Chapter 9 - Cellular Respiration and Fermentation

Question 1

How do catabolic pathways yield energy?

Answer 1

By oxidizing organic fuels; catabolic pathways release stored energy by breaking down complex molecules - electron transfer plays a major role in these pathways.

Question 2

What types of compounds can act as fuels?

Answer 2

Compounds that can participate in exergonic reactions. The breakdown of organic molecules is exergonic.

Question 3

What is fermentation?

Answer 3

A catabolic process, which is a partial degradation of sugars or other organic fuel that occurs without the use of oxygen.

Question 4

What is aerobic respiration?

Answer 4

The most efficient catabolic process, in which oxygen is consumed as a reactant along with the organic fuel.

Question 5

What is anaerobic respiration?

Answer 5

Harvests chemical energy without oxygen and consumes compounds other than O2; some prokaryotes use substances other than oxygen as reactants.

Question 6

What is cellular respiration?

Answer 6

The catabolic pathways of aerobic and anaerobic respiration (however, it is often synonymous with the aerobic process), which break down organic molecules and use an electron transport chain for the production of ATP.

Question 7

What types of molecules can be consumed as fuel?

Answer 7

Carbohydrates, fats, and proteins from food can all be processed and consumed as fuel.

Question 8

How do the catabolic pathways that decompose glucose and other organic fuels yield energy?

Answer 8

The transfer of electrons during chemical reactions releases energy stored in organic molecules, and this energy is ultimately used to synthesize ATP.

Question 9

What is a redox reaction?

Answer 9

Chemical reactions that transfer electrons between reactants are known as oxidation-reduction reactions, or redox reactions.

Question 10

What is oxidation?

Answer 10

In a redox reaction, the loss of electrons from one substance.

Question 11

What is reduction?

Answer 11

In a redox reaction, the addition of electrons to another substance.

*Adding negatively charged electrons to an atom reduces the amount of positive charge of that atom.

Question 12

What is a general formula for a redox reaction?

Answer 12

Xe- + Y —> X + Ye-

Question 13

What is the reducing agent?

Answer 13

The electron donor (Xe-)

Question 14

What is the oxidizing agent?

Answer 14

The electron acceptor (Y)

Question 15

What is one of the most potent of all oxidizing agents?

Answer 15

Oxygen, because it is so electronegative.

An electron loses potential energy when it shifts from a less electronegative atom toward a more electronegative one (just as a ball loses potential energy when it rolls downhill). A redox reaction that moves electrons closer to oxygen, such as the burning (oxidation) of methane, therefore releases energy that can be put to work.

Question 16

In general, which type of organic molecules make for excellent fuel sources?

Answer 16

Organic molecules that have an abundance of hydrogen; their bonds are a source of “hilltop” electrons, whose energy may be released as these electrons “fall” down an energy gradient when they are transferred to oxygen.

Hydrogen is transferred from glucose to oxygen. The oxidation of glucose transfers electrons to a lower energy state, liberating energy that becomes available for ATP synthesis.

Question 17

What is the most versatile electron acceptor in cellular respiration?

Answer 17

NAD+, as it functions in several of the redox steps during the breakdown of glucose.

As an electron acceptor, NAD+ functions as an oxidizing agent during respiration.

Question 18

What is NAD+?

Answer 18

Nicotinamide adenine dinucleotide.

Question 19

What is the reduced form of NAD+?

Answer 19

NADH; The enzymatic transfer of 2 negatively charged electrons and 1 positively charged proton (H+) from an organic molecule in food to NAD+ reduces the NAD+ to NADH.

Most of the electrons removed from food are transferred initially to NAD+, forming NADH.

*Electrons lose very little of their potential energy when they are transferred from glucose to NAD+. Each NADH molecule formed during respiration represents stored energy. This energy can be tapped to make ATP when the electrons complete their “fall” in a series of steps down an energy gradient from NADH to oxygen.

Question 20

What is the electron transport chain?

Answer 20

Respiration uses an electron transport chain to break the fall of electrons to oxygen into several energy-releasing steps. An electron transport chain consists of a number of molecules, mostly proteins, built into the inner membrane of the mitochondria of eukaryotic cells. Electrons removed from glucose are shuttled by NADH to the “top”, higher energy end of the chain. At the lower energy end, “bottom”, O2 captures these electrons along with hydrogen nuclei (H+) forming water.

