Chapter 9 Questions: Glucose Metabolism

Why is the electron transport chain also called oxidative phosphorylation?

As electrons are transferred, a carrier is oxidized. The process of electron transfer assists in setting up the electrochemical gradient that drives phosphorylation of ATP from ADP.

A cell has enough available ATP to meet its needs for about 30 seconds. So what happens in the athlete when he exhausts his ATP supply?

Catabolic processes are activated that generate ATP.

What mode of ATP production would you expect most organisms to use in a deep-sea environment with low oxygen? What mode would they use on land?

Deep sea—anaerobic respiration; on land—aerobic respiration

Which molecule is reduced in the following chemical reaction? Glyceraldehyde phosphate + NAD+ ---> diphosphoglycerate + NADH + H+

NAD+

In the energy-yielding phase of glycolysis, energy is extracted in the form of:

NADH and ATP

Following glycolysis and Krebs cycle, the carbon skeleton of glucose has been broken down to CO2. Most of the energy from the original glucose is in the form of:

high-energy electrons that are associated with electron carriers

Where does the Krebs cycle occur in eukaryotes?

in the matrix of mitochondria

If a cell is treated with a drug that inhibits ATP synthase, the pH in the mitochondrial matrix will:

increase

If oxygen is removed from a human muscle cell, the concentration of lactate will:

increase

The metabolic pathway, glycolysis is active when cellular energy levels are ____________; the regulatory enzyme, phosphofructokinase is ____________ by ATP.

low; inhibited

In the electron transport chain, ATP is produced by:

movement of protons down their gradient through a proton channel that is coupled to ATP synthase

Which of the following chemical reactions does not occur during glycolysis?

oxidation of NADH

What characteristic of this molecule (ATP) is responsible for its high energy level?

the closely spaced negative charges associated with the phosphate groups

At the end of the Krebs cycle, but before the electron transport chain, the oxidation of glucose has produced a net yield of:

6 CO2, 10 NADH, 2 FADH2, and 4 ATP

Most agricultural societies have come up with ways to ferment the sugars in barley, wheat, rice, corn, or grapes to produce alcoholic beverages. Historians argue that this was an effective way for farmers to preserve the chemical energy in grains and fruits in a form that would not be eaten by rats or spoiled by bacteria or fungi. Why does a great deal of chemical energy remain in the products of fermentation pathways?

A great deal of energy remains in the products of the fermentation pathway because pyruvate is not a very strong oxidizing agent. This means that the potential energy drop between glucose and pyruvate is relatively small. Thus much of the potential energy left after the oxidation of pyruvate during fermentation is stored in the alcohol or other fermentation products.

There is a great deal of energy in the glucose molecule in its:

C-H bonds

What is the relationship between cellular respiration and fermentation? Why does cellular respiration produce so much more ATP than fermentation does?

Cellular respiration and fermentation are two different catabolic processes that cells can use to harness the chemical energy in glucose to fuel the production of ATP. Both processes begin with glycolysis. But in cellular respiration pyruvate, the product of glycolysis, is further metabolized through the Krebs cycle. Most of the energy in the glucose molecule is transferred (by electrons) to NADH and FADH2, which go on to donate electrons to the electron transport chain, which indirectly fuels the production of ATP by chemiosmosis and ATP synthase. Cellular respiration can take place only in the presence of oxygen, because it is the final electron acceptor in the electron transport chain. But in the absence of oxygen there is no final electron acceptor, so all members of the electron transport chain become stuck in a reduced state, since the next molecule in the pathway/chain is unable to accept its electron. In a very short time, all the NAD+ in the cell becomes reduced to NADH. Without NAD+, glycolysis cannot continue. In this situation, pyruvate can be used as an oxidizing agent by the enzymes of fermentation pathway to oxidize NADH to NAD+, allowing glycolysis to continue. During these reactions, by-products such as lactate or ethanol are produced from the reduction of pyruvate. Cellular respiration produces substantially more ATP than fermentation does, because oxygen is a much stronger oxidizing agent than pyruvate. This means that the potential energy drop between glucose and oxygen is much greater than the potential energy drop between glucose and pyruvate.

Glucose can be converted to fatty acid, but fatty acid cannot be converted to glucose. What does this result suggest?

Conversion of acetyl CoA to pyruvate does not occur significantly in a cell.

What does the chemiosmotic hypothesis claim?

Electron transport chains generate ATP indirectly, by the creation of a proton-motive force.

Glycolysis, an energy-generating metabolic pathway, takes place in the mitochondria of all cells. True or false?

False

Prokaryotes use a different series of metabolic reactions to break down glucose to two pyruvate molecules than do eukaryotes. True or false?

False

A substrate-level phosphorylation occurs in the Krebs cycle where:

GDP is phosphorylated to produce GTP.

