chapter 7 study modules

If significant amounts of materials are removed from the citric acid cycle to produce amino acids for protein synthesis, which of the following will result?

1) less ATP will be produced by the cell because carbon is diverted from the oxidative reactions in the citric acid cycle 2) less CO2 will be produced by the cell 3) the four-carbon compound that combines with acetyl CoA will have to be made by some other process.

Which of the following best describes the electron transport chain?

Electrons are passed from one carrier to another, releasing a little energy at each step.

After completion of the citric acid cycle, most of the usable energy from the original glucose molecule is in the form of __________.

NADH

The energy given up by electrons as they move through the electron transport chain is used in which of the following processes?

Pumping H+ across a membrane

In preparing pyruvate to enter the citric acid cycle, which of the following steps occurs?

Pyruvate is oxidized and decarboxylated, and the removed electrons are used to reduce an NAD+ to an NADH.

Most of the electrons removed from glucose by cellular respiration are used for which of the following processes?

Reducing NAD+ to NADH in glycolysis and the citric acid cycle AND producing a proton gradient for ATP synthesis in the mitochondria.

Which of the following accompanies the conversion of pyruvate to acetyl CoA before the citric acid cycle?

Release of CO2 and synthesis of NADH The carboxyl group of pyruvate is removed as a carbon dioxide molecule. The remaining two-carbon fragment is oxidized to acetate and NAD+ is reduced to NADH. The cycle functions as a metabolic furnace that oxidizes organic fuel derived from pyruvate. Pyruvate is broken down to three CO2 molecules, including the molecule of CO2 released during the conversion of pyruvate to acetyl CoA. The cycle has eight steps, each catalyzed by a specific enzyme. For each turn of the citric acid cycle, two carbons enter in the relatively reduced form of an acetyl group, and two different carbons leave in the completely oxidized form of CO2 molecules.

Which part of the catabolism of glucose by cellular respiration requires molecular oxygen (O2) and produces CO2?

The combination of the citric acid cycle and electron transport CO2 is produced in the citric acid cycle, and O2 is taken up in electron transport. In eukaryotes, pyruvate enters the mitochondrion and is oxidized to a compound called acetyl CoA, which enters the citric acid cycle. There, the breakdown of glucose to carbon dioxide is completed. (In prokaryotes, these processes take place in the cytosol.) Thus, the carbon dioxide produced by respiration represents fragments of oxidized organic molecules.

The ATP synthase in a human cell obtains energy for synthesizing ATP directly from which of the following processes?

The flow of H+ across the inner mitochondrial membrane through the ATP synthase enzyme!

Why is the citric acid cycle called a cycle?

The four-carbon acid that accepts the acetyl CoA in the first step of the cycle is regenerated by the last step of the cycle. Acetyl CoA binds to oxaloacetate and this compound is restored at the end of the cycle.

Where do the reactions of the citric acid cycle occur in eukaryotic cells?

The matrix of the mitochondrion

When electrons flow along the electron transport chains of mitochondria, which of the following changes occurs?

The pH of the matrix increases

In glycolysis, there is no production of carbon dioxide as a product of the pathway. Which of the following is the best explanation for this?

The products of glycolysis contain the same total number of carbon atoms as in the starting material.

A small amount of ATP is made in glycolysis by which of the following processes?

Transfer of a phosphate group from a fragment of glucose to ADP by substrate-level phosphorylation

When protein molecules are used as fuel for cellular respiration, __________ are produced as waste.

amino acid groups The amino group is a residual product of amino acid catabolism. Proteins can also be used for fuel, but first they must be digested to their constituent amino acids. Many of the amino acids are used by the organism to build new proteins. Amino acids present in excess are converted by enzymes to intermediates of glycolysis and the citric acid cycle. Before amino acids can enter into glycolysis or the citric acid cycle, their amino groups must be removed, a process called deamination. The nitrogenous refuse is excreted from the animal in the form of ammonia (NH3), urea, or other waste products.

