Chapter 13 - How Cells Obtain Energy from Food

Question 1

In cells that cannot carry out fermentation, which products derived from glycolysis will accumulate under anaerobic conditions?

• glucose and NADH
• pyruvate and NAD+
• pyruvate and NADH
• lactate and NAD+
• glucose 6-phosphate and NADH

Answer 1

Pyruvate and NADH

(Without oxygen, NADH would be unable to donate its electrons to the electron transport chain and the pyruvate produced by glycolysis would not be removed by fermentation.)

Question 2

Most of the energy released by oxidizing glucose is saved in the high-energy bonds of what molecules?

• ATP and other activated carriers
• O2
• H2O and CO2
• GDP and other activated carriers
• ADP and other activated carriers

Answer 2

ATP and other activated carriers

(Much of the energy released by the oxidative breakdown of glucose is saved in the high-energy bonds of ATP and the high-energy electrons of NADH.)

Question 3

Useful energy is obtained by cells when sugars derived from food are broken down by which processes?

• glycolysis, the citric acid cycle, and oxidative phosphorylation
• glycolysis, the Calvin cycle, and oxidative phosphorylation
• gluconeogenesis, fermentation, and oxidative phosphorylation
• glycolysis, the citric acid cycle, and gluconeogenesis
• gluconeogenesis, the citric acid cycle, and oxidative phosphorylation

Answer 3

Glycolysis, the citric acid cycle, and oxidative phosphorylation

(Together, these processes capture the energy released from the oxidative breakdown of sugars.)

Question 4

Where does the oxidative (oxygen-dependent) stage of the breakdown of food molecules occur in a eukaryotic cell?

• mitochondrion
• Golgi apparatus
• endoplasmic reticulum
• cytosol

Answer 4

Mitochondrion

(The oxidative stage of the breakdown of food molecules takes place entirely in the mitochondrion.

The citric acid cycle, which requires oxygen to proceed, occurs in the mitochondrial matrix. And oxidative phosphorylation, which consumes a large amount of oxygen, takes place on the inner mitochondrial membrane.)

Question 5

Which of the following processes generates the largest number of ATP molecules?

• gluconeogenesis
• fermentation
• electron transport chain
• glycolysis
• citric acid cycle

Answer 5

Electron transport chain

(The electron transport chain in the inner mitochondrial membrane generates large amounts of ATP from electrons donated by the active carriers produced during glycolysis and the citric acid cycle.)

Question 6

Under anaerobic conditions, which metabolic pathway regenerates the supply of NAD+ needed for glycolysis?

• citric acid cycle
• fermentation
• breakdown of amino acids
• breakdown of fats
• formation of acetyl CoA

Answer 6

Fermentation

(Fermentation reactions convert the pyruvate produced during glycolysis into lactate or ethanol. In the process, NADH gives up its electrons, thereby producing NAD+.

Without replenishing NAD+, glycolysis could not continue.)

Question 7

Although the citric acid cycle itself does not use O2, it requires a functioning electron transport chain (which uses O2) in order to regenerate which molecule for further use in the citric acid cycle?

• NADH
• ADP
• ATP
• NAD+
• FADH2

Answer 7

NAD+

(Like glycolysis, the citric acid cycle uses NAD+ as an electron acceptor. This molecule—along with FAD—must be regenerated for the citric acid cycle to continue. Generating NAD+ requires oxygen (or an ability to carry out fermentation reactions). Oxygen allows NADH to hand off its high-energy electrons, regenerating the NAD+ needed to keep the citric acid cycle going.)

Question 8

The NADH generated during glycolysis and the citric acid cycle feeds its high-energy electrons to which of the following?

• FAD
• the electron transport chain
• ADP
• H2O
• the citric acid cycle

Answer 8

The electron transport chain

(The NADH generated during glycolysis (and the NADH and FADH2 produced by the citric acid cycle) feeds its high-energy electrons to the electron transport chain.)

Question 9

In eukaryotic cells, what is the final electron acceptor in the electron transport chain?

• O2
• CO2
• ATP
• FADH2
• NAD+

Answer 9

O2

(High-energy electrons, donated to the electron transport chain by NADH and FADH2, are ultimately passed on to O2, which serves as the final electron acceptor in the chain.)

Question 10

In the electron transport chain, the oxygen atoms in O2 become part of which of the following molecules?

• glucose (C6H12O6)
• NADH
• H2O
• ATP
• CO2

Answer 10

H2O

The electron transport chain donates electrons to O2, producing H2O.

Question 11

Which two-carbon molecule enters the citric acid cycle?

