Cellular Respiration and Metabolic Pathways

Free Energy and Chemical Equilibrium

Understanding how free energy and chemical equilibrium relate to cellular reactions is essential for grasping how cells perform work and maintain life processes.

• Free Energy (G): The energy available to do work in a system. Changes in free energy () determine whether a reaction is spontaneous.

• Direction and Rate of Reactions: The concentrations of reactants and products, along with their free energies, influence the direction and speed of a reaction as it approaches equilibrium.

• Equilibrium Characteristics: At equilibrium, the rates of the forward and reverse reactions are equal, and there is no net change in the concentrations of reactants and products.

• Cellular Prevention of Equilibrium: Cells prevent reactions from reaching equilibrium by continuously removing products or supplying reactants, allowing them to perform work and maintain metabolic processes.

• Example: In cellular respiration, glucose is constantly broken down, and products like CO2 are removed, preventing equilibrium.

Catabolic Pathways and Oxidation of Organic Fuels

Cellular respiration is a catabolic pathway that breaks down organic molecules to release energy, primarily through redox reactions.

• Chemical Equation for Cellular Respiration:

• Free Energy Change: Cellular respiration is highly exergonic (), releasing energy that cells use to synthesize ATP.

• Redox Reactions:

  • Oxidation: Loss of electrons from a molecule.

  • Reduction: Gain of electrons by a molecule.

  • Oxidizing Agent: Accepts electrons (e.g., O2 in respiration).

  • Reducing Agent: Donates electrons (e.g., glucose in respiration).

  • Examples: NAD+ is reduced to NADH; NADH is oxidized in the electron transport chain.

• Role of NADH: NADH carries electrons from catabolic reactions to the electron transport chain, facilitating ATP production.

• Free Energy Change in NAD+/NADH:

  • NADH has higher free energy; its oxidation releases energy used for ATP synthesis.

Major Stages of Cellular Respiration

Cellular respiration consists of several stages, each occurring in specific cellular locations and involving distinct reactants and products.

• Stages:

  • Glycolysis: Cytosol; glucose → pyruvate, ATP, NADH.

  • Citric Acid Cycle (Krebs Cycle): Mitochondrial matrix; acetyl-CoA → CO2, ATP, NADH, FADH2.

  • Oxidative Phosphorylation: Inner mitochondrial membrane; NADH/FADH2 → ATP, H2O.

• Mitochondrion Structure:

  • Outer membrane

  • Inner membrane (folded into cristae)

  • Intermembrane space

  • Matrix

  • Function: The inner membrane's structure supports the electron transport chain and ATP synthesis by creating a proton gradient.

• ATP Generation Mechanisms:

  • Substrate-level phosphorylation: Direct transfer of phosphate to ADP during glycolysis and the citric acid cycle.

  • Oxidative phosphorylation: ATP synthesis powered by the electron transport chain and chemiosmosis.

Stages of Cellular Respiration: Glycolysis, Citric Acid Cycle, Oxidative Phosphorylation

Each stage of cellular respiration involves specific events, energy transformations, and contributions to ATP production.

• Glycolysis:

  • Events: Glucose is split into two pyruvate molecules.

  • Free Energy: Some energy is captured as ATP and NADH; overall exergonic.

  • Matter/Energy Flow: Glucose → pyruvate; ATP and NADH produced.

  • ATP Generation: Substrate-level phosphorylation.

• Citric Acid Cycle:

  • Events: Acetyl-CoA is oxidized; CO2 released.

  • Free Energy: Energy captured in NADH, FADH2, and ATP.

  • Matter/Energy Flow: Acetyl-CoA → CO2; NADH, FADH2, ATP produced.

  • ATP Generation: Substrate-level phosphorylation.

• Oxidative Phosphorylation:

  • Events: Electrons from NADH/FADH2 pass through the electron transport chain; proton gradient drives ATP synthesis.

  • Free Energy: Energy from electron transfer used to pump protons and synthesize ATP.

  • Matter/Energy Flow: Electrons → O2; ATP and H2O produced.

  • ATP Generation: Oxidative phosphorylation via ATP synthase.

ATP Production Without Oxygen: Fermentation

When oxygen is limited, cells use fermentation to generate ATP, allowing survival under anaerobic conditions.

• Cellular Respiration Without Oxygen: Electron transport chain cannot operate; NADH cannot be oxidized.

• Fermentation: Glycolysis continues; NAD+ is regenerated by converting pyruvate to lactate (in animals) or ethanol (in yeast).

• ATP Yield: Fermentation produces much less ATP than aerobic respiration.

• Example: Muscle cells perform lactic acid fermentation during intense exercise.

Metabolic Pathway Alterations

Changes in one part of a metabolic pathway can affect both upstream and downstream reactions, impacting overall cellular function.

• Disruption Effects: Blocking a step (e.g., inhibiting an enzyme) can cause accumulation of reactants and depletion of products.

• Example: Inhibiting the electron transport chain prevents NADH oxidation, halting ATP production and affecting glycolysis and the citric acid cycle.

Summary Table: Stages of Cellular Respiration

Additional info: The study guide references connections to other metabolic pathways but does not require detailed coverage. For exam preparation, focus on the main stages, energy transformations, and regulatory mechanisms of cellular respiration.