In the overall process of glycolysis and cellular respiration, __________ is oxidized and __________ is reduced.

glucose; oxygen

Of the metabolic pathways listed below, which is the only pathway found in all organisms?

glycolysis

Which metabolic pathway is common to both fermentation and cellular respiration of a glucose molecule?

glycolysis

A chemist has discovered a drug that blocks phosphoglucoisomerase, an enzyme that catalyzes the second reaction in glycolysis. He wants to use the drug to kill bacteria in people with infections. However, he cannot do this because __________.

human cells must also perform glycolysis; the drug might also poison them

During respiration in eukaryotic cells, the electron transport chain is located in or on the __________.

inner membrane of the mitochondrion

Each ATP molecule contains about 1% of the amount of chemical energy available from the complete oxidation of a single glucose molecule. Cellular respiration produces about 32 ATP from one glucose molecule. What happens to the rest of the energy in glucose?

it is converted to heat We humans use some of this heat to maintain our relatively high body temperature (37°C), and we dissipate the rest through sweating and other cooling mechanisms.

Sports physiologists at an Olympic training center wanted to monitor athletes to determine at what point their muscles were functioning anaerobically. They could do this by checking for a buildup of which of the following compounds?

lactate

A molecule becomes more oxidized when it __________.

loses an electron

The overall efficiency of respiration (the percentage of the energy released that is saved in ATP) is approximately __________.

34% 34% of the potential chemical energy in glucose has been transferred to ATP; the actual percentage is bound to vary as ΔG varies under different cellular conditions. Cellular respiration is remarkably efficient in its energy conversion. By comparison, the most efficient automobile converts only about 25% of the energy stored in gasoline to energy that moves the car.

Which of the following substances is/are involved in oxidative phosphorylation?

ADP, Oxygen, ATP

During aerobic respiration, molecular oxygen (O2) is used for which of the following purposes?

At the end of the electron transport chain to accept electrons and form H2O

Which of the following is the source of the energy that produces the chemiosmotic gradient in mitochondria?

Electrons moving down the electron transport chain

A gram of fat oxidized by respiration produces approximately twice as much ATP as a gram of carbohydrate. Which of the following best explains this observation?

Fats are better electron donors to oxygen than are sugars. Fats contain more hydrogen and less oxygen than sugars and it is the transfer of electrons from good donors, such as hydrogen atoms, to good acceptors, such as oxygen, that provides the energy in respiration. Catabolism can also harvest energy stored in fats obtained either from food or from storage cells in the body. After fats are digested to glycerol and fatty acids, the glycerol is converted to glyceraldehyde 3-phosphate, an intermediate of glycolysis. Most of the energy of a fat is stored in the fatty acids. A metabolic sequence called beta oxidation breaks the fatty acids down to two-carbon fragments, which enter the citric acid cycle as acetyl CoA. NADH and FADH2 are also generated during beta oxidation; they can enter the electron transport chain, leading to further ATP production. Fats make excellent fuel, in large part due to their chemical structure and the high energy level of their electrons (equally shared between carbon and hydrogen) compared to those of carbohydrates. Unfortunately, the high energy yield of fats also means that a person trying to lose weight must work hard to use up fat stored in the body because so many calories are stockpiled in each gram of fat.

Which of the following represents the major (but not the only) energy accomplishment of the citric acid cycle?

Formation of NADH and FADH2

In glycolysis in the absence of oxygen, cells need a way to regenerate which compound?

NAD+

When a poison such as cyanide blocks the electron transport chain, glycolysis and the citric acid cycle also eventually stop working. Which of the following is the best explanation for this?

NAD+ and FAD are not available for glycolysis and the citric acid cycle to continue. Unless the electron transport molecules (NADH and FADH2) can recycle back to their oxidized states (NAD+ and FAD), they will be unable to receive electrons in the other steps of cellular respiration. Without some mechanism to recycle NAD+ from NADH, glycolysis would soon deplete the cell's pool of NAD+ by reducing it all to NADH and would shut itself down for lack of an oxidizing agent. Under aerobic conditions, NAD+ is recycled from NADH by the transfer of electrons to the electron transport chain. An anaerobic alternative is to transfer electrons from NADH to pyruvate, the end product of glycolysis.

