Cellular Respiration: Free Energy, Chemical Equilibrium, and Metabolic Pathways

Concept 7.0 – Free Energy and Chemical Equilibrium

Introduction to Free Energy and Chemical Equilibrium

Understanding free energy and chemical equilibrium is essential for analyzing how biological reactions proceed and how cells regulate metabolic pathways. Free energy changes determine whether reactions are spontaneous, while equilibrium concepts help explain reaction direction and rates.

• Free Energy (G): The energy in a system available to do work. Changes in free energy ($\Delta G$) predict whether a reaction will occur spontaneously.

• Chemical Equilibrium: The state in which the forward and reverse reactions occur at equal rates, so the concentrations of reactants and products remain constant.

• Direction and Rate of Reactions: The concentrations of reactants and products, as well as their free energies, influence the direction and rate of chemical reactions.

• Equilibrium Constant (Keq): Indicates the ratio of product to reactant concentrations at equilibrium.

• Cellular Reactions: Cells prevent reactions from reaching equilibrium by removing products or supplying reactants, allowing continuous metabolic activity.

Example: In cellular respiration, glucose is continuously broken down, and products like CO2 are removed, preventing equilibrium and enabling ongoing ATP production.

Concept 7.1 – Catabolic Pathways Oxidize Organic Fuels

Overview of Catabolic Pathways

Catabolic pathways break down organic molecules, releasing energy that cells capture for work. Cellular respiration is a key catabolic process that oxidizes glucose to produce ATP.

• Cellular Respiration Equation: The overall chemical equation is: $\text{C}_6\text{H}_{12}\text{O}_6 + 6\text{O}_2 \rightarrow 6\text{CO}_2 + 6\text{H}_2\text{O} + \text{Energy (ATP + heat)}$

• Energy Release: Energy is released during oxidation reactions as electrons are transferred from organic molecules to oxygen.

• Redox Reactions: Oxidation is the loss of electrons; reduction is the gain of electrons. These reactions are coupled in cellular respiration.

• NAD+ and NADH: NAD+ acts as an electron carrier, accepting electrons during glycolysis and the citric acid cycle to become NADH, which then donates electrons to the electron transport chain.

• Major Stages of Cellular Respiration:

  1. Glycolysis

  2. Pyruvate oxidation

  3. Citric acid cycle

  4. Oxidative phosphorylation (electron transport chain and chemiosmosis)

• Mitochondrial Structure: The mitochondrion has an outer membrane, inner membrane, intermembrane space, and matrix. The inner membrane contains proteins for the electron transport chain and ATP synthesis.

• ATP Generation: ATP is produced by substrate-level phosphorylation (direct transfer of phosphate to ADP) and oxidative phosphorylation (using energy from electrons transferred to oxygen).

Example: During glycolysis, glucose is split into two pyruvate molecules, producing ATP and NADH.

Concepts 7.2 / 7.3 / 7.4 – Cellular Respiration Stages

Stages of Cellular Respiration

Cellular respiration consists of several stages, each contributing to the breakdown of glucose and the production of ATP. The main stages are glycolysis, citric acid cycle, and oxidative phosphorylation.

• Glycolysis:

  • Occurs in the cytoplasm.

  • Converts one glucose molecule into two pyruvate molecules.

  • Produces 2 ATP (substrate-level phosphorylation) and 2 NADH.

• Citric Acid Cycle (Krebs Cycle):

  • Occurs in the mitochondrial matrix.

  • Completes the oxidation of glucose derivatives.

  • Produces 2 ATP, 6 NADH, and 2 FADH2 per glucose molecule.

• Oxidative Phosphorylation:

  • Includes electron transport chain and chemiosmosis.

  • Occurs in the inner mitochondrial membrane.

  • Uses NADH and FADH2 to generate a proton gradient, driving ATP synthesis via ATP synthase.

  • Produces about 26-28 ATP per glucose molecule.

• ATP Yield Table:

Stage	Main Products	ATP Produced
Glycolysis	2 Pyruvate, 2 NADH	2
Citric Acid Cycle	6 NADH, 2 FADH2	2
Oxidative Phosphorylation	H2O, ATP	~26-28

Example: The electron transport chain uses electrons from NADH and FADH2 to pump protons, creating a gradient that powers ATP synthase.

Concept 7.5 – ATP Production Without Oxygen

Anaerobic Pathways and Fermentation

Cells can produce ATP without oxygen through anaerobic pathways such as fermentation. These processes allow glycolysis to continue by regenerating NAD+.

• Fermentation: Occurs when oxygen is limited; pyruvate is converted to lactate (lactic acid fermentation) or ethanol and CO2 (alcohol fermentation).

• ATP Yield: Only 2 ATP per glucose, produced during glycolysis.

• Regeneration of NAD+: Fermentation recycles NADH back to NAD+, allowing glycolysis to continue.

• Comparison to Aerobic Respiration: Aerobic respiration yields much more ATP than fermentation.

Example: Muscle cells use lactic acid fermentation during intense exercise when oxygen is scarce.

Concept 7.6 – Connections to Other Metabolic Pathways

Integration of Metabolic Pathways

Cellular respiration is interconnected with other metabolic pathways, allowing cells to use various molecules for energy and biosynthesis.

• Other Fuels: Proteins, fats, and carbohydrates can enter cellular respiration at different points.

• Metabolic Flexibility: Cells can adjust metabolic pathways based on nutrient availability and energy needs.

• Pathway Regulation: Changes in one pathway can affect upstream and downstream reactions, impacting overall metabolism.

Example: Fatty acids are broken down via beta-oxidation and enter the citric acid cycle as acetyl-CoA.

Additional info: The integration of metabolic pathways is crucial for maintaining homeostasis and responding to environmental changes.