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Chapter 7

How Cells Release

Chemical Energy

7.1 Mighty Mitochondria

• More than forty disorders related to defective mitochondria

are known (such as Friedreich’s ataxia); many of those

afflicted die young

A Mitochondrion

Two Main Metabolic Pathways

• Aerobic metabolic pathways (using oxygen) are used by

most eukaryotic cells

• Anaerobic metabolic pathways (which occur in the absence

of oxygen) are used by prokaryotes and protists in anaerobic

habitats

Aerobic Respiration

• In modern eukaryotic cells, most of the aerobic respiration

pathway takes place inside mitochondria

• Like chloroplasts, mitochondria have an internal folded

membrane system that allows them to make ATP efficiently

• Electron transfer chains in this membrane set up hydrogen

ion gradients that power ATP synthesis

• At the end of these chains, electrons are transferred to

oxygen molecules

INTERACTION: Structure of a

mitochondrion

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7.2 Overview of

Carbohydrate Breakdown Pathways

• Photoautotrophs make ATP during photosynthesis and use it

to synthesize glucose and other carbohydrates

• Most organisms, including photoautotrophs, make ATP by

breaking down glucose and other organic compounds

Figure 7-2 p118

energy

Photosynthesis

CO2 glucose

H2O O2

Aerobic Respiration

energy

Overview of Aerobic Respiration

• Three stages

• Glycolysis

• Acetyl-CoA formation and Krebs cycle

• Electron transfer phosphorylation (ATP formation)

C6H12O6 (glucose) + O2 (oxygen) →

CO2 (carbon dioxide) + H2O (water)

• Coenzymes NADH and FADH2 carry electrons and

hydrogen

Figure 7-3 p119

Aerobic Respiration

glucose

Glycolysis

2 NADH 2 pyruvate

Krebs

Cycle

8 NADH, 2 FADH2

Electron Transfer

Phosphorylation oxygen

2 ATP 4 ATP (2

net)

6 CO2

2 ATP

H2O

32 ATP

In the Cytoplasm

In the Mitochondrion

ANIMATED FIGURE: Overview of aerobic

respiration

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Aerobic Respiration vs.

Anaerobic Fermentation

• Aerobic respiration and fermentation both begin with

glycolysis, which converts one molecule of glucose into two

molecules of pyruvate

• After glycolysis, the two pathways diverge

• Fermentation is completed in the cytoplasm, yielding 2

ATP per glucose molecule

• Aerobic respiration is completed in mitochondria, yielding

36 ATP per glucose molecule

Figure 7-4 p119

Carbohydrate

breakdown pathways

start in the cytoplasm,

with glycolysis.

Glycolysis

Fermentation

concludes in

cytoplasm.

In eukaryotes,

aerobic respiration

concludes inside

mitochondria.

ANIMATED FIGURE: Where pathways start

and finish

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Take-Home Message: How do cells access the

chemical energy in carbohydrates?

• Most cells convert the chemical energy of carbohydrates to

chemical energy of ATP by aerobic respiration or fermentation

• Aerobic respiration and fermentation pathways start in

cytoplasm, with glycolysis

• Fermentation is anaerobic and ends in the cytoplasm

• Aerobic respiration requires oxygen. In eukaryotes, it ends in

mitochondria

3D ANIMATION: Cellular Respiration

7.3 Glycolysis –

Glucose Breakdown Starts

• The reactions of glycolysis convert one molecule of glucose to

two molecules of pyruvate for a net yield of two ATP

• An energy investment of ATP is required to start glycolysis

Glycolysis

• Two ATP are used to split glucose and form 2 PGAL, each

with one phosphate group

• Enzymes convert 2 PGAL to 2 PGA, forming 2 NADH

• Four ATP are formed by substrate-level phosphorylation

(net 2 ATP)

• Glycolysis ends with the formation of two three-carbon

pyruvate molecules

ANIMATED FIGURE: Glycolysis

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ATP-Requiring Steps

An enzyme (hexokinase) transfers a phosphate group from ATP to glucose, forming glucose-6-phosphate. 1

A phosphate group from a second ATP is transferred to the glucose-6phosphate. The resulting molecule is unstable, and it splits into two three carbon molecules. The molecules are interconvertible, so we will call them both PGAL (phosphoglyceraldehyde). Two ATP have now been invested in the reactions.

2

ATP-Generating Steps

Enzymes attach a phosphate to the two PGAL, and transfer two electrons and a hydrogen ion from each PGAL to NAD+. Two PGA (phosphoglycerate) and two NADH are the result.

3

Enzymes transfer a phosphate group from each PGA to ADP. Thus, two ATP have formed by substrate-level phosphorylation. The original energy investment of two ATP has now been recovered.

4

Enzymes transfer a phosphate group from each of two intermediates to ADP. Two more ATP have formed by substrate-level phosphorylation. Two molecules of pyruvate form at this last reaction step.

