s) for its substrate(s) (Figure 4.9).

Figure 4.9 Allosteric inhibition works by indirectly inducing a conformational change to the active site such that the substrate no longer fits. In contrast, in allosteric activation, the activator molecule modifies the shape of the active site to allow a better fit of the substrate.

CAREERS IN ACTION

Pharmaceutical Drug Developer

Figure 4.10 Have you ever wondered how pharmaceutical drugs are developed? (credit: Deborah Austin)

Enzymes are key components of metabolic pathways. Understanding how enzymes work and how they can be regulated are key principles behind the development of many of the pharmaceutical drugs on the market today. Biologists working in this field collaborate with other scientists to design drugs (Figure 4.10).

Consider statins for example—statins is the name given to one class of drugs that can reduce cholesterol levels. These compounds are inhibitors of the enzyme HMG-CoA reductase, which is the enzyme that synthesizes cholesterol from lipids in the body. By inhibiting this enzyme, the level of cholesterol synthesized in the body can be reduced. Similarly, acetaminophen, popularly marketed under the brand name Tylenol, is an inhibitor of the enzyme cyclooxygenase. While it is used to provide relief from fever and inflammation (pain), its mechanism of action is still not completely understood.

How are drugs discovered? One of the biggest challenges in drug discovery is identifying a drug target. A drug target is a molecule that is literally the target of the drug. In the case of statins, HMG-CoA reductase is the drug target. Drug targets are identified through painstaking research in the laboratory. Identifying the target alone is not enough scientists also need to know how the target acts inside the cell and which reactions go awry in the case of disease. Once the target and the pathway are identified, then the actual process of drug design begins. In this stage, chemists and biologists work together to design and synthesize molecules that can block or activate a particular reaction. However, this is only the beginning: If and when a drug prototype is successful in performing its function, then it is subjected to many tests from in vitro experiments to clinical trials before it can get approval from the U.S. Food and Drug Administration to be on the market.

Many enzymes do not work optimally, or even at all, unless bound to other specific non-protein helper molecules. They may bond either temporarily through ionic or hydrogen bonds, or permanently through stronger covalent bonds. Binding to these molecules promotes optimal shape and function of their respective enzymes. Two examples of these types of helper molecules are cofactors and coenzymes. Cofactors are inorganic ions such as ions of iron and magnesium. Coenzymes are organic helper molecules, those with a basic atomic structure made up of carbon and hydrogen. Like enzymes, these molecules participate in reactions without being changed themselves and are ultimately recycled and reused. Vitamins are the source of coenzymes. Some vitamins are the precursors of coenzymes and others act directly as coenzymes. Vitamin C is a direct coenzyme for multiple enzymes that take part in building the important connective tissue, collagen. Therefore, enzyme function is, in part, regulated by the abundance of various cofactors and coenzymes, which may be supplied by an organism’s diet or, in some cases, produced by the organism.

Feedback Inhibition in Metabolic Pathways

Molecules can regulate enzyme function in many ways. The major question remains, however: What are these molecules and where do they come from? Some are cofactors and coenzymes, as you have learned. What other molecules in the cell provide enzymatic regulation such as allosteric modulation, and competitive and non-competitive inhibition? Perhaps the most relevant sources of regulatory molecules, with respect to enzymatic cellular metabolism, are the products of the cellular metabolic reactions themselves. In a most efficient and elegant way, cells have evolved to use the products of their own reactions for feedback inhibition of enzyme activity. Feedback inhibition involves the use of a reaction product to regulate its own further production (Figure 4.11). The cell responds to an abundance of the products by slowing down production during anabolic or catabolic reactions. Such reaction products may inhibit the enzymes that catalyzed their production through the mechanisms described above.

Figure 4.11 Metabolic pathways are a series of reactions catalyzed by multiple enzymes. Feedback inhibition, where the end product of the pathway inhibits an upstream process, is an important regulatory mechanism in cells.

The production of both amino acids and nucleotides is controlled through feedback inhibition. Additionally, ATP is an allosteric regulator of some of the enzymes involved in the catabolic breakdown of sugar, the process that creates ATP. In this way, when ATP is in abundant supply, the cell can prevent the production of ATP. On the other hand, ADP serves as a positive allosteric regulator (an allosteric activator) for some of the same enzymes that are inhibited by ATP. Thus, when relative levels of ADP are high compared to ATP, the cell is triggered to produce more ATP through sugar catabolism.


Biology 171


Virtually every task performed by living organisms requires energy. Organisms require energy to perform heavy labor and exercise, but humans also use considerable energy while thinking, and even during sleep. Every organism’s living cells constantly use energy. Organisms import nutrients and other molecules. They metabolize (break down) and possibly synthesize into new molecules. If necessary, molecules modify, move around the cell and may distribute themselves to the entire organism. For example, the large proteins that make up muscles are actively built from smaller molecules. Complex carbohydrates break down into simple sugars that the cell uses for energy. Just as energy is required to both build and demolish a building, energy is required to synthesize and break down molecules. Additionally, signaling molecules such as hormones and neurotransmitters transport between cells. Cells ingest and break down bacteria and viruses. Cells must also export waste and toxins to stay healthy, and many cells must swim or move surrounding materials via the beating motion of cellular appendages like cilia and flagella.

The cellular processes that we listed above require a steady supply of energy. From where, and in what form, does this energy come? How do living cells obtain energy, and how do they use it? This chapter will discuss different forms of energy and the physical laws that govern energy transfer. This chapter will also describe how cells use energy and replenish it, and how chemical reactions in the cell perform with great efficiency.