• oxaloacetate
• citrate
• pyruvate
• acetyl CoA
• carbon dioxide

Answer 11

Acetyl CoA

(In the first step of the citric acid cycle, acetyl CoA donates a two-carbon acetyl group to oxaloacetate to form citrate. These carbons are then oxidized to produce CO2.)

Question 12

What occurs in the first step of the citric acid cycle?

• ATP is consumed.
• A two-carbon molecule is combined with a four-carbon molecule to form citrate.
• CO2 is released.
• NADH is produced.
• Two molecules of acetyl CoA combine to form oxaloacetate.

Answer 12

A two-carbon molecule is combined with a four-carbon molecule to form citrate.

(In the initial reaction of the citric acid cycle, a two-carbon acetyl group combines with a four-carbon oxaloacetate molecule to form the six-carbon citrate after which the cycle is named.)

Question 13

CO2 is released in which steps of the citric acid cycle, as shown below?

• Steps 1 and 8
• Steps 2, 3, and 4
• Steps 1 and 5
• Steps 3 and 4
• Steps 2 and 4

Answer 13

Steps 3 and 4

(The steps of the citric acid cycle that release CO2 are those in which an intermediate loses a carbon.

In step 3, a six-carbon substrate (isocitrate) is converted into a five-carbon product (α-ketoglutarate). The lost carbon is released as CO2. In step 4, the five-carbon α-ketoglutarate reacts with a molecule of coenzyme A to yield the four-carbon succinyl CoA. Again, the missing carbon is accounted for by CO2, which is released in this step.)

Question 14

The ethanol in wine and beer is produced from metabolic reactions carried out by the yeast Saccharomyces cerevisiae. Since it is of great commercial value, researchers have studied factors that influence ethanol production. To maximize ethanol yield, which environmental factor should be limiting?

• sunlight
• carbon dioxide
• glucose
• oxygen

Answer 14

Oxygen

(In the absence of oxygen, yeast cannot perform aerobic respiration and instead switch to fermentation. Fermentation products in yeast include CO2 and ethanol.)

Question 15

How do enzymes maximize the energy harvested from the oxidation of food molecules?

• They allow the stepwise oxidation of food molecules, which releases energy in small amounts.
• They guarantee that each reaction involved in the oxidation of food molecules proceeds in just one direction.
• They allow a larger amount of energy to be released from food molecules such as glucose.
• They allow oxidation reactions to take place without an input of activation energy.
• They allow what would otherwise be an energetically unfavorable oxidation reaction to occur.

Answer 15

They allow the stepwise oxidation of food molecules, which releases energy in small amounts.

(Enzymes allow cells to carry out the oxidation of sugars in a tightly controlled stepwise series of reactions. These reactions pay out energy in small packets to activated carriers, which allows cells to capture much of the energy released by the oxidative breakdown of glucose in the high-energy bonds of ATP and other activated carriers.)

Question 16

What happens to the energy captured during glycolysis and the citric acid cycle by the activated carriers NADH and FADH2.

• It is passed to an electron transport chain that uses it to generate a proton gradient across the inner mitochondrial membrane.
• It is passed to an electron transport chain that uses it to oxidize food molecules.
• It is used to drive biosynthetic reactions.
• It is passed to ADP to form ATP.
• It is passed to an electron transport chain that uses it to produce oxygen.

Answer 16

It is passed to an electron transport chain that uses it to generate a proton gradient across the inner mitochondrial membrane.

(This proton gradient then serves as a source of energy (like a battery) that can be tapped to drive a variety of energy-requiring reactions, including the phosphorylation of ADP to generate ATP on the matrix side of the inner membrane.)

Question 17

What does the term “gluconeogenesis” refer to?

• the transport of glucose across a cell membrane
• the breakdown of glucose during glycolysis
• the synthesis of glucose from small organic molecules such as pyruvate
• the breakdown of glucose during fermentation
• the release of glucose from molecules such as glycogen

Answer 17

The synthesis of glucose from small organic molecules such as pyruvate

(These reactions sometimes occur during periods of fasting, when glucose reserves are consumed faster than they can be replenished.)

Question 18

When food is plentiful, animals can store glucose as what?

• starch
• acetyl CoA
• glucose 6-phosphate
• glycogen
• glycogen or starch

Answer 18

Glycogen

Glycogen and starch are both branched polysaccharides made of glucose, but only glycogen is made by animal cells.

Question 19

When nutrients are plentiful, plants can store glucose as what?

• glucose 6-phosphate
• starch
• glycogen and starch
• fats
• glycogen

Answer 19

Starch

(This energy-rich material serves as a major food source for plants—and for the many animals that eat them, including humans.)