Summing up, glycolysis yields two NADH, two ATP (net), and two pyruvate for each glucose molecule. Depending on the type of cell and environmental conditions, the pyruvate may enter the second stage of aerobic respiration or it may be used in other ways, such as in fermentation.

5

6

Stepped Art Figure 7-5 p121

Take-Home Message:

What is glycolysis?

• Glycolysis is the first stage of carbohydrate breakdown in both

aerobic respiration and fermentation

• The reactions of glycolysis occur in the cytoplasm

• Glycolysis converts one molecule of glucose to two molecules

of pyruvate, with a net energy yield of two ATP; two NADH

also form

3D ANIMATION: Cellular Respiration

ANIMATION: Energy inputs and release in

glycolosis

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7.4 Second Stage of Aerobic Respiration

• The second stage of aerobic respiration completes the

breakdown of glucose that began in glycolysis

• Occurs in mitochondria

• Includes two sets of reactions: acetyl CoA formation and the

Krebs cycle (each occurs twice in the breakdown of one

glucose molecule)

Acetyl CoA Formation

• In the inner compartment of the mitochondrion, enzymes split

pyruvate, forming acetyl CoA and CO2 (which diffuses out of

the cell)

• NADH is formed

The Krebs Cycle

• Krebs cycle

• A sequence of enzyme-mediated reactions that break

down 1 acetyl CoA into 2 CO2

• Oxaloacetate is used and regenerated

• 3 NADH and 1 FADH2 are formed

• 1 ATP is formed

Second Stage of Aerobic Respiration

cytoplasm

outer

membrane

inner

membrane The breakdown of 2

pyruvate to 6 CO2 yields 2

ATP and 10 reduced

coenzymes (8 NADH, 2

FADH2). The coenzymes will

carry their cargo of

electrons and hydrogen

ions to the third stage of

aerobic respiration.

matrix

ANIMATED FIGURE: The Krebs Cycle -

details

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Acetyl–CoA Formation and the Krebs Cycle

Krebs

Cycle

The final steps of the Krebs cycle regenerate oxaloacetate.

8

NAD+ combines with hydrogen ions and electrons, forming NADH.

7

The coenzyme FAD combines with hydrogen ions and electrons, forming FADH2.

6

One ATP forms by substrate-level phosphorylation.

5

An enzyme splits a pyruvate coenzyme A NAD+ molecule into a two-carbon acetyl group and CO2. Coenzyme A binds the acetyl group (forming acetyl–CoA). NAD+ combines with released hydrogen ions and electrons, forming NADH.

1

The Krebs cycle starts as one carbon atom is transferred from acetyl–CoA tooxaloacetate. Citrate forms, and coenzyme A is regenerated.

2

A carbon atom is removed from an intermediate and leaves the cell as CO2. NAD+ combines with released hydrogen ions and electrons, forming NADH.

3

A carbon atom is removed from another intermediate and leaves the cell as CO2, and another NADH forms.

Pyruvate’s three carbon atoms have now exited the cell, in CO2.

4

Stepped Art

Figure 7-7 p123

Take-Home Message: What happens during the

second stage of aerobic respiration?

• The second stage of aerobic respiration, acetyl–CoA

formation and the Krebs cycle, occurs in the inner

compartment (matrix) of mitochondria

• The pyruvate that formed in glycolysis is converted to acetyl–

CoA and CO2; the acetyl–CoA enters the Krebs cycle, which

breaks it down to CO2

• For two pyruvate molecules broken down in the second-stage

reactions, two ATP form, and ten coenzymes (eight NAD+;

two FAD) are reduced

7.5 Aerobic Respiration’s Big Energy Payoff

• Many ATP are formed during the third and final stage of

aerobic respiration

• Electron transfer phosphorylation

• Occurs in mitochondria

• Results in attachment of phosphate to ADP to form ATP

Electron Transfer Phosphorylation

• Coenzymes NADH and FADH2 donate electrons and H+ to electron transfer chains

• Active transport forms a H+ concentration gradient in the outer mitochondrial compartment

• H+ follows its gradient through ATP synthase, which attaches a phosphate to ADP

• Finally, oxygen accepts electrons and combines with H+, forming water

Electron Transfer Phosphorylation

Summary: The Energy Harvest

• Typically, the breakdown of one glucose molecule yields 36

ATP

• Glycolysis: 2 ATP

• Acetyl CoA formation and Krebs cycle: 2 ATP

• Electron transfer phosphorylation: 32 ATP

Figure 7-9 p125

ANIMATED FIGURE: Third-stage reactions

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Take-Home Message: What happens during

the third stage of aerobic respiration?

• In electron transfer phosphorylation, energy released by

electrons flowing through electron transfer chains is captured

in the attachment of phosphate to ADP; a typical net yield of

aerobic respiration is thirty-six ATP per glucose

• The reactions begin when coenzymes that were reduced in

the first and second stages of reactions deliver electrons and

hydrogen ions to electron transfer chains in the inner

mitochondrial membrane

Take-Home Message: (cont.)

