How Cells Release Chemical Energy

Chapter 7

7.1 Overview of

Carbohydrate Breakdown Pathways

All organisms (including photoautotrophs)

convert chemical energy of organic compounds

to chemical energy of ATP

ATP is a common energy currency that drives

metabolic reactions in cells

Pathways of Carbohydrate Breakdown

Start with glycolysis in the cytoplasm

• Convert glucose and other sugars to pyruvate

Fermentation pathways

• End in cytoplasm, do not use oxygen, yield 2 ATP

per molecule of glucose

Aerobic respiration

• Ends in mitochondria, uses oxygen, yields up to

36 ATP per glucose molecule

Pathways of Carbohydrate Breakdown

Fig. 7.2b, p.108

a All carbohydrate breakdown pathways

start in the cytoplasm, with glycolysis.

b Fermentation pathways

are completed in the semifluid

matrix of the cytoplasm.

c In eukaryotes, aerobic

respiration is completed

inside mitochondria.

Overview of Aerobic Respiration

Three main stages of aerobic respiration:

1. Glycolysis

2. Krebs cycle

3. Electron transfer phosphorylation

Summary equation:

C6H12O6 + 6O2 → 6CO2 + 6 H2O

Overview of Aerobic Respiration

Key Concepts: ENERGY FROM

CARBOHYDRATE BREAKDOWN

All organisms produce ATP by degradative

pathways that extract chemical energy from

glucose and other organic compounds

Aerobic respiration yields the most ATP from

each glucose molecule

In eukaryotes, aerobic respiration is completed

inside mitochondria

7.2 Glycolysis –

Glucose Breakdown Starts

Enzymes of glycolysis use two ATP to convert

one molecule of glucose to two molecules of

three-carbon pyruvate

Reactions transfer electrons and hydrogen

atoms to two NAD+ (reduces to NADH)

4 ATP form by substrate-level phosphorylation

Products of Glycolysis

Net yield of glycolysis:

• 2 pyruvate, 2 ATP, and 2 NADH per glucose

Pyruvate may:

• Enter fermentation pathways in cytoplasm

• Enter mitochondria and be broken down further in

aerobic respiration

Glycolysis

Glycolysis

Key Concepts:

GLYCOLYSIS

Glycolysis is the first stage of aerobic respiration

and of anaerobic routes (fermentation pathways)

As enzymes break down glucose to pyruvate,

the coenzyme NAD+ picks up electrons and

hydrogen atoms

Net energy yield is two ATP

7.3 Second Stage of Aerobic Respiration

The second stage of aerobic respiration takes

place in the inner compartment of mitochondria

It starts with acetyl-CoA formation and proceeds

through the Krebs cycle

Second Stage of Aerobic Respiration

Acetyl-CoA Formation

Two pyruvates from glycolysis are converted to

two acetyl-CoA

Two CO2 leave the cell

Acetyl-CoA enters the Krebs cycle

Krebs Cycle

Each turn of the Krebs cycle, one acetyl-CoA is

converted to two molecules of CO2

After two cycles

• Two pyruvates are dismantled

• Glucose molecule that entered glycolysis is fully

broken down

Energy Products

Reactions transfer electrons and hydrogen

atoms to NAD+ and FAD

• Reduced to NADH and FADH2

ATP forms by substrate-level phosphorylation

• Direct transfer of a phosphate group from a

reaction intermediate to ADP

Net Results

Second stage of aerobic respiration results in

• Six CO2, two ATP, eight NADH, and two FADH2

for every two pyruvates

Adding the yield from glycolysis, the total is

• Twelve reduced coenzymes and four ATP for

each glucose molecule

Coenzymes deliver electrons and hydrogen to

the third stage of reactions

Second Stage Reactions

Fig. 7.6b, p.113

7.4 Third Stage:

Aerobic Respiration’s Big Energy Payoff

Coenzymes deliver electrons and hydrogen ions

to electron transfer chains in the inner

mitochondrial membrane

Energy released by electrons flowing through

the transfer chains moves H+ from the inner to

the outer compartment

Hydrogen Ions and Phosphorylation

H+ ions accumulate in the outer compartment,

forming a gradient across the inner membrane

H+ ions flow by concentration gradient back to

the inner compartment through ATP synthases

(transport proteins that drive ATP synthesis)

