Concept 7.0 – Free Energy and Chemical Equilibrium

Free Energy and Chemical Equilibrium in Biological Systems

Understanding free energy and chemical equilibrium is essential for analyzing how cells regulate metabolic reactions. These concepts explain the direction and rate of biochemical processes.

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

• 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: Reactions proceed in the direction that decreases free energy until equilibrium is reached.

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

• Cellular Implications: Cells avoid equilibrium in metabolic pathways to continue performing work, often by removing products or using them in subsequent reactions.

Equation:

where is change in enthalpy, is temperature in Kelvin, and is change in entropy.

Example: ATP hydrolysis is a spontaneous reaction with a negative , driving cellular work.

Concept 7.1 – Catabolic Pathways Oxidize Organic Fuels

Overview of Cellular Respiration and Catabolic Pathways

Catabolic pathways break down organic molecules, releasing energy that cells capture for work. Cellular respiration is a key catabolic process in most organisms.

• Cellular Respiration: The process by which cells extract energy from glucose and other organic molecules.

• General Equation:

• Stages: Glycolysis, Pyruvate Oxidation, Citric Acid Cycle, Electron Transport Chain.

• Redox Reactions: Oxidation (loss of electrons) and reduction (gain of electrons) are central to energy extraction.

• NAD+ and NADH: NAD+ acts as an electron carrier, accepting electrons during catabolic reactions to form NADH.

• Major Stages: Glycolysis (cytoplasm), Pyruvate Oxidation and Citric Acid Cycle (mitochondrial matrix), Electron Transport Chain (inner mitochondrial membrane).

• Mitochondrial Structure: The double membrane and cristae increase surface area for reactions; compartments separate different stages.

• ATP Generation: Substrate-level phosphorylation and oxidative phosphorylation are two mechanisms for ATP synthesis.

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 and ATP Generation

Cellular respiration consists of several interconnected stages, each contributing to the production of ATP and the flow of energy in the cell.

• Glycolysis: Occurs in the cytoplasm; breaks glucose into pyruvate, producing ATP and NADH.

• Citric Acid Cycle (Krebs Cycle): Occurs in the mitochondrial matrix; oxidizes acetyl-CoA to CO2, generating NADH, FADH2, and ATP.

• Electron Transport Chain (ETC): Located in the inner mitochondrial membrane; uses electrons from NADH and FADH2 to create a proton gradient, driving ATP synthesis via chemiosmosis.

• Oxidative Phosphorylation: ATP is produced as protons flow back into the matrix through ATP synthase.

Equation for ATP Yield:

• Contribution to ATP Generation: Glycolysis and the citric acid cycle produce ATP directly (substrate-level phosphorylation), while most ATP is generated by oxidative phosphorylation.

Example: The ETC uses oxygen as the final electron acceptor, forming water.

Concept 7.5 – ATP Production Without Oxygen

Fermentation and Anaerobic Respiration

Cells can generate ATP in the absence of oxygen through fermentation or anaerobic respiration, though less efficiently than aerobic respiration.

• Fermentation: Glycolysis followed by conversion of pyruvate to lactate (lactic acid fermentation) or ethanol and CO2 (alcohol fermentation).

• Anaerobic Respiration: Uses an electron transport chain with a final electron acceptor other than oxygen (e.g., sulfate).

• ATP Yield: Only 2 ATP per glucose via substrate-level phosphorylation in glycolysis.

• Limitation: Fermentation regenerates NAD+ but does not produce additional ATP beyond glycolysis.

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 adapt to changing energy demands and nutrient availability.

• Catabolic and Anabolic Pathways: Intermediates from respiration can be used for biosynthesis (anabolism), while other molecules can enter respiration for energy (catabolism).

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

• Example: Fatty acids can be broken down to acetyl-CoA and enter the citric acid cycle.

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