Chapter 4 Glossary, Summary, and Practice Questions

KEY TERMS

acetyl CoA the combination of an acetyl group derived from pyruvic acid and coenzyme A which is made from pantothenic acid (a B-group vitamin)

activation energy the amount of initial energy necessary for reactions to occur

active site a specific region on the enzyme where the substrate binds

allosteric inhibition the mechanism for inhibiting enzyme action in which a regulatory molecule binds to a second site (not the active site) and initiates a conformation change in the active site, preventing binding with the substrate

anabolic describes the pathway that requires a net energy input to synthesize complex molecules from simpler ones

anaerobic cellular respiration the use of an electron acceptor other than oxygen to complete metabolism using electron transport-based chemiosmosis

ATP (also, adenosine triphosphate) the cell’s energy currency

ATP synthase a membrane-embedded protein complex that regenerates ATP from ADP with energy from protons diffusing through it

bioenergetics the concept of energy flow through living systems

catabolic describes the pathway in which complex molecules are broken down into simpler ones, yielding energy as an additional product of the reaction

chemiosmosis the movement of hydrogen ions down their electrochemical gradient across a membrane through ATP synthase to generate ATP

citric acid cycle a series of enzyme-catalyzed chemical reactions of central importance in all living cells that harvests the energy in carbon-carbon bonds of sugar molecules to generate ATP; the citric acid cycle is an aerobic metabolic pathway because it requires oxygen in later reactions to proceed

competitive inhibition a general mechanism of enzyme activity regulation in which a molecule other than the enzyme’s substrate is able to bind the active site and prevent the substrate itself from binding, thus inhibiting the overall rate of reaction for the enzyme

electron transport chain a series of four large, multi-protein complexes embedded in the inner mitochondrial membrane that accepts electrons from donor compounds and harvests energy from a series of chemical reactions to generate a hydrogen ion gradient across the membrane

endergonic describes a chemical reaction that results in products that store more chemical potential energy than the reactants

enzyme a molecule that catalyzes a biochemical reaction

exergonic describes a chemical reaction that results in products with less chemical potential energy than the reactants, plus the release of free energy

feedback inhibition a mechanism of enzyme activity regulation in which the product of a reaction or the final product of a series of sequential reactions inhibits an enzyme for an earlier step in the reaction series

fermentation the steps that follow the partial oxidation of glucose via glycolysis to regenerate NAD+; occurs in the absence of oxygen and uses an organic compound as the final electron acceptor

glycolysis the process of breaking glucose into two three-carbon molecules with the production of ATP and NADH

heat energy the energy transferred from one system to another that is not work

kinetic energy the type of energy associated with objects in motion

metabolism all the chemical reactions that take place inside cells, including those that use energy and those that release energy

noncompetitive inhibition a general mechanism of enzyme activity regulation in which a regulatory molecule binds to

a site other than the active site and prevents the active site from binding the substrate; thus, the inhibitor molecule does not compete with the substrate for the active site; allosteric inhibition is a form of noncompetitive inhibition

oxidative phosphorylation the production of ATP by the transfer of electrons down the electron transport chain to

create a proton gradient that is used by ATP synthase to add phosphate groups to ADP molecules

potential energy the type of energy that refers to the potential to do work

substrate a molecule on which the enzyme acts

thermodynamics the science of the relationships between heat, energy, and work

CHAPTER SUMMARY

Energy and Metabolism

Cells perform the functions of life through various chemical reactions. A cell’s metabolism refers to the combination of chemical reactions that take place within it. Catabolic reactions break down complex chemicals into simpler ones and are associated with energy release. Anabolic processes build complex molecules out of simpler ones and require energy.

In studying energy, the term system refers to the matter and environment involved in energy transfers. Entropy is a measure of the disorder of a system. The physical laws that describe the transfer of energy are the laws of thermodynamics. The first law states that the total amount of energy in the universe is constant. The second law of thermodynamics states that every energy transfer involves some loss of energy in an unusable form, such as heat energy. Energy comes in different forms: kinetic, potential, and free. The change in free energy of a reaction can be negative (releases energy, exergonic) or positive (consumes energy, endergonic). All reactions require an initial input of energy to proceed, called the activation energy.

Enzymes are chemical catalysts that speed up chemical reactions by lowering their activation energy. Enzymes have an active site with a unique chemical environment that fits particular chemical reactants for that enzyme, called substrates. Enzymes and substrates are thought to bind according to an induced-fit model. Enzyme action is regulated to conserve resources and respond optimally to the environment.

