In-Depth Notes on Photosynthesis

Overview of Photosynthesis

  • Photosynthesis is the process by which plants, algae, and some bacteria capture sunlight energy.
  • It is essential for acquiring energy which fuels nearly all living cells.
  • Only about 1% of available sunlight energy is utilized.

Components of a Leaf

  • Layers of a Leaf: Light must pass through the following layers:
    • Cuticle: Outermost waxy layer, provides protection.
    • Epidermis: One-cell thick layer acting as a protective skin.
    • Mesophyll: Contains multiple layers of chloroplasts where photosynthesis occurs.

Chloroplast Structure

  • Chloroplasts are found in leaf cells and contain the following:
    • Thylakoids: Internal membranes stacked in columns called grana.
    • Stroma: Semi-liquid substance surrounding the thylakoids.

Function of Photosystems

  • Photosystems within the thylakoid membranes are essential for capturing sunlight.
    • Chlorophyll is the primary pigment, absorbing blue and red light.
    • Secondary pigments (carotenoids) assist by absorbing wavelengths not captured by chlorophyll.

Stages of Photosynthesis

  • Photosynthesis consists of three main stages:
    1. Capturing energy from sunlight.
    2. Using captured energy to produce ATP and NADPH.
    3. Using ATP and NADPH to synthesize carbohydrates from CO2 (Calvin Cycle).

Light-Dependent Reactions

  • Definition: Occurs in the presence of light, producing ATP and NADPH.
    • Five key stages:
    1. Capturing Light: Light is absorbed by chlorophyll, energizing electrons.
    2. Exciting Electrons: Electrons are transferred to an electron acceptor.
    3. Electron Transport: Excited electrons move through an electron transport chain, pumping protons.
    4. Making ATP: Protons diffuse through ATP synthase, phosphorylating ADP to ATP.
    5. Making NADPH: Electrons re-energized and used to form NADPH.

Photosystem II and Water Splitting

  • Photosystem II: Captures light and produces O2 by splitting H2O for electron replenishment.
    • Reaction center involves multiple proteins facilitating electron transfer.

Electron Transport System (ETS)

  • Composed of proteins in the thylakoid membrane, it transports electrons following their excitation.
  • Protons are pumped creating a gradient, leading to ATP production via chemiosmosis.

Chemiosmosis

  • The build-up of protons creates a concentration gradient, enabling ATP production through ATP synthase as protons re-enter the stroma.

Photosystem I

  • Receives electrons from ETS, absorbs light, and produces NADPH for the Calvin Cycle.

Light-Independent Reactions: The Calvin Cycle

  • Calvin Cycle: Operates using products from light-dependent reactions to synthesize glucose from CO2.
    • Must turn six times to generate one glucose molecule.
    • Utilizes ATP for energy and NADPH for hydrogen atoms and electrons to form carbohydrates.