Learning Objectives

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

  • Explain metabolic pathways and describe the two major types
  • Discuss how chemical reactions play a role in energy transfer

Scientists use the term bioenergetics to discuss the concept of energy flow ((Figure)) through living systems, such as cells. Cellular processes such as building and breaking down complex molecules occur through stepwise chemical reactions. Some of these chemical reactions are spontaneous and release energy whereas, others require energy to proceed. Just as living things must continually consume food to replenish what they have used, cells must continually produce more energy to replenish that which the many energy-requiring chemical reactions that constantly take place use. All of the chemical reactions that transpire inside cells, including those that use and release energy, are the cell’s metabolism .

Carbohydrate Metabolism

Sugar (chemical reactions) metabolism (a simple carbohydrate) is a classic example of the many cellular processes that use and produce energy. Living things consume sugar as a major energy source, because sugar molecules have considerable energy stored within their bonds. The following equation describes the breakdown of glucose, a simple sugar:

Consumed carbohydrates have their origins in photosynthesizing organisms like plants ((Figure)). During photosynthesis, plants use the energy of sunlight to convert carbon dioxide gas (CO2) into sugar molecules, like glucose (C6H12O6). Because this process involves synthesizing a larger, energy-storing molecule, it requires an energy input to proceed. The following equation (notice that it is the reverse of the previous equation) describes the synthesis of glucose:

During photosynthesis chemical reactions, energy is in the form of a very high-energy molecule scientists call ATP, or adenosine triphosphate. This is the primary energy currency of all cells. Just as the dollar is the currency we use to buy goods, cells use ATP molecules as energy currency to perform immediate work. The sugar (glucose) is stored as starch or glycogen. Energy-storing polymers like these break down into glucose to supply ATP molecules.

Solar energy is required to synthesize a glucose molecule during the photosynthesis reactions. In photosynthesis, light energy from the sun initially transforms into chemical energy that temporally stores itself in the energy carrier molecules ATP and NADPH (nicotinamide adenine dinucleotide phosphate). Photosynthesis later uses the stored energy in ATP and NADPH to build one glucose molecule from six molecules of CO2. This process is analogous to eating breakfast in the morning to acquire energy for your body that you can use later in the day. Under ideal conditions, energy from 18 molecules of ATP is required to synthesize one glucose molecule during photosynthesis reactions. Glucose molecules can also combine with and convert into other sugar types. When an organism consumes sugars, glucose molecules eventually make their way into each organism’s living cell. Inside the cell, each sugar molecule breaks down through a complex series of chemical reactions. The goal of these reactions is to harvest the energy stored inside the sugar molecules. The harvested energy makes high-energy ATP molecules, which perform work, powering many chemical reactions in the cell. The amount of energy needed to make one glucose molecule from six carbon dioxide molecules is 18 ATP molecules and 12 NADPH molecules (each one of which is energetically equivalent to three ATP molecules), or a total of 54 molecule equivalents required for synthesizing one glucose molecule. This process is a fundamental and efficient way for cells to generate the molecular energy that they require.

Metabolic Pathways

The processes of making and breaking down sugar molecules illustrate two types of metabolic pathways. A metabolic pathway is a series of interconnected biochemical reactions that convert a substrate molecule or molecules, step-by-step, through a series of metabolic intermediates, eventually yielding a final product or products. In the case of sugar metabolism, the first metabolic pathway synthesized sugar from smaller molecules, and the other pathway broke sugar down into smaller molecules. Scientists call these two opposite processes—the first requiring energy and the second producing energy—anabolic (building) and catabolic (breaking down) pathways, respectively. Consequently, building (anabolism) and degradation (catabolism) comprise metabolism.

Evolution of Metabolic Pathways There is more to the complexity of metabolism than understanding the metabolic pathways alone. Metabolic complexity varies from organism to organism. Photosynthesis is the primary pathway in which photosynthetic organisms like plants (planktonic algae perform the majority of global synthesis) harvest the sun’s energy and convert it into carbohydrates. The by-product of photosynthesis is oxygen, which some cells require to carry out cellular respiration. During cellular respiration, oxygen aids in the catabolic breakdown of carbon compounds, like carbohydrates. Among the products are CO2 and ATP. In addition, some eukaryotes perform catabolic processes without oxygen (fermentation) that is, they perform or use anaerobic metabolism.

Organisms probably evolved anaerobic metabolism to survive (living organisms came into existence about 3.8 billion years ago, when the atmosphere lacked oxygen). Despite the differences between organisms and the complexity of metabolism, researchers have found that all branches of life share some of the same metabolic pathways, suggesting that all organisms evolved from the same ancient common ancestor ((Figure)). Evidence indicates that over time, the pathways diverged, adding specialized enzymes to allow organisms to better adapt to their environment, thus increasing their chance to survive. However, the underlying principle remains that all organisms must harvest energy from their environment and convert it to ATP to carry out cellular functions.

Anabolic and Catabolic Pathways

Anabolic pathways require an input of energy to synthesize complex molecules from simpler ones. Synthesizing sugar from CO2 is one example. Other examples are synthesizing large proteins from amino acid building blocks, and synthesizing new DNA strands from nucleic acid building blocks. These biosynthetic processes are critical to the cell’s life, take place constantly, and demand energy that ATP and other high-energy molecules like NADH (nicotinamide adenine dinucleotide) and NADPH provide ((Figure)).

