Chapter 6: How Cells Harvest Chemical Energy

What is the role of oxygen in cellular respiration?

It accepts electrons from the electron transport chain

What happens to the energy that is given up by electrons as they move through the electron transport chain?

It pumps H+ through a membrane.

Animals store most of their energy reserves as fats, not as polysaccharides. What is the advantage of this mode of storage for an animal?

Most animals are mobile and benefit from a compact and concentrated form of energy storage. Also, because fats are hydrophobic, they can be stored without extra water associated with them.

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

NAD+

Your body makes NAD+ and FAD from two B vitamins, niacin and riboflavin. The Recommended Dietary Allowance for niacin is 20 mg and for riboflavin, 1.7 mg. These amounts are thousands of times less than the amount of glucose your body needs each day to fuel its energy needs. Why is the daily requirement for these vitamins so small?

NAD+ and FAD are coenzymes that are not used up during the oxidation of glucose. NAD+ and FAD are recycled when NADH and FADH2 pass the electrons they are carrying to the electron transport chain. We need a small additional supply to replace those that are damaged.

The electron transport chain is, in essence, a series of redox reactions that conclude cellular respiration. During these redox reactions, _____.

NAD+ is reduced, which then oxidizes an electron acceptor in the electron transport chain

Once the citric acid cycle has been completed, most of the usable energy from the original glucose molecule is in the form of _____.

NADH

Which of the following is a true distinction between cellular respiration and fermentation?

NADH is oxidized by the electron transport chain in respiration only

For each glucose molecule processed, what are the net molecular products of glycolysis?

Two molecules of pyruvate, two molecules of ATP, and two molecules of NADH

Through respiration, humans breathe in O2 and breathe out CO2. However, what would happen if we did not breathe in O2?

We would not make enough ATP to meet our energy requirements

What effect would an absence of oxygen have on oxidative phosphorylation?

Without oxygen to "pull" electrons down the electron transport chain, the energy stored in NADH could not be harnessed for ATP synthesis.

Besides sugars and fats, organisms can use other molecules as fuel for cellular respiration. When protein molecules are used, _____ are produced as waste.

amino groups

During aerobic respiration, molecular oxygen (O2) is used _____.

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

Carbon monoxide is a toxic gas because it directly _____.

blocks the transfer of electrons to the final electron acceptor

A small amount of ATP is made in glycolysis _____.

by the transfer of a phosphate group from a fragment of glucose to ADP (substrate-level phosphorylation)

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

carbon dioxide Cellular respiration, or the oxidization of glucose, involves the release of CO2.

Where does most of the ATP produced in cellular respiration come from?

chemiosmosis

When growing in a nutrient-rich environment, bacteria can use the provided amino acids to synthesize their proteins. However, should the environment change, they can synthesize their amino acids using _____.

citrate

A single glucose molecule produces about 38 molecules of ATP through the process of cellular respiration. However, this only represents approximately 38% of the chemical energy present in this molecule. The rest of the energy from glucose is _____.

converted to heat

In a eukaryotic cell, the electron transport chain is precisely located in or on the _____.

cristae of the mitochondrion

Where in bacterial cells does the citric acid cycle occur?

cytoplasm Glycolysis and the citric acid cycle both occur in the cytoplasm in bacterial cells.

What is the mechanism of action for the enzyme ATP synthase? ATP is formed _____.

due to the potential energy of a concentration gradient of hydrogen ions across a membrane

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

electrons Energy from the flow of electrons along the electron transport chain is used to pump hydrogen ions (protons) across the inner mitochondrial membrane, creating the chemiosmotic gradient.

The function of cellular respiration is to _____.

extract usable energy from glucose

The major (but not sole) energy accomplishment of the citric acid cycle is the _____.

formation of NADH and FADH2 However, the citric acid cycle forms ATP.