Which of the following statements is the best explanation of what happens to the temperature and carbon dioxide concentration during a one-hour class period in a classroom of 300 students if the heating and air conditioning is turned off and all doors are kept closed?

Temperature and the level of carbon dioxide rise as heat and carbon dioxide are by-products of cellular respiration.

If muscle cells in the human body consume O2 faster than it can be supplied, which of the following is likely to result?

The cells will not be able to carry out oxidative phosphorylation. The cells will consume glucose at an increased rate. The muscle cells will have more trouble making enough ATP to meet their energy requirements. During lactic acid fermentation, pyruvate is reduced directly by NADH to form lactate as an end product, with no release of CO2. (Lactate is the ionized form of lactic acid.) Human muscle cells make ATP by lactic acid fermentation when oxygen is scarce. This occurs during strenuous exercise, when sugar catabolism for ATP production outpaces the muscle's supply of oxygen from the blood. Under these conditions, the cells switch from aerobic respiration to fermentation. The lactate that accumulates was previously thought to cause muscle fatigue and pain, but recent research suggests instead that increased levels of potassium ions (K+) may be to blame, whereas lactate appears to enhance muscle performance. In any case, the excess lactate is gradually carried away by the blood to the liver, where it is converted back to pyruvate by liver cells. Because oxygen is available, this pyruvate can then enter the mitochondria in liver cells and complete cellular respiration.

In an experiment, mice were fed glucose (C6H12O6) containing a small amount of radioactive oxygen. The mice were closely monitored, and after a few minutes radioactive oxygen atoms showed up in __________.

carbon dioxide Food provides the fuel for respiration, and the exhaust is carbon dioxide and water. Glycolysis, which occurs in the cytosol, begins the degradation process by breaking glucose into two molecules of a compound called pyruvate. In eukaryotes, pyruvate enters the mitochondrion and is oxidized to a compound called acetyl CoA, which enters the citric acid cycle. There, the breakdown of glucose to carbon dioxide is completed. Thus, the carbon dioxide produced by respiration represents fragments of oxidized organic molecules.

Fermentation is essentially glycolysis plus an extra step in which pyruvate is reduced to form lactate or alcohol and carbon dioxide. This last step __________.

enables the cell to recycle the reduced NADH to oxidized NAD+ Normally, the reduced NADH delivers its electrons to the electron transport chain, enabling NAD+ to be recycled to glycolysis. As an alternative to respiratory oxidation of organic nutrients, fermentation is an extension of glycolysis that allows continuous generation of ATP by the substrate-level phosphorylation of glycolysis. For this to occur, there must be a sufficient supply of NAD+ to accept electrons during the oxidation step of glycolysis. Without some mechanism to recycle NAD+ from NADH, glycolysis would soon deplete the cell's pool of NAD+ by reducing it all to NADH and would shut itself down for lack of an oxidizing agent. Under aerobic conditions, NAD+ is recycled from NADH by the transfer of electrons to the electron transport chain. An anaerobic alternative is to transfer electrons from NADH to pyruvate, the end product of glycolysis.

The function of cellular respiration is to __________.

extract usable energy from glucose

How many molecules of ATP are gained by substrate-level phosphorylation from the complete breakdown of a single molecule of glucose in the presence of oxygen?

four Glycolysis and the citric acid cycle produce only 4 ATP molecules per glucose molecule, all by substrate-level phosphorylation: 2 net ATP from glycolysis and 2 ATP from the citric acid cycle. Molecules of NADH (and FADH2) account for most of the energy extracted from the glucose. These electron escorts link glycolysis and the citric acid cycle to the machinery of oxidative phosphorylation, which uses energy released by the electron transport chain to power most of the ATP synthesis.

