Cellular Respiration Problem Set

NAD+ : Oxidized or Reduced? Higher Energy/Lower Energy?

Oxidized, Lower Energy

Which component (X or Y) is oxidized and which is reduced.

Xe- is the reducing agent in this reaction, and Y is the oxidizing agent.

in the citric acid cycle, How many FADH2 have been formed?

1

NAD+ is a coenzyme. What is a coenzyme?

An organic molecule serving as a cofactor. Most vitamins function as coenzymes in metabolic reactions.

Explain how AMP stimulates cellular respiration while citrate and ATP inhibit it.

If the cell is working hard and its ATP concentration begins to drop, respiration speeds up. When there is plenty of ATP to meet demand, respiration slows down, sparing valuable organic molecules for other functions.

What three organic macromolecules are often utilized to make ATP by cellular respiration?

Possible examples include fats, proteins, sucrose, and starch.

Notice that glycolysis occurs in the cystol of the cell. Is oxygen required?

No

Oxygen is the ultimate electron acceptor. Why is this?

Oxygen is extremely electronegative.

What is the meaning of glycolysis? What occurs in this step of cellular respiration?

Glycosis is a series of reactions that ultimately splits glucose into pyruvate. Glycolysis occurs in almost all living cells, serving as the starting point for fermentation or cellular respiration.

Both cellular respiration and photosynthesis are redox reactions. In redox, reactions pay attention to the flow of electrons. What is the difference between oxidation and reduction?

In a redox reaction, the loss of electrons from one substance is called oxidation, and the addition of electrons to another substance is known as reduction.

Electron transport involves a series of electron carriers. Where are these found in eukaryotic cells?

Membrane of the mitochondria

Give the formula (with names) for the catabolic degradation of glucose by cellular respiration.

Organic Compounds + Oxygen Carbon dioxide + Water + Energy

What strongly electronegative atom, pulling electrons down the electron transport chain, is the final electron acceptor?

Oxygen

To enter the citric acid cycle, pyruvate must enter the mitochondria by active transport. Three things are necessary to convert pyruvate to acetyl CoA. Explain the three steps in the conversion process. (1)

Pyruvate's carboxyl group, which is already fully oxidized and thus has little chemical energy, is removed and given off as a molecule of CO2 .

To enter the citric acid cycle, pyruvate must enter the mitochondria by active transport. Three things are necessary to convert pyruvate to acetyl CoA. Explain the three steps in the conversion process. (2)

The remaining two-carbon fragment is oxidized, forming acetate (the ionized form of acetic acid). The extracted electrons are transferred to NAD+, storing energy in the form of NADH.

Oxidative phosphorylation involves two components: the electron transport chain and ATP synthesis. Referring to Figure 9.13, notice that each member of the electron transport chain is lower in free _______than the preceding member of the chain, but higher in ____________. The molecule at zero free energy, which is _________, is lowest of all the molecules in free energy and highest in electronegativity.

energy, electronegativity, oxygen

When compounds lose electrons, they _______energy; when compounds gain electrons, they ________energy

lose, gain

in the citric acid cycle, How many NADHs are formed?

3

How many times does the citric acid cycle occur for each molecule of glucose?

2

in the citric acid cycle, How many total carbons are lost as pyruvate is oxidized?

2

At this point, you should be able to account for the total number of ATPs that could be formed from a glucose molecule. To accomplish this, we have to add the ATPs formed by substrate-level phosphorylation in glycolysis and the citric acid cycle to the ATPs formed by chemiosmosis. Each NADH can form a maximum of ___ ATP molecules. Each FADH2, which donates electrons that activate only two proton pumps, makes ____ ATP molecules.

3, 2

Lactic acid fermentation starts with glucose and yields lactate. Explain this process, and be sure to describe how NAD+ is recycled.

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.) Lactic acid fermentation by certain fungi and bacteria is used in the dairy industry to make cheese and yogurt. Pyruvate, the end product of glycolysis, serves as an electron acceptor for oxidizing NADH back to NAD+, which can then be reused in glycolysis.

To enter the citric acid cycle, pyruvate must enter the mitochondria by active transport. Three things are necessary to convert pyruvate to acetyl CoA. Explain the three steps in the conversion process. (3)

Finally, coenzyme A, a sulfur compound derived from a B vitamin, is attached via its sulfur atom to the acetate, for acetyl CoA, which has a high potential energy. This molecule will now feed its acetyl group into the citric acid cycle for further oxidation.

Explain what has happened to each of the six carbons found in the original glucose molecule.

For each pyruvate molecule formed from the original glucose molecule, the pyruvate is broken down to three CO2 molecules, including the molecule ofCO2 released during the conversion of pyruvate to acetyl CoA.

Explain why pyruvate is a key juncture in metabolism.

Glycolysis is common to fermentation and cellular respiration. The end product of glycolysis, pyruvate, represents a fork in the catabolic pathways of glucose oxidation. In a facultative anaerobe or a muscle cell, which are capable of both aerobic cellular respiration and fermentation, pyruvate is committed to one of those two pathways, usually depending on whether or not oxygen is present.

Oxygen stabilizes the electrons by combining with two hydrogen ions to form what compound?

H2O

In cellular respiration, electrons are not transferred directly from glucose to oxygen. Following the movement of hydrogens allows you to follow the flow of electrons. The hydrogens are held in the cell temporarily by what electron carrier? What electron carrier is hydrogen transferred to first?

