Chapter 6 – Energy, Metabolism, and Enzymes

Energy in Biological Systems

Energy is a fundamental concept in biology, driving all cellular processes. Understanding the forms and transformations of energy is essential for studying metabolism and enzyme function.

• Energy: The capacity to cause change, do work, or move matter against opposing forces. Exists in various forms, such as kinetic and potential energy.

• Kinetic energy: Energy of motion (e.g., movement of molecules).

• Potential energy: Stored energy due to position or structure (e.g., chemical bonds).

• 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): Indicates whether a reaction is spontaneous. For exergonic reactions, ΔG is negative; for endergonic reactions, ΔG is positive.

Example: Diffusion of ions through a channel is driven by kinetic energy.

Endergonic vs. Exergonic Reactions

Chemical reactions in cells can either release or require energy.

• Exergonic reaction: Releases energy; ΔG is negative; spontaneous.

• Endergonic reaction: Requires energy input; ΔG is positive; non-spontaneous.

• Cellular respiration: Exergonic (ΔG negative).

• Photosynthesis: Endergonic (ΔG positive).

• Energy source for cellular work: ATP hydrolysis.

Catabolic vs. Anabolic Pathways

Metabolic pathways are classified based on whether they break down or build up molecules.

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

• Anabolic pathways: Build complex molecules from simpler ones, requiring energy (e.g., protein synthesis).

Energy Coupling in Cells

Cells couple exergonic and endergonic reactions to drive necessary processes.

• Energy coupling: The use of energy released from exergonic reactions to drive endergonic reactions.

• ATP hydrolysis: ; ΔG ≈ -7.3 kcal/mol.

• Biosynthetic reactions: Often have positive ΔG; coupling with ATP hydrolysis makes them spontaneous.

Enzymes and Activation Energy

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

• Catalyst: Substance that increases reaction rate without being consumed.

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

• Enzyme effect: Lowers E_a, does not affect ΔG.

Graph: Progress of the reaction (x-axis) vs. Free Energy (y-axis). Enzyme lowers the peak (E_a).

Enzyme Structure and Function

Enzymes have specific structures that determine their function and specificity.

• Active site: Region on enzyme where substrate binds.

• Induced fit: Enzyme changes shape to fit substrate upon binding.

• Protein structure: Determines enzyme specificity; primary, secondary, tertiary, and quaternary structures all contribute.

• Cofactors: Non-protein helpers (e.g., metal ions).

• Coenzymes: Organic cofactors (e.g., NAD+, FAD).

Enzyme Regulation

Enzyme activity is regulated by various mechanisms to control metabolic pathways.

• Competitive inhibitors: Bind to active site, blocking substrate.

• Noncompetitive inhibitors: Bind elsewhere, changing enzyme shape.

• Feedback inhibition: End product inhibits an earlier step in the pathway.

• Environmental factors: pH, temperature, substrate concentration affect enzyme activity.

• Enzymes are globular proteins: Extremes of pH or temperature can denature them.

Equilibrium in Cells

Cells avoid chemical equilibrium to maintain life.

• Equilibrium: No net change in reactants/products; cells avoid this by constantly removing products and supplying reactants.

Chapter 7 – Cellular Respiration and Fermentation

Fermentation vs. Aerobic Respiration

Cells extract energy from glucose by two main processes: fermentation and aerobic respiration.

• Fermentation: Anaerobic; produces less ATP; end products include lactate or ethanol.

• Aerobic respiration: Requires oxygen; produces much more ATP; end products are CO2 and H2O.

Redox Reactions in Metabolism

Cellular respiration and photosynthesis involve electron transfer (redox reactions).

• Oxidation: Loss of electrons.

• Reduction: Gain of electrons.

• OIL RIG: Oxidation Is Loss, Reduction Is Gain.

• When compounds lose electrons, they lose energy; when they gain electrons, they gain energy.

Electron Carriers and Transport Chain

Electron carriers transfer electrons during cellular respiration.

• NAD+: Oxidized form; NADH: Reduced form.

• Electron transport chain (ETC): Series of proteins in the inner mitochondrial membrane; transfers electrons to oxygen, the final electron acceptor.

• Oxidative phosphorylation: ATP synthesis powered by ETC.

• Substrate-level phosphorylation: Direct transfer of phosphate to ADP.

Stages of Cellular Respiration

Cellular respiration occurs in several stages, each in specific cellular locations.

• Glycolysis: Cytoplasm; breaks glucose into pyruvate.

• Pyruvate oxidation: Mitochondrial matrix; converts pyruvate to Acetyl-CoA.

• Citric Acid Cycle (Krebs Cycle): Mitochondrial matrix; completes glucose oxidation.

• Oxidative phosphorylation: Inner mitochondrial membrane; produces most ATP.

Glycolysis

Glycolysis is the first step in glucose metabolism.

• Starting product: Glucose (6 carbons).

• Ending products: Two pyruvate (3 carbons each).

• Energy investment phase: Uses ATP.

• Energy payoff phase: Produces ATP and NADH.

• Net gain: 2 ATP, 2 NADH per glucose.

• Glycolysis is ancient: Evidence includes its presence in nearly all organisms, its anaerobic nature, and its cytoplasmic location.

Citric Acid Cycle (Krebs Cycle)

The citric acid cycle completes the oxidation of glucose.

• NADH formed: 3 per cycle.

• FADH2 formed: 1 per cycle.

• ATP formed: 1 per cycle (via substrate-level phosphorylation).

• CO2 released: 2 per cycle.

• Most energy: Stored in NADH and FADH2 for use in ETC.

Electron Transport Chain and ATP Synthesis

The ETC uses electron carriers to create a proton gradient, driving ATP synthesis.

• Oxygen: Ultimate electron acceptor; forms water.

• Proton gradient: Drives ATP synthase to produce ATP from ADP and inorganic phosphate.

• Final process: Chemiosmosis.

• ATP yield: Aerobic respiration yields ~30-32 ATP per glucose; fermentation yields 2 ATP per glucose.

Fermentation

Fermentation allows ATP production without oxygen.

• Regenerates NAD+ for glycolysis.

• End products: Lactate (animals), ethanol (yeast).

Organic Macromolecules in Respiration

Cells can use carbohydrates, fats, and proteins to make ATP.

• Carbohydrates: Glucose is primary substrate.

• Fats: Broken down into fatty acids and glycerol.

• Proteins: Amino acids enter metabolic pathways.

Regulation of Cellular Respiration

Cellular respiration is regulated by feedback mechanisms.

• Inhibitors: Molecules that slow or stop respiration (e.g., ATP, citrate).

• Activators: Molecules that promote respiration (e.g., ADP).

Summary Table: Comparison of Fermentation and Aerobic Respiration

Key Equations

• ATP hydrolysis:

• Cellular respiration:

Additional info: Some explanations and definitions have been expanded for clarity and completeness, based on standard biology curriculum.