• Energy released by electrons as they pass through electron

transfer chains is used to pump H+ from the mitochondrial

matrix to the intermembrane space

• The H+ gradient that forms across the inner mitochondrial

membrane drives the flow of hydrogen ions through ATP

synthases, which results in ATP formation

ANIMATION: Mitochondrial chemiosmosis

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7.6 Fermentation

• Fermentation pathways break down carbohydrates without

using oxygen

• The final steps in these pathways regenerate NAD+ but do not

produce ATP

Fermentation

• Glycolysis is the first stage of fermentation

• Forms 2 pyruvate, 2 NADH, and 2 ATP

• Pyruvate is converted to other molecules, but is not fully

broken down to CO2 and water

• Regenerates NAD+ but doesn’t produce ATP

• Provides enough energy for some single-celled anaerobic

species

Two Fermentation Pathways

• Alcoholic fermentation

• Pyruvate is split into acetaldehyde and CO2

• Acetaldehyde receives electrons and hydrogen from

NADH, forming NAD+ and ethanol

• Lactate fermentation

• Pyruvate receives electrons and hydrogen from NADH,

forming NAD+ and lactate

Figure 7-10a p127

Glycolysis glucose

2 2

4

pyruvate

Alcoholic

Fermentation acetaldehyde

2 CO2

2 NAD+

ethanol

2

2 NAD+

Figure 7-10b p127

Figure 7-11a p127

Glycolysis glucose

2

2

pyruvate

Lactate

Fermentation

2

lactate

2 CO2

2 NAD+

2 NAD+

4

ANIMATED FIGURE: Fermentation

pathways

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Red and White Muscle Fibers

• Red muscle fibers make ATP by aerobic respiration

• Have many mitochondria

• Myoglobin stores oxygen

• Sustain prolonged activity

• White muscle fibers make ATP by lactate fermentation

• Have few mitochondria and no myoglobin

• Sustain short bursts of activity

Figure 7-11b p127

Figure 7-11c p127

Take-Home Message:

What is fermentation?

• ATP can form by carbohydrate breakdown in fermentation

pathways, which are anaerobic

• The end product of lactate fermentation is lactate. The end

product of alcoholic fermentation is ethanol

• Both pathways have a net yield of two ATP per glucose

molecule; the ATP forms during glycolysis

• Fermentation reactions regenerate the coenzyme NAD+,

without which glycolysis (and ATP production) would stop

7.7 Alternative Energy Sources in Food

• Aerobic respiration can produce ATP from the breakdown of

complex carbohydrates, fats, and proteins

• As in glucose metabolism, many coenzymes are reduced,

and the energy of the electrons they carry ultimately drives

the synthesis of ATP in electron transfer phosphorylation

Energy From Complex Carbohydrates

• Enzymes break starch and other complex carbohydrates

down to monosaccharide subunits

• Monosaccharides are taken up by cells and converted to

glucose-6-phosphate, which continues in glycolysis

• A high concentration of ATP causes glucose-6-phosphate to

be diverted away from glycolysis and into a pathway that

forms glycogen

Energy From Fats

• Enzymes cleave fats into glycerol and fatty acids

• Glycerol products enter glycolysis

• Fatty acids are converted to acetyl Co-A and enter the

Krebs cycle

• Compared to carbohydrates, fatty acid breakdown yields

more ATP per carbon atom

• When blood glucose level is high, acetyl CoA is diverted from

the Krebs cycle and into a pathway that makes fatty acids

Energy from Proteins

• Enzymes split dietary proteins into amino acid subunits, which

are used to build proteins or other molecules

• The amino group is removed and converted into ammonia

(NH3), a waste product eliminated in urine

• Acetyl–CoA, pyruvate, or an intermediate of the Krebs cycle

forms, depending on the amino acid

Figure 7-12a p128

starch (a complex carbohydrate) glucose

A Complex carbohydrates are broken down to their

monosaccharide subunits, which can enter glycolysis. 1

Figure 7-12b p128

a triglyceride (fat)

glycerol head

fatty acid tails

Figure 7-12b p128

Food

Fats Complex Carbohydrates Proteins

fatty acids glycerol glucose, other simple

sugars amino acids

acetyl–CoA PGAL acetyl–CoA

Glycolysis

NADH pyruvate

intermediate of

Krebs cycle

Krebs Cycle

NADH, FADH2

Electron Transfer Phosphorylation

2 3 1 4

Figure 7-12c p128

alanine (an amino acid) pyruvate

ANIMATED FIGURE: Alternative energy

sources

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Take-Home Message: Can organic molecules

other than glucose be used for energy?

• Complex carbohydrates, fats, and proteins can be oxidized in

aerobic respiration to yield ATP

• First the digestive system and then individual cells convert

molecules in food into intermediates of glycolysis or the Krebs

cycle