Question 20

In what form do plant and animal cells store fat?

• triacylglycerol
• glycogen
• phospholipids
• starch
• nitroglycerin

Answer 20

Triaclyglycerol

(The triacylglycerol in plants and animals differs only in the types of fatty acids that predominate: plant oils contain unsaturated fatty acids (with one or more double bonds) and animal fats are saturated.)

Question 21

You are packing for a hiking trip during which you’ll be burning a lot of calories with physical activity. You want to pack as efficiently as possible since you need to carry a tent and all your food. You can get the most calories out of 5kg of food if it is in the form of ______.

• starch
• glycogen
• fat
• glucose

Answer 21

Fat

Carbohydrates provide 4 calories per gram, and fat provides 9 calories per gram.

Question 22

In the absence of oxygen, in cells that cannot carry out fermentation, glycolysis would halt at which step?

• the oxidation of glyceraldehyde 3-phosphate to 1,3-bisphosphoglycerate
• the phosphorylation of fructose 6-phosphate to fructose 1,6-bisphosphate
• the transfer of a phosphate group from 1,3-bisphosphoglycerate to ADP to form ATP
• the reversible rearrangement of glucose 6-phosphate to fructose 6-phosphate

Answer 22

The oxidation of glyceraldehyde 3-phosphate to 1,3-bisphosphoglycerate

Question 23

In the absence of oxygen, in cells that cannot carry out fermentation, glycolysis would halt at which step?

• the oxidation of glyceraldehyde 3-phosphate to 1,3-bisphosphoglycerate
• the phosphorylation of fructose 6-phosphate to fructose 1,6-bisphosphate
• the transfer of a phosphate group from 1,3-bisphosphoglycerate to ADP to form ATP
• the reversible rearrangement of glucose 6-phosphate to fructose 6-phosphate

Answer 23

The oxidation of glyceraldehyde 3-phosphate to 1,3-bisphosphoglycerate

(Cells that cannot carry out fermentation will run out of NAD+ under anaerobic conditions. The oxidation of glyceraldehyde 3-phosphate to 1,3-bisphosphoglycerate is the only reaction in glycolysis that requires NAD+. Oxidation of glyceraldehyde 3-phosphate involves the transfer of a hydrogen atom, along with an electron, to NAD+.)

Question 24

Which molecules are required for the citric acid cycle to fully oxidize the carbons donated by acetyl CoA?

• ATP
• GTP
• oxaloacetate
• NAD+
• GDP
• O2

Answer 24

• oxaloacetate
• NAD+
• GDP
• O2

(At the start of the citric acid cycle, a molecule of oxaloacetate accepts a two-carbon acetyl group from acetyl CoA to form citrate.

Oxidation of citrate yields energy that is used to produce GTP, NADH, and FADH2. Therefore, GDP and NAD+ (as well as FAD) are also needed for the cycle to continue.

Although molecular oxygen does not participate in the citric acid cycle directly, it is required for the reactions to continue as O2 is the ultimate acceptor for electrons donated by NADH and FADH2 to the electron transport chain. This electron transfer regenerates the NAD+ and FAD needed for the cycle to continue.)

Question 25

Experiments performed by Hans Krebs in the 1930s revealed that the set of reactions that oxidize food molecules and produce CO2 occur in a cycle. In one experiment, Krebs exposed pigeon muscles to malonate, a compound that inhibits succinate dehydrogenase—the enzyme that converts succinate to fumarate, indicated in red in the linear representation of the reactions of the citric acid cycle (below).

Which of the following observations, made in malonate-treated muscle, led Krebs to believe that this set of reactions is cyclical?

• If succinate were added, fumarate would accumulate.
• If fumarate were added, succinate would accumulate.
• Regardless of what he added, large amounts of oxygen would be produced.
• Regardless of what he added, oxaloacetate would accumulate.
• If citrate were added, succinate would accumulate.

Answer 25

If fumarate were added, succinate would accumulate.

(If fumarate were added, succinate would indeed accumulate. Malonate blocks the conversion of succinate to fumarate. But because the reaction is a cycle, the addition of any compound “downstream” of this blockage would go on to produce oxaloacetate, which combines with the two carbons from acetyl CoA to produce citrate at the start of the cycle.

This citrate would continue on until it was converted to succinate, at which point malonate would block its further conversion to fumarate. Thus, succinate would accumulate.

Krebs found that addition of fumarate, malate, or oxaloacetate would result in an accumulation of succinate in muscles treated with the inhibitor malonate.)

