Energy and Metabolism

Exergonic vs Endergonic Reactions

Biological reactions can be classified based on their energy changes:

• Exergonic reactions: Release energy; spontaneous; ΔG < 0. Example: Cellular respiration.

• Endergonic reactions: Require energy input; non-spontaneous; ΔG > 0. Example: Photosynthesis.

Catabolic reactions break down molecules and are usually exergonic. Anabolic reactions build molecules and are typically endergonic.

Entropy and Free Energy (ΔG)

• Entropy (S): Measure of disorder; increases in spontaneous reactions.

• Free Energy (G): Energy available to do work.

• Equation:

• ΔG: Change in free energy; negative for exergonic, positive for endergonic.

First and Second Law of Thermodynamics

• First Law: Energy cannot be created or destroyed, only transformed.

• Second Law: Every energy transfer increases entropy (disorder) of the universe.

Types of Energy

• Potential energy: Stored energy (e.g., chemical bonds, water behind a dam).

• Kinetic energy: Energy of motion (e.g., moving water, muscle contraction).

• Chemical energy: Potential energy in chemical bonds (e.g., glucose, ATP).

Enzymes and Their Function

How Enzymes Work

Enzymes are biological catalysts that speed up reactions by lowering activation energy.

• Do not change ΔG or equilibrium.

• Bind substrates at the active site.

• Form enzyme-substrate complex.

Factors Affecting Enzyme Activity

• Temperature: Too high/low can denature or slow enzyme.

• pH: Each enzyme has optimal pH.

• Substrate concentration: Higher concentration increases rate until saturation.

• Enzyme concentration: More enzyme increases rate if substrate is available.

Enzyme Inhibition

• Competitive inhibitors: Bind active site, block substrate.

• Non-competitive inhibitors: Bind elsewhere, change enzyme shape.

• Feedback inhibition: End product inhibits pathway, regulates metabolism.

Free Energy Diagram

Enzymes lower the activation energy barrier, making reactions proceed faster. The overall ΔG remains unchanged.

ATP: Structure and Function

ATP and Energy

• ATP (Adenosine Triphosphate): Main energy currency of the cell.

• Energy comes from hydrolysis of phosphate bonds.

• ATP drives endergonic reactions via coupling.

Redox Reactions

Oxidation vs Reduction

• Oxidation: Loss of electrons.

• Reduction: Gain of electrons.

• Follow electron carriers (e.g., NAD+, FAD) in respiration and photosynthesis.

In cellular respiration, glucose is oxidized; oxygen is reduced. In photosynthesis, water is oxidized; CO2 is reduced.

Cellular Respiration

Stages of Cellular Respiration

• Glycolysis: Cytoplasm; glucose → 2 pyruvate, 2 ATP, 2 NADH.

• Pyruvate Oxidation: Mitochondrial matrix; pyruvate → acetyl-CoA, CO2, NADH.

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

• Electron Transport Chain (ETC): Inner mitochondrial membrane; NADH/FADH2 → ATP, H2O.

Major Players

• Electron carriers: NADH, FADH2

• ATP synthase: Makes ATP via chemiosmosis.

Overall Flow of Electrons

Electrons flow from glucose to NADH/FADH2 to ETC to oxygen (final electron acceptor).

Aerobic vs Anaerobic Respiration

Purpose of Oxygen

• Oxygen is the final electron acceptor in ETC.

• We breathe to supply oxygen for ATP production.

• CO2 waste comes from Krebs cycle and pyruvate oxidation.

Fermentation

• Purpose: Regenerate NAD+ for glycolysis when oxygen is absent.

• Types:

  • Lactic acid fermentation: Pyruvate → lactic acid (muscle cells).

  • Alcohol fermentation: Pyruvate → ethanol + CO2 (yeast).

Electron Transport Chain and Chemiosmosis

• ETC location: Inner mitochondrial membrane.

• What is pumped: H+ ions (protons) across membrane, creating gradient.

• Chemiosmosis: H+ flows back through ATP synthase, making ATP.

Photosynthesis

Overall Reaction

• Water is oxidized; CO2 is reduced.

• Equation:

Primary Producers

• Organisms (plants, algae, cyanobacteria) that produce organic molecules from CO2 using light energy.

Photosynthetic Pigments

• Absorption: Pigments absorb specific wavelengths (chlorophyll absorbs red/blue).

• Reflection: Pigments reflect others (chlorophyll reflects green).

Stages of Photosynthesis

Photosystems: PSII vs PSI

• PSII: Source of electrons is water (splitting releases O2); electrons go to ETC.

• PSI: Receives electrons from ETC; electrons used to reduce NADP+ to NADPH.

• H+ gradient is created in thylakoid space for ATP synthesis.

RUBISCO

• RUBISCO: Enzyme that catalyzes CO2 fixation in Calvin cycle.

• Most abundant enzyme on Earth.

Photorespiration, C4, and CAM Plants

• Photorespiration: RUBISCO binds O2 instead of CO2, wasting energy.

• C4 plants: Spatial separation of carbon fixation; minimize photorespiration (e.g., corn).

• CAM plants: Temporal separation; fix carbon at night, reduce water loss (e.g., cacti).

Additional info: Chemiosmosis is a universal mechanism for ATP synthesis in both mitochondria (respiration) and chloroplasts (photosynthesis).