• Electrons cascade down the chain from one carrier molecule to the next in a series of redox reactions, losing a small amount of energy with each step until they finally reach oxygen, the terminal electron acceptor, which as a very great affinity for electrons.
• Oxygen pulls electrons down the chain.

Question 21

What “downhill” route do most electrons travel during cellular respiration?

Answer 21

glucose —> NADH —> electron transport chain —> oxygen

Question 22

The harvesting of energy from glucose by cellular respiration is a cumulative function of what three metabolic stages?

Answer 22

1. Glycolysis - breaks down glucose into 2 molecules of pyruvate
2. Pyruvate Oxidation and the CITRIC ACID CYCLE - completes the breakdown of glucose
3. Oxidative Phosphorylation - accounts for most of the ATP synthesis

Question 23

What is glycolysis?

Answer 23

Occurs in the cytosol and begins the degradation process by breaking glucose into two molecules of a compound called pyruvate. In eukaryotes, pyruvate enters the mitochondrion and is oxidized to a compound called acetyl CoA.

Question 24

What is the citric acid cycle?

Answer 24

Also called the Krebs cycle, completes the breakdown of pyruvate to CO2.

Question 25

What is oxidative phosphorylation?

Answer 25

In the third stage of respiration, the electron transport chain accepts electrons (most often via NADH) from the breakdown products of the first two stages and passes these electrons from one molecule to another. At the end of the chain, the electrons are combined with molecular oxygen and hydrogen ions (H+), forming water. The energy released at each step of the chain is stored in a form the mitochondrion can use to make ATP from ADP. This mode of ATP synthesis is powered by the redox reactions of the electron transport chain.

Question 26

What is the site of oxidative phosphorylation?

Answer 26

The inner membrane of the mitochondrion.

*Oxidative phosphorylation accounts for almost 90% of the ATP generated by respiration.

Question 27

What is substrate-level phosphorylation?

Answer 27

A smaller amount of ATP is formed in glycolysis and the citric acid cycle by substrate-level phosphorylation.

A mode of ATP synthesis that occurs when an enzyme transfers a phosphate group from a substrate molecule to ADP, rather than adding an inorganic phosphate to ADP as in oxidative phosphorylation. “Substrate molecule” here refers to an organic molecule generated as an intermediate during the catabolism of glucose.

Question 28

For each molecule of glucose degraded to CO2 and H20 by respiration, how many molecules of ATP does the cell make up?

Answer 28

32 - oxidative phosphorylation (more than 90%)

2 - glycolysis
2 - citric acid cycle

Total 36ish

Question 29

How does glycolysis harvest chemical energy?

Answer 29

By oxidizing glucose to pyruvate

Question 30

What does glycolysis mean?

Answer 30

“Sugar splitting” - glucose, a six carbon sugar, is split into two three carbon sugars. These smaller sugars are then oxidized and their remaining atoms rearranged to form two molecules of pyruvate (pyruvate is the ionized form of pyruvic acid).

Question 31

What are the two phases of glycolysis?

Answer 31

1. Energy investment phase - the cell “spends” ATP (2 ATP used)
2. Energy payoff phase - ATP is produced by substrate level phosphorylation and NAD+ is reduced to NADH by electrons released from the oxidation of glucose

Question 32

What is the net energy yield from glycolysis per glucose molecule?

Answer 32

2 ATP plus 2 NADH

Glucose —> 2 pyruvate + 2 H20

4 ATP formed - 2 ATP used —> 2 ATP

2 NAD+ + 4e- + 4 H+ —> 2 NADH + 2 H+

Question 33

Does O2 need to be present for glycolysis to occur?

Answer 33

Glycolysis occurs whether or not O2 is present

Question 34

What happens to pyruvate in the presence of oxygen?

Answer 34

In the presence of O2, pyruvate enters the mitochondrion where the oxidation of glucose is completed

Question 35

What must happen before the citric acid cycle can begin?

Answer 35

Before the citric acid cycle can begin, pyruvate must be converted to acetyl Coenzyme A (acetyl CoA), which links glycolysis to the citric acid cycle

*This step is carried out by a multi-enzyme complex that catalyses three reactions

Question 36

What are the three steps that converts pyruvate to acetyl CoA?

