biol lecture 6

Photosynthesis and Control Systems

Overview of Photosynthesis and Control Systems

  • Photosynthesis is crucial for life on Earth, comprising various processes like:
    • Chloroplast Structure
    • Light Reactions
    • Calvin Cycle
    • Photorespiration
    • C4 and CAM Photosynthesis
  • Control Systems and Hormones affect plant responses to various stimuli, encompassing:
    • Overview of Control Systems
    • Discussion of 7 Hormones

Importance of Photosynthesis

  • Source of Oxygen in the atmosphere:
    • 50% of oxygen produced by terrestrial plants.
    • 50% produced by marine phytoplankton and macroalgae.
  • Photosynthesis is the first step in moving energy into the living world and is the source of all energy in ecosystems.

Photosynthetic Organisms

  • Different types of photosynthetic organisms:
    • (a) Plants (~40 μm)
    • (b) Multicellular Alga
    • (c) Unicellular Protists (Diatoms, Dinoflagellates) (~10 μm)
    • (d) Cyanobacteria (~1 μm)
    • (e) Purple Sulfur Bacteria (Use sulfur instead of water; do NOT produce O2)

Structure of Leaf and Chloroplast

  • **Leaf Anatomy and Structure: **
    • Mesophyll: Contains chloroplasts, where photosynthesis occurs.
    • **Cross-section of Leaf: **
    • Stomata: Openings for gas exchange (CO2 in, O2 out).
    • Veins: Transport nutrients and water.
    • Chloroplast: Contains thylakoids and stroma, primary site for photosynthesis.

Photosynthesis Process

Main Inputs and Outputs
  • Key compounds involved in photosynthesis:
    • Reactants:
    • Carbon Dioxide (CO2)
    • Water (H2O)
    • Products:
    • Glucose (C6H12O6)
    • Oxygen (O2)
  • The overall balanced equation for photosynthesis:
    ext{6 CO}2 + ext{6 H}2 ext{O} + ext{light energy}
    ightarrow ext{C}6 ext{H}{12} ext{O}6 + ext{6 O}2

Electron Transfer in Photosynthesis

Mechanism of Electron Transfer
  • Electron Transfer:
    • Electron Acceptor: Gets reduced by gaining electrons.
    • Electron Donor: Gets oxidized by losing electrons.
  • The formation of Glucose from water involves:
    • Water splitting via photolysis: ext{H}_2 ext{O}
      ightarrow 2 ext{H}^+ + 2 ext{e}^- + ext{O}
Two Stages of Photosynthesis

  1. Light Reactions

    • Occur in the thylakoid membranes of chloroplasts.
    • Convert light energy to chemical energy (NADPH and ATP).
    • Release O2 as a byproduct.
    • Process can be summarized as:
      ext{H}2 ext{O} + ext{light} + ext{NADP}^+ + ext{ADP} + ext{Pi} ightarrow ext{O}2 + ext{NADPH} + ext{ATP}

  2. Calvin Cycle (Dark Reactions)

    • Occurs in the stroma of chloroplasts.
    • Utilizes NADPH and ATP from light reactions to convert CO2 to sugar (G3P).
    • Returns ADP and NADP+ to light reactions.
    • Involves three main phases:
      • Phase 1: Carbon fixation (Rubisco enzyme adds CO2 to RuBP).
      • Phase 2: Reduction of 3-phosphoglycerate to glyceraldehyde-3-phosphate (G3P).
      • Phase 3: Regeneration of ribulose bisphosphate (RuBP).

The Role of Chlorophyll

Chlorophyll Absorption Spectra
  • Chlorophyll a:
    • Absorbs violet-blue and red light.
    • Reflects or transmits green light, which is why plants appear green.
  • Accessory Pigments: Broaden the absorption spectrum.
Photosynthesis Action Spectrum
  • Matches the combined absorption spectrum of chlorophyll a and accessory pigments.

Calvin Cycle Details

  • Calvin cycle requires:
    • NADPH: Provides reducing power for sugar production.
    • ATP: Supplies energy for reactions.

Photorespiration

  • Photorespiration occurs when Rubisco fixes O2 instead of CO2, leading to:
    • Consumption of O2, release of CO2, and no ATP production.
    • This reduces overall photosynthetic output.
  • Solution: PEP Carboxylase (PEPC) can be used in certain plants to fix CO2 efficiently.

C4 Photosynthesis

  • C4 Plants (~3% of species): Adaptation to hot and dry climates.
    • Distinct leaf anatomy (Kranz anatomy) with:
    • PEP Carboxylase: adds CO2 to phosphoenolpyruvate (PEP) in mesophyll cells.
    • Generated four-carbon compounds (oxaloacetate, malate).
    • Malate is converted back to CO2 in bundle-sheath cells for Calvin cycle.

CAM (Crassulacean Acid Metabolism)

  • CAM Plants (~10% of species): Another adaptation mechanism.
    • Fix CO2 at night reducing water loss, with stomata open at night and closed during the day.

Overview of Control Systems in Plants

  • Plants Response Mechanisms:
    • Suffering from herbivores leads to:
    • Physical defenses: Thorns, trichomes.
    • Chemical defenses: Toxic compounds (e.g., canavanine) that discourage herbivory.
    • Some plants recruit predatory insects to deter herbivores through signaling pathways.

Plant Hormones

  • Key Plant Hormones include:
    • Abscisic Acid (ABA): Stimulates stomatal closure in dry conditions.
    • Auxins: Promotes stem elongation, phototropism, and apical dominance.
    • Cytokinins: Promote cell division and modify apical dominance.
    • Ethylene: Controls fruit ripening and leaf abscission.
Hormone Action Mechanism
  • Hormones bind to specific receptors, causing a change in receptor conformation, stimulating production of relay molecules that trigger various responses in the plant.

Ethylene and Its Effects

  • Ethylene, known as a gas hormone, plays various roles:
    • Promotes leaf abscission, ripening of fruit, and root hair formation.
    • Discovered to cause premature ripening and is extensively produced for commercial purposes.