The Aerobic Part of Aerobic Respiration

Oxygen combines with electrons and H+ at the

end of the transfer chains, forming water

Overall, aerobic respiration yields up to 36 ATP

for each glucose molecule

Electron Transfer Phosphorylation

Fig. 7.7a, p.114

NADH FADH2

ATP

Fig. 7.7b, p.114

ADP + Pi

H+

H+

H+ H+

H+ H+

H+ H+ H+

H+ H+ H+

INNER

MITOCHONDRIAL

MEMBRANE

OUTER

COMPARTMENT

INNER

COMPARTMENT

H2O

1/2 O2

H+

Key Concepts:

HOW AEROBIC RESPIRATION ENDS

In the Krebs cycle (and a few steps before)

• Pyruvate is broken down to carbon dioxide

• Coenzymes pick up electrons and hydrogen atoms

In electron transfer phosphorylation

• Coenzymes deliver electrons to transfer chains

that set up conditions for ATP formation

Oxygen accepts electrons at end of chains

Summary: Aerobic Respiration

Fig. 7.8, p.115

glucose

Glycolysis

2 pyruvate

Krebs

Cycle

Electron Transfer

Phosphorylation

(2 net)

2 NADH

ATP

2 FADH2

6 NADH

2 acetyl-CoA

32

ADP + Pi

H+

ATP

2 NADH

2 NADH

ATP

ATP

H+ H+ H+ H+

2 NAD+

2 CO2

4 CO2

2

oxygen

INNER MITOCHONDRIAL COMPARTMENT

OUTER MITOCHONDRIAL COMPARTMENT

CYTOPLASM

water

2

7.5 Anaerobic

Energy-Releasing Pathways

Different fermentation pathways begin with

glycolysis and end in the cytoplasm

• Do not use oxygen or electron transfer chains

• Final steps do not produce ATP; only regenerate

oxidized NAD+ required for glycolysis to continue

Anaerobic Pathways

Lactate fermentation

• End product: Lactate

Alcoholic fermentation

• End product: Ethyl alcohol (or ethanol)

Both pathways have a net yield of 2 ATP per

glucose (from glycolysis)

Alcoholic and Lactate Fermentation

Fig. 7.9a, p.116

Fig. 7.9b, p.116

glucose Glycolysis

2

pyruvate

NADH

2 CO2

2 NAD+

2 ATP

ATP 4

Fig. 7.9b, p.116

NADH

2 NAD+

2

Alcoholic

Fermentation acetaldehyde

ethanol

Fig. 7.9c, p.116

Fig. 7.9c, p.116

glucose Glycolysis

2

pyruvate

NADH

2 NAD+

2 ATP

ATP 4

NADH

2 NAD+

2

Lactate

Fermentation

lactate

Alcoholic Fermentation

7.6 The Twitchers

Slow-twitch and fast-twitch skeletal muscle fibers

can support different activity levels

Aerobic respiration and lactate fermentation

proceed in different fibers of muscles

Muscles and Lactate Fermentation

Key Concepts:

HOW ANAEROBIC PATHWAYS END

Fermentation pathways start with glycolysis

Substances other than oxygen are the final

electron acceptor

Compared with aerobic respiration, net yield of

ATP is small

7.7 Alternative Energy Sources

in the Body

In humans and other mammals, foods enter

aerobic respiration at various steps

• Simple sugars from carbohydrates

• Glycerol and fatty acids from fats

• Carbon backbones of amino acids from proteins

Disposition of Organic Compounds

Fig. 7.12a, p.119

FOOD

fats COMPLEX CARBOHYDRATES PROTEINS

glucose, other simple sugars amino acids

Glycolysis

glycerol fatty acids

pyruvate

acetyl-coA

NADH

Krebs

Cycle

NADH, FADH2

PGAL acetyl-coA

oxaloacetate or another intermediate of the Krebs

Electron Transfer

Phosphorylation

Key Concepts:

OTHER METABOLIC PATHWAYS

Molecules other than glucose are common

energy sources

Different pathways convert lipids and proteins to

substances that may enter glycolysis or the

Krebs cycle

7.8 Life’s Unity

Photosynthesis and aerobic respiration are

interconnected on a global scale

In its organization, diversity, and continuity

through generations, life shows unity at the

bioenergetic and molecular levels

Energy, Photosynthesis, and

Aerobic Respiration

Key Concepts:

PERSPECTIVE AT UNIT’S END

Life shows unity in its molecular and cellular

organization and in its dependence on a one-

way flow of energy