Cellular respiration completely breaks down a glucose molecule through glycolysis and the citric acid cycle. However, these two processes yield only a few ATPs. The majority of the energy the cell derives from glucose is _____.

found in NADH and FADH2

Rotenone is a poison that blocks the electron transport chain. When it does so, glycolysis and the citric acid cycle eventually halt as well. This is because _____.

they run out of NAD+ and FAD

The principal molecules involved in transporting electrons to the electron transport chain are composed of _____.

nucleotides At key steps in cellular respiration, hydrogen atoms are stripped from glucose and passed to a coenzyme called NAD+, or nicotinamide adenine dinucleotide.

Bacteria have no membrane-enclosed organelles. However, some still generate ATP through cellular respiration. Where is the electron transport chain found in these organisms?

plasma membrane The electron transport chain requires a membrane that will act as a barrier to hydrogen ions that are being pumped across this space.

In which of the following is the first molecule becoming reduced to the second molecule?

pyruvate ---> lactate (at the same time NADH is oxidized to NAD+)

A muscle cell deprived of molecular oxygen will convert glucose to lactic acid to _____.

recycle NADH through fermentation

In eukaryotes, most of the high-energy electrons released from glucose by cell respiration _____.

reduce NAD+ to NADH, which then delivers them to the electron transport chain

What is the total number of NADH molecules generated during the complete breakdown of one glucose molecule to six molecules of carbon dioxide?

10 NADH: 2 from glycolysis; 2 from the oxidation of pyruvate; 6 from the citric acid cycle (remember to double the output due to the sugar-splitting step of glycolysis)

Carbon monoxide is a colorless, odorless, and toxic gas. Theoretically, if you were breathing carbon monoxide, how many ATP molecules could you produce from one glucose molecule?

2 ATP Carbon monoxide will not block fermentation.

During glycolysis, a molecule of glucose is partially oxidized. What is the net gain of ATP and NADH for each glucose molecule?

2 ATP and 2 NADH

Sodium fluoroacetate, or compound 1080, is a rodent poison that probably disrupts the citric acid cycle. In the body, the molecule is converted to a compound that prevents citrate from being used. Initially, when first exposed to this molecule, what is the theoretical energy produced per glucose molecule through the process of cellular respiration?

2 ATP, 4 NADH, and 0 FADH2

The energy production per glucose molecule through the citric acid cycle is _____.

2 ATP, 6 NADH, 2 FADH2

Substrate-level phosphorylation directly generates ATP during a chemical reaction. As a single molecule of glucose is completely oxidized, in the presence of oxygen, how many molecules of ATP are gained by substrate-level phosphorylation?

4 ATP Substrate-level phosphorylation produces 2 ATP (net) in glycolysis and 2 ATP in the citric acid cycle.

What would a cell's net ATP yield per glucose be in the presence of the poison DNP?

4 ATP, all from substrate-level phosphorylation. The uncoupler would destroy the H+ concentration gradient necessary for chemiosmosis

The overall efficiency of respiration is approximately _____.

40% Thirty-eight molecules of ATP are produced.

How many kilocalories are captured in ATP from 1.5 moles of glucose?

420 kcal 1.5 moles is equivalent to 1,029 kcal, 40% of which is captured during cellular respiration as ATP.

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

A compound called coenzyme A binds to a two-carbon fragment to form acetyl coenzyme A.

In cellular respiration, which of the following is performed directly by the electron transport chain (or its components)?

A proton gradient is formed The energy released by the electron transport chain is used to create a proton gradient across the inner mitochondrial membrane.

Why is the citric acid cycle called a cycle?

Acetyl CoA binds to oxaloacetate that is restored at the end of the cycle.

Explain how your body can convert excess carbohydrates in the diet to fats. Can excess carbohydrates be converted to protein? What else must be supplied?

As carbohydrates are broken down in glycolysis and the oxidation of pyruvate, glycerol can be made from G3P and fatty acids can be made from acetyl CoA. Amino groups, containing N atoms, must be supplied to various intermediates of glycolysis and the citric acid cycle to produce amino acids.

In the first stage of cellular respiration, two molecules of pyruvate are produced. In the remaining stages, a number of products are produced, including _____. These stages occur in the _____.