Answer 36

1. Pyruvate’s carboxyl group (which is already fully oxidized and thus has little chemical energy) is removed and given off as CO2
2. The remaining two-carbon fragment is oxidized, forming acetate (CH3COO-, which is the ionized form of acetic acid). The extracted electrons are transferred to NAD+, storing energy in the form of NADH
3. CoA, a sulfur-containing compound derived from a B vitamin, is attached via its sulfur atom to the acetate, forming acetyl CoA, which has a high potential energy. This molecule will now feed its acetyl group into the citric acid cycle for further oxidation

Question 37

What does the citric acid cycle function as?

Answer 37

A metabolic furnace that oxidizes organic fuel derived from pyruvate, generating 1 ATP, 3 NADH and 1 FADH2 per turn. The citric acid cycle has 8 steps, each catalyzed by a specific enzyme.

The acetyl group of acetyl CoA joins the cycle by combining with oxaloacetate, forming citrate.

The next seven steps decompose the citrate back to oxaloacetate, making the process a cycle.

The NADH and FADH2 produced by the cycle relay electrons extracted from food to the electron transport chain.

Question 38

What happens during oxidative phosphorylation?

Answer 38

Chemiosmosis couples electron transport to ATP synthesis

Question 39

Following glycolysis and the citric acid cycle, what accounts for most of the energy extracted from food?

Answer 39

NADH and FADH2;

These electron escorts link glycolysis and the citric acid cycle to the machinery of oxidative phosphorylation, which uses energy released by the electron transport chain to power ATP synthesis

Question 40

What is the electron transport chain?

Answer 40

A collection of molecules embedded in the inner membrane of the mitochondrion (in eukaryotic cells)

Most components of the chain are proteins, which exist in multi-protein complexes

The carriers alternate between reduced and oxidized states as they accept and then donate electrons

Electrons drop in free energy as they go down the chain and are finally passed to O2 forming H2O

Electrons are transferred from NADH or FADH2 to the electron transport chain

Question 41

What are cytochromes?

Answer 41

An iron containing protein that is a component of electron transport chains in the mitochondria and chloroplasts of eukaryotic cells; the electron transport chain has several types of cytochromes, each a different protein with a slightly different electron-carrying heme (an iron atom that accepts and donates electrons) group

Question 42

Does the electron transport chain make ATP directly?

Answer 42

No; the electron transport chain makes no ATP directly. Instead, it eases the fall of electrons from food to oxygen, breaking a large free energy drop into a series of smaller steps that release energy in manageable amounts, step by step.

Question 43

How does the mitochondrion couple this electron transport and energy release to ATP synthesis?

Answer 43

Chemiosmosis; the use of energy in an H+ gradient to drive cellular work, such as the synthesis of ATP

• Electron transfer in the electron transport chain causes proteins to pump H+ from the mitochondrial matrix to the intermembrane space
• H+ then moves back across the membrane, passing through the protein complex, ATP synthase
• ATP synthase uses the exergonic flow of H+ to drive phosphorylation of ATP

Question 44

What is ATP synthase?

Answer 44

The enzyme that makes ATP from ADP and inorganic phosphate; functions as a mill, powered by the flow of hydrogen ions.

Multiple ATP synthases reside in eukaryotic mitochondrial and chloroplast membranes and in prokaryotic plasma membranes. Each part of the complex consists of a number of polypeptide subunits.

*ATP synthase is the smallest molecular rotary motor known in nature

Question 45

What is the H+ gradient?

Answer 45

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.

The energy stored in a H+ gradient across a membrane couples the redox reactions of the electron transport chain to ATP synthesis.

Question 46

What makes up oxidative phosphorylation?

Answer 46

The electron transport chain and chemiosmosis

Question 47

What sequence does most energy flow during respiration?

Answer 47

glucose —> NADH —> electron transport chain —> proton-motive force —> ATP

Question 48

What percentage of the energy in a glucose molecule is transferred to ATP during cellular respiration?

Answer 48

About 34%; making about 32 ATP / glucose molecule

Glycolysis - 2 ATP; 2 NADH

Citric acid cycle - 2 ATP; 6 NADH; 2 FADH2

Oxidative phosphorylation - 26-28ish ATP

Question 49

What enables cells to produce ATP without oxygen?

Answer 49

Fermentation and anaerobic respiration;

Most cellular respiration requires O2 to produce ATP - without O2, the electron transport chain would cease to operate; in that case, glycolysis couples with anaerobic respiration or fermentation to produce ATP.

Question 50

What is one difference between fermentation and anaerobic respiration?

Answer 50

The electron transport chain is used in anaerobic respiration but not in fermentation

Question 51

How does anaerobic respiration use the electron transport chain without O2?

