How Cells Release Chemical Energy

Chapter 7

Biology Concepts and Applications, Eight Edition, by Starr, Evers, Starr. Brooks/Cole, Cengage Learning 2011.

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

Photoautotrophs Photosynthetic autotrophs• Produce sugar

Fermentation pathways anaerobic pathway• End in cytoplasm, do not use oxygen, yield 2 ATP

per molecule of glucose Aerobic respiration oxygen-requiring pathway

that breaks down carbohydrates to produce ATP• Ends in mitochondria, uses oxygen, yields up to

36 ATP per glucose molecule

• **Occurs in the presence of OXYGEN**

Pathways of Carbohydrate Breakdown

Overview of Aerobic Respiration

Three main stages of aerobic respiration:1. Glycolysis in the cytoplasm

•Convert glucose and other sugars to (2) pyruvate and (2) ATP

•Pyruvate is 3-carbon end product of 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

7.3 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• Transfers a phosphate group directly from a

substrate to ADP to form ATP

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

ATP

ATP

glucose

ADP

ADP

PP

glucose–6–phosphate

fructose–1,6–bisphosphate

DHAP

Fig. 7.4c1, p.111

Glycolysis

ENERGY REQUIRING PHASE

ATP

2 ADP

2 NAD+ + 2 Pi

2 PGA

NADH

2 PGAL

Fig. 7.4c2, p.111

ATP

2 pyruvate

2 PEP

2 ADP

to second stage Net 2 ATP + 2 NADH

2 ATP producedby substrate-levelphosphorylation

2 ATP producedby substrate-levelphosphorylation

2 reduced coenzymes

ENERGY PRODUCINGPHASE

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

7.4 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• Kreb cycle cyclic pathway that, along with

acetyl-CoA formation, breaks down pyruvate to carbon dioxide

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

NADH

NADH

FADH2

ATP

KrebsCycle

Fig. 7.6a, p.113

KrebsCycle

NADH

NADH

coenzyme A

coenzyme A

acetyl–CoA

oxaloacetate citrate

pyruvate

Acetyl–CoAFormation

CO2

CO2

NAD+

NAD+

NAD+

FAD

NAD+

CO2

ADP + Pi

Fig. 7.6b, p.113

7.5 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

NADHFADH2

ATP

Fig. 7.7b, p.114

ADP + Pi

H+

H+

H+

H+

H+H+

H+H+H+

H+ H+ H+

INNERMITOCHONDRIAL

MEMBRANE

OUTERCOMPARTMENT

INNERCOMPARTMENT

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

Fig. 7.8, p.115

glucose

Glycolysis

2 pyruvate

KrebsCycle

Electron TransferPhosphorylation

(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 MITOCHONDRIALCOMPARTMENT

OUTER MITOCHONDRIALCOMPARTMENT

CYTOPLASM

water

2

7.6 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: ATP & Lactate

• Bacteria break down lactose in milk produce buttermilk, cheese, and yogurt

• Yeast preserve pickles, cored beef, and sauerkraut Alcoholic fermentation

• End product: ATP & Ethyl alcohol (or ethanol)

• Yeast to make bread dough rises as CO2 forms bubbles

Both pathways have a net yield of 2 ATP per glucose (from glycolysis) and NAD+

Fig. 7.9a, p.116

Fig. 7.9b, p.116

Fig. 7.9c, p.116

glucoseGlycolysis

2

pyruvate

NADH

2 NAD+

2ATP

ATP4

NADH

2 NAD+

2

LactateFermentation

lactate

Alcoholic Fermentation

Animal Skeletal Muscle

Red fibers (legs of chicken)• A lot of mitochondria and Myoglobin (stores oxygen)

• Produce ATP by aerobic respiration

• Sustain prolonged activity (marathon runs) White fibers (wings of chicken)

• Few mitochondria and no myoglobin can not carry out a lot of aerobic respiration

• Most ATP produced by lactate fermentation•ATP produced quick by not for long

• Short strenuous activity (Sprinting and weight lifting) Humans Mixed fibers

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

Alternative Energy Sources

Fig. 7.12a, p.119

FOOD

fats COMPLEX CARBOHYDRATES PROTEINS

glucose, other simple sugars amino acids

Glycolysis

glycerolfatty acids

pyruvate

acetyl-coA

NADH

KrebsCycle

NADH, FADH2

PGALacetyl-coA

oxaloacetateor anotherintermediateof the Krebs

Electron TransferPhosphorylation

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

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

Animation: Alternative energy sources

Animation: Fermentation pathways

Animation: Functional zones in mitochondria

Animation: Glycolysis

Animation: Overview of aerobic respiration

Animation: The Krebs Cycle - details

Animation: Third-stage reactions

Animation: Where pathways start and finish