7 Pathways That Harvest Chemical Energy. 7 Pathways That Harvest Chemical Energy 7.1 How Does Glucose Oxidation Release Chemical Energy? 7.2 What Are

7Pathways That Harvest

Chemical Energy

7 Pathways That Harvest Chemical Energy

• 7.1 How Does Glucose Oxidation Release Chemical Energy?

• 7.2 What Are the Aerobic Pathways of Glucose Metabolism?

• 7.3 How Is Energy Harvested from Glucose in the Absence of Oxygen?

• 7.4 How Does the Oxidation of Glucose Form ATP?

• 7.5 Why Does Cellular Respiration Yield So Much More Energy Than Fermentation?

• 7.6 How Are Metabolic Pathways Interrelated and Controlled?

7.1 How Does Glucose Oxidation Release Chemical Energy?

Fuels: molecules whose stored energy can be released for use.

The most common fuel in organisms is glucose. Other molecules are first converted into glucose or other intermediate compounds.


Principles governing metabolic pathways:

• Complex chemical transformations occur in a series of reactions.

• Each reaction is catalyzed by a specific enzyme.

• Metabolic pathways are similar in all organisms.

• In eukaryotes, metabolic pathways are compartmentalized in organelles.

• Each pathway is regulated by key enzymes.


Burning or metabolism of glucose:

Glucose metabolism pathway traps the free energy in ATP:

energyfreeOHCOOOHC 2226126 666

ATPenergyfreePADP i


ΔG from complete combustion of glucose = –686 kcal/mole

Highly exergonic; drives endergonic formation of many ATP

Figure 7.1 Energy for Life

Three metabolic pathways involved in harvesting the energy of glucose


If O2 is present, four pathways operate:

• Glycolysis, pyruvate oxidation, citric acid cycle, and electron transport chain.

If O2 is not present, pyruvate is metabolized in fermentation.

Figure 7.2 Energy-Producing Metabolic Pathways


Redox reactions: one substance transfers electrons to another substance

Reduction: gain of one or more electrons by an atom, ion, or molecule

Oxidation: loss of one or more electrons

Also applies if hydrogen atoms are gained or lost.


Oxidation and reduction always occur together.

The reactant that becomes reduced is the oxidizing agent.

The reactant that becomes oxidized is the reducing agent.

Figure 7.3 Oxidation and Reduction Are Coupled


In combustion of glucose, glucose is the reducing agent, O2 is the oxidizing agent.

Energy is transferred in a redox reaction.

Energy in the reducing agent becomes associated with the reduced product.


Coenzyme NAD is an electron carrier in redox reactions.

Two forms:

NAD+ (oxidized)

NADH + H+ (reduced)

Figure 7.4 NAD Is an Energy Carrier in Redox Reactions (A)


A hydride ion (H–) is transferred, leaving a free H+

H– : a proton with two electrons

HNADHHNAD 2

Figure 7.4 NAD Is an Energy Carrier in Redox Reactions (B)


Oxygen accepts electrons from NADH:

exergonic—ΔG = –52.4 kcal/mole

Oxidizing agent is molecular oxygen—O2

OHNADOHNADH 2221

7.2 What Are the Aerobic Pathways of Glucose Metabolism?

Glycolysis takes place in the cytosol.

Involves 10 enzyme-catalyzed reactions

Results in: 2 molecules of pyruvate

4 molecules ATP

2 molecules NADH

Figure 7.5 Glycolysis Converts Glucose into Pyruvate (Part 1)

probably 4 parts?


probably 4 parts?


probably 4 parts?


probably 4 parts?


probably 4 parts?


probably 4 parts?


A kinase is an enzyme that catalyzes transfer of a phosphate group from ATP to another substrate.

In the first half of glycolysis, the glucose molecule is split into two 3-carbon molecules (G3P).


Phosphorylation: addition of a phosphate group

Enzyme-catalyzed transfer of a phosphate group to ADP is called substrate-level phosphorylation.

Figure 7.6 Changes in Free Energy During Glycolysis


Pyruvate Oxidation:

• Links glycolysis and the citric acid cycle

• Pyruvate is converted to acetyl CoA

• Takes place in the mitochondrial matrix

Figure 7.8 Pyruvate Oxidation and the Citric Acid Cycle (Part 1)

1st part – pyruvate oxidation


Acetyl CoA is the starting point of the citric acid cycle:

• Coenzyme A is removed in the first reaction and can be reused

• The cycle is in steady state: the concentrations of the intermediates don’t change

• Outputs: CO2, reduced electron carriers, and ATP

Figure 7.8 Pyruvate Oxidation and the Citric Acid Cycle (Part 2)

citric acid cycle

Figure 7.7 The Citric Acid Cycle Releases Much More Free Energy Than Glycolysis Does


The electron carriers that are reduced during the citric acid cycle must be reoxidized to take part in the cycle again.

Fermentation—if no O2 is present

Oxidative phosphorylation—O2 is present

7.3 How Is Energy Harvested from Glucose in the Absence of Oxygen?

Fermentation occurs in the cytosol.