ATP is an important molecule for cells to have in sufficient supply at all times. The breakdown of sugars illustrates how a single glucose molecule can store enough energy to make a great deal of ATP, 36 to 38 molecules. This is a catabolic pathway. Catabolic pathways involve degrading (or breaking down) complex molecules into simpler ones. Molecular energy stored in complex molecule bonds release in catabolic pathways and harvest in such a way that it can produce ATP. Other energy-storing molecules, such as fats, also break down through similar catabolic reactions to release energy and make ATP ((Figure)).

It is important to know that metabolic pathway chemical reactions do not take place spontaneously. A protein called an enzyme facilitates or catalyzes each reaction step. Enzymes are important for catalyzing all types of biological reactions—those that require energy as well as those that release energy.

Section Summary

Cells perform the functions of life through various chemical reactions. A cell’s metabolism refers to the chemical reactions that take place within it. There are metabolic reactions that involve breaking down complex chemicals into simpler ones, such as breaking down large macromolecules. Scientists refer to this process as catabolism, and we associate such reactions an energy release. On the other end of the spectrum, anabolism refers to metabolic processes that build complex molecules out of simpler ones, such as macromolecule synthesis. Anabolic processes require energy. Glucose synthesis and glucose breakdown are examples of anabolic and catabolic pathways, respectively.

Free Response

Does physical exercise involve anabolic and/or catabolic processes? Give evidence for your answer.

Physical exercise involves both anabolic and catabolic processes. Body cells break down sugars to provide ATP to do the work necessary for exercise, such as muscle contractions. This is catabolism. Muscle cells also must repair muscle tissue damaged by exercise by building new muscle. This is anabolism.

Name two different cellular functions that require energy that parallel human energy-requiring functions.

Energy is required for cellular motion, through beating of cilia or flagella, as well as human motion, produced by muscle contraction. Cells also need energy to perform digestion, as humans require energy to digest food.

Glossary


Aerobic Respiration

There are four stages of aerobic cellular respiration that occur to produce ATP (the energy cells need to do their work):

Stage 1 Glycolysis (also known as the breakdown of glucose)

This occurs in the cytoplasm and involves a series of chain reactions known as glycolysis to convert each molecule of glucose (a six-carbon molecule) into two smaller units of pyruvate (a three-carbon molecule). During the formation of pyruvate, two types of activated carrier molecules (small diffusible molecules in cells that contain energy rich covalent bonds) are produced, these are ATP and NADH (reduced nicotinamide adenine dinucleotide).This stage produces 4 molecules of ATP and 2 molecules of NADH from glucose but uses 2 molecules of ATP to get there,- so it actually results in 2 ATP + 2 NADH and pyruvate. The pyruvate then passes into the mitochondria.

Stage 2 The Link reaction

This links glycolysis with stage 3 the Citric acid/ Krebs cycle, which is explained below. At this point, one carbon dioxide molecule and one hydrogen molecule are removed from the pyruvate (called oxidative decarboxylation) to produce an acetyl group, which joins to an enzyme called CoA (Coenzyme A) to form acetyl-CoA, which is then ready to be used in the Citric acid/Krebs cycle. Acetyl-CoA is essential for the next stage.

Stage 3 The Citric Acid/Krebs Cycle

Taking place in the mitochondria, the acetyl-CoA (which is a two-carbon molecule) combines with oxaloacetate (a four-carbon molecule) to form citrate (a six-carbon molecule). The citrate molecule is then gradually oxidized, allowing the energy of this oxidation to be used to produce energy-rich activated carrier molecules. The chain of eight reactions forms a cycle because, at the end, the oxaloacetate is regenerated and can enter a new turn of the cycle. The cycle provides precursors including certain amino acids as well as the reducing agent NADH that are used in numerous biochemical reactions.

Each turn of the cycle produces two molecules of carbon dioxide, three molecules of NADH, one molecule of GTP (guanosine triphosphate) and one molecule of FADH2 (reduced flavin adenine dinucleotide).

Because two acetyl-CoA molecules are produced from each glucose molecule utilised, two cycles are required per glucose molecule.

Stage 4 Electron Transport Chain

In this final stage, the electron carriers NADH and FADH2, which gained electrons when they were oxidizing other molecules, transfer these electrons to the electron transport chain. This is found in the inner membrane of the mitochondria. This process requires oxygen and involves moving these electrons through a series of electron transporters that undergo redox reactions (reactions where both oxidation and reduction take place). This causes hydrogen ions to accumulate in the intermembrane space.

A concentration gradient then forms where hydrogen ions diffuse out of this space by passing through ATP synthase. The current of hydrogen ions powers the catalytic conversion of ATP synthase, which, in turn, phosphorylates ADP (adds a phosphate group) therefore producing ATP. The endpoint of the chain occurs when the electrons reduce molecular oxygen, which results in the production of water.

Although there is a theoretical yield of 38 ATP from the breakdown of one glucose molecule, realistically it is thought 30-32 ATP molecules are actually generated.

This process of aerobic respiration takes place when the body requires sufficient energy just to live, as well as to carry out everyday activities and perform cardio exercise. While this process yields more energy than the anaerobic systems, it is also less efficient and can only be used during lower-intensity activities.

So, if you have SLOW and STEADY energy requirements, your NET ENERGY PRODUCTION from aerobic respiration equals 30-32 Molecules of ATP.

Glucose + Oxygen → Carbon dioxide + Water + Energy (as 30-32 ATP)

The body releases carbon dioxide and water in this process. This will theoretically burn the highest number of calories.

Under other physiological conditions the body can still acquire its energy in other ways:

There are further energy processes the body uses to create ATP, they depend on the speed at which the energy is required and whether they have access to oxygen or not.