Answer 51

Anaerobic respiration uses an electron transport chain with a final electron receptor other than O2; for example - sulfate

Question 52

What is fermentation and what does it use to generate ATP?

Answer 52

A way of harvesting chemical energy without using either oxygen or any electron transport chain.

Fermentation is an extension of glycolysis that allows continuous generation of ATP by use of the substrate level phosphorylation of glycolysis. Consists of glycolysis plus reactions that generate NAD+ by transferring electrons from NADH to pyruvate or derivatives of pyruvate. The NAD+ can then be reused to oxidize sugar by glycolysis, which nets 2 molecules of ATP by substrate level phosphorylation.

Question 53

What are the two types of fermentation?

Answer 53

1. Alcohol fermentation - pyruvate is converted to ethanol in two steps; the first step releases CO2 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. (i.e. yeast)
2. Lactic acid fermentation - pyruvate is reduced directly by NADH to form lactate (the ionized form of lactic acid) as an end product, with no release of CO2. (i.e. lactic acid fermentation by certain fungi and bacteria is used in the dairy industry to make cheese and yogurt)
   * Human muscle cells use lactic acid fermentation to generate ATP when O2 is scarce; occurs during strenuous exercise, when sugar catabolism for ATP production outpaces the muscle’s supply of oxygen from the blood.

Question 54

Compare fermentation with anaerobic and aerobic respiration.

Answer 54

All three; use glycolysis to oxidize glucose and other organic fuels to pyruvate, with a net production of 2 ATP by substrate level phosphorylation; NAD+ is the oxidizing agent that accepts electrons from food during glycolysis.

Different mechanisms for oxidizing NADH;

Fermentation - the final electron acceptor is an organic molecule such as Pyruvate (lactic acid fermentation) or acetaldehyde (alcohol fermentation) - yields 2 molecules of ATP

Cellular respiration - electrons carried by NADH are transferred to an electron transport chain, which regenerates the NAD+ required for glycolysis - yields 32 molecules of ATP

Question 55

What are obligate anaerobes?

Answer 55

Carry out only fermentation or anaerobic respiration and cannot survive in the presence of oxygen

Question 56

What are facultative anaerobes?

Answer 56

Can make enough ATP to survive using either fermentation or cellular respiration; i.e. yeasts and many bacteria; on the cellular level, our muscles cells can behave as facultative anaerobes - in such cells, pyruvate s a fork in the metabolic road that leads to two alternative catabolic routes

Question 57

Discuss the evolutionary significance of glycolysis.

Answer 57

Ancient prokaryotes are thought to have used glycolysis long before O2 was present in the earths atmosphere; very little O2 was available in the atmosphere until about 2.7 billion years ago, so early prokaryotes likely used glycolysis to generate ATP; glycolysis is a very ancient process

Question 58

What is deamination?

Answer 58

Before amino acids can feed into glycolysis or the citric acid cycle, their amino groups must be removed, a process called deamination.

Question 59

What is the versatility of catabolism?

Answer 59

Catabolic pathways funnel electrons from many different kinds of organic molecules into cellular respiration; i.e. carbohydrates, fats, and proteins…

Glycolysis accepts a wide range of carbohydrates; proteins must first be digested to amino acids - amigo groups can feed glycolysis or the citric acid cycle; fats are digested to glycerol (used in glycolysis) and fatty acids (used in generating acetyl CoA; fatty acids are broken down by by beta oxidation and yield acetyl CoA) - an oxidized gram of fat produces more than twice as much ATP as an oxidized gram of carbohydrate

Question 60

What is biosynthesis (anabolic pathways)?

Answer 60

The body uses small molecules to build other substances; i.e. food must also provide the carbon skeletons that cells require to make their own molecules; some organic monomers obtained from digestion can be used directly; these small molecules may come directly from food, from glycolysis, or from the citric acid cycle.

Question 61

What feedback mechanisms regulate cellular respiration?

Answer 61

Feedback inhibition is the most common mechanism for metabolic control;

If ATP concentration begins to drop, respiration speeds up; when there is plenty of ATP, respiration slows down

Control of catabolism is based mainly on regulating the activity of enzymes at strategic points in the catabolic pathway

*Example - phosphofructokinase is an allosteric enzyme, which catalyzes an early step in glycolysis; it is stimulated by AMP by is inhibited by ATP and by citrate; this feedback regulation adjusts the rate of respiration as the cell’s catabolic and anabolic demands change.