Photosynthesis Overview and Key Concepts

Chapter Overview

  • Photosynthesis: Process converting light energy to chemical energy.
  • Chloroplasts: Sites of photosynthesis in plants.

Key Points

  • Photosynthesis Equation:

6CO2 + 6H2O + ext{Light Energy}
ightarrow C6H{12}O6 + 6O2

  • Reactants: 6CO2 and 6H2O.
  • Products: C6H12O6 (glucose) and 6O2.
  • Redox Reaction: Water is split, electrons (e-) transferred to CO2 to form sugar.
  • OIL RIG:
    • Oxidation: loss of electrons
    • Reduction: gain of electrons

Anatomy Related to Photosynthesis

  • Leaf and Chloroplast Structure:
    • Mesophyll: Cells where chloroplasts are mostly found.
    • Stomata: Pores in leaf for gas exchange (CO2 enters, O2 exits).
    • Chlorophyll: Green pigment in thylakoid membranes, absorbs light.
  • Chloroplast Structure:
    • Outer membrane, thylakoids, granum (stack of thylakoids), and stroma.

Photosystems and Light Reactions

  • Photosystems: Complexes of proteins and pigments (chlorophyll a & b, carotenoids) that capture light energy.
  • Light Reactions:
    • Steps:
    1. Light absorption splits H2O, releasing O2.
    2. Excited electrons generate ATP and NADPH through electron transport chain (ETC).
    • Cyclic vs Noncyclic Electron Flow:
    • Noncyclic: Produces ATP, NADPH, and O2.
    • Cyclic: Produces ATP only (no O2 or NADPH) utilises PSI only.

Calvin Cycle

  • Occurs in Stroma: Uses ATP, NADPH, and CO2 to produce G3P (3-C sugar).
  • Phases:
    1. Carbon Fixation:
    • RuBP + CO2 → 3-Phosphoglycerate (catalyzed by Rubisco).
    1. Reduction:
    • Use ATP and NADPH to convert 3-Phosphoglycerate to G3P.
    1. Regeneration:
    • Use ATP to regenerate RuBP for the cycle.

C4 and CAM Plants Adaptations

  • C4 Plants:
    • Example: Corn, sugarcane.
    • Fix CO2 into a 4-C compound, reducing photorespiration.
    • Adaptation: Mesophyll cells fix CO2; bundle-sheath cells use it in Calvin Cycle.
  • CAM Plants:
    • Example: Cacti, pineapples.
    • Stomata open at night to fix CO2 into organic acids that release CO2 during the day for Calvin Cycle, minimizing water loss.

Importance of Photosynthesis

  • Produces glucose for respiration and builds plant structures like cellulose.
  • Generates oxygen (O2) as a byproduct, essential for aerobic life.
  • Global Importance: Photosynthesis is the foundation for food sources in ecosystems.