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Chapter 6: Harvesting Chemical Energy – Study Guide and Key Concepts

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Chapter 6: Harvesting Chemical Energy

Major Themes and Learning Objectives

This chapter explores how living organisms acquire and utilize energy through chemical reactions, focusing on the processes of cellular respiration and fermentation. The following study notes cover essential definitions, mechanisms, and comparisons relevant to energy metabolism in cells.

Key Concepts and Definitions

Metabolic Intermediates

  • Metabolic intermediate (intermediate): A compound formed in one step of a metabolic pathway and used in the next step. Intermediates connect the starting molecule (substrate) to the final product.

Phosphorylation Mechanisms

  • Substrate-level phosphorylation: The direct transfer of a phosphate group from a substrate molecule to ADP, forming ATP. Occurs during glycolysis and the citric acid cycle.

  • Oxidative phosphorylation: ATP synthesis powered by the transfer of electrons through the electron transport chain (ETC) and the resulting chemiosmotic gradient. Occurs in mitochondria.

  • Chemiosmosis: The movement of protons (H+) across a membrane, generating a proton gradient that drives ATP synthesis via ATP synthase.

Electron Transport Chain (ETC) and ATP Synthase

  • Electron transport chain (ETC): A series of protein complexes in the inner mitochondrial membrane that transfer electrons from NADH and FADH2 to oxygen, releasing energy used to pump protons and create a gradient.

  • ATP synthase: An enzyme complex that synthesizes ATP from ADP and inorganic phosphate, powered by the flow of protons down their gradient.

Energy Pathways in Living Organisms

Photosynthesis vs. Cellular Respiration

  • Photosynthesis: Converts light energy into chemical energy, producing glucose and oxygen from carbon dioxide and water.

  • Cellular respiration: Breaks down glucose in the presence of oxygen to produce ATP, carbon dioxide, and water.

  • Comparison: Photosynthesis stores energy; cellular respiration releases energy. The products of one are the reactants of the other.

Redox Reactions in Cells

  • Oxidation: Loss of electrons from a molecule.

  • Reduction: Gain of electrons by a molecule.

  • Redox reactions: Chemical reactions involving the transfer of electrons; essential for energy transfer in cells.

  • Example: In cellular respiration, glucose is oxidized to CO2, and oxygen is reduced to H2O.

Summary Equation of Cellular Respiration

  • The overall reaction is:

  • Reactants: Glucose (C6H12O6), Oxygen (O2)

  • Products: Carbon dioxide (CO2), Water (H2O), ATP

  • Cellular sites: Glycolysis (cytoplasm), Pyruvate oxidation and Citric Acid Cycle (mitochondrial matrix), ETC and chemiosmosis (inner mitochondrial membrane)

  • Oxidized: Glucose

  • Reduced: Oxygen

Electron Carriers in Metabolism

  • NAD+ (Nicotinamide adenine dinucleotide): Accepts electrons to become NADH.

  • FAD (Flavin adenine dinucleotide): Accepts electrons to become FADH2.

  • These carriers shuttle electrons to the ETC.

Mitochondrial Structure and Function

  • Matrix: Innermost compartment; site of the citric acid cycle and pyruvate oxidation.

  • Inner mitochondrial membrane: Contains the ETC and ATP synthase; site of oxidative phosphorylation.

  • Intermembrane space: Space between inner and outer membranes; accumulates protons during ETC activity.

Main Reaction Sequences of Cellular Respiration

Stage

Location

Reactants

Products

Glycolysis

Cytoplasm

Glucose, 2 NAD+, 2 ADP

2 Pyruvate, 2 NADH, 2 ATP

Pyruvate Oxidation

Mitochondrial matrix

2 Pyruvate, 2 NAD+, 2 CoA

2 Acetyl-CoA, 2 NADH, 2 CO2

Citric Acid Cycle

Mitochondrial matrix

2 Acetyl-CoA, 6 NAD+, 2 FAD, 2 ADP

4 CO2, 6 NADH, 2 FADH2, 2 ATP

Oxidative Phosphorylation

Inner mitochondrial membrane

NADH, FADH2, O2

NAD+, FAD, H2O, ~28 ATP

Oxidative Phosphorylation: Electron Transport and Chemiosmosis

  • Electron transport: Electrons from NADH and FADH2 pass through the ETC, releasing energy to pump protons into the intermembrane space.

  • Chemiosmosis: Protons flow back into the matrix through ATP synthase, driving ATP production.

  • Reactants: NADH, FADH2, O2

  • Products: NAD+, FAD, H2O, ATP

ATP Yield from Cellular Respiration

Stage

ATP Produced (per glucose)

Glycolysis

2

Pyruvate Oxidation

0

Citric Acid Cycle

2

Oxidative Phosphorylation

~28

Total

~32

Additional info: Actual ATP yield may vary depending on cell type and conditions.

Fermentation

  • Role: Allows ATP production in the absence of oxygen by regenerating NAD+ from NADH.

  • Location: Cytoplasm

  • Reactants: Glucose, NAD+

  • Products: ATP, NAD+, and end products such as lactate (lactic acid fermentation) or ethanol and CO2 (alcoholic fermentation)

  • Conditions: Occurs when oxygen is scarce or absent.

Energy Flow in Cellular Respiration

  • Energy flows from organic molecules (e.g., glucose) to electron carriers (NADH, FADH2), then to the ETC, and finally to ATP.

Fermentation vs. Cellular Respiration

Feature

Fermentation

Cellular Respiration

Oxygen Required?

No

Yes

ATP Yield (per glucose)

2

~32

End Products

Lactate or ethanol + CO2

CO2 + H2O

Electron Acceptor

Organic molecule

Oxygen

Regulation of Cellular Respiration

  • Feedback inhibition: The end product of a pathway inhibits an enzyme involved earlier in the pathway, preventing overproduction of ATP.

Alternative Energy Sources

  • Polysaccharides: Broken down to glucose and enter glycolysis.

  • Lipids: Glycerol enters glycolysis; fatty acids are converted to acetyl-CoA for the citric acid cycle.

  • Proteins: Amino acids are deaminated and enter glycolysis or the citric acid cycle.

Biosynthesis from Food Molecules

  • Organic molecules from food can be used as building blocks for synthesizing cellular components, tissues, and organisms, not just for energy.

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