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Inquiry Questions
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Self Test

1). In cellular respiration, energy-depleted electrons are donated to an inorganic molecule. In fermentation, what molecule accepts these electrons?
    a). oxygen
    b). an organic molecule
    c). sulfur
    d). an inorganic molecule other than O2

2). Which of the following is not a stage of aerobic respiration?
    a). glycolysis
    b). pyruvate oxidation
    c). the Krebs cycle
    d). electron transport chain

3). Which steps in glycolysis require the input of energy?
    a). the glucose priming steps
    b). the phosphorylation of glucose
    c). the phosphorylation of fructose 6-phosphate
    d). All of these steps require the input of energy.

4). Pyruvate dehydrogenase is a multienzyme complex that catalyzes a series of reactions. Which of the following is not carried out by pyruvate dehydrogenase?
    a). a decarboxylation reaction
    b). the production of ATP
    c). producing an acetyl group from pyruvate
    d). combining the acetyl group with a cofactor

5). How many molecules of CO2 are produced for each molecule of glucose that passes through glycolysis and the Krebs cycle?
    a). 2
    b). 3
    c). 6
    d). 7

6). The electrons generated from the Krebs cycle are transferred to ____________ and then are shuttled to _______________.
    a). NAD+ / oxygen
    b). NAD+ / electron transport chain
    c). NADH / oxygen
    d). NADH / electron transport chain

7). The electron transport chain pumps protons
    a). out of the mitochondrial matrix.
    b). out of the intermembrane space and into the matrix.
    c). out of the mitochondrion and into the cytoplasm.
    d). out of the cytoplasm and into the mitochondrion.

8). What process of cellular respiration generates the most ATP?
    a). glycolysis
    b). oxidation of pyruvate
    c). Krebs cycle
    d). chemiosmosis

9). Oxidizing which of the following substances yields the most energy?
    a). proteins
    b). glucose
    c). fatty acids
    d). water

10). The final electron acceptor in lactic acid fermentation is:
    a). pyruvate
    b). NAD+
    c). lactic acid
    d). O2

Test Your Visual Understanding

Answer the following questions related to this figure, which shows the process of chemiosmosis.
1). In the figure, proton pumps are transporting hydrogen ions across the membrane. What is the driving force of this pump? What type of membrane transport is the proton pump? Explain.
Answer: The driving force of the proton pump is the reduction of the protein with electrons transferred to the protein from NADH. These electrons carry energy and the energy is transferred to the pump and allows it to transport hydrogen ions (protons) across the inner mitochondrial membrane. The proton pump is a type of active transport. The pump requires energy to transport the protons that it receives, not from ATP, but rather from electrons transferred from NADH. It pumps protons across the membrane against their concentration gradient. The proton pump can also be viewed as couple transport because the actions of the proton pump are coupled to the actions of ATP synthase so even though both processes are not functioning in the same protein pump, they are coupled transport systems.

2). Why is this process called chemiosmosis? What is the force driving the synthesis of ATP? How could the process of ATP synthesis be inhibited or shut down?
Answer: This process is called chemiosmosis because the chemical formation of ATP is driven by a diffusion force similar to osmosis, the protons diffuse from an area of higher concentration to an area of lower concentration. The force driving the synthesis of ATP is the steep electrochemical gradient established by the actions of the proton pump. The proton pump pumps hydrogen ions across the membrane establishing a chemical gradient, the protons are higher in the intermembrane space than in the matrix. On top of the chemical gradient, an electrical gradient is also established. The pumping of the hydrogen ions across the membrane makes the intermembrane space more positively-charged compared to the matrix, such that the positively-charged hydrogen ions are driven toward the more negatively-charged matrix. This strong electrochemical gradient is a very strong driving force–energizing the ATP synthase protein to form ATP.

The process of ATP synthesis could be inhibited or shut down with the depletion of NADH in the cell. If there is no NADH carrying electrons, the proton pumps can not function. That is why it is important that the cell recycles NAD+ so there is a constant supply of NADH.

Apply Your Knowledge
1). How much energy would be generated in the cells of a person who consumed a diet of pyruvate instead of glucose? Calculate the energy generated on a per molecule basis.
Answer: The person would not receive the benefits of energy generated from glycolysis because the pyruvate would enter directly into pyruvate oxidation. For each molecule of pyruvate consumed:

The oxidation of pyruvate to acetyl-CoA would produce:
1 molecule of NADH

One round of the Krebs cycle would produce:
    3 molecules of NADH
    1 molecule of FADH2
    1 molecule of ATP

The electron transport chain would generate:
    4 NADH x 2.5 ATP = 9 ATPs
    1 FADH2 x 1.5 = 1.5 ATPs

For a total of 11.5 ATP molecules per each pyruvate molecule consumed.

2). As explained in chapter 5, mitochondria are thought to have evolved from bacteria that were engulfed by and lived symbiotically within early eukaryotic cells. Why haven't present-day eukaryotic cells dispensed with mitochondria, placing all of the mitochondrial genes in the nucleus and carrying out all of the metabolic functions of the mitochondria within the cytoplasm?
Answer: The evolution of the eukaryotic cell tended toward compartmentalizing functions as a means of increasing efficiency. By compartmentalizing a function or process, all of the enzymes and substrates for the process could be sequestered in one area increasing the efficiency of reactions taking place. The sequestering of aerobic metabolic functions to the mitochondria and the sequestering of mitochondrial genes help to improve efficiency. The redundancy of some mitochondrial genes to the nucleus is probably more of a "backup" function than of a switching over of the location of the function. It is simply more efficient for all aerobic metabolic functions to be sequestered in the mitochondria.

3). Why do plants typically store their excess energy as carbohydrates rather than fat?
Answer: Because plants are able to produce their own food, it is not necessary for them to have long-term storage of energy provided by fatty acids. Carbohydrate storage is sufficient for plants. When they need more carbohydrates, they just make them. Animals, on the other hand, need to consume their food therefore their bodies have evolved the ability to store energy in more energy-rich molecules for times of famine.

4). If you poke a hole in a mitochondrion, can it still perform oxidative respiration? Can fragments of mitochondria perform oxidative respiration?
Answer: Because oxidative respiration requires the establishment of a concentration gradient of hydrogen ions (protons) a hole in the mitochondrion would allow leakage of protons out of the mitochondrion and would disrupt the proton concentration gradient. Without this gradient, oxidative respiration could not occur. Using that same argument, a fragment of mitochondrial membrane could not perform oxidative respiration. A concentration gradient of protons could not be established across a fragment of membrane, it requires a "closed" system.

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