Question 26

The chemistry of many metabolic reactions was deciphered using molecules labeled with radioactive isotopes. If acetyl CoA labeled with radioactive 14C in both carbon positions were fed into the citric acid cycle, where would the radioactivity be after one turn of the cycle?

• in citrate
• in CO2
• in oxaloacetate and in CO2
• in acetyl CoA
• in oxaloacetate

Answer 26

in oxaloacetate

(Both of the labeled carbons (highlighted in red in the diagram) wind up in oxaloacetate at the end of the first turn of the cycle. They will be released as CO2 in subsequent turns of the cycle.)

Question 27

If acetyl CoA labeled with radioactive 14C in both carbon positions were fed into the citric acid cycle, where would the radioactivity be after two turns of the cycle?

• in malate
• in CO2
• in oxaloacetate and in CO2
• in acetyl CoA
• in oxaloacetate

Answer 27

In oxaloacetate and in CO2

(In the first turn of the cycle, the labeled acetyl carbons picked up in step 1 will remain in the oxaloacetate regenerated in step 8.

As that oxaloacetate reenters the cycle, it will accept another acetyl group to form citrate.

One of the labeled carbons will now be in a position to be released as CO2 in step 3 of the second cycle. The other will remain in oxaloacetate. It will take two more turns of the cycle for this carbon to be oxidized to CO2.)

Question 28

Your friends are on a low-fat, high-carbohydrate diet, which they claim will prevent fat accumulation within their bodies. They eat tons of pasta and bread without worrying about calorie count. What can you correctly say to your friends about their potential to accumulate lipids on their low-fat diet?

• They will accumulate fats because cells have no way of storing carbohydrates.
• They will not accumulate fats because cells have no way of storing fats.
• They will not accumulate fats because carbohydrates have less energy per gram than fats.
• They will accumulate fats because cells can convert glycolytic metabolites into lipids.

Answer 28

They will accumulate fats because cells can convert glycolytic metabolites into lipids.

(Glycolysis and the citric acid cycle provide precursors to synthesize many organic molecules, including lipids.)

Question 29

Glycolysis occurs in a series of 10 steps using 10 different enzymes.
What are the 4 broad events that occur during glycolysis?

Answer 29

1. Energy is invested by using up some ATP
2. The six-carbon sugar is split into two smaller molecules
3. Energy is captured as ATP and NADH
4. Two molecules of pyruvate are generated

(The first major phase of glycolysis occurs in steps 1–3, during which two ATP are invested to prepare glucose for breakdown. The second major phase occurs in steps 4 and 5, during which the six-carbon sugar is cleaved into two three-carbon sugars and converted to glyceraldehyde 3-phosphate. The final major stage is where energy is harvested in the form of NADH in step 6 and ATP in steps 7 and 10. The net result is the production of two NADH, two ATP, and two pyruvate molecules per starting glucose.)

Question 30

The main regulatory step of glycolysis occurs in step 3. Choose all of the following that correctly describe some aspect of step 3 in glycolysis.

• The enzyme that catalyzes step 3 is phosphofructokinase.
• The reaction generates the product fructose 1,6-bisphosphate.
• The enzyme uses an ATP.
• The reaction is an irreversible reaction.

Answer 30

• The enzyme that catalyzes step 3 is phosphofructokinase.
• The reaction generates the product fructose 1,6-bisphosphate.
• The enzyme uses an ATP.
• The reaction is an irreversible reaction.

(Step 3 of glycolysis is catalyzed by the enzyme phosphofructokinase, which phosphorylates the substrate fructose 6-phosphate to the product fructose 1,6-bisphosphate. The phosphate for phosphorylation comes from ATP and uses up one ATP per glucose. Because this step is irreversible, it is one of the major regulatory steps of glycolysis.)

Question 31

Which statements below accurately describe an aspect of the citric acid cycle?

• The two carbons that enter as acetyl CoA are released in the same cycle as CO2.
• NADH is generated in steps 3, 4, and 8.
• The citric acid cycle produces two kinds of high-energy molecules, GTP and NADH.
• Oxaloacetate is regenerated by the end of the citric acid cycle.

Answer 31

• NADH is generated in steps 3, 4, and 8.

- Oxaloacetate is regenerated by the end of the citric acid cycle.

Question 32

Which statements below accurately describe an aspect of the citric acid cycle?

• The two carbons that enter as acetyl CoA are released in the same cycle as CO2.
• NADH is generated in steps 3, 4, and 8.
• The citric acid cycle produces two kinds of high-energy molecules, GTP and NADH.
• Oxaloacetate is regenerated by the end of the citric acid cycle.