CO2 ... mitochondria ATP, CO2, and H2O are the products of cellular respiration. As pyruvate is "groomed," it is moved into a mitochondrion where the remaining stages of cellular respiration occur.

When pyruvate is converted to acetyl CoA, _____.

CO2 and NADH are formed The carboxyl group of pyruvate is removed as a molecule of carbon dioxide, which diffuses out of the cell. The remaining two-carbon fragment is oxidized and NAD+ is reduced to NADH.

Which of the following best describes the electron transport chain?

Electrons pass from one carrier to another, releasing a little energy at each step. NADH and FADH2 deliver electrons from the breakdown products of glycolysis and the citric acid cycle to the electron transport chain, which passes these electrons from one molecule to another, releasing a small amount of energy at each step.

Which of the following serves primarily as a hydrogen-atom carrier molecule in cells?

FAD Like NAD+, FAD accepts hydrogen ions that have been stripped from glucose and delivers them to the electron transport chain.

An electron carrier, such as _____, acts as an energy-storage molecule when it is _____.

FADH2 ... reduced

List some of the characteristics of glycolysis that indicate that it is an ancient metabolic pathway.

Glycolysis occurs universally (functioning in both fermentation and respiration), does not require oxygen, and does not occur in a membrane-bounded organelle

Explain how someone can gain weight and store fat even when on a low-fat diet.

If caloric intake is excessive, body cells use metabolic pathways to convert the excess to fat. The glycerol and fatty acids of fats are made from G3P and acetyl CoA, respectively, both produced from the oxidation of carbohydrates.

How is your breathing related to your cellular respiration?

In breathing, carbon dioxide and oxygen are exchanged between your lungs and the air. In cellular respiration, cells use the oxygen obtained through breathing to break down fuel, releasing carbon dioxide as a waste product.

Compare and contrast fermentation as it occurs in your muscle cells and as it occurs in yeast cells.

In lactic acid fermentation, pyruvate is reduced by NADH to form lactate, and NAD+ is recycled. In alcohol fermentation, pyruvate is broken down to carbon dioxide and ethanol as NADH is oxidized to NAD+. Both types of fermentation allow glycolysis to continue to produce 2 ATP per glucose by recycling NAD+.

Of the three main stages of cellular respiration, which one uses oxygen to extract chemical energy from organic compounds?

Oxidative phosphorylation, using the electron transport chain, which eventually transfers electrons to oxygen.

What chemical characteristic of the element oxygen accounts for its function in cellular respiration?

Oxygen is very electronegative, making it very powerful in pulling electrons down the electron transport chain.

Explain in terms of cellular respiration why you need oxygen and why you exhale carbon dioxide.

Oxygen picks up electrons from the oxidation of glucose at the end of the electron transport chain. Caron dioxide results from the oxidation of glucose. It is released int he oxidation of pyruvate and in the citric acid cycle.

A glucose-fed yeast cell is moved from an aerobic environment to an anaerobic one. For the cell to continue generating ATP at the same rate, how would its rate of glucose consumption need to change?

The cell would have to consume glucose at a rate about 16 times the consumption rate in the aerobic environment (2 ATP per glucose molecule is made by fermentation versus 32 ATP by cellular respiration)

Why are sweating and other body-cooling mechanisms necessary during vigorous exercise?

The demand for ATP is supported by an increased rate of cellular respiration, but about 66 percent of the energy from food produces heat instead of ATP.

What is the purpose of embedding the electron transport chain in the membrane of a cell?

The membrane is impermeable to H+. As observed when the membrane is uncoupled, the process relies on the properties of the membrane that exclude hydrogen ions (H+) from flowing with their concentration gradient across the membrane. Hydrogen ions are permitted to reenter, but the potential energy that they generate is captured by ATP synthase and used to synthesize ATP.

ATP synthase enzymes are found in the prokaryotic plasma membrane and in the inner membrane of a mitochondrion. What does this suggest about the evolutionary relationship of this eukaryotic organelle to prokaryotes?

