Chapter 7 How Cells Release Chemical Energy (Sections 7.1 - 7.4)

Albia Dugger • Miami Dade College

Cecie StarrChristine EversLisa Starr

www.cengage.com/biology/starr

Chapter 7 How Cells Release

Chemical Energy(Sections 7.1 - 7.4)

7.1 When Mitochondria Spin Their Wheels

• Mitochondria produce ATP by aerobic respiration

• Electron transfer chains in mitochondrial membranes set up H+ gradients that power ATP formation

• Many disorders related to defective mitochondria are known:• Example: Friedreich’s ataxia, an enzyme-building protein

(frataxin) does not work properly, causing loss of coordination (ataxia), weak muscles, and heart problems

The Mitochondrion• Mitochondria have an internal folded membrane system that

allows them to make ATP

7.2 Extracting Energy From Carbohydrates

• Most organisms convert chemical energy of carbohydrates to chemical energy of ATP

• Anaerobic and aerobic pathways of carbohydrate breakdown start in the cytoplasm with glycolysis, which converts glucose and other sugars to pyruvate

• Anaerobic fermentation pathways end in cytoplasm and yield two ATP per molecule of glucose

Key Terms

• anaerobic • Occurring in the absence of oxygen

• aerobic • Involving or occurring in the presence of oxygen

Key Terms

• glycolysis • Set of reactions in which glucose or another sugar is

broken down to 2 pyruvate for a net yield of 2 ATP

• pyruvate • Three-carbon end product of glycolysis

• fermentation • An anaerobic pathway by which cells harvest energy from

carbohydrates to produce ATP

Evolution of Earth’s Atmosphere

• The first cells on Earth did not use sunlight for energy

• Ancient organisms extracted energy and carbon from simple molecules such as methane and hydrogen sulfide—gases that were plentiful in Earth’s early atmosphere

Evolution of Earth’s Atmosphere

• Aerobic respiration, which uses oxygen and yields much more ATP than fermentation, evolved after O2 released by early photoautotrophs changed Earth’s atmosphere

• aerobic respiration • Oxygen-requiring pathway that breaks down carbohydrates

to produce ATP

• photoautotroph • Photosynthetic autotroph

Then and Now• Artist’s conception of how Earth was permanently altered by

the evolution of photosynthesis and aerobic respiration

Aerobic Respiration

• This equation summarizes aerobic respiration:

C6H12O6 (glucose) + O2 (oxygen) →

CO2 (carbon dioxide) + H2O (water)

• Note that aerobic respiration requires oxygen (a product of photosynthesis), and produces carbon dioxide and water (the raw materials of photosynthesis)

Energy, Photosynthesis, and Respiration

• The connection between photosynthesis and aerobic respiration

• Note the cycling of materials, and one-way flow of energy

Fig, 7.3, p. 108

energy

glucose

H2O

energy

CO2

O2

Energy, Photosynthesis, and Respiration

Animation: Links with Photosynthesis

Carbohydrate Breakdown Pathways

• Both fermentation and aerobic respiration begin in the cytoplasm with glycolysis

• After glycolysis, pathways of fermentation and aerobic respiration diverge: • Aerobic respiration continues inside mitochondria, and

ends when oxygen accepts electrons at the end of electron transfer chains

• Fermentation ends in the cytoplasm, where a molecule other than oxygen accepts electrons

Overview: Aerobic Respiration

Fig, 7.4, p. 109

Aerobic Respiration

In the Cytoplasm

A The first stage, glycolysis, occurs in the cell’s cytoplasm. Enzymes convert a glucose molecule to 2 pyruvate for a net yield of 2 ATP. 2 NAD+ combine with electrons and hydrogen ions during the

reactions, so 2 NADH also form.

B The second stage occurs in mitochondria. The 2 pyruvate are converted to a molecule that enters the Krebs cycle. CO2 forms and leaves the cell. 2 ATP, 8 NADH, and 2 FADH2 form during the reactions.

C The third and final stage, electron transfer phosphorylation, occurs inside mitochondria. 10 NADH and 2 FADH2 give up electrons and hydrogen ions to electron transfer chains. Electron flow through the chains sets up hydrogen oxygen ion gradients that drive ATP formation. Oxygen accepts electrons at the end of the chains.

