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 reduce this barrier, allowing reactions to proceed faster.

• Enzyme-Substrate Interaction: Enzymes bind to specific substrates at their active site, forming an enzyme-substrate complex. The 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: In a reaction energy diagram, the activation energy is lower for enzyme-catalyzed reactions compared to uncatalyzed reactions.

Factors Affecting Enzyme Activity

The rate of enzyme-catalyzed reactions is influenced by 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. Two main types:

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

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

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

• Allosteric Activators: Bind to sites other than the active site, increasing enzyme 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) and inorganic phosphate.

• Energy Storage: ATP formation from ADP stores energy; ATP breakdown releases energy.

Overview 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 during glycolysis.

• Krebs/Citric Acid Cycle: Occurs in the mitochondrial matrix. Pyruvate is converted to Acetyl CoA, releasing CO2. The cycle runs twice per glucose molecule, producing NADH, FADH2, ATP, and CO2.

• Oxidative Phosphorylation/Electron Transport Chain (ETC): Occurs on the inner mitochondrial membrane. NADH and FADH2 deliver electrons to the ETC, which powers ATP synthesis. Oxygen is the final electron acceptor, forming water.

Mitochondrial Structure and Function

• Double Membrane: Creates the matrix and intermembrane space. The inner membrane has a large surface area for ETC and ATP production.

• ATP Production: Most ATP is produced during oxidative phosphorylation due to the high surface area of the inner membrane.

Electron Carriers

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

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

Fermentation and Anaerobic Respiration

• Aerobic Respiration: Requires oxygen; produces much more ATP.

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

• Fermentation: Recycles NADH to NAD+ to keep glycolysis running. Two types:

  • Alcohol Fermentation: Produces ethanol and CO2.

  • Lactic Acid Fermentation: Produces lactic acid.

Summary Table: Cellular Respiration Stages

Photosynthesis

Overview and Importance

Photosynthesis is the process by which organisms use sunlight to synthesize sugars from carbon dioxide and water. It is essential for life on Earth, providing energy and organic molecules for most living organisms.

• Reactants: CO2, H2O, sunlight

• Products: Glucose, O2

• Organisms: Plants, some protists, and bacteria perform photosynthesis.

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: Pores on leaf undersides for gas exchange (CO2 in, O2 out).

Stages of Photosynthesis

• Light Reactions: Occur in thylakoid membranes. Use pigments to absorb light, excite electrons, and produce ATP and NADPH.

• Calvin Cycle (Dark Reactions): Occurs in the stroma. Uses ATP and NADPH to convert CO2 into glucose.

Electron Carriers in Photosynthesis

• NADP+: Electron carrier that becomes NADPH after accepting electrons in light reactions.

Light Reactions Details

• Pigments: Multiple pigments allow plants to absorb a wider range of light wavelengths.

• Photosystems: Light excites electrons in photosystems, fueling electron transport chains that produce ATP and NADPH.

• 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 Comparison

• Cellular Respiration: ETC pumps H+ ions across the inner mitochondrial membrane, powering ATP synthase.

• Photosynthesis: Light reactions pump H+ ions across thylakoid membrane, powering ATP synthase.

Calvin Cycle Details

• Location: Stroma of chloroplast.

• CO2 Source: Enters leaves via stomata from the atmosphere.

• Process: Builds glucose from CO2 using ATP and NADPH.

• Energy Molecules: ATP and NADPH from light reactions; after energy is used, they become ADP and NADP+ and return to light reactions.

• Product: Glucose, which is used in cellular respiration.

Summary Table: Photosynthesis Stages

Photosynthesis and Cellular Respiration: The Matter Cycle

Interconnection of Processes

Photosynthesis and cellular respiration are interconnected, forming a cycle that sustains life on Earth. 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

• Photosynthesis:

• Cellular Respiration:

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