Chapter 6 – Energy, Metabolism, and Enzymes

Introduction to Energy in Biological Systems

• Energy is the capacity to cause change, do work, or move matter against opposing forces. It exists in various forms, such as kinetic and potential energy.

• Kinetic energy is the energy of motion (e.g., diffusion of ions through a channel).

• Potential energy is stored energy due to position or structure (e.g., water behind a dam, chemical bonds in glucose).

• Example: The energy stored in a mole of glucose is potential energy; when glucose is metabolized, this energy is released as kinetic energy for cellular work.

Thermodynamics and Free Energy

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

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

• Free energy (G): The portion of a system's energy that can perform work at constant temperature and pressure. Symbol: G.

• ΔG (Change in Free Energy):

• For an exergonic reaction (energy-releasing), is negative.

• For an endergonic reaction (energy-requiring), is positive.

Metabolic Pathways: Catabolic vs. Anabolic

• Catabolic pathways break down complex molecules into simpler ones, releasing energy (e.g., cellular respiration).

• Anabolic pathways build complex molecules from simpler ones, consuming energy (e.g., protein synthesis).

• Example: Glycolysis is a catabolic pathway; photosynthesis is an anabolic pathway.

Energy Coupling and ATP

• Energy coupling uses exergonic processes to drive endergonic ones, often via ATP hydrolysis.

• ATP Hydrolysis: ; kcal/mol.

• Coupling ATP hydrolysis (exergonic) to an endergonic reaction can make the overall process spontaneous.

Enzymes and Catalysis

• Catalyst: A substance that speeds up a reaction without being consumed.

• Enzymes are biological catalysts, usually proteins, that lower the activation energy () of reactions.

• Activation energy (): The initial energy input required to start a reaction.

• Enzymes do not affect ; they only lower .

Enzyme Structure and Function

• Active site: The region on the enzyme where the substrate binds.

• Induced fit: The enzyme changes shape slightly to fit the substrate more snugly.

• Enzyme specificity is determined by the shape and chemical environment of the active site, which is a result of the enzyme's protein structure.

Cofactors and Coenzymes

• Cofactors: Non-protein helpers required for enzyme activity (e.g., metal ions like Mg2+).

• Coenzymes: Organic cofactors, often derived from vitamins (e.g., NAD+, FAD).

Enzyme Regulation

• Competitive inhibitors bind to the active site, blocking substrate binding.

• Noncompetitive inhibitors bind elsewhere, changing the enzyme's shape and reducing activity.

• Feedback inhibition: The end product of a pathway inhibits an earlier step, regulating the pathway.

Factors Affecting Enzyme Activity

• Enzyme activity is affected by pH, temperature, and substrate concentration.

• Extreme pH or high temperatures can denature enzymes, reducing activity.

• Optimal conditions vary for each enzyme (e.g., pepsin in the stomach works best at pH 2).

ATP and Cellular Metabolism

• ATP is the main energy currency of the cell, providing energy for cellular work.

• Cells avoid chemical equilibrium by constantly using and regenerating ATP.

Chapter 7 – Cellular Respiration and Fermentation

Overview of Cellular Respiration

• Cellular respiration is the process by which cells extract energy from glucose to produce ATP.

• It includes glycolysis, pyruvate oxidation, the citric acid cycle, and oxidative phosphorylation.

• Fermentation is an anaerobic process that allows ATP production without oxygen.

Redox Reactions in Respiration

• Oxidation: Loss of electrons.

• Reduction: Gain of electrons.

• In respiration, glucose is oxidized and oxygen is reduced.

• OIL RIG: Oxidation Is Loss, Reduction Is Gain (of electrons).

Electron Carriers

• NAD+ and FAD are electron carriers that shuttle electrons during respiration.

• NAD+ is reduced to NADH; FAD is reduced to FADH2.

Stages of Cellular Respiration

• Glycolysis: Occurs in the cytoplasm; breaks glucose into two pyruvate molecules.

• Pyruvate oxidation: Converts pyruvate to acetyl-CoA in the mitochondria.

• Citric Acid Cycle (Krebs Cycle): Completes the breakdown of glucose, producing NADH, FADH2, and ATP.

• Oxidative phosphorylation: Uses the electron transport chain and chemiosmosis to produce most ATP.

ATP Production and Substrate-Level Phosphorylation

• ATP is produced by substrate-level phosphorylation (direct transfer of phosphate) and oxidative phosphorylation (via ATP synthase and proton gradient).

• Most ATP is generated during oxidative phosphorylation.

Electron Transport Chain and Chemiosmosis

• The electron transport chain (ETC) is a series of protein complexes in the inner mitochondrial membrane.

• Electrons are transferred from NADH and FADH2 to oxygen, the final electron acceptor.

• Proton gradient across the membrane drives ATP synthase to produce ATP (chemiosmosis).

• Equation:

Fermentation

• Occurs when oxygen is absent; regenerates NAD+ for glycolysis.

• Produces lactic acid (in animals) or ethanol and CO2 (in yeast).

• Yields 2 ATP per glucose molecule.

Summary Table: Comparison of Aerobic Respiration and Fermentation

Key Points and Applications

• Oxygen is the final electron acceptor in the ETC; without it, the chain stops and ATP production ceases.

• ATP synthase uses the proton gradient to synthesize ATP from ADP and inorganic phosphate.

• Most of the energy from glucose is stored in NADH and FADH2 until the ETC.

• Cells regulate respiration via feedback inhibition and allosteric enzymes.

Example Calculation

• For one glucose molecule:

  • Glycolysis: 2 ATP (net), 2 NADH

  • Pyruvate oxidation: 2 NADH

  • Citric Acid Cycle: 2 ATP, 6 NADH, 2 FADH2

  • Oxidative phosphorylation: ~26-28 ATP

Additional info: These notes are based on standard college-level General Biology content and expand on the study guide questions provided in the file.