Harvesting Chemical Energy

Major Themes and Learning Objectives

This chapter explores how living organisms acquire and utilize energy through evolved sequences of chemical reactions. These processes enable biological work and are fundamental to life.

• Energy Acquisition: Organisms perform complex chemical reactions to harvest energy from their environment.

• Evolution of Metabolic Pathways: These pathways have developed over time to maximize energy efficiency.

Key Terms and Definitions

• Metabolic Intermediate: A compound formed during a metabolic pathway that is neither the initial substrate nor the final product.

• Substrate-Level Phosphorylation: The direct transfer of a phosphate group to ADP from a phosphorylated intermediate, producing ATP.

• Oxidative Phosphorylation: ATP synthesis powered by the transfer of electrons through the electron transport chain and chemiosmosis.

• Chemiosmosis: The movement of ions across a semipermeable membrane, generating ATP via ATP synthase.

• Electron Transport Chain (ETC): A series of protein complexes in the mitochondrial membrane that transfer electrons and pump protons to create a gradient.

• ATP Synthase: An enzyme that synthesizes ATP from ADP and inorganic phosphate, driven by the proton gradient.

Energy Sources in Living Organisms

Organisms utilize two main energy sources:

• Organic Molecules: Such as glucose, used in cellular respiration.

• Light Energy: Used by photosynthetic organisms to produce organic molecules.

• Comparison: Cellular respiration breaks down organic molecules for energy, while photosynthesis builds them using light.

Oxidation-Reduction (Redox) Reactions

Redox reactions are central to energy transfer in cells.

• Oxidation: Loss of electrons from a molecule.

• Reduction: Gain of electrons by a molecule.

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

Summary Reaction of Cellular Respiration

The overall equation for cellular respiration is:

• Reactants: Glucose and oxygen

• Products: Carbon dioxide, water, and ATP

• Cellular Sites: Glycolysis (cytoplasm), Krebs cycle and ETC (mitochondria)

• Energy Levels: Glucose (high energy), CO2 and H2O (low energy)

• Oxidized: Glucose

• Reduced: Oxygen

Electron Carriers in Metabolism

• NAD+: Accepts electrons to become NADH.

• FAD: Accepts electrons to become FADH2.

• Role: Shuttle electrons to the ETC for ATP production.

Main Reaction Sequences of Cellular Respiration

• Glycolysis: Converts glucose to pyruvate (cytoplasm).

• Transition Step: Converts pyruvate to acetyl-CoA (mitochondrial matrix).

• Krebs (Citric Acid) Cycle: Converts acetyl-CoA to CO2 (mitochondrial matrix).

• Reactants and Products: Each step has specific reactants and products.

Chemiosmosis in Oxidative Phosphorylation

• Location: Inner mitochondrial membrane

• Process: Protons pumped by ETC create a gradient; ATP synthase uses this gradient to produce ATP.

• Reactants: ADP, inorganic phosphate, oxygen, NADH, FADH2

• Products: ATP, water

ATP Yield per Glucose

Additional info: Actual ATP yield may vary due to inefficiencies and transport costs.

Fermentation in Metabolism

• Role: Allows ATP production without oxygen.

• Location: Cytoplasm

• Reactants: Glucose

• Products: Lactic acid or ethanol, ATP

• Conditions: Occurs when oxygen is unavailable.

Fermentation vs. Cellular Respiration

Alternative Energy Sources

• Polysaccharides: Broken down to glucose for cellular respiration.

• Lipids: Fatty acids enter Krebs cycle after conversion to acetyl-CoA.

• Proteins: Amino acids are deaminated and enter metabolic pathways.

• Application: Cells can use these macromolecules for energy when glucose is scarce.