Anaerobic Respiration

Human muscle can respire anaerobically, a process that does not require oxygen. The process is relatively inefficient as it has a net energy production of 2 molecules of ATP.

This is effective for vigorous exercise of between 1-3 minutes duration, such as short sprints. If the intense exercise requires more energy than can be supplied by the oxygen available, your body will partially burn glucose without oxygen (anaerobic). Without the presence of oxygen, the electron transport chain cannot work. Therefore, the usual number of ATP molecules cannot be made. The anaerobic pathway uses pyruvate, the final product from the glycolysis stage. Pyruvate is reduced to lactic acid by NADH, leaving NAD + after the reduction. This reaction is catalysed by an enzyme (lactate dehydrogenase) and leads to the recycling of NAD + . This then allows the process of glycolysis to continue.

This glycolysis pathway yields 2 molecules ATP, which can be used for energy to drive muscle contraction. Anaerobic glycolysis occurs faster than aerobic respiration as less energy is produced for every glucose molecule broken down, so more has to be broken down at a faster rate to meet demands.

Lactic acid (the by-product from anaerobic respiration) builds up in the muscles causing the “burn” felt during strenuous activity. If more than a few minutes of this activity are used to generate ATP, lactic acid acidity increases, causing painful cramps. The extra oxygen you breath in following intensive exercise, reacts with the lactic acid in your muscles, breaking it down to make carbon dioxide and water.

So, summing up: Exercises that are performed at maximum rates for between 1 and 3 minutes depend heavily on anaerobic respiration for ATP energy. Also, in some performances, such as running 1500 meters or a mile, the lactic acid system is used predominately for the “kick” at the end of a race.

Therefore, if you are doing VIGOUROUS EXERCISE for 1-3 minutes, there will be NO TISSUE OXYGEN AVAILABLE so you will see a NET ENERGY PRODUCTION from anaerobic respiration equal to 2 molecules of ATP.

Beta Oxidation/Gluconeogenesis or Fat Burning (Aerobic Lipolysis)

A fat molecule consists of a glycerol backbone and three fatty acid tails. They are called triglycerides. In the body, they are stored primarily in fat cells called adipocytes making up the adipose tissue. To obtain energy from fat, the triglyceride molecules are broken down into fatty acids in a process called ‘Lipolysis’ occurring in the cytoplasm. These fatty acids are oxidized into acetyl- CoA, which is used in the Citric acid/Krebs cycle. Because one triglyceride molecule yields three fatty acid molecules with 16 or more carbons in each one, fat molecules yield more energy than carbohydrates and are an important source of energy for the human body (over 100 molecules of ATP generated per molecule of fatty acid). Therefore, when glucose levels are low, triglycerides can be converted into acetyl-CoA molecules and used to generate ATP through aerobic respiration.

This need arises after any period of not eating even with a normal overnight fast, mobilization of fat occurs, so that by the morning most of the acetyl-CoA entering the Citric acid/Krebs cycle comes from fatty acids rather than from glucose. Following a meal, however, most of the acetyl-CoA entering the Citric acid/Krebs cycle comes from glucose from food, with any excess glucose being used to replenish depleted glycogen stores or to synthesize fats.

This is a SLOW, NOT IMMEDIATE ENERGY SOURCE but has a NET ENERGY PRODUCTION of over 100 molecules of ATP.

ATP Phosphocreatine (ATP-PC)

This energy system consists of ATP (all muscle cells have a little ATP in them) and phosphocreatine (PC), which provide immediate energy from the breakdown of these high energy substrates.

Firstly, ATP that is stored in the myosin cross-bridges (within the muscle) gets broken down producing adenosine diphosphate (ADP) and one single phosphate molecule. Then, an enzyme, known as creatine kinase, breaks down phosphocreatine (PC) to creatine and a phosphate molecule. This breakdown of phosphocreatine (PC) releases energy, which allows the adenosine diphosphate (ADP) and phosphate molecule to re-join forming more ATP. This newly formed ATP can then be broken down to release energy to fuel activity. This will continue until creatine phosphate stores are depleted.

Short, sharp explosive bursts of exercise (10-30 secs) use this system. It doesn’t require oxygen but is very limited to short periods of explosive exercise, such as a sprint or weight/power lifting. This is why creatine supplementation helps this sort of exercise, ensuring there is adequate creatine phosphate to provide those required phosphates. The ATP-CP system usually recovers 100% in 3 mins so, the recommended rest time in between high intensity training is 3 minutes.

In short, for sharp explosive bursts of exercise needing FAST, IMMEDIATE energy this system produces COPIUS AMOUNTS OF ATP until the creatine phosphate in muscles runs out.

Different forms of exercise use different systems to produce ATP

  • For short distance sprinters/ weight lifters the energy system used would be ATP-PC as its fast and only few seconds
  • During intense, intermittent exercise and throughout prolonged physical activity the energy system used would typically be via the glycogen route (fat burning /no oxygen) https://www.ncbi.nlm.nih.gov/pmc/articles/PMC6019055/
  • In endurance events like marathon running or rowing etc., which lasts for unlimited time would use the energy process of aerobic respiration.

Role of gut bacteria in energy regulation

Gut bacteria plays an important role in nutrient and energy extraction and energy regulation. The bacteria makes a multitude of small molecules (known as metabolites) that can act as signals that can modulate appetite, energy uptake, storage and expenditure, something which is explored in the review article Gut Microbiota-Dependent Modulation of Energy Metabolism.

Gut bacteria influences the bioavailability of polysaccharides and how this occurs is unclear but it is an increasing area of research, with this 2016 paper, on the causality of small and large intestinal microbiota in weight regulation and insulin resistance, investigating the subject at length.