Two ATP molecules are expended in the energy investment phase. Why is this energy necessary to begin the process of glucose catabolism?

Glucose contains a great deal of energy in its chemical bonds, but it is a stable molecule; thus, some energy must be invested to make the molecule unstable and begin the process of catabolism.

Which of the following is accomplished during the reactions of glycolysis?

Glucose is converted to pyruvate.

Diagram the relationships among the three components of cellular respiration: glycolysis, the Krebs cycle, and electron transport. What molecules connect these three processes? Where does each process occur in a eukaryotic cell?

Glycolysis—Glucose: pyruvate -2 ATP, 2 NADH released per glucose -This process occurs in the cytoplasm of a eukaryotic cell. Pyruvate oxidation—Pyruvate: acetyl CoA -2 NADH released per glucose -This process occurs in the cytoplasm of a eukaryotic cell. Krebs cycle—Acetyl CoA + citrate: oxaloacetate -2 ATP, 6 NADH, 2 FADH2 released per glucose -6 CO2 released per glucose; these are the six fully oxidized carbons of the original glucose molecule. -This process occurs in the mitochondrial matrix of a eukaryotic cell. Electron transport chain—The NADH and FADH2 molecules from glycolysis, pyruvate oxidation, and the Krebs cycle all donate electrons to the electron transport chain that generated the energy for chemiosmosis (the creation of the proton-motive force) and the synthesis of ATP by ATP synthase.

An athlete is running a marathon. His muscles are using ATP faster than it can be produced by aerobic respiration. How does the runner continue the race?

His muscle cells use lactic acid fermentation to continue producing ATP by substrate-level phosphorylation.

What would happen to the glycolytic pathway if a cell ran completely out of ATP?

If a cell ran completely out of ATP, it could not perform glycolysis, because the first and third steps require ATP input before ATP is made in the final steps.

Malate + NAD+ oxaloacetate + NADH + H+ Which of the following statements concerning the above reaction is true?

Malate is more reduced than oxaloacetate.

Describe the relationships among carbohydrate metabolism, the catabolism of proteins and fats, and anabolic pathways.

In the absence of sufficient glucose, stored carbohydrates can be broken down into glucose and fructose monomers that can be used by the glycolytic pathway. In the absence of sufficient carbohydrate stores, both fats and proteins can be used to fuel the production of ATP. Fats can be broken down into glycerol, which enters the glycolytic pathway after being converted to glyceraldehyde 3-phosphate via phosphorylation, and acetyl CoA, which can enter the Krebs cycle. The amino acids released from the metabolism of proteins can be enzymatically converted to pyruvate, acetyl CoA, and other intermediates of glycolysis and the Krebs cycle. In the presence of an excess of ATP, these reactions can be reversed; and carbohydrates, fats, and proteins can be synthesized from the intermediates of glycolysis and the Krebs cycle.

When phosphofructokinase is inactivated by high levels of ATP, what can the cell do with its glucose?

It can make glycogen (if animal) or starch (if plant), or use the glucose in RNA and DNA synthesis.

Fatty acids are typically an even number of carbons. They are catabolized by a process called beta-oxidation. The end-products of the metabolic pathway are two-carbon fragments called acetyl CoA. What is the most likely fate of acetyl CoA?

It enters the Krebs cycle.

How does transferring a phosphate group to a protein affect the function of the protein?

It increases the energy level of the protein.

Pyruvate, the end-product of glycolysis, is actively transported into the mitochondrion. This means:

It is an energy-requiring process.

What is the function of coenzyme A in the Krebs cycle?

It is the coenzyme of acetylation reactions.

What is the function of NAD+ in glycolysis?

It serves as an electron carrier.

Which of the following statements concerning cellular metabolism is false?

Krebs cycle activity is dependent solely on availability of substrate; otherwise it is unregulated.

Explain why NADH and FADH2 are called electron carriers. Where do these molecules get electrons, and where do they deliver them? In eukaryotes, what molecule do these electrons reduce?

NADH and FADH2 are called electron carriers because they can readily donate electrons to more oxidized molecules. They are said to have reducing power because when they donate electrons to another molecule, they reduce that molecule. NADH and FADH2 get their electrons from the intermediates of glycolysis (specifically glyceraldehyde phosphate) and the Krebs cycle (specifically isocitrate, α-ketoglutarate, succinate, and malate), and they deliver them to the proteins of the electron transport chain. Specifically in eukaryotes, NADH reduces a flavin-containing protein called flavin mononucleotide (FMN), and FADH2 reduces an iron- and sulfur-containing protein. Each of these proteins then reduces ubiquinone, which reduces the next complex in the electron transport chain.

Why does NADH donate electrons to the beginning of the electron transport chain, whereas FADH2 donates electrons to the middle of the chain?

NADH has more potential energy than FADH2.