Pyruvate is reduced by NADH + H+ and NAD+ is regenerated.


Lactic acid fermentation:

• Occurs in microorganisms, some muscle cells.

• Pyruvate is the electron acceptor.

Figure 7.9 Lactic Acid Fermentation


Alcoholic fermentation:

• Yeasts and some plant cells

• Pyruvate is converted to acetaldehyde, CO2 is released

• Acetaldehyde is reduced by NADH + H+, producing NAD+ and ethyl alcohol

Figure 7.10 Alcoholic Fermentation

7.4 How Does the Oxidation of Glucose Form ATP?

Oxidative phosphorylation: ATP is synthesized as electron carriers are reoxidized in the presence of O2.

Two stages:

Electron transport chain

Chemiosmosis


Why does the electron transport chain have so many steps?

Why not

in one step?

OHNADOHNADH 2221


Too much free energy would be released all at once—it could not be harvested by the cell.

In a series of reactions, each releases a small amount of energy that can be captured by an endergonic reaction.


The electron transport chain:

• On the inner mitochondrial membrane

• 4 protein complexes: I, II, III, IV

• Cytochrome c

• Ubiquinone (Q)—a lipid

Figure 7.11 The Oxidation of NADH + H+ (Part 1)

Figure 7.11 The Oxidation of NADH + H+ (Part 2)

Figure 7.12 The Complete Electron Transport Chain


The electron transport chain results in the active transport of protons (H+) across the inner mitochondrial membrane.

The transmembrane complexes act as proton pumps.

Figure 7.13 A Chemiosmotic Mechanism Produces ATP (Part 1)

Figure 7.13 A Chemiosmotic Mechanism Produces ATP (Part 2)


The proton pump results in a proton concentration gradient and an electric charge difference across the inner membrane: potential energy!

Proton-motive force


The protons must pass through a protein channel—ATP synthase—to flow back into the mitochondrial matrix.

Chemiosmosis is the coupling of the proton-motive force and ATP synthesis.


ATP synthase allows protons to diffuse back to the mitochondrial matrix, and uses the energy of that diffusion to make ATP from ADP and Pi.

Figure 7.14 Two Experiments Demonstrate the Chemiosmotic Mechanism (Part 1)

Figure 7.14 Two Experiments Demonstrate the Chemiosmotic Mechanism (Part 2)


ATP synthesis can be uncoupled: if a different H+ diffusion channel is inserted into the mitochondrial membrane, the energy of the diffusion is lost as heat.

The protein thermogenin occurs in human infants and hibernating animals.


ATP synthase:

• F0 subunit—transmembrane

• F1 subunit—projects into the mitochondrial matrix, rotates to expose active sites for ATP synthesis

7.5 Why Does Cellular Respiration Yield So Much More Energy Than Fermentation?

Energy yields:

Glycolysis and fermentation: 2 ATP

Glycolysis and cellular respiration: 32 ATP

Fermentation by-products have a lot of energy remaining.

Figure 7.15 Cellular Respiration Yields More Energy Than Glycolysis Does (Part 1)

Figure 7.15 Cellular Respiration Yields More Energy Than Glycolysis Does (Part 2)

7.6 How Are Metabolic Pathways Interrelated and Controlled?

Metabolic pathways are interrelated. Interchange of molecules occurs between the pathways.


Catabolic interconversions:

Polysaccharides → hydrolyzed to glucose, enters glycolysis

Lipids broken down toglycerol → DAPfatty acids → acetyl CoA

Proteins → hydrolyzed to amino acids—feed into glycolysis or the citric acid cycle at various points

Figure 7.16 Relationships among the Major Metabolic Pathways of the Cell


Anabolic interconversions:

• Most catabolic reactions are reversible

• Gluconeogenesis: glucose from citric acid cycle and glycolysis intermediates

Figure 7.17 Coupling Metabolic Pathways


Metabolic homeostasis: concentrations of the biochemical molecules remain constant (e.g., glucose concentration in blood)

• Allosteric control of enzymes in catabolic pathways

• Negative and positive feedback controls

Figure 7.18 Regulation by Negative and Positive Feedback

Figure 7.19 Allosteric Regulation of Glycolysis and the Citric Acid Cycle (Part 1)

Figure 7.19 Allosteric Regulation of Glycolysis and the Citric Acid Cycle (Part 2)


The main control point in glycolysis is phosphofructokinase—allosterically inhibited by ATP.

The main control point in the citric acid cycle is isocitrate dehydrogenase—inhibited by NADH + H+ and ATP.


Acetyl CoA—accumulation of citrate diverts acetyl CoA to fatty acid synthesis.

Cell differentiation: Slow twitch muscle cells have many mitochondria that catabolize aerobically, providing a steady supply of ATP.

Documents

7 Pathways That Harvest Chemical Energy. 7 Pathways That Harvest Chemical Energy 7.1 How Does Glucose Oxidation Release Chemical Energy? 7.2 What Are