Side effects with low energy levels

Not properly managing your energy levels can result in both physical and cognitive functions being affected.

Physical signs can include: reduced stamina, reduced strength and less ability to recover from exercise.

Performance related effects can include: loss of focus, slow reaction times, low mood, poor working memory, poor decision making and decreased reaction times.


Chapter 4: Introduction to How Cells Obtain Energy

Figure 4.1 A hummingbird needs energy to maintain prolonged flight. The bird obtains its energy from taking in food and transforming the energy contained in food molecules into forms of energy to power its flight through a series of biochemical reactions. (credit: modification of work by Cory Zanker)

Virtually every task performed by living organisms requires energy. Energy is needed to perform heavy labor and exercise, but humans also use energy while thinking, and even during sleep. In fact, the living cells of every organism constantly use energy. Nutrients and other molecules are imported into the cell, metabolized (broken down) and possibly synthesized into new molecules, modified if needed, transported around the cell, and possibly distributed to the entire organism. For example, the large proteins that make up muscles are built from smaller molecules imported from dietary amino acids. Complex carbohydrates are broken down into simple sugars that the cell uses for energy. Just as energy is required to both build and demolish a building, energy is required for the synthesis and breakdown of molecules as well as the transport of molecules into and out of cells. In addition, processes such as ingesting and breaking down pathogenic bacteria and viruses, exporting wastes and toxins, and movement of the cell require energy. From where, and in what form, does this energy come? How do living cells obtain energy, and how do they use it? This chapter will discuss different forms of energy and the physical laws that govern energy transfer. This chapter will also describe how cells use energy and replenish it, and how chemical reactions in the cell are performed with great efficiency.


The way in which a root cell obtains energy. Introduction: Photosynthesis is a process by which carbon dioxide and water are used to produce glucose molecules by using energy from sunlight. The process of conversion of light energy to the chemical energy occurs in the two phases of reactions, the light-dependent reaction and carbon fixation reaction (dark reactions).

To determine: The way in which a root cell obtains energy.

Introduction: Photosynthesis is a process by which carbon dioxide and water are used to produce glucose molecules by using energy from sunlight. The process of conversion of light energy to the chemical energy occurs in the two phases of reactions, the light-dependent reaction and carbon fixation reaction (dark reactions).

To determine: The way in which organic molecules obtains energy.

Introduction: Photosynthesis is a process by which carbon dioxide and water are used to produce glucose molecules by using energy from sunlight. The process of conversion of light energy to the chemical energy occurs in the two phases of reactions, the light-dependent reaction and carbon fixation reaction (dark reactions).


Introduction

Virtually every task performed by living organisms requires energy. Energy is needed to perform heavy labor and exercise, but humans also use energy while thinking, and even during sleep. In fact, the living cells of every organism constantly use energy. Nutrients and other molecules are imported into the cell, metabolized (broken down) and possibly synthesized into new molecules, modified if needed, transported around the cell, and possibly distributed to the entire organism. For example, the large proteins that make up muscles are built from smaller molecules imported from dietary amino acids. Complex carbohydrates are broken down into simple sugars that the cell uses for energy. Just as energy is required to both build and demolish a building, energy is required for the synthesis and breakdown of molecules as well as the transport of molecules into and out of cells. In addition, processes such as ingesting and breaking down pathogenic bacteria and viruses, exporting wastes and toxins, and movement of the cell require energy. From where, and in what form, does this energy come? How do living cells obtain energy, and how do they use it? This chapter will discuss different forms of energy and the physical laws that govern energy transfer. This chapter will also describe how cells use energy and replenish it, and how chemical reactions in the cell are performed with great efficiency.


Cellular Respiration: Definition, Equation and Stages

The process of obtaining energy in order to produce ATP molecules is called cellular respiration.

More Bite-Sized Q&As Below

2. What compound is phosphorylated for ATP formation? What is the resulting compound when ATP releases energy?

ATP, or adenosine triphosphate, is formed after the binding of one phosphate molecule (phosphorylation) to one ADP (adenosine diphosphate) molecule. This is a process that stores energy in the produced ATP molecule.

When ATP provides energy to the cellular metabolism, it releases one of its phosphate ions and ADP reappears.

ADP can also release more phosphate ions and generate AMP (adenosine monophosphate) or even non-phosphorylated adenosine. Adenosine production from ATP is used in tissues that need an urgent supply oxygen, such as in the heart during a myocardial infarction (heart attack). This is because adenosine creates a local vasodilator effect, thus providing faster vasodilation than other physiological methods.

Aerobic and Anaerobic Cell Respiration

3. What are the types of cell respiration?

There are two types of cell respiration: aerobic cell respiration, a reaction with the participation of molecular oxygen (O₂) and anaerobic cell respiration, without the participation of molecular oxygen and which uses other inorganic molecules as an oxidant instead. There are several varieties of anaerobic cell respiration. The main one is fermentation.

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Fermentation

4. Under which conditions do aerobic cells use fermentation?

Some cells that usually obtain energy from aerobic cellular respiration can use fermentation when oxygen is not available.

There are bacteria and fungi that, in the absence of oxygen, use their anaerobic metabolic capability for energy supply. Muscle cells also use fermentation when oxygen is scarce.

5. What is the difference between aerobic and anaerobic organisms?

Aerobic organisms are those whose cells do not survive without oxygen, since they depend on aerobic cell respiration to obtain energy for ATP production. Anaerobic organisms are those that live or can live in environments devoid of oxygen.