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. Because it is used by nearly all organisms on earth, it must have evolved early in the history of life. Glycolysis consists of two parts: The first part prepares the six-carbon ring of glucose for separation into two three-carbon sugars. Energy from ATP is invested into the molecule during this step to energize the separation. The second half of glycolysis extracts ATP and high-energy electrons from hydrogen atoms and attaches them to NAD+. Two ATP molecules are invested in the first half and four ATP molecules are formed during the second half. This produces a net gain of two ATP molecules per molecule of glucose for the cell.

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. The electrons are passed through a series of chemical reactions, with a small amount of free energy used at three points to transport hydrogen ions across the membrane. This contributes to the gradient used in chemiosmosis. As the electrons are passed from NADH or FADH₂ down the electron transport chain, they lose energy. The products of the electron transport chain are water and ATP. A

number of intermediate compounds can be diverted into the anabolism of other biochemical molecules, such as nucleic acids, non-essential amino acids, sugars, and lipids. These same molecules, except nucleic acids, can serve as energy sources for the glucose pathway.

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.

Connections to Other Metabolic Pathways

The breakdown and synthesis of carbohydrates, proteins, and lipids connect with the pathways of glucose catabolism. The carbohydrates that can also feed into glucose catabolism include galactose, fructose, and glycogen. These connect with glycolysis. The amino acids from proteins connect with glucose catabolism through pyruvate, acetyl CoA, and components of the citric acid cycle. Cholesterol synthesis starts with acetyl CoA, and the components of triglycerides are picked up by acetyl CoA and enter the citric acid cycle.

ART CONNECTION QUESTIONS

• Figure 4.6 Look at each of the processes shown and decide if it is endergonic or exergonic.
• Figure 4.15 Cyanide inhibits cytochrome c oxidase, a component of the electron transport chain. If cyanide poisoning occurs, would you expect the pH of the intermembrane space to increase or decrease? What affect would cyanide have on ATP synthesis?

REVIEW QUESTIONS

• Which of the following is not an example of an energy transformation?
• Heating up dinner in a microwave
• Solar panels at work
• Formation of static electricity
• None of the above
• Which of the following is not true about enzymes?

• They are consumed by the reactions they catalyze.
• They are usually made of amino acids.
• They lower the activation energy of chemical reactions.
• Each one is specific to the particular substrate(s) to which it binds.
• Energy is stored long-term in the bonds of and used short-term to perform work from a(n) molecule.
• ATP : glucose
• an anabolic molecule : catabolic molecule
• glucose : ATP
• a catabolic molecule : anabolic molecule
• The energy currency used by cells is .
• ATP
• ADP
• 
  Figure 4.16 Tremetol, a metabolic poison found in white snake root plant, prevents the metabolism of lactate. When cows eat this plant, Tremetol is concentrated in the milk. Humans who consume the milk become ill. Symptoms of this disease, which include vomiting, abdominal pain, and tremors, become worse after exercise. Why do you think this is the case?

• AMP
• adenosine
• The glucose that enters the glycolysis pathway is split into two molecules of .
• ATP
• phosphate
• NADH
• pyruvate
• What do the electrons added to NAD+ do?
• They become part of a fermentation pathway.
• They go to another pathway for ATP production.
• They energize the entry of the acetyl group into the citric acid cycle.
• They are converted into NADP.
• Chemiosmosis involves
• the movement of electrons across the cell membrane
• the movement of hydrogen atoms across a mitochondrial membrane
• the movement of hydrogen ions across a mitochondrial membrane
• the movement of glucose through the cell membrane
• Which of the following fermentation methods can occur in animal skeletal muscles?

• lactic acid fermentation
• alcohol fermentation
• mixed acid fermentation
• propionic fermentation
• The cholesterol synthesized by cells uses which component of the glycolytic pathway as a starting point?

• glucose

CRITICAL THINKING QUESTIONS

• Does physical exercise to increase muscle mass involve anabolic and/or catabolic processes? Give evidence for your answer.
• Explain in your own terms the difference between a spontaneous reaction and one that occurs instantaneously, and what causes this difference.
• With regard to enzymes, why are vitamins and minerals necessary for good health? Give examples.
• Both prokaryotic and eukaryotic organisms carry out some form of glycolysis. How does that fact support or not
• 
  acetyl CoA
• pyruvate
• carbon dioxide
• Beta oxidation is .
• the breakdown of sugars
• the assembly of sugars
• the breakdown of fatty acids
• the removal of amino groups from amino acids

support the assertion that glycolysis is one of the oldest metabolic pathways?

• We inhale oxygen when we breathe and exhale carbon dioxide. What is the oxygen used for and where does the carbon dioxide come from?
• When muscle cells run out of oxygen, what happens to the potential for energy extraction from sugars and what pathways do the cell use?
• Would you describe metabolic pathways as inherently wasteful or inherently economical, and why?