Biology 111 Chapter 9

Carbohydrates and Fats

Main energy yielding foods Reservoirs of electrons associated with hydrogen

Oxidizing Agen

electron acceptor

Electron Transport Chain

breaks the fall of electrons to oxygen into several energy-releasing steps that is used to make ATP

The final electron acceptor of the electron transport chain that functions in aerobic oxidative phosphorylation is a. oxygen. b. water. c. NAD+. d. pyruvate.

A

9.6 3. What will happen in a muscle cell that has used up its supply of oxygen and ATP?

AMP will accumulate, stimulating phosphofruc- tokinase, and thus increasing 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.

Reduction

Addition of electrons to another substance Becomes more negative

In mitochondria, exergonic redox reactions a. are the source of energy driving prokaryotic ATP synthesis. b. provide the energy that establishes the proton gradient. c. reduce carbon atoms to carbon dioxide. d. are coupled via phosphorylated intermediates to endergonic processes.

B

Most CO2 from catabolism is released during a. glycolysis. b. the citric acid cycle. c. lactate fermentation. d. electron transport.

B

Compare and contrast aerobic and anaerobic respiration.

Both processes include glycolysis, the citric acid cycle, and oxidative phosphorylation. In aerobic respiration, the final electron acceptor is molecular oxygen (O2); in anaerobic respiration, the final electron acceptor is a different substance.

The immediate energy source that drives ATP synthesis by ATP synthase during oxidative phosphorylation is the a. oxidation of glucose and other organic compounds. b. flow of electrons down the electron transport chain. c. H+ concentration gradient across the membrane holding ATP synthase. d. transfer of phosphate to ADP.

C

Which metabolic pathway is common to both fermentation and cellular respiration of a glucose molecule? a. the citric acid cycle b. the electron transport chain c. glycolysis d. reduction of pyruvate to lactate

C

If the following redox reaction occurred, which compound would be oxidized? Reduced? C4H6O5 + NAD+ → C4H4O5 + NADH + H+

C4H6O5 would be oxidized and NAD+ would be reduced.

What processes in your cells produce the CO2 that you exhale?

CO2 is released from the pyruvate that is the end product of glycolysis, and CO2 is also released during the citric acid cycle.

What is the oxidizing agent in the following reaction? Pyruvate + NADH + H+ ---> Lactate + NAD+ a. oxygen b. NADH c. lactate d. pyruvate

D

9.4 2. In the absence of O2, what do you think would happen if you decreased the pH of the inter membrane space of the mitochondrium?

Decreasing the pH means addition of H+. This would establish a proton gradient even without the function of the electron transport chain, and we would expect ATP synthase to function and synthesize ATP. (In fact, it was experiments like this that provided support for chemiosmosis as an energy-coupling mechanism.)

Oxidation

Loss of electrons from one substance Becomes more positive

Describe the difference between the two processes in cellular respiration that produce ATP: oxidative phosphorylation and substrate-level phosphorylation.

Most of the ATP produced in cellular respiration comes from oxidative phos- phorylation, in which the energy released from redox reactions in an electron transport chain is used to produce ATP. In substrate-level phosphorylation, an enzyme directly transfers a phosphate group to ADP from an intermediate sub- strate. All ATP production in glycolysis occurs by substrate-level phosphorylation; this form of ATP production also occurs at one step in the citric acid cycle.

During the redox reaction in glycolysis, 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.

Name the molecules that conserve most of the energy from the redox reactions of the citric acid cycle. How is this energy converted to a form that can be used to make ATP?

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

NAD+

Nicotinamide Adenine Dinucleotide Coenzyme, electron carrier Cycles between the oxidized (NAD+) and reduced (NADH) states Functions as an oxidizing agent

Glycolysis

Occurs in cytosol Begins degradation process by breaking glucose into tow molecules of the pyruvate compound In Eukaryotes, pyruvate enters the mitochondrion and is oxidized to a compound called acetyl CoA

Citric Acid Cycle

Occurs in mitochondria (cytosol in prokaryotes) Breakdown of glucose to carbon dioxide is completed

9.4 3. Membranes must be fluid to function properly. How does the operation of the electron transport chain support that assertion?

One of the components of the electron transport chain, ubiquinone (Q), must be able to diffuse within the membrane. It could not do so if the membrane were locked rigidly into place.

Redox Reactions

Transfer of electrons from one reactant to another

9.6 4. During intense exercise, can a muscle cell use fat as a concentrated source of chemical energy? Explain.

When oxygen is present, the fatty acid chains containing most of the energy of a fat are oxidized and fed into the citric acid cycle and the electron trans- port chain. During intense exercise, however, oxygen is scarce in muscle cells, so ATP must be generated by glycolysis alone. A very small part of the fat molecule, the glycerol backbone, can be oxidized via glycolysis, but the amount of energy released by this portion is insignificant compared to that released by the fatty acid chains. (This is why moderate exercise, staying below 70% maximum heart rate, is better for burning fat—because enough oxygen remains available to the muscles.)