6. What is the difference between facultative anaerobes and obligate anaerobes?

Facultative anaerobes, like the fungi Saccharomyces cerevisiae, a brewing yeast, can survive in oxygen-poor environments by using fermentation. However, when oxygen is available, these organisms use aerobic respiration.

Obligate anaerobes are those that cannot survive when oxygen is present. Some fungi, some bacteria (like the agent of botulism Clostridium botulinum, and the agent of tetanus, Clostridium tetani) and some protozoans are examples of obligate anaerobes.

7. What are the two types of fermentation? What are their chemical equations?

The two main types of fermentation are alcoholic fermentation and lactic fermentation.

In alcoholic fermentation, pyruvic acid, an intermediate molecule, is converted into ethanol with the release of carbon dioxide. The equation for alcoholic fermentation is as follows:

C₆H₁₂O₆ + 2 ADP + P --> 2 C₂H₅OH + 2 CO₂ + 2 ATP

In lactic fermentation, pyruvic acid is transformed into lactic acid and there is no production of carbon dioxide. The equation for lactic fermentation is:

C₆H₁₂O₆ + 2 ADP + P --> 2 C₃H₅OOH + 2 ATP

8. In general, what are the reagents and products of fermentation?

In fermentation, glucose (sugar) is broken down into pyruvic acid (each glucose molecule forms two pyruvic acid molecules). In this process, two molecules of ATP are produced.

Depending on to the type of fermentation, pyruvic acid can produce ethanol and carbon dioxide (in alcoholic fermentation) or lactic acid (in lactic fermentation). There are other varieties of fermentation in which pyruvic acid can generate acetic acid (acetic fermentation), propionic acid, isopropanol (also a type of alcohol), etc. The type of fermentation depends on the species of the involved organisms.

9. Why are alcoholic fermenting organisms and not lactic fermenting organisms used in the cooking of cakes and breads?

Fermentation causes cakes and breads to grow. This is accomplished by the release of carbon dioxide in alcoholic fermentation, as the gas passes through the dough and makes it grow. In lactic fermentation, there is no release of carbon dioxide and the desired result would not be obtained.

10. What substance causes the acidic flavor of fermented milk?

Some bacteria ferment milk lactose via lactic fermentation, producing lactic acid. This product is responsible for the acidic flavor of yogurt, curds and milk. 

11. How can knowledge pf fermentation explain the origin of muscle cramps and pains after intense physical exercise?

A typical fermentation process due to oxygen scarcity happens in muscle tissue. Under intense use, muscles require too much energy (ATP) and consume much more oxygen to produce that energy. High consumption leads to oxygen scarcity and the muscle cells begin to use lactic fermentation to satisfy their energy needs. In this situation, muscle pain, cramps and fatigue are due to the lactic acid released by fermentation.

Aerobic Respiration

12. How many ATP molecules are produced for each glucose molecule used in fermentation? How many ATP molecules are produced for each glucose molecule used in aerobic respiration?

In fermentation, two ATP molecules are produced from one glucose molecule. In aerobic respiration, a much more productive process, 36 ATP molecules are made from one glucose molecule.

13. What cell organelle is specialized in aerobic respiration?

The cell organelles specialized in aerobic respiration are the mitochondria. 

14. What are the main components of the mitochondrion structure?

Mitochondria are organelles enclosed by two lipid membranes. The inner membrane invaginates to the interior of the organelle, forming cristae and delimiting an internal space known as the mitochondrial matrix. 

15. What are the three phases of cell respiration?

The three phases of aerobic cell respiration are glycolysis, the Krebs cycle and the respiratory chain (also known as the electron transport chain).

Glycolysis

16. What is glycolysis? What are the products of this process?

Glycolysis, the first stage of aerobic cell respiration, is a process in which glucose is broken down to form two pyruvic acid molecules along with the formation of two ATP and two NADH molecules.

Glycolysis is a complex reaction that involves the formation of several intermediate molecules until pyruvic acid molecules are made. Although two ATP molecules are consumed in the reaction, four molecules of ATP are also produced therefore, a positive balance of two ATP molecules is obtained. Two NADH molecules are also produced. In glycolysis, the 6-carbon structure of glucose is broken down and two organic chains of three carbons each are produced. These chains produce two pyruvic acid molecules. 

17. Does glycolysis occur within the mitochondria?

Glycolysis happens in the cytosol and not within the mitochondria. Pyruvic acid molecules later enter the mitochondria to participate in the next phase of aerobic cell respiration.

18. How many ATP molecules are made after glycolysis?

Glycolysis is a process similar to the breaking down of glucose in fermentation. It produces (a final balance of) two molecules of ATP for each glucose broken down. 

19. What is NAD? What is the role of the NAD molecule in glycolysis?

NAD (nicotinamide adenine dinucleotide) is a hydrogen acceptor and necessary reductant (to receive hydrogen) in some reactions, in which it is reduced and converted into NADH₂. During glycolysis, two NAD molecules gain hydrogen ions released after an intermediate reaction, thus forming NADH₂. 

The Krebs Cycle

20. What happens to the pyruvic acid molecules made by glycolysis during aerobic respiration? What is the sequence of reactions that follows?

The pyruvic acid molecules made in the cytosol via glycolysis enter the mitochondria.

Within the mitochondria, each pyruvic acid molecule is converted into one molecule of acetyl-CoA (acetyl coenzyme A), releasing one carbon dioxide molecule. The Krebs Cycle (also known as the citric acid cycle), the second stage of aerobic respiration, then begins.

21. What is the official name of pyruvic acid?

Pyruvic acid is 2-oxopropanoic acid. Therefore, it is composed of three linearly bound carbon atoms with one extremity forming the organic acid function (COOH) and the middle carbon atom binding to an oxygen atom by double bond.