What would have been the result of Krebs' experiment if the Krebs cycle were linear instead of circular?

Oxaloacetate + pyruvate would have produced no products.

Explain the relationship between electron transport and oxidative phosphorylation. What does ATP synthase look like, and how does it work?

Oxidative phosphorylation refers to the synthesis of ATP by the enzyme ATP synthase. This enzyme couples the energy-releasing diffusion of protons down their concentration gradient to the phosphorylation of ADP to form ATP. The energy to create the proton-motive force (the concentration gradient that drives the diffusion of protons through ATP synthase) comes from the movement of electrons through a series of increasingly oxidized molecules, of which oxygen is the final electron acceptor. Many of the molecules in the electron transport chain use the energy released by oxidation to pump protons from the mitochondrial matrix to the intermembrane space. The structure of the ATP synthase protein highlights its ability to facilitate the diffusion of protons across the inner mitochondrial membrane and its ability to catalyze the phosphorylation of ADP to make ATP. The ATP synthase is a complex of polypeptide subunits, assembled into a protein that forms a cylindrical proton-specific channel in the inner mitochondrial membrane, with a knob portion that protrudes into the mitochondrial matrix. The channel and the knob are connected by a stalk region. As protons diffuse through the channel, it rotates clockwise in the inner membrane. This rotates the stalk, which activates the ATPase activity in the knob portion—which in turn phosphorylates ADP to form ATP.

If you isolated intact mitochondria and added ADP to the medium:

Oxygen consumption by the mitochondria would increase.

Why do reduced molecules tend to have a lot of C-H bonds?

Reduced molecules usually gain protons along with electrons.

In the Buchner experiment, why did boiling of the yeast extract prevent the conversion of ethanol from glucose?

Proteins were denatured.

Cyanide poisons an electron transport chain protein with less free energy than Q. Which of the following would occur during cyanide poisoning?

Q would stay reduced and would stop moving protons across the mitochondrial membrane.

Compare and contrast substrate-level phosphorylation and oxidative phosphorylation.

Substrate-level phosphorylation and oxidative phosphorylation result in the formation of ATP by the addition of an inorganic phosphate to a molecule of ADP. Both reactions are catalyzed by enzymes that couple the formation of ATP to an exergonic reaction that creates the energy for ATP synthesis. Substrate-level phosphorylation occurs when enzymes involved in glycolysis, the Krebs cycle, or fermentation remove a phosphate from a substrate in the pathway and directly transfer it to ADP. Removal of the phosphate from the substrate is the exergonic reaction that fuels endergonic synthesis of ATP. Substrate-level phosphorylation during glycolysis and fermentation occurs in the cytoplasm, while substrate-level phosphorylation during the Krebs cycle occurs in the mitochondrial matrix. On the other hand, oxidative phosphorylation refers to the synthesis of ATP by the enzyme ATP synthase. This enzyme couples the energy-releasing diffusion of protons down their concentration gradient to the phosphorylation of ADP to form ATP. It is called oxidative because the energy to create the proton-motive force (the concentration gradient that drives the diffusion through ATP synthase) comes from the movement of electrons through a series of increasingly oxidized molecules, of which oxygen is the final electron acceptor. Many of the molecules in the electron transport chain use the energy released by oxidation to pump protons from the mitochondrial matrix to the intermembrane space.

Glucose is used by nearly every living organism as a nutrient source of energy. Why?

The ability to harvest energy from glucose appeared very early in biological evolution.

Explain why it is advantageous for the inner mitochondrial membrane to be highly folded.

The folds provide more surface area—more area for electron transport chain components and ATPases.

Why might adding inorganic phosphate to a reaction mixture where glycolysis was rapidly proceeding sustain the metabolic pathway?

The metabolic intermediates of glycolysis are phosphorylated.

The presence of many saclike cristae results in a large amount of membrane inside mitochondria. Suppose that some mitochondria had few cristae. How would their output of ATP compare with that of mitochondria with many cristae? Explain your answer.

The mitochondria with unfolded cristae would necessarily have fewer electron transport chains and ATP synthase molecules on the inner mitochondrial membrane. This would cause these particular mitochondria to have a smaller ATP output than that of the mitochondria with folded cristae.

What is the purpose in having several steps in glycolysis or the Krebs cycle rather than a single step from glucose and oxygen to carbon dioxide and water?

The multistep approach makes better use of the potential energy in the reaction.

A compound called dinitrophenol (DNP) can be used to poke holes in the inner mitochondrial membrane. What would this do to ATP synthesis by oxidative phosphorylation?

When DNP puts holes in the inner mitochondrial membrane, no ATP can be produced by ATPase because the proton gradient disappears.

When one of the eight Krebs cycle intermediates is added to the respiration medium of living cells, like yeast, what happens to the rates of ATP and carbon dioxide production?

The rates of ATP production and carbon dioxide production both increase.

