Biology Chapter 9 Concept Checks

Concept Checks Chapter 9

9.1

In the following redox reaction, which compound is oxidized and which is reduced?

C₄H₆O₅ + NAD⁺ à C₄H₄O₅ + NADH + H+

Oxidized: C₄H₆O₅

Reduced: NAD⁺

9.2

During the redox reaction in Glycolysis (step 6 in Figure 9.9), which molecule acts as the oxidizing agent? The reducing agent?

NAD⁺ acts as the oxidizing agent in step 6, accepting electrons from glyceraldehyde-3-phosphate, which thus acts as the reducing agent.

9.3

1. In which molecules is most of the energy from the citric acid cycle’s redox reactions conserved? How will these molecules convert their energy to a form that can be used to make ATP?

NADH and FADH₂; they will donate electrons to the electron transport chain.

2. What cellular processes produce the carbon dioxide that you exhale?

CO₂ is removed from pyruvate, which is produced by glycolysis, and CO₂ is produced by the citric acid cycle.

9.4

1. What effect would an absence of O₂ have on the process shown in Figure 9.15?

Oxidative phosphorylation would stop entirely, resulting in no ATP production. Without oxygen to “pull” electrons down the electron transport chain, H⁺ would not be pumped into the mitochondrion’s intermembrane space and chemiosmosis would not occur.

2. In the absence of O2, as above, what do you think would happen if you decreased the pH of the intermembrane space of the mitochondrion? Explain your answer.

Because addition of H+ (lowering the pH) would establish a proton gradient even without the function of the electron transport chain, we would expect ATP synthase to function and synthesize ATP.

9.5

1. Consider the NADH formed by glycolysis. What is the final acceptor for its electrons during fermentation? What is the final acceptor for its electrons during respiration?

A derivative of pyruvate—either acetaldehyde during alcohol fermentation or pyruvate itself during lactic acid fermentation; oxygen.

2. 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 need to consume glucose at a rate about 19 times the consumption rate in the aerobic environment. (2 ATP are generated by fermentation per molecule of glucose versus up to 38 ATP by cellular respiration.)

9.6

1. Compare the structure of a fat (Fig. 5.11) with that of a carbohydrate (Fig. 5.3). What features of their structures make a fat a much better fuel?

The fat is much more reduced; it has many –CH₂—units. The electrons present in a carbohydrate molecule are already somewhat oxidized, as some of them are bound to oxygen.

2. Under what circumstances might your body synthesize fat molecules

When we eat more food then we need for metabolic processes, our bodies synthesize fat as a way of storing energy for later.

3. What will happen in a muscle cell that has used up its supply of oxygen and ATP? (See Fig. 9.20)

AMP will accumulate, stimulating phosphofructokinase, which increases the rate of glycolysis. Since oxygen is not present, the cell will convert pyruvate to lactate in lactic acid fermentation, providing a supply of ATP.

Chapter 9 Test

1. What is the reducing agent in the following reaction?

Pyruvate + NADH + H⁺ à Lactate + NAD⁺

Answer: NADH

2. The immediate energy source that drives ATP synthesis by ATP synthase during oxidative phosphorylation is:

Answer: The H+ concentration gradient across the inner mitochondrial membrane.

3. Which metabolic pathway is common to both fermentation and cellular respiration?

Answer: Glycolysis

4. In mitochondria, exergonic redox reactions:

Answer: provide the energy to establish the proton gradient.

5. The final electron acceptor of the electron transport chain that functions in oxidative phosphorylation is:

Answer: oxygen

6. When electrons flow along the electron transport chains of mitochondria, which of the following changes occur?

Answer: The pH of the matrix increases

7. In the presence of a metabolic poison that specifically and completely inhibits all function of the mitochondrial ATP synthase, which would you expect?

Answer: An increase in the pH difference across the inner mitochondrial membrane.

8. Cells do not catabolize carbon dioxide because:

Answer: CO₂ is already completely oxidized.

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

Answer: NADH is oxidized by te electron transport chain in respiration only.