Of the following molecules in the glycolytic pathway (the process of glycolysis), the one with the most chemical energy is __________.

fructose-1,6-bisphosphate ...is one of the molecules in the glycolytic pathway with the most chemical energy because this molecule has received two phosphate groups from ATP as an energy investment. Consider that glycolysis can be divided into two phases: energy investment and energy payoff. Fructose-1,6-bisphosphate is the molecule in the energy investment phase when the cell actually spends ATP. This investment is eventually repaid with interest during the energy payoff phase.

If a compound that allows protons to freely diffuse across membranes is added to cells that are actively metabolizing glucose via cellular respiration, which of the following processes would stop?

none. If a compound that allows protons to freely diffuse across membranes is added to cells that are actively metabolizing glucose via cellular respiration, the chemiosmotic synthesis of ATP would stop in the mitochondria, but substrate-level phosphorylation could continue as would glycolysis and electron transport.

In the citric acid cycle, for each pyruvate that enters the cycle,

one ATP, three NADH, and one FADH2 are produced.

Which of the following is a correct description of the events of cellular respiration and the sequence of events in cellular respiration?

oxidation of glucose to pyruvate; oxidation of pyruvate; oxidation of acetyl-coA; oxidative phosphorylation

Most of the ATP produced in cellular respiration comes from which of the following processes?

oxidative phosphorylation

During the reaction C6H12O6 + 6 O2 → 6 CO2 + 6 H2O, which compound is reduced as a result of the reaction?

oxygen

In mitochondria, exergonic redox reactions __________.

provide the energy that establishes the proton gradient

Muscle tissues make lactate from pyruvate to do which of the following?

regenerate NAD+ Human muscle cells make ATP by lactic acid fermentation when oxygen is scarce. This allows them to regenerate NAD+ so that glycolysis can continue. On the cellular level, our muscle cells behave as facultative anaerobes. In such cells, pyruvate is a fork in the metabolic road that leads to two alternative catabolic routes. Under aerobic conditions, pyruvate can be converted to acetyl CoA, and oxidation continues in the citric acid cycle via aerobic respiration. Under anaerobic conditions, lactic acid fermentation occurs: Pyruvate is diverted from the citric acid cycle, serving instead as an electron acceptor to recycle NAD+. To make the same amount of ATP, a facultative anaerobe has to consume sugar at a much faster rate when fermenting than when respiring.

Most CO2 from catabolism is released during __________.

the citric acid cycle

Most of the NADH that delivers electrons to the electron transport chain comes from which of the following processes?

the citric acid cycle

Where do the reactions of glycolysis occur in a eukaryotic cell?

the cytosol

Oxygen gas (O2) is one of the strongest oxidizing agents known. The explanation for this is that __________.

the oxygen atom is very electronegative The high electronegativity of the oxygen atom is the reason that oxygen gas (O2) is such a good oxidizing agent (why it can oxidize so many compounds). The two atoms of the oxygen molecule (O2) share their electrons equally but, for example, when oxygen reacts with the hydrogen from methane, forming water, the electrons of the covalent bonds spend more time near the oxygen than the hydrogen. In effect, each oxygen atom has partially "gained" electrons, so the oxygen molecule has been reduced. Because oxygen is so electronegative, it is one of the most potent of all oxidizing agents. Energy must be added to pull an electron away from an atom, just as energy is required to push a ball uphill. The more electronegative the atom (the stronger its pull on electrons), the more energy is required to take an electron away from it. An electron loses potential energy when it shifts from a less electronegative atom toward a more electronegative one, just as a ball loses potential energy when it rolls downhill. A redox reaction that moves electrons closer to oxygen, such as the burning (oxidation) of methane, therefore releases chemical energy that can be put to work.

In the citric acid cycle, for each pyruvate that enters the cycle, one ATP, three NADH, and one FADH2 are produced. For each glucose molecule that enters glycolysis, how many ATP, NADH, and FADH2 are produced in the citric acid cycle?

two ATP, six NADH, two FADH2 because glucose molecule = 2 pyruvate