How Cells Release Stored Energy

Chapter 8

8.1 Main Types of Energy-Releasing Pathways

Aerobic pathways

• Evolved later• Require oxygen• Start with glycolysis

in cytoplasm• Completed in

mitochondria

Anaerobic pathways

• Evolved first• Don’t require oxygen• Start with glycolysis in

cytoplasm• Completed in

cytoplasm

Summary Equation for Aerobic Respiration

C6H1206 + 6O2 6CO2 + 6H20 glucose oxygen carbon water

dioxide

Overview of Aerobic Respiration

CYTOPLASM

Glycolysis

Electron Transfer

Phosphorylation

KrebsCycle ATP

ATP

2 CO2

4 CO2

2

32

water

2 NADH

8 NADH

2 FADH2

2 NADH 2 pyruvate

e- + H+

e- + oxygen

(2 ATP net)

glucose

Typical Energy Yield: 36 ATP

e-

e- + H+

e- + H+

ATP

H+

e- + H+

ATP2 4

Figure 8.3Page 135

The Role of Coenzymes

• NAD+ and FAD accept electrons and

hydrogen

• Become NADH and FADH2

• Deliver electrons and hydrogen to the

electron transfer chain

• A simple sugar

(C6H12O6)

• Atoms held together by covalent bonds

Glucose

In-text figurePage 136

8.2 GLYCOLYSIS

Glycolysis Occurs in Two Stages

• Energy-requiring steps

– ATP energy activates glucose and its six-carbon

derivatives

• Energy-releasing steps

– The products of the first part are split into three-

carbon pyruvate molecules

– ATP and NADH form

Energy-Requiring Steps 2 ATP invested

Energy-Requiring Steps of Glycolysis

glucose

PGAL PGALPP

ADP

P

ATP

glucose-6-phosphate

Pfructose-6-phosphate

ATP

fructose1,6-bisphosphateP P

ADP

Figure 8.4(2)Page 137

Energy-Releasing

Steps

ADPATP

pyruvate

ADPATP

pyruvate

H2OP

PEP

H2OP

PEP

P

2-phosphoglycerate

P

2-phosphoglycerate

ADPATP

P3-phosphoglycerate

ADPATP

P3-phosphoglycerate

NAD+

NADHPi

1,3-bisphosphoglycerateP P

NAD+

NADHPi

1,3-bisphosphoglycerateP P

PGALP

PGALP

Figure 8.4 Page 137

Glycolysis: Net Energy Yield

Energy requiring steps: 2 ATP invested

Energy releasing steps:2 NADH formed 4 ATP formed

Net yield is 2 ATP and 2 NADH

8.3 Second Stage Reactions

• Preparatory reactions– Pyruvate is oxidized into two-carbon acetyl

units and carbon dioxide– NAD+ is reduced

• Krebs cycle– The acetyl units are oxidized to carbon

dioxide– NAD+ and FAD are reduced

Preparatory Reactions

pyruvate

NAD+

NADH

coenzyme A (CoA)

O O carbon dioxide

CoAacetyl-CoA

Krebs Cycle

NAD+

NADH

=CoAacetyl-CoA

oxaloacetate citrate

CoA

H2O

malate isocitrate

H2O

H2O

FAD

FADH2

fumarate

succinate

ADP + phosphate groupATP

succinyl-CoA

O O

CoANAD+

NADH

O ONAD+

NADH

-ketoglutarate

Figure 8.6Page 139

The Krebs Cycle

Overall Products

• Coenzyme A

• 2 CO2

• 3 NADH

• FADH2

• ATP

Overall Reactants

• Acetyl-CoA• 3 NAD+

• FAD

• ADP and Pi

Results of the Second Stage

• All of the carbon molecules in pyruvate end up in carbon dioxide

• Coenzymes are reduced (they pick up electrons and hydrogen)

• One molecule of ATP forms

• Four-carbon oxaloacetate regenerates

Coenzyme Reductions during First Two Stages

• Glycolysis 2 NADH• Preparatory

reactions 2 NADH• Krebs cycle 2 FADH2 + 6 NADH

• Total 2 FADH2 + 10 NADH

• Occurs in the mitochondria

• Coenzymes deliver electrons to electron transfer chains

• Electron transfer sets up H+ ion gradients

• Flow of H+ down gradients powers ATP formation

8.4 Electron Transfer Phosphorylation

Creating an H+ Gradient

NADH

OUTER COMPARTMENT

INNER COMPARTMENT

Making ATP: Chemiosmotic Model

ATP

ADP+Pi

INNER COMPARTMENT

Importance of Oxygen

• Electron transport phosphorylation requires the presence of oxygen

• Oxygen withdraws spent electrons from the electron transfer chain, then combines with H+ to form water

Summary of Energy Harvest(per molecule of glucose)

• Glycolysis– 2 ATP formed by substrate-level phosphorylation

• Krebs cycle and preparatory reactions– 2 ATP formed by substrate-level phosphorylation

• Electron transport phosphorylation– 32 ATP formed

Energy Harvest Varies

• NADH formed in cytoplasm cannot enter mitochondrion

• It delivers electrons to mitochondrial membrane

• Membrane proteins shuttle electrons to NAD+ or FAD inside mitochondrion

• Electrons given to FAD yield less ATP than those given to NAD+

• 686 kcal of energy are released

• 7.5 kcal are conserved in each ATP

• When 36 ATP form, 270 kcal (36 X 7.5) are

captured in ATP

• Efficiency is 270 / 686 X 100 = 39 percent

• Most energy is lost as heat

Efficiency of Aerobic Respiration

• Do not use oxygen

• Produce less ATP than aerobic pathways

• Two types

– Fermentation pathways

– Anaerobic electron transport

8.5 Anaerobic Pathways

Fermentation Pathways

• Begin with glycolysis

• Do not break glucose down completely to

carbon dioxide and water

• Yield only the 2 ATP from glycolysis

• Steps that follow glycolysis serve only to

regenerate NAD+

Lactate Fermentation

C6H12O6

ATP

ATPNADH

2 lactate

electrons, hydrogen from NADH

2 NAD+

2

2 ADP

2 pyruvate

2

4

energy output

energy input

GLYCOLYSIS

LACTATE FORMATION

2 ATP net

Alcoholic Fermentation

C6H12O6

ATP

ATPNADH

2 acetaldehyde

electrons, hydrogen from NADH

2 NAD+

2

2 ADP

2 pyruvate

2

4

energy output

energy input

GLYCOLYSIS

ETHANOL FORMATION

2 ATP net

2 ethanol

2 H2O

2 CO2

Anaerobic Electron Transport

• Carried out by certain bacteria

• Electron transfer chain is in bacterial plasma membrane

• Final electron acceptor is compound from environment (such as nitrate), not oxygen

• ATP yield is low

FOOD

complex carbohydrates

simple sugars

pyruvate

acetyl-CoA

glycogenfats proteins

amino acids

carbon backbones

fatty acids

glycerol

NH3

PGAL

glucose-6-phosphate

GLYCOLYSIS

KREBS CYCLE

urea

Figure 8.11Page 145

8.6 ALTERNATIVE

ENERGY SOURCES

• When life originated, atmosphere had little

oxygen

• Earliest organisms used anaerobic pathways

• Later, noncyclic pathway of photosynthesis

increased atmospheric oxygen

• Cells arose that used oxygen as final

acceptor in electron transport

Evolution of Metabolic Pathways

8.7 Processes Are Linked

sunlight energy

water+

carbondioxide

PHOTOSYNTHESIS

AEROBICRESPIRATION

sugarmolecules oxygen

In-text figurePage 146