Answer 32

• The citric acid cycle produces two kinds of high-energy molecules, GTP and NADH.
• Oxaloacetate is regenerated by the end of the citric acid cycle.

(During the cycle, NADH is produced in steps 3, 4, and 8 as the citrate is converted to oxaloacetate to regenerate the beginning product.)

Question 33

Assume five molecules of FADH2 are made in the citric acid cycle in a given amount of time. How many NADH are made during the same time interval?

Answer 33

15

A total of 15 NADH are produced if five FADH2 are produced.

Question 34

When ATP and food molecules such as fatty acids are abundant, which will occur?

• When food and ATP are plentiful, both glycolysis and gluconeogenesis will occur.
• When food molecules are plentiful, neither glycolysis nor gluconeogenesis will occur.
• Enzymes involved in glycolysis will operate in the reverse direction, using pyruvate to produce glucose.
• Enzymes involved in glycolysis will break down glucose to generate pyruvate.
• Enzymes involved in gluconeogenesis will use energy to produce glucose.

Answer 34

Enzymes involved in gluconeogenesis will use energy to produce glucose.

(One of the key enzymes in gluconeogenesis is inhibited by AMP—a molecule that accumulates when energy reserves are low. When ATP is plentiful, this inhibition is relieved and gluconeogenesis can proceed.

The production of glucose in the presence of abundant ATP allows the cell to store glucose in the form of glycogen for use when energy is needed and food is scarce.)

Question 35

After an overnight fast, most of the acetyl CoA entering the citric acid cycle is derived from what type of molecule?

• amino acids
• fatty acids
• pyruvate
• glycogen
• glucose

Answer 35

Fatty acids

(Fats produce more energy when burned than glucose; after a fast, fats tend to be mobilized and converted to acetyl CoA.)

Question 36

In cells, pyruvate can be converted to which of the following?
Choose one or more:

• lactate
• acetyl CoA
• oxaloacetate
• glucose
• alanine

Answer 36

• lactate
• acetyl CoA
• oxaloacetate
• glucose
• alanine

(Pyruvate is a substrate for more than half a dozen different enzymes, each of which modifies it chemically in a different way. All of these molecules can be made using pyruvate as a starting material.)

Question 37

Which statement is true of glycogen phosphorylase?

• It stimulates gluconeogenesis.
• It is inhibited by glucose 6-phosphate, but activated by ATP.
• It is activated by glucose 6-phosphate and by ATP.
• It is activated by glucose 6-phosphate, but inhibited by ATP.
• It is inhibited by glucose 6-phosphate and by ATP.

Answer 37

It is inhibited by glucose 6-phosphate and by ATP.

(When more ATP is needed than can be generated from food-derived molecules available in the bloodstream, cells break down glycogen in a reaction that is catalyzed by glycogen phosphorylase.

The enzyme is inhibited by glucose 6-phosphate, as well as by ATP. This regulation helps to prevent glycogen breakdown when ATP and food molecules are plentiful.

Glycogen breakdown produces glucose 1-phosphate, which is then converted to the glucose 6-phosphate that feeds into the glycolytic pathway.)

Question 38

Weight loss can occur when glucose is oxidized to CO2 rather than being stored as glycogen. The first step in glucose oxidation is glycolysis. A 1930s diet drug, DNP, made the inner mitochondrial membrane permeable to protons, increasing the rate of glycolysis. What is the explanation for the DNP-induced increase in glycolysis?

• DNP also makes the inner mitochondrial membrane leaky to glucose, increasing the ability of the mitochondria to perform glycolysis.
• High ADP activates phosphofructokinase.
• High AMP inhibits phosphofructokinase.
• High ATP activates phosphofructokinase.

Answer 38

High ATP activates phosphofructokinase.

Phosphofructokinase is inhibited by high levels of ATP and activated by products of ATP hydrolysis (ADP, AMP, and Pi).

Question 39

Below is a table listing the reactions that constitute the 10 steps of glycolysis, along with the change in free energy (ΔG°) for each step. Based on the data, which steps in glycolysis are effectively irreversible?

• Steps 1, 3, 7, and 10
• Steps 5, 7, 8, and 9
• Step 1
• Steps 1, 3, and 10
• Steps 1, 2, 3, 4, 6, and 10
• Steps 6 and 7

Answer 39

Steps 1, 3, and 10

(The large negative ΔG for these three reactions indicates that they are energetically highly favorable. Hence, they effectively operate in only the forward direction, favoring glucose breakdown.

The enzymes that catalyze these reactions—hexokinase, phosphofructokinase, and pyruvate kinase—are key points in the regulation of glycolysis.)