22. Why can it be said that each glucose molecule moves the Krebs cycle twice?

Each glucose molecule “cycles” the Krebs cycle twice because, after glycolysis, each used glucose molecule has generated two pyruvic acid molecules and each pyruvic acid molecule is converted in a 1:1 proportion into acetyl CoA. Each acetyl CoA molecule then goes through a Krebs cycle.

23. Why is the Krebs cycle also called the final common pathway of the breaking down of organic compounds?

The Krebs cycle is called the final common pathway of the breaking down of organic compounds because it is also possible to generate acetyl CoA from the breaking down of lipids and proteins. Since acetyl CoA is the substrate that triggers the Krebs cycle, this process is called the final common pathway because it is activated by other organic molecules (lipids and proteins) and not only by glucose.

The body uses its energy reserves of fat and protein to cycle the Krebs cycle when experiencing malnutrition or when there is no glucose available for the cells. 

24. What are the final energy products of each round of the Krebs cycle? Where is the majority of the useful energy found at the end of Krebs cycle?

After each round of the Krebs cycle, two carbon dioxide molecules, eight protons (hydrogen ions) captured by NAD and FAD (also a hydrogen acceptor) and one ATP molecule are produced.

During the Krebs cycle, acetyl CoA is broken down. At the end, the useful energy is incorporated into hydrogen atoms transported by FADH₂ਊnd NADH₂ molecules. 

25. How many carbon dioxide molecules are released after each cycle of the Krebs cycle? For a single glucose molecule, how many carbon dioxide molecules were already released by aerobic respiration up to that point?

Each round of the Krebs cycle releases two carbon dioxide molecules.

At the end of the cycle, all carbon atoms from the original glucose molecule broken down ਍uring glycolysis are already released, incorporated in carbon dioxide molecules. That occurs because, for each glucose, two pyruvic acid molecules were made during glycolysis. Each of these two pyruvic acid molecules is then converted into acetyl CoA with the release of one carbon dioxide molecule (two in total). Since each of the two produced acetyl CoA molecules cycles the Krebs cycle once, the initial glucose molecule triggers two rounds of the Krebs cycle and, as a result, four other carbon dioxide molecules are produced.

All of the six carbons atoms of the glucose molecule are then incorporated into six carbon dioxide molecules (two made during acetyl CoA formation and four during the two cycles of the Krebs cycle).

The Respiratory Chain

26. Where in the mitochondria does the process called the respiratory chain occur? Which products of the Krebs cycle are used in that final phase of aerobic respiration?

The respiratory chain, or the electron transport chain, is performed by protein systems located in the inner membrane of the mitochondria. Energized electrons of hydrogen atoms transported by NADH₂ਊnd FADH₂ਊre the products of the preceding phases which are used in the respiratory chain.

27. What are cytochromes?

Cytochromes are proteins of the internal mitochondrial membrane that are specialized in electron transfer and which participate in the respiratory chain. Energized electrons released by the hydrogen donors NADH₂ਊnd FADH₂ (then reconverted into NAD and FAD) pass through a sequence of cytochromes, losing energy each time. The energy is then used in the synthesis of ATP. 

28. During what part of the respiratory chain do electrons from FADH₂ਊnd NADH₂ passing through cytochromes release energy for ATP synthesis? What is this ATP synthesis called?

FADH₂ਊnd NADH₂ਊre oxidized into FAD and NAD and release hydrogen ions and highly energized electrons at the beginning of the respiratory chain.

The energy lost by electrons passing through the cytochromes is used to pump protons (hydrogen ions) out of the inner mitochondrial membrane (to the region between the inner and the outer membranes of the mitochondrion). The concentration gradient of hydrogen between the inner and the outer spaces separated by the inner membrane forces protons (hydrogen ions) to return to the mitochondrial matrix (the region inside the inner membrane). However, that return is only possible if the hydrogen ions pass through an enzymatic complex called ATP synthetase, which is embedded in the inner membrane. During that passage, the ATP synthetase phosphorylates ADP and ATP molecules are produced.

Hydrogen released in the mitochondrion then bonds with oxygen to form water. As a reaction that depends on oxygen, this type of ATP synthesis is called oxidative phosphorylation.

29. Until the Krebs cycle, aerobic respiration can be described without mentioning oxygen, the chemical element after which the reaction gets its name. In which part of the process does this chemical element take part? What is its importance?

Oxygen is involved in aerobic respiration in its final phase, the respiratory chain. It is of fundamental importance because it is responsible for the maintenance of the hydrogen concentration gradient between the spaces separated by the inner mitochondrial membrane. This gradient promotes the functioning of ATP synthetase and therefore the phosphorylation of ADP to form ATP. In the space inside the inner membrane, oxygen binds to free hydrogens atoms to form water. This hydrogen consumption maintains the hydrogen gradient and the traffic of protons through the ATP synthetase.

The entire aerobic respiration process takes place to make ATP synthetase work. Aerobic organisms, like us humans, for example, need to breathe oxygen to maintain the hydrogen concentration gradient and to keep ATP synthetase working.

30. What is the effect of the poison cyanide on aerobic respiration?

Cyanide is a poison that inhibits the last cytochrome of the respiratory chain, interrupting ATP formation and thus causing the cell to die.

31. What is anoxia?

Anoxia is a situation in which there is no available oxygen in the cell. Without oxygen, the respiratory chain stops, there is no ATP production, the cell does not obtain energy and it dies.