NAD+

Electron transport involves a series of electron carriers. Where are these found in prokaryotic cells?

Plasma membrane

For aerobic respiration to continue, the cell must be supplied with oxygen—the ultimate electron acceptor. What is the electron acceptor in fermentation?

NAD+

NADH : Oxidized or Reduced? Higher Energy/Lower Energy?

Reduced, Higher Energy

Fermentation allows for the production of ATP without using either ______or any ____________________

oxygen, electron transport chain

What is the function of the electron transport chain in cellular respiration?

The electron transport chain shuttles electrons down a series of redox reactions that release energy used to make ATP.

Describe what happens when NAD+ is reduced. What enzyme is involved?

The enzymatic transfer of 2 electrons and 1 proton (H+) from an organic molecule in food to NAD+ reduces the NAD+ to NADH; the second proton (H+) is released. The enzyme involved is dehydrogenase.

Why is the total count about 36 or 38 ATP molecules rather than a specific number?

There are three reasons we cannot state an exact number of ATP molecules generated by the breakdown of one molecule of glucose. First, phosphorylation and the redox reactions are not directly coupled to each other, so the ratio of the number of NADH molecules to the number of ATP molecules is not a whole number. Second, the ATP yield varied slightly, depending on the type of shuttle used to transport electrons from the cytosol into the mitochondrion. A third variable that reduces the yield of ATP is the use of the proton-motive force generated by the redox reactions of respiration to drive other kinds of work.

Explain how the electron transport chain is utilized in oxidative phosphorylation

This mode of ATP synthesis is powered by the redox reactions of the electron transport chain.

The following is a generalized formula for a redox reaction:

Xe-+ Y --> X + Ye-

in the citric acid cycle, How many ATPs are formed?

1

Phosphofructokinase is an allosteric enzyme that catalyzes an important step in glycolysis. Explain how this step is a control point in cellular respiration.

As ATP accumulates, inhibition of the enzyme slows down glycolysis. The enzyme becomes active again as cellular work converts ATP to ADP (and AMP) faster than ATP is being regenerated. This mechanism helps synchronize the rates of glycolysis and the citric acid cycle.

in the citric acid cycle, The carbons have been lost in the molecule ________

Carbon Dioxide

Explain the difference in energy usage between the catabolic reactions of cellular respiration and anabolic pathways of biosynthesis.

Catabolic pathways funnel electrons from many kinds of organic molecules into cellular respiration. Many carbohydrates can enter glycolysis, most often after conversion to glucose. Amino acids of proteins must be deaminated before being oxidized. The fatty acids of fats undergo beta oxidation to two-carbon fragments and then enter the citric acid cycle as acetyl CoA. Anabolic pathways can use small molecules from food directly or build other substances using intermediates of glycolysis or the citric acid cycle.

What is the role of the electron transport chain in forming the H+ gradient across the inner mitochondrial membrane?

Certain members of the electron transport chain accept and release protons (H+) along with electrons. (The aqueous solutions inside and surrounding the cell are a ready source of H+.) At certain steps along the chain, electron transfers cause H+ to be taken up and released into the surrounding solution. In eukaryotic cells, the electron carriers are spatially arranged in the inner mitochondrial membrane in such a way that H+ is accepted from the mitochondrial matrix and deposited in the intermembrane space. The H+ gradient that results is referred to as a proton-motive force, emphasizing the capacity of the gradient to perform work. The force drives H+ back across the membrane through the H+channels provided by ATP synthases.

Two key terms are chemiosmosis and proton-motive force. Relate both of these terms to the process of oxidative phosphorylation.

Chemiosmosis refers to the energy-coupling mechanism that uses energy stored in the form of a hydrogen ion gradient across a membrane to drive cellular work, such as the synthesis of ATP. Proton-motive force refers to the potential energy stored in the form of a proton electrochemical gradient, generated by the pumping of hydrogen ions (H+) across a biological membrane during chemiosmosis.

Explain the difference between fermentation and cellular respiration

Fermentation is a partial degradation of sugars or other organic fuel that occurs without the use of oxygen. Cellular respiration consumes oxygen as a reactant along with the organic fuel.

Explain the overall concept of how ATP synthase uses the flow of hydrogen ions to produce ATP.

H+ ions flow down their gradient and enter binding sites within a rotor, changing the shape of each subunit so that the rotor spins within the membrane, before leaving the rotor and passing through a second half channel into the mitochondrial matrix. The spinning of the rotor causes an internal rod to spin, activating catalytic sites in the knob that produce ATP.

Alcohol fermentation starts with glucose and yields ethanol. Explain this process, and be sure to describe how NAD+ is recycled

In alcohol fermentation pyruvate is converted to ethanol (ethyl alcohol) in two steps. The first step releases carbon dioxide from the pyruvate, which is converted to the two-carbon compound acetaldehyde. In the second step, acetaldehyde is reduced by NADH to ethanol. This regenerates the supply of NAD needed for the continuation of glycolysis. Many bacteria carry out alcohol fermentation under anaerobic conditions. Yeast (a fungus) also carries out alcohol fermentation.

The step that converts pyruvate to acetyl CoA at the top of the diagram occurs twice per glucose. This oxidation of pyruvate accounts for two additional reduced _______molecules and two molecules of CO2.

NADH

The two electron carrier molecules that feed electrons into the electron transport system are....

NADH and FADH2.