In the Mitochondrion

Animation: Overview of Aerobic Respiration

Key Concepts

• Energy From Carbohydrates • Various pathways convert chemical energy of glucose and

other organic compounds to chemical energy of ATP • Aerobic respiration yields the most ATP from each glucose

molecule • In eukaryotes, this pathway ends in mitochondria

Animation: Where Pathways Start and Finish

7.3 Glycolysis: Glucose Breakdown Starts

• Glycolysis, the first stage of aerobic respiration and fermentation, occurs in cytoplasm

• Enzymes use 2 ATP to convert 1 molecule of glucose or another six-carbon sugar to 2 molecules of pyruvate

Energy Transfer

• Electrons and hydrogen ions are transferred to 2 NAD+, which are reduced to NADH

• 4 ATP form by substrate-level phosphorylation

• substrate-level phosphorylation • A reaction that transfers a phosphate group from a

substrate directly to ADP, thus forming ATP

5 Steps of Glycolysis

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

2. A phosphate group from a second ATP is transferred to glucose-6-phosphate • The resulting unstable molecule splits into 2 three-carbon

molecules of PGAL (phosphoglyceraldehyde) • 2 ATP have been invested in the reactions

5 Steps of Glycolysis

3. Enzymes attach a phosphate to the 2 PGAL, and transfer 2 electrons and 1 hydrogen ion from each PGAL to NAD+ • 2 PGA (phosphoglycerate) and 2 NADH form

4. Enzymes transfer a phosphate group from each PGA to ADP • 2 ATP have formed by substrate-level phosphorylation• The original investment of 2 ATP has been recovered

5 Steps of Glycolysis

• 5. Enzymes transfer a phosphate group from each of 2 intermediates to ADP• 2 molecules of pyruvate form at this last step• 2 more ATP have formed by substrate-level

phosphorylation

Steps of Glycolysis

Fig, 7.5, p. 110

ATP-Requiring Steps

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

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.

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.

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

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.

1

2

3

4

5

6

Fig, 7.5, p. 110

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-levelphosphorylation. 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

Animation: Glycolysis

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Products of Glycolysis

• Glycolysis yields 2 NADH, 2 ATP (net), and 2 pyruvate for each glucose molecule

• Depending on the type of cell and environmental conditions, pyruvate may enter the second stage of aerobic respiration or may be used in other ways, such as in fermentation

Key Concepts

• Glycolysis • Glycolysis, the first stage of aerobic respiration and of

anaerobic fermentation pathways, occurs in cytoplasm • Enzymes of glycolysis convert one molecule of glucose to

two molecules of pyruvate for a net yield of two ATP

Animation: Cellular Respiration–Glycolysis

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7.4 Second Stage of Aerobic Respiration

• The second stage of aerobic respiration, Acetyl–CoA formation and the Krebs cycle, takes place in the mitrochondrion

• A mitochondrion’s inner membrane divides its interior into two fluid-filled spaces: the inner compartment (matrix) and the intermembrane space

From Pyruvate to CO2

• 2 pyruvate from glycolysis are converted to 2 acetyl–CoA and 2 CO2

• Acetyl–CoA enters the Krebs cycle – it takes two cycles of Krebs reactions to dismantle 2 acetyl–CoA

• At the end of the Krebs cycle, all carbon atoms in the glucose molecule that entered glycolysis have left the cell in CO2

Acetyl-CoA Formation and the Krebs Cycle

Fig, 7.6, p. 112

a mitochondrion

matrix

cytoplasm

outer membrane

inner membrane

Acetyl-CoA Formation and the Krebs Cycle

8 Steps of the Second Stage

1. An enzyme splits a pyruvate 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

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

8 Steps of the Second Stage

3. A carbon atom is removed and leaves the cell as CO2

• NAD+ combines with H+ and electrons, forming NADH

4. Another carbon atom is removed and leaves the cell as CO2, and another NADH forms

• Pyruvate’s 3 carbon atoms have exited the cell, in CO2

5. 1 ATP forms by substrate-level phosphorylation

8 Steps of the Second Stage

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

7. NAD+ combines with H+ and electrons, forming NADH

8. The final steps of the Krebs cycle regenerate oxaloacetate

Steps of the Second Stage

Fig, 7.7, p. 112

Acetyl–CoA Formation and the Krebs Cycle

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.

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

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

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.

One ATP forms by substrate-level phosphorylation.

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

Krebs Cycle

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

The final steps of the Krebs cycle regenerateoxaloacetate.

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2

3

4

8

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6

5

Fig, 7.7, p. 112

Acetyl–CoA Formation and the Krebs Cycle

Krebs Cycle

The final steps of the Krebs cycle regenerateoxaloacetate.

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

Animation: The Krebs Cycle–Details

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Products of the Second Stage

• Electrons and hydrogen ions are transferred to NAD+ and FAD, which are reduced to NADH and FADH2

• ATP forms by substrate-level phosphorylation

• TOTAL: Breakdown of 2 pyruvate molecules yields 10 reduced coenzymes and 2 ATP

Animation: Functional Zones in Mitochondria

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Animation: Cellular Respiration–Krebs Cycle

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Animation: Krebs Cycle Overview