Reduction

addition of electron

aerobic respiration

catabolic process in which oxygen is consumed as a reactant with the organic fuel

Fermentation

degradation of sugars or other organic fuel that occurs without the use of oxygen

Oxidizing agent

electron acceptor

NAD+

electron carrier, coenzyme, nicotinamide adenine dinucleotide Cycles easily between oxidized and reduced state Enzymes remove a pair of H atoms from substrate, enzyme delivers both e- and one proton to NAD + while other proton is released

Reducing Agent

electron donor

Reducing agent

electron donor

Which reactions in glycolysis are the source of energy for the formation of ATP and NADH?

he oxidation of the three-carbon sugar, glyceraldehyde 3-phosphate, yields energy. In this oxidation, electrons and H+ are transferred to NAD+, forming NADH, and a phosphate group is attached to the oxidized substrate. ATP is then formed by substrate-level phosphorylation when this phosphate group is trans- ferred to ADP.

Oxidation

loss of electrons

Catabolic pathways

metabolic pathways that release stored energy by breaking down complex molecules

anaerobic respiration

process used by some prokaryotes that uses substances other than oxygen as reactants

Respiration

the oxidation of glucose and other molecules in food

When electrons flow along the electron transport chains of mi- tochondria, which of the following changes occurs? a. The pH of the matrix increases. b. ATP synthase pumps protons by active transport. c. The electrons gain free energy. d. NAD+ is oxidized.

A

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

A derivative of pyruvate, such as acetaldehyde during alcohol fermentation, or pyruvate itself during lactic acid fermentation; oxygen

Electron Transport Chain

A number of molecules, mostly proteins, built into the inner membrane of the mitochondria of eukaryotic cells

Oxidative Phosphorylation

ATP synthesis powered by the redox reactions of the electron transport chain

Which process yields more ATP, fermentation or anaerobic respiration? Explain.

Anaerobic respiration yields more ATP. The 2 ATP produced by substrate-level phosphorylation in glycolysis represent the total en- ergy yield of fermentation. NADH passes its "high-energy" electrons to pyruvate or a derivative of pyruvate, recycling NAD+ and allowing glycolysis to continue. In anaerobic respiration, the NADH produced during glycolysis, as well as addi- tional molecules of NADH produced as pyruvate is oxidized, are used to generate ATP molecules. An electron transport chain captures the energy of the electrons in NADH via a series of redox reactions; ultimately, the electrons are transferred to an electronegative molecule other than oxygen.

Oxidation During Cellular Respiration

Fuel is oxidized Oxygen is reduced

Considering the overall result of glycolysis, would you expect ATP to inhibit or stimulate activity of this enzyme? Explain.

Since the overall process of glycolysis results in net production of ATP, it would make sense for the process to slow down when ATP levels have increased sub- stantially. Thus, we would expect ATP to allosterically inhibit phosphofructoki- nase.

Describe how the catabolic pathways of glycolysis and the citric acid cycle intersect with anabolic pathways in the metabolism of a cell.

The ATP produced by catabolic pathways is used to drive anabolic pathways. Also, many of the interme- diates of glycolysis and the citric acid cycle are used in the biosynthesis of a cell's molecules.

9.5 2. A glucose fed yeast cell is moved from an aerobic environment to an anaerobic one. How would its rate of glucose consumption change if ATP were to be generated at the same rate?

The cell would need to consume glucose at a rate about 16 times the consumption rate in the aerobic environment (2 ATP are generated by fermentation versus up to 32 ATP by cellular respiration).

9.6 1. Compare the structure of a fat with that of a carbohydrate. What features of their structures make fat a much better fuel?

The fat is much more reduced; it has many ¬CH2¬ units, and in all these bonds the electrons are equally shared. The electrons present in a carbohydrate molecule are already somewhat oxidized (shared unequally in bonds), as quite a few of them are bound to oxygen. Electrons that are equally shared, as in fat, have a higher energy level than electrons that are unequally shared, as in carbohydrates. Thus, fat is a much better fuel than carbohydrate.

Briefly explain the mechanism by which ATP synthase produces ATP. List three locations in which ATP synthases are found.

The flow of H+ through the ATP synthase complex causes the rotor and attached rod to rotate, exposing catalytic sites in the knob portion that produce ATP from ADP and ~P . ATP synthases are found in the inner mi- i tochondrial membrane, the plasma membrane of prokaryotes, and membranes within chloroplasts.

What molecular products indicate the complete oxidation of glucose during cellular respiration?

The release of six molecules of CO2 represents the complete oxidation of glucose. During the processing of two pyruvates to acetyl CoA, the fully oxidized carboxyl groups (¬COO-) are given off as 2 CO2. The remaining four carbons are released as CO2 in the citric acid cycle as citrate is oxidized back to oxaloacetate.

9.6 2. Under what circumstances might your body synthesize fat molecules?

Whenweconsumemore food than necessary for metabolic processes, our body synthesizes fat as a way of storing energy for later use.

Glycolysis

begins degradation process by breaking glucose down into two molecules of a compounds called pyruvate.

cellular respiration

breaking down organic molecules and using an electron transport chain for production of ATP, aerobic/anaerobic process exergonic (-deltaG, spontaneous, stores less energy)

Redox Reactions

transfer of one or more electrons from one reactant to another