3.5 bio notes

Cellular Respiration vs. Photosynthesis

  • Overview: Photosynthesis and cellular respiration are interconnected processes that can be viewed as opposites in terms of reactions and energy flow.

    • Respiration:
    • Reactants: Glucose and oxygen.
    • Products: Carbon dioxide and water.
    • Energy Change: Releases energy (negative ΔG).
    • Entropy Change: Increases entropy (disorder).
    • Photosynthesis:
    • Reactants: Carbon dioxide and water.
    • Products: Glucose and oxygen.
    • Energy Change: Absorbs energy (positive ΔG).
    • Entropy Change: Decreases entropy (order).

  • Energy Capture: Photosynthesis transforms light energy into the chemical energy stored in glucose.

Reactions in Photosynthesis and Cellular Respiration

  • Cellular respiration consists of two main processes:

    1. Glucose Breakdown:
    • Processes Involved: Glycolysis, pyruvate processing, and the citric acid cycle break glucose into carbon dioxide and hydrogen ions (NADH).
    1. Electron Transport Chain (ETC):
    • Electrons react with oxygen and hydrogen ions to form water.

  • Photosynthesis also involves two reactions:

    1. Light Capture:
    • In chloroplasts, chlorophyll splits water to produce electrons, hydrogen ions (NADPH), and oxygen gas (O2).
    1. Carbon Dioxide Capture:
    • Uses carbon dioxide and hydrogen ions to synthesize glucose.

Chloroplast Structure

  • Chloroplasts: Organelles with a double membrane.
    • Composed of an outer membrane and an inner membrane.
    • Contains stroma (aqueous interior) and thylakoids (folded sacs).
    • Thylakoid lumen: Space inside the thylakoids.

Light Capture in Photosynthesis

  • Photosystems:

    • Two large protein complexes (Photosystem I and II) containing chlorophyll pigments.
    • Chlorophyll can energize electrons when exposed to light.

  • Photosystem II Process:

    1. Light captured in the light harvesting complex.
    2. Energy transferred to reaction center, energizing electrons.
    3. Energized electrons removed by pheophytin and enter the photosynthetic ETC.
    4. H+ ions are pumped from the stroma into the lumen creating a proton gradient.

ATP Production

  • ATP Synthase:
    • Allows H+ ions to diffuse back into the stroma down their concentration gradient.
    • The energy from this process is used to synthesize ATP, similar to electron transport in cellular respiration, termed photophosphorylation.

Photolysis

  • Water-Splitting Reaction:
    • Photosystem II splits water to replenish lost electrons,
    • Producing 2 electrons, 2 hydrogen ions (for NADPH), and oxygen gas (O2).
    • This process is known as photolysis.

Photosystem I Process

  1. Pheophytin passes electrons to plastocyanin (PC) which delivers electrons to Photosystem I.
  2. Light strikes the light harvesting complex in PS I, re-energizing electrons.
  3. Electrons are transferred to ferredoxin, leading to two potential pathways:
    • Cyclic Electron Flow:
      • Recycles electrons back into the ETC for more ATP production.
    • Non-Cyclic Electron Flow:
      • Electrons are transferred to NADP+ reductase to produce NADPH from NADP+, H+, and electrons from photolysis.

NADP+ and NADPH

  • NADP+ vs NADPH:
    • NADP+ is the unenergized form; NADPH is the energized form functioning as an electron carrier in anabolic reactions.
    • Separation allows simultaneous catabolic (NAD+/NADH) and anabolic (NADP+/NADPH) reactions, maintaining distinct pools of electrons for cellular metabolism.

Study Guide Reminders

  • Be familiar with both cellular respiration and photosynthesis and their simultaneous reactions.
  • Understand chloroplast structure and the roles of photosystems.
  • Remember the step-by-step mechanism of Photosystem II and its components (light harvesting complex, reaction center, pheophytin, ETC, ATP synthase).
  • Know the significance and process of photolysis.
  • Grasp the function of Photosystem I and related components (plastocyanin, light harvesting complex, reaction center, ferredoxin).
  • Recognize the similarities and differences in the roles of NAD+/NADH and NADP+/NADPH and their importance in metabolic processes.