Cell Biology ATP and Metabolic Reactions

ATP: The Cell's Energy Currency

• ATP (adenosine triphosphate) is composed of adenine, ribose, and three phosphate groups.
• The unstable phosphate tail releases energy when broken by hydrolysis.
• Hydrolysis equation: ATP → ADP + Pi + Energy
• Phosphorylation: A phosphate group is added to another molecule, making it more reactive.
• ATP is constantly regenerated through phosphorylation of ADP, using energy from catabolic reactions.
• ATP couples exergonic (energy-releasing) reactions to endergonic (energy-requiring) reactions.

Metabolic Reactions

• Catabolic Reactions: Break down molecules, releasing energy (exergonic). Examples include glucose breakdown in cellular respiration.
• Anabolic Reactions: Build complex molecules, requiring energy (endergonic). Examples include protein synthesis and photosynthesis.
• Exergonic: Reactions that release energy; ∆G < 0; spontaneous.
• Endergonic: Reactions that absorb energy; ∆G > 0; non-spontaneous.

Photosynthesis

• Key Equation: 6CO₂ + 6H₂O + light energy → C₆H₁₂O₆ + 6O₂
• Stages:
  • Light-Dependent Reactions (Thylakoid Membranes): Convert light energy into ATP and NADPH.
  • Calvin Cycle (Stroma): Uses ATP and NADPH to fix CO₂ into sugars.
• Light-Dependent Reactions Inputs: Light, water, ADP, Pi, NADP⁺
• Light-Dependent Reactions Outputs: O₂, ATP, NADPH
• Light-Dependent Reactions Steps: Light excites electrons in Photosystem II, electrons pass through an ETC, pumping H⁺ ions into thylakoid lumen, electrons replenish Photosystem I, electrons reduce NADP⁺ to NADPH, ATP is generated via chemiosmosis.
• Calvin Cycle Inputs: CO₂, ATP, NADPH
• Calvin Cycle Outputs: G3P (3-carbon sugar), ADP, NADP⁺, Pi
• Calvin Cycle Steps: Carbon fixation (CO₂ binds to RuBP), Reduction (ATP and NADPH reduce 3-PGA to G3P), Regeneration (ATP regenerates RuBP from G3P).
• Noncyclic Electron Flow: Involves Photosystem II and I; produces ATP, NADPH, and O₂; replenishes PSII electrons from water.
• Cyclic Electron Flow: Involves Photosystem I only; produces ATP but not NADPH or O₂; helps balance ATP/NADPH ratio for the Calvin Cycle.
• Chemiosmosis in Photosynthesis: H⁺ gradient across the thylakoid membrane drives ATP synthesis. H⁺ flows through ATP synthase.

Photosynthetic Pigments

• Absorb light energy, exciting electrons to drive photosynthesis.
• Key Pigments: Chlorophyll a (main), chlorophyll b, and carotenoids (accessory pigments).

Role of Water in Photosynthesis

• Splits to provide electrons to Photosystem II.
• Produces O₂ as a byproduct.
• Supplies H⁺ ions for the H⁺ gradient.

Cellular Respiration

• Stages: Glycolysis, Pyruvate Oxidation, Krebs Cycle, Electron Transport Chain.
• Glycolysis: Glucose → 2 Pyruvate, 2 ATP (net), 2 NADH (cytoplasm).
• Pyruvate Oxidation: Pyruvate → Acetyl-CoA + CO₂ (mitochondria).
• Krebs Cycle: Produces 2 ATP, 6 NADH, 2 FADH₂, CO₂ (mitochondrial matrix).
• Electron Transport Chain: Uses NADH and FADH₂ to pump H⁺ ions, generating up to 34 ATP (inner mitochondrial membrane).
• Aerobic vs. Anaerobic: Aerobic requires O₂. Anaerobic uses fermentation which produces less ATP.

Fermentation

• Allows NAD⁺ regeneration in the absence of oxygen.
• Alcoholic Fermentation: Ethanol + CO₂
• Lactic Acid Fermentation: Lactate

Key Molecules

• NADH vs. FADH₂: NADH contributes more to ATP production than FADH₂ which enters the ETC later.
• Cytochrome: A protein in the ETC that helps transfer electrons and pump H⁺ ions.
• Oxygen’s Purpose: Final electron acceptor in aerobic respiration, forming water.

This quiz covers the fundamentals of ATP as the energy currency of the cell, detailing its composition, hydrolysis, and phosphorylation. It also explores metabolic reactions, distinguishing between catabolic and anabolic processes and their energy dynamics. Test your understanding of these key concepts in cellular biology.