10. Most CO₂ from catabolism is released during:

Answer: The citric acid cycle

Biology Chapter 9 Review

Chapter 9 Review

Concept 9.1

Catabolic Pathways Yield Energy By Oxidizing Organic Fuels

Catabolic Pathways & the Production of ATP: The breakdown of glucose and other organic fuels is exergonic. Starting with glucose or another organic molecule and using O₂, cellular respiration yields H₂0, CO₂ and energy in the form of ATP & heat. To keep working, a cell must regenerate ATP.

Redox Reactions-Oxidation & Reduction: The cell taps the energy stored in food molecules through redox reactions, in which one substance partially or totally shifts electrons to another. The substance receiving the electrons is reduced; the substance losing electrons is oxidized. During cellular respiration, glucose (C₆H₁₂O₆) is oxidized to CO₂ and O₂ is reduced to H₂0. Electrons lose potential energy during their transfer from organic compounds to oxygen. Electrons from organic compounds are usually passed first to NAD^+, reducing it to NADH. NADH passes the electrons to an electron transport chain, which conducts them to O₂ in energy-releasing steps. The energy released is used to make ATP.

The Stages of Cellular Respiration- A Preview: Glycolysis and the citric acid cycle supply electrons (via NADH or FADH₂ ) to the electron transport chain, which drives oxidative phosphorylation. Oxidative phosphorylation generates ATP.

Concept 9.2

Glycolysis harvests chemical energy by oxidizing glucose to pyruvate.

Glycolysis breaks down glucose into two pyruvate molecules and nets 2 ATP and 2 NADH per glucose molecule.

Concept 9.3

The citric acid cycle completes the energy-yielding oxidation of organic molecules.

The import of pyruvate into the mitochondrion and its conversion to acetyl CoA links Glycolysis to the citric acid cycle. The two-carbon acetyl CoA joins the four-carbon oxaloacetate, forming the six carbon citrate, which is degraded back to oxaloacetate. The cycle releases 2 CO₂ forms 1 ATP and passes electrons to NAD⁺ and FAD, yielding 3 NADH and 1 FADH₂ per turn.

Concept 9.4

During oxidative phosphorylation, chemiosmosis couples electron transport to ATP synthesis.

NADH and FADH₂ donate electrons to the electron transport chain, which powers ATP synthesis via oxidative phosphorylation.

The Pathway of Electron Transport: In the electron transport chain, electrons from NADH and FADH₂ lose energy in several energy releasing steps. At the end of the chain, electrons are passed to O₂ reducing it to H₂O.

Chemiosmosis- The Energy-Coupling Mechanism: At certain steps along the electron transport chain, electron transfer causes protein complexes to move H+ from the mitochondrial matrix to the intermembrane space string energy as a proton-motive force (H+ gradient). As H+ diffuses back into the matrix through ATP synthase, its passage drives phosphorylation of ADP.

An Accounting of ATP Production by Cellular Resperation: About 40% of the energy stored in a glucose molecule is transferred to ATP during cellular respiration, producing a maximum of about 38 ATP.

Concept 9.5

Fermentation enables some cells to produce ATP without the use of oxygen.

Types of Fermentation: Glycolysis nets two ATP by substrate-level phosphorylation whether oxygen is present or not. Under anaerobic conditions, the electrons from NADH are passed to pyruvate, regenerating the NAD+ required to oxidize more glucose. Two common types of fermentation are alcohol fermentation and lactic acid fermentation.

Fermentation & Cellular Respiration Compared: Both use Glycolysis to oxidize glucose, but differ in their final electron acceptor. Respiration yields more ATP.

The Evolutionary Significance of Glycolysis: Glycolysis occurs in nearly all organisms and probably evolved in ancient prokaryotes before there was O₂ in the atmosphere.

Concept 9.6

Glycolysis and the citric acid cycle connect to many other metabolic pathways.

The Versatility of Catabolism: Catabolic pathways funnel electrons from many kinds of organic molecules into cellular respiration.

Biosynthesis (Anabolic Pathways): The body can use small molecules from food directly or use them to build other small substances through Glycolysis or the citric acid cycle.

Regulation of Cellular Respiration via Feedback Mechanisms: Cellular respiration is controlled by allostericenzymes at keypoints in Glycolysis and the citric acid cycle.