Exploring Cellular Energy: Glycolysis, Aerobic Respiration, and Fermentation, Study notes of Biology

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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

Overview of Aerobic Respiration

Three main stages of aerobic respiration:

1. Glycolysis
2. Krebs cycle
3. Electron transfer phosphorylation

Summary equation: C 6 H 12 O 6 + 6O 2 → 6CO 2 + 6 H 2 O

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

Animation: Glycolysis

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Animation: Fermentation pathways

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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

Animation: Functional zones in

mitochondria

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Acetyl-CoA Formation

Two pyruvates from glycolysis are converted to two acetyl-CoA

Two CO 2 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 CO 2

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 FADH 2

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 CO 2 , two ATP, eight NADH, and two FADH 2 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

Animation: The Krebs Cycle - details

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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.

Animation: Third-stage reactions

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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

Animation: Overview of aerobic

respiration

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Summary: Aerobic Respiration

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

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

Animation: Where pathways start and

finish

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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

Animation: Alternative energy sources

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Disposition of Organic Compounds

Alternative Energy Sources

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

Animation: Links with photosynthesis

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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