Anoxia can be caused, for example, by pulmonary insufficiency (drowning, extensive pulmonary injuries, etc.), by obstructions, stoppages and deficiencies in tissue circulation (atherosclerosis of the coronary arteries that irrigate the myocardium, tourniquets, cardiac arrest), by hemolysis (lysis of red blood cell) or hemoglobin diseases (anemia, fetal erythroblastosis), etc. 

The Efficiency of Aerobic Respiration

32. How many ATP molecules are produced after in aerobic respiration and what is the net energy gain of the process?

After aerobic respiration, 38 ATP molecules are produced from the consumption of one glucose molecule (but two of these ATP molecules are consumed by glycolysis). The net gain of the process is then 36 ATP molecules per glucose molecule. 

33. What is the general equation for aerobic respiration (also representing ADP and phosphate)?

The general equation for aerobic respiration is:

C₆H₁₂O₆ + 6 O₂ + 36 ADP + 36 P --> 6 CO₂ + 6 H₂O + 36 ATP 

34. Why can the consumption of molecular oxygen indicate the metabolic rate of aerobic organisms?

The consumption of molecular oxygen (O₂) has a direct relationship with metabolic rate of aerobic cells and therefore the metabolic rate of organisms. Cells with a greater metabolic activity require more energy and this energy comes from ATP molecules. As ATP production is required, the intensity of aerobic cell respiration is also higher and more oxygen is consumed.

Now that you have finished studying Cell Respiration, these are your options:


Branches of Biology

· Agriculture – the study of producing crops from the land, with an emphasis on practical applications

· Anatomy – the study of form and function, in plants, animals, and other organisms, or specifically in humans

· Biochemistry – the study of the chemical reactions required for life to exist and function, usually a focus on the cellular level

· Botany – the study of plants

· Cell biology – the study of the cell as a complete unit, and the molecular and chemical interactions that occur within a living cell

· Ecology – the study of the interactions of living organisms with one another and with the non-living elements of their environment

· Microbiology – the study of microscopic organisms (microorganisms) and their interactions with other living things

· Molecular biology – the study of biology and biological functions at the molecular level

· Physiology – the study of the functioning of living organisms and the organs and parts of living organisms

· Zoology – the study of animals, including classification, physiology, development, and behavior.

Living Thing and Non-Living Thing

Living thing is any organism or a living form that possesses or shows the characteristics of life . Thus, they have an organized structure being made up of a cell or cells , requires energy to survive, ability to reproduce, grow, metabolize, respond to stimuli , move, respire and to adapt to the environment.

Examples of living things include the bacteria , protozoa , plants , fungi , animals , humans , etc. Viruses are not absolutely living or non-living. When outside their host , viruses are inactive and seemingly inanimate. When inside their host, they became active and alive, capable of utilizing the host cell ‘s structures and replicate.

A non-living thing is one that lacks or has stopped displaying the characteristics of life. Thus, they lack or no longer displaying the capability for growth , reproduction , respiration , metabolism , and movement . They also are not capable of responding to stimuli and adapt to their environment . They also do not require energy to continue existing. Examples of non-living things are rock, water, and sun.

Characteristics of Living Things

1. Feeding : All living organisms need to take substances from their environment to obtain energy, to grow and to stay healthy.

2. Movement : All living organisms show movement of one kind or another. All living organisms have internal movement, which means that they have the ability of moving substances from one part of their body to another. Some living organisms show external movement as well – they can move from place to place by walking, flying or swimming.

3. Breathing or Respiration : All living things exchange gases with their environment. Animals take in oxygen and breathe out carbon dioxide.

4. Excretion : Excretion is the removal of waste from the body. If this waste was allowed to remain in the body it could be poisonous. Humans produce a liquid waste called urine. We also excrete waste when we breathe out. All living things need to remove waste from their bodies.

5. Growth : When living things feed they gain energy. Some of this energy is used in growth. Living things become larger and more complicated as they grow.

6. Sensitivity or Irritability : Living things react to changes around them. We react to touch, light, heat, cold and sound, as do other living things.

7. Reproduction : All living things produce young. Humans make babies, cats produce kittens and pigeons lay eggs. Plants also reproduce. Many make seeds which can germinate and grow into new plants.

Differences between Plants and Animals

1) Plants generally are rooted in one place and do not move on their own (locomotion).

Most animals have the ability to move fairly freely.

2) Plants contain chlorophyll and can make their own food, this is called Photosynthesis.

Animals cannot make their own food and are dependent on plants and other animals for food.

3) Plants give off oxygen and take in carbon dioxide given off by animals.

Animals give off carbon dioxide which plants need to make food and take in oxygen which they need to breathe.

4) Plants cells have cell walls and other structures differ from those of animals.

Animal cells do not have cell walls and have different structures than plant cells.

5) Plants have either no or very basic ability to sense.

Animals have a much more highly developed sensory and nervous system.

Growth occurs equally on all parts.

1. Which of the following is NOT a characteristic of life? (a) Movement (b) Respiration (c) Tissue (d) Reproduction Answer: Tissue

2. ————————- is the removal of waste from the body. (a) Irritability (b) Excretion (c) Adaptation (d) Growth Answer: Excretion

3. Plants contain chlorophyll and can make their own food by ———————– (a) Respiration (b) Mouth (c) Photosynthesis (d) Preying Answer: Photosynthesis

4. The ability to respond to stimuli is called ——————- (a) Irritability (b) Excretion (c) Adaptation (d) Growth Answer: Irritability

5. The following are the branches of biology except —————– (a) Ecology (b) Botany (c) Zoology (d) philosophy Answer: Philosophy