Energy and Metabolism

Key Energy Terms and Concepts

Understanding energy and its transformations is essential for studying metabolism in biological systems. Metabolism encompasses all chemical reactions in a cell, divided into catabolic (breaking down) and anabolic (building up) pathways.

• Endergonic vs. Exergonic Reactions: Endergonic reactions require energy input (ΔG > 0), while exergonic reactions release energy (ΔG < 0).

• Catabolism vs. Anabolism: Catabolism breaks down molecules, releasing energy; anabolism builds molecules, consuming energy.

• ΔG (Gibbs Free Energy): Indicates the spontaneity of a reaction.

• Spontaneous vs. Nonspontaneous: Spontaneous processes occur without energy input (ΔG < 0); nonspontaneous require energy (ΔG > 0).

• Energy Coupling: Using energy released from exergonic reactions to drive endergonic reactions.

• Entropy (S): A measure of disorder; increases in spontaneous processes.

• Energetically Favorable Processes: Processes with negative ΔG; products have lower energy and higher stability than reactants.

• ATP Hydrolysis: The breakdown of ATP to ADP and Pi releases energy:

• Energy Released in Catabolism: Used to drive anabolic processes and regenerate ATP.

Enzymes and Their Functions

Enzyme Structure and Activity

Enzymes are biological catalysts that speed up chemical reactions by lowering activation energy without being consumed.

• Function: Accelerate reactions by stabilizing the transition state.

• Effect on Free Energy: Enzymes do not change ΔG; they only lower activation energy.

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

• Specificity: Enzymes are highly specific for their substrates.

• Consumption: Enzymes are not consumed in reactions; they can be reused.

• Phosphorylation/Dephosphorylation: Addition/removal of phosphate groups, often regulating enzyme activity.

• Kinases: Enzymes that add phosphate groups.

• Phosphatases: Enzymes that remove phosphate groups.

Cellular Respiration and Fermentation

Overview and Energy Flow

Cellular respiration is a catabolic, exergonic process that converts glucose into ATP, using oxygen as the final electron acceptor.

• Equation for Cellular Respiration:

• Phases: Glycolysis, Pyruvate Oxidation, Citric Acid Cycle, Electron Transport Chain (ETC) & Chemiosmosis.

• Energy Capture: Electrons from glucose are transferred to electron carriers (NADH, FADH2), then to the ETC, generating a proton motive force for ATP synthesis.

• ATP Synthase: Enzyme that synthesizes ATP using the proton gradient.

Phases of Cellular Respiration

• Glycolysis: Occurs in the cytosol; glucose (6C) → 2 pyruvate (3C each), producing ATP and NADH.

• Pyruvate Oxidation: In mitochondrial matrix; pyruvate → acetyl-CoA, producing NADH and CO2.

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

• Electron Transport Chain & Chemiosmosis: Inner mitochondrial membrane; electrons transferred through complexes, pumping protons to intermembrane space, creating a gradient used by ATP synthase to make ATP.

• Integration: Products of one phase feed into the next (e.g., NADH from glycolysis and Krebs cycle to ETC).

• Electron Carriers: NADH and FADH2 produced in earlier phases deliver electrons to the ETC.

Redox Reactions and ATP Production

• Oxidation/Reduction: Oxidation = loss of electrons; reduction = gain of electrons.

• Oxygen's Role: Final electron acceptor in ETC; forms water.

• Dehydrogenases: Enzymes that remove hydrogen atoms (electrons) from substrates.

• Substrate-Level vs. Oxidative Phosphorylation: Substrate-level: direct transfer of phosphate to ADP; oxidative: ATP synthesis powered by ETC and chemiosmosis.

• Carbon Accounting: Glucose (6C) → 2 pyruvate (3C each); two cycles needed for complete oxidation.

• Key Glycolysis Steps: Step 3 (phosphofructokinase) and Step 10 (pyruvate kinase) are regulatory points.

• Proton Gradient: Highest in intermembrane space; ATP made in matrix.

• Electron Transport Chain & Chemiosmosis: Electrons flow through complexes, protons pumped, ATP produced as protons flow back via ATP synthase.

Fermentation

• Types: Lactic acid fermentation (produces lactate, no CO2 released), Alcohol fermentation (produces ethanol and CO2).

• Oxygen Requirement: Fermentation occurs without oxygen (anaerobic).

• When Used: When oxygen is scarce or absent.

Metabolic Integration and Regulation

• Integration: Carbohydrates, fats, and proteins can enter cellular respiration at various points.

• Anabolism Link: Intermediates from respiration can be used to build macromolecules.

• Feedback Inhibition: Enzymes (e.g., phosphofructokinase in glycolysis) are inhibited by ATP and citrate when products accumulate.

• Enzyme Regulation: Allosteric activators/inhibitors modulate enzyme activity.

• Electron Flow: Electrons (hydrogen) flow from glucose to NADH/FADH2 to ETC to oxygen.

Photosynthesis

Overview and Energy Concepts

Photosynthesis is an anabolic, endergonic process in which light energy is used to synthesize glucose from CO2 and H2O. It occurs in plants, algae, and some bacteria.

• Equation for Photosynthesis:

• Oxygen Source: O2 produced comes from water, not CO2.

• Carbon Incorporation: Carbon from CO2 is fixed into glucose.

• Energy Terms: Photosynthesis is anabolic (builds molecules) and endergonic (requires energy input).

• Organismal Differences: Plants perform both photosynthesis and respiration; animals only perform respiration.

Phases of Photosynthesis

• Light Reactions: Occur in thylakoid membranes; convert light energy to chemical energy (ATP, NADPH), release O2.

• Calvin Cycle (Dark Reactions): Occur in stroma; use ATP and NADPH to fix CO2 into glucose.

• Integration: ATP and NADPH from light reactions power the Calvin cycle.

• Electron Carrier: NADP+ is reduced to NADPH during light reactions; NADPH delivers electrons to the Calvin cycle.

• Light Activation: Light excites electrons in chlorophyll, initiating electron transport through photosystems II and I.

• Electron Flow: Electrons move from water through photosystems, generating a proton gradient for ATP synthesis (chemiosmosis).

• ATP Synthase: Produces ATP as protons flow back into the stroma.

• Comparison: Chemiosmosis occurs in both mitochondria (respiration) and chloroplasts (photosynthesis), but with different energy sources (food vs. light).

The Calvin Cycle

• Three Phases: Carbon fixation, reduction, regeneration of RuBP.

• Rubisco: Enzyme that fixes CO2 to RuBP in the first step.

• Final Output: G3P (glyceraldehyde-3-phosphate), which can be used to form glucose and other carbohydrates.

C4 and CAM Adaptations

• C4 Plants: Spatial separation of carbon fixation and Calvin cycle; adaptation to hot, dry environments.

• CAM Plants: Temporal separation; carbon fixation at night, Calvin cycle during the day; adaptation to arid conditions.

Photosynthesis and Cellular Respiration: Comparison

Example: In muscle cells, when oxygen is low, lactic acid fermentation provides ATP. In plants, C4 and CAM pathways allow photosynthesis under stress conditions.

Additional info: The above notes expand on the study guide by providing definitions, explanations, and context for all major concepts listed, ensuring a comprehensive review for exam preparation.