The presence of ATP synthase enzymes in prokaryotic plasma membranes and the inner membrane of mitochondria provides support for the theory of endosymbiosis -- that mitochondria evolved from an engulfed prokaryote that used aerobic respiration.

What is misleading about the following statement? "Plant cells perform photosynthesis, and animal cells perform cellular respiration."

The statement implies that cellular respiration does not occur in plant cells. In fact, almost all eukaryotic cells use cellular respiration to obtain energy for their cellular work.

What is the fate of the electrons that are stripped from glucose during cellular respiration?

They are used to form water. At the end of the electron transport chain, the electrons and hydrogen ions are used to reduce oxygen to form water.

When the poison cyanide blocks the electron transport chain, glycolysis and the citric acid cycle soon grind to a halt as well. Why do you think they stop?

They run out of NAD+ and FAD (NAD+ and FAD, which are recycled by electron transport, are in limited supply in a cell)

During cellular respiration _____ is oxidized and _____ is reduced.

glucose ... oxygen

In glycolysis, ________ is oxidized and _________ is reduced.

glucose...NAD+

During cellular respiration, glucose is oxidized. However, an intermediate, _____, can be siphoned off and used to synthesize fats.

glyceraldeyhyde-3-phosphate Glyceraldeyhyde-3-phosphate is used in the synthesis of glycerol.

Of the metabolic pathways listed below, the only pathway found in almost all organisms is _____.

glycolysis

What is the name of the process in which glucose is converted to pyruvate?

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 treat people with bacterial infections. However, he can't do this because _____.

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

Where does glycolysis occur in a eukaryotic cell?

in the cytoplasmic fluid

Sports physiologists at an Olympic training center want to monitor athletes to determine at what point their muscles begin to function anaerobically. They could do this by checking for a buildup of _____.

lactic acid

A molecule is oxidized when it _____.

loses an electron

Primarily, cellular respiration serves to _____.

make ATP to power the cell's activities

The electron transport chain is a series of electron carrier molecules. In eukaryotes, where can this structure be found?

mitochondria The electron transport chain molecules are embedded in the inner mitochondrial membrane.

Muscle tissues make lactic acid from pyruvate so that you can _____.

regenerate (oxidized) NAD+

Most of the NADH that delivers high-energy electrons to the electron transport chain comes from _____.

the citric acid cycle However, a small quantity of NADH is produced during glycolysis.

Which of the following directly requires molecular oxygen (O2)?

the electron transport chain

The ATP synthase in a human cell gets energy for making ATP directly from _____.

the flow of H+ through a membrane

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

the mitochondrion

Which of the following is the most immediate source of energy for making most of the ATP in your cells?

the movement of H+ across a membrane down its concentration gradient

The enzyme ATP synthase catalyzes the phosphorylation of ADP to form ATP. In eukaryotic cells, the energy needed for this endergonic reaction is derived from _____.

the movement of hydrogen ions across the mitochondrial membrane The electron transport chain is an energy converter that uses the exergonic flow of electrons to pump hydrogen ions across the inner mitochondrial membrane from the matrix to the intermembrane space. Hydrogen ions diffuse back into the matrix via a channel in ATP synthase.

Using the ATP generated during cellular respiration, the intermediates of glycolysis and the citric acid cycle can be siphoned off and used _____.

to power the biosynthesis of amino acids, fats, and sugars

A gram of fat oxidized by cellular respiration produces approximately _____ as much ATP as a gram of carbohydrate.

twice

A biochemist wanted to study how various substances were used in cellular respiration. In one experiment, he allowed a mouse to breathe air containing oxygen "labeled" by a particular isotope. In the mouse, the labeled oxygen first showed up in

water

Fat is the most efficient molecule for long-term energy storage, even compared to carbohydrates, because _____.

with their numerous hydrogen atoms, fats provide an abundant source of high-energy electrons Because oxidative phosphorylation depends on a source of electrons (all of which are extracted from hydrogen atoms), fats, which contain primarily hydrogen and carbon atoms, are an abundant source of high-energy electrons that can be harvested for energy production.