In-Depth Notes on Photosynthesis

Photosynthesis Overview

  • Photosynthesis: Process through which plants, some bacteria, and protistans use sunlight to produce glucose from carbon dioxide and water.
  • Word Equation: Carbon Dioxide + Water → Glucose + Oxygen
  • Chlorophyll: The green pigment involved in capturing sunlight; has various modifications across species.

Chlorophyll and Accessory Pigments

  • Chlorophyll a: The primary pigment in photosynthetic organisms (all have it).
  • Accessory Pigments: Absorb light wavelengths not absorbed by chlorophyll a.
  • Examples: Chlorophyll b, c, d, e (in algae), xanthophylls, carotenoids (e.g., beta-carotene).
  • Light Absorption:
  • Chlorophyll a absorbs mainly from violet-blue and reddish-orange light; little from green-yellow-orange.
  • Structure: Lipid-soluble hydrocarbon tail (C20H39 -) + flat hydrophilic head with magnesium.

Leaf Structure and Function

  • Leaves: Solar collectors with photosynthetic cells. Some plants may not have leaves.
  • Water Transport:
  • Water enters roots → transported to leaves via xylem vessels.
  • Gas Exchange:
  • Stomata (small openings) allow CO2 in and O2 out, regulated by guard cells to prevent water loss.
  • Example: Cottonwood trees lose about 100 gallons of water per hour in heat.

Chloroplast Structure

  • Thylakoids: Flattened sacs within chloroplasts, stacked in grana, containing photosynthetic chemicals.
  • Membranes: Chloroplasts have three membranes, creating three compartments (inner, outer, thylakoid).

Stages of Photosynthesis

  1. Light-dependent Reactions (in grana):
  • Chlorophyll absorbs light → Excited electrons are transferred to primary electron acceptor.
  • Photolysis: Water splitting produces O2, H+ ions, electrons.
  • ATP (via photophosphorylation) and NADPH (reduction of NADP+) generated.
  • Equation: 2H₂O → 4H⁺ + O₂ + 4e⁻
  1. Light-independent Reactions (Calvin Cycle) (in stroma):
  • Utilizes ATP and NADPH to convert CO₂ into carbohydrates (initially forming glyceraldehyde 3-phosphate).

Light-dependent Reactions Details

  • Photoexcitation: Absorption of light causes electrons to move to higher energy levels.
  • Photoionization: Electrons are freed from chlorophyll, forming positively charged chlorophyll ions.
  • Photosystems:
  • Photosystem II (PSII) (P680): Takes place first, absorbs light at 680 nm.
  • Photosystem I (PSI) (P700): Absorbs light at 700 nm.
  • Z Scheme: The process of electron transfer forms a Z shape.

Chemiosmosis and ATP Synthesis

  • Electrochemical Gradient: H⁺ ions pumped across thylakoid membrane, creating a concentration difference.
  • H⁺ ions diffuse back, powering ATP synthesis via ATP synthase.

Cyclic Phosphorylation

  • Involves only PSI and produces ATP without generating NADPH, recycling electrons back to PSI.

Calvin Cycle Steps

  • Carbon Fixation: CO₂ combines with RuBP (ribulose 1,5-biphosphate). Unstable six-carbon molecule breaks to form two GP (glycerate 3-phosphate) molecules.
  • Reduction: GP is phosphorylated to produce glycerate diphosphate, reduced to two molecules of glyceraldehyde 3-phosphate (GALP).
  • Output: Out of each pair of GALP, one is converted to glucose, the other regenerates RuBP.

Factors Affecting Photosynthesis

  • Light Intensity: As intensity increases, photosynthesis rate increases until limited by another factor.
  • Carbon Dioxide Concentration: Increased concentration raises the incorporation rate into carbohydrates.
  • Temperature: Optimal for enzyme activity; extremes can reduce the rate significantly.