The energy of electron transport serves to move (translocate) protons to the outer mitochondrial compartment. How does this help the mitochondrion to produce energy?

The translocation of protons sets up the electrochemical gradient that drives ATP synthesis in the mitochondria.

Why are NADH and FADH2 said to have "reducing power?"

They donate electrons to components of the ETC, reducing those components.

Which of the following is true regarding ATP synthesis in prokaryotes?

They oxidize NADH on the cell membrane.

Why are fermentation reactions important for cells?

They regenerate NAD+ so that glycolysis can continue.

Glycolysis takes place in both eukaryotic and prokaryotic organisms. True or false?

True

It takes more energy to reduce NAD+ to NADH than to produce ATP by substrate-level phosphorylation. True or false?

True

Cyanide (C≡N-) blocks complex IV of the electron transport chain. Suggest a hypothesis for what happens to the ETC when complex IV stops working. Your hypothesis should explain why cyanide poisoning is fatal.

When cyanide blocks complex IV of the electron transport chain, all the upstream components of the electron transport chain become stuck in a reduced state because the next molecule in the pathway/chain is unable to accept its electron. In a very short time, all of the NAD+ in the cell becomes reduced to NADH. Without NAD+, glycolysis and the Krebs cycle cannot continue. This stops the production of ATP in the cell unless a fermentation pathway is available to the cell. However, cyanide poisoning is fatal because brain cells do not have the enzymes needed for fermentation. Even in cells that can undergo fermentation, there would not be enough ATP molecules to fuel the cell's energy needs over the long term.

When yeast cells are placed into low-oxygen environments, the mitochondria in the cells become reduced in size and number. Suggest an explanation for this observation.

When yeast cells are placed in a low-oxygen environment, they begin switching to fermentation instead of cellular respiration. Because the fermentation pathway is predominant in this situation, there is no need for the cell to devote the energy needed for the production and upkeep of the mitochondria.

In the late 1890s, two scientists were experimenting with yeast in efforts to develop a substance(s) that would have therapeutic use. Sucrose, a disaccharide made up of fructose and glucose, was often used as a preservative during the late 1800s. When the Buchners added sucrose to the yeast-containing mixture, what would have happened?

Yeast catabolized the glucose.

What would happen if you followed a recipe for making wine, but then bubbled oxygen through your wine container instead of letting it sit undisturbed?

You would produce carbon dioxide, water, and lots more yeast; you would produce very little if any alcohol, because the yeast would use aerobic respiration instead of fermentation.

What is the electron carrier that functions in the Krebs cycle?

both NAD+ and FAD

Substrate-level phosphorylation occurs within a metabolic pathway where sufficient energy is released by a given chemical reaction to drive the synthesis of ATP from ADP and phosphate. Substrate-level phosphorylation is seen in which metabolic pathway(s)?

both glycolysis and the Krebs cycle

If glucose is labeled with 14C, what molecule will become radioactive as glycolysis and the Krebs cycle are completed?

carbon dioxide

Glycolysis occurs in the ________; the Krebs cycle occurs in the ________; and the ETC occurs in the ________.

cytosol; mitochondrial matrix; mitochondrial inner membrane

C6H12O6 (glucose) + 6O2 6 CO2 + 6H2O Where is the majority of the water in this reaction produced?

electron transport chain

Canine phosphofructokinase (PFK) deficiency afflicts springer spaniels, affecting an estimated 10% of the breed. PFK is the glycolytic enzyme that phosphorylates fructose-1-phosphate and catalyzes the committed step in glycolysis. Given its critical role in glycolysis, one implication of the genetic defect resulting in PFK deficiency in dogs is:

exercise intolerance

The metabolic pathway shown below is a five-step pathway in which threonine is converted to isoleucine. When the end-product, isoleucine, accumulates, it inhibits the initial enzyme of the pathway, threonine deaminase. This is an example of:

feedback inhibition

The electron transport chain:

is a series of redox reactions

The inner mitochondrial membrane:

is virtually impermeable to hydrogen ions

An example of feedback inhibition is:

the effect of high concentrations of ATP on phosphofructokinase

Which of the following takes place in the electron transport chain?

the extraction of energy from high-energy electrons remaining from glycolysis and the Krebs cycle

The enzyme phosphofructokinase is the major regulatory enzyme of glycolysis. It catalyzes:

the phosphorylation of fructose 6-phosphate

What is the function of the reactions in a fermentation pathway?

to generate NAD+ from NADH, so glycolysis can continue

If oxygen is labeled with 18O, what molecule will become radioactive as glycolysis and the Krebs cycle are completed?

water

When does feedback inhibition occur?

when an enzyme that is active early in a metabolic pathway is inhibited by a product of the pathway

When do cells switch from cellular respiration to fermentation?

when electron acceptors are not available