Metabolism & Enzymes

Enzyme Function and Activation Energy

Enzymes are biological catalysts that accelerate metabolic reactions by lowering the activation energy required for those reactions. This enables cells to carry out complex biochemical processes efficiently.

• Activation Energy: The minimum energy required to initiate a chemical reaction. Enzymes lower this barrier, making reactions proceed faster.

• Enzyme-Substrate Interaction: Enzymes have a specific active site where substrates bind, forming an enzyme-substrate complex. This specificity is due to the unique shape and chemical environment of the active site.

• Enzyme Cycle: Enzymes bind substrates, catalyze the reaction, release products, and are then available to bind more substrate.

• Graph Interpretation: Enzyme-catalyzed reactions show a lower peak (activation energy) compared to uncatalyzed reactions.

Factors Affecting Enzyme Activity

The rate of enzyme-catalyzed reactions depends on several factors:

• Substrate Concentration: Increasing substrate concentration increases reaction rate until the enzyme becomes saturated.

• Enzyme Concentration: More enzyme molecules increase the reaction rate, provided there is enough substrate.

• Temperature: Higher temperatures generally increase reaction rates, but extreme heat can denature enzymes.

• pH: Each enzyme has an optimal pH; deviations can reduce activity or denature the enzyme.

Regulation of Enzyme Activity

Cells regulate metabolism by controlling enzyme activity through various mechanisms.

• Activators: Molecules that increase enzyme activity.

• Inhibitors: Molecules that decrease enzyme activity.

• Cofactors: Non-protein helpers (often metal ions) required for enzyme function.

• Allosteric Activators: Bind to sites other than the active site to enhance enzyme activity.

• Competitive Inhibitors: Bind to the active site, blocking substrate access.

• Noncompetitive (Allosteric) Inhibitors: Bind elsewhere, changing enzyme shape and reducing activity.

Cellular Respiration and Fermentation

ATP: The Energy Currency of the Cell

ATP (adenosine triphosphate) is the primary molecule used by cells to perform work, such as active transport and cell division.

• Structure: ATP contains three negatively charged phosphate groups, which store energy.

• ATP-ADP Cycle: Energy is released when the bond between the 2nd and 3rd phosphate is broken, forming ADP (adenosine diphosphate).

• Energy Storage and Release: ATP formation stores energy; ATP hydrolysis releases energy for cellular use.

Stages of Cellular Respiration

Cellular respiration is a metabolic pathway that breaks down glucose to produce ATP. It occurs in three main stages:

• Glycolysis: Occurs in the cytoplasm; breaks down glucose (6C) into two pyruvate (3C) molecules. Requires ATP to start, produces ATP and NADH. No CO2 is released.

• Pyruvate to Acetyl CoA: Pyruvate is modified in the mitochondria, releasing CO2 and forming Acetyl CoA.

• Krebs/Citric Acid Cycle: Occurs in the mitochondrial matrix; Acetyl CoA is broken down, releasing CO2, producing NADH and FADH2. Runs twice per glucose molecule.

• Oxidative Phosphorylation/Electron Transport Chain (ETC): Located in the inner mitochondrial membrane; uses NADH and FADH2 to drive ATP production. Oxygen is the final electron acceptor, forming water.

Summary Table: Stages of Cellular Respiration

Electron Carriers

• NADH and FADH2: Carry electrons from glycolysis and Krebs cycle to the ETC.

• NAD+ and FAD: Low-energy forms; pick up electrons and become NADH/FADH2.

ATP Production and Mitochondrial Structure

• Inner Membrane: Has a large surface area for ETC and ATP synthase, maximizing ATP production.

• ATP Synthase: Powered by H+ gradient across the inner membrane, produces ATP.

• Majority of ATP: Produced during oxidative phosphorylation.

Aerobic vs. Anaerobic Respiration

• Aerobic Respiration: Requires oxygen; oxygen is the final electron acceptor in the ETC.

• Anaerobic Respiration: Occurs without oxygen; produces much less ATP.

• Fermentation: Recycles NADH to NAD+ to keep glycolysis running. Types include alcohol fermentation (produces ethanol) and lactic acid fermentation (produces lactic acid).

Photosynthesis

Overview and Organisms

Photosynthesis is the process by which organisms use sunlight to build sugars from carbon dioxide and water. It is performed by plants, some protists, and bacteria.

• Reactants: CO2, H2O, sunlight

• Products: Glucose, O2

Chloroplast Structure

• Thylakoid: Membrane-bound compartment where light reactions occur.

• Granum: Stack of thylakoids.

• Stroma: Fluid surrounding thylakoids; site of Calvin cycle.

• Membranes: Outer and inner membranes enclose the chloroplast.

• Stomata: Openings on leaf undersides for gas exchange.

Stages of Photosynthesis

• Light Reactions: Occur in thylakoid membranes; use pigments to absorb light, produce ATP and NADPH.

• Calvin Cycle (Dark Reactions): Occurs in stroma; uses ATP and NADPH to build glucose from CO2.

Summary Table: Stages of Photosynthesis

Electron Carriers in Photosynthesis

• NADP+: Electron carrier; becomes NADPH during light reactions, delivers energy to Calvin cycle.

Light Reactions: Pigments and Electron Transport

• Pigments: Absorb light energy; multiple pigments allow absorption of a broader spectrum.

• Photosystems: Complexes in thylakoid membrane where light excites electrons.

• Electron Transport Chain: Excited electrons fuel ATP and NADPH production.

• Water: Splitting water replaces electrons lost from photosystem II; produces O2 as a waste product.

• ATP and NADPH: Produced in light reactions; used in Calvin cycle.

ATP Synthase in Photosynthesis and Respiration

• Hydrogen Ion Gradient: Both processes pump H+ across membranes to power ATP synthase.

• Comparison: In respiration, gradient is across inner mitochondrial membrane; in photosynthesis, across thylakoid membrane.

Calvin Cycle: Building Glucose

• Location: Stroma of chloroplast.

• CO2 Intake: Enters through stomata; provides carbon for glucose synthesis.

• Process: Calvin cycle builds molecules (anabolic).

• Energy Molecules: ATP and NADPH from light reactions drive the cycle; after use, revert to ADP and NADP+.

• Product: Glucose; used in cellular respiration.

Photosynthesis and Cellular Respiration: The Matter Cycle

Connection Between Processes

Photosynthesis and cellular respiration are interconnected, forming a cycle vital for life. The products of one process are the reactants of the other.

• Photosynthesis: Converts CO2 and H2O into glucose and O2 using sunlight.

• Cellular Respiration: Breaks down glucose and O2 to produce CO2, H2O, and ATP.

Cycle Diagram (Described)

CO2 + H2O + sunlight → Photosynthesis → Glucose + O2 → Cellular Respiration → CO2 + H2O + ATP

Key Equations

• Cellular Respiration:

• Photosynthesis:

Example: Plants produce glucose and oxygen via photosynthesis; animals use these for cellular respiration, producing carbon dioxide and water, which plants then use again.