OCR (A) Biology A-Level: Topic 5.2 - Energy for Biological Processes

Photosynthesis

• Definition: Light energy splits water molecules (photolysis), combining hydrogen with CO2 to produce glucose. Oxygen is released as a by-product.
• Rate Influencers: Affected by CO2 concentration, light intensity, and temperature.
• Chloroplast Adaptations:
  • Contains grana (stacks of thylakoid membranes) with photosynthetic pigments (e.g., chlorophyll).
  • Stroma surrounds grana; contains enzymes for the light-independent stage.
• Photosynthesis Stages:
  • Light-dependent Reaction:
    • Electrons excited by captured light energy.
    • Electrons travel down the electron transport chain, generating ATP (from ADP and inorganic phosphate) in photophosphorylation.
    • Produces reduced NADP (NADPH).
  • Light-independent Reaction (Calvin Cycle):
    • Uses ATP and NADPH to produce glucose.
    • Steps involve RuBP, GP, and TP:
      1. Carbon fixation: RuBP + CO2, catalyzed by RUBISCO.
      2. RuBP converts to two GP molecules.
      3. GP to TP using ATP and NADPH.
      4. TP used for glucose and essential organic compounds.
      5. Remaining TP reforms RuBP with ATP.

Factors Affecting Photosynthesis

• Limiting Factors: The factor in the lowest supply limits the reaction rate.
  • Light Intensity: Affects ATP and NADPH production, influencing Calvin cycle.
  • CO2 Concentration: Affects the speed of the light-independent reaction.
  • Temperature: Affects kinetic energy and enzyme activity, e.g., RUBISCO.

Respiration

• Aerobic Respiration: Splits respiratory substrate, releases CO2, and combines hydrogen with oxygen, releasing energy.
• Glycolysis:
  • Occurs in cytoplasm; glucose phosphorylated to produce pyruvate, NADH, and ATP.
• Link Reaction:
  • Pyruvate converts to acetyl-CoA; forms NADH and releases CO2.
• Krebs Cycle:
  • Acetyl-CoA oxidized; produces CO2, ATP, NADH, FADH2.
  • Each glucose turns the cycle twice.

Oxidative Phosphorylation

• Process:
  • Occurs in mitochondria's electron transport chain.
  • Reduced coenzymes (NADH, FADH2) transport electrons and hydrogen ions.
  • Electrons move through carriers, creating a proton gradient.
  • ATP synthesized using ATP synthase as H+ diffuses back into the matrix.
  • Oxygen is the final electron acceptor, forming water.
  • Theoretical ATP yield: 38 per glucose.
• Respiratory Substrates: Carbohydrates, lipids, proteins vary in energy release.
• Respiratory Quotient (RQ): CO2 produced/O2 consumed; varies by substrate.

Anaerobic Respiration

• Occurs with low oxygen concentration.
• ATP Production: Continues through glycolysis (net 2 ATP per glucose).
• In Mammals:
  • Pyruvate converts to lactate, reoxidizing NADH to NAD.
  • Lactate can revert to pyruvate when oxygen increases.
• In Yeast/Plants:
  • Alcoholic fermentation: Pyruvate decarboxylated to ethanal, then to ethanol, reoxidizing NAD.
  • Produces CO2; irreversible reaction.