Photosynthesis

PHOTOSYNTHESIS

Overview of Photosynthesis

• Definition: Photosynthesis is a process that converts light energy into chemical energy stored in glucose (C₆H₁₂O₆).
• Importance: It provides organic molecules necessary for life on Earth and is the foundation for food chains.

1. Major Types of Organisms

• Autotrophs:

  • Organisms that sustain themselves without consuming other living beings.
  • Produce organic molecules from inorganic substances (e.g., CO2).
  • Types of Autotrophs:
  • Photoautotrophs: Use light as an energy source (e.g., plants, algae).
  • Prokaryotes: Certain bacteria can perform photosynthesis.

• Heterotrophs:

  • Organisms that obtain food by consuming other organisms.
  • Break down decaying organic matter.
  • Depend on autotrophs for food supply.

2. The Site of Photosynthesis

• Chloroplasts:
  • Main site of photosynthesis in plants, located in the mesophyll tissues of the leaves.
  • Function:
  • Absorb light and convert it into chemical energy.
  • Stomata facilitate gas exchange (O₂ enters, CO₂ exits).
  • Veins transport water from roots and sugar to roots.

3. Photosynthesis as a Redox Process

• Chemical Equation:
  • $6 ext{CO}<em>2 + 6 ext{H}_2 ext{O} + ext{light energy} ightarrow ext{C}_6 ext{H}</em>{12} ext{O}_6 + 6 ext{O}_2$
  • Oxidation and Reduction:
  • $ ext{CO}_2$ is reduced to glucose.
  • Water ($ ext{H}_2 ext{O}$) is oxidized to oxygen ($ ext{O}_2$).
  • The process is endergonic as it requires energy input (light).

4. Stages of Photosynthesis

4.1 Light Reactions

• Location: Thylakoid membranes in chloroplasts.
• Function: Convert solar energy to chemical energy (ATP and NADPH).
• Process:
  • Light energy excites electrons in chlorophyll.
  • Electrons are transferred through an electron transport chain, leading to ATP production through chemiosmosis.
  • Water molecules are split ($ext{H}_2 ext{O} <br /> ightarrow 2 ext{e}^- + 2 ext{H}^+ + ext{O}$) releasing oxygen as a byproduct.

4.2 Calvin Cycle

• Location: Stroma of chloroplasts.

• Function: Uses ATP and NADPH from the light reactions to synthesize glucose from carbon dioxide.

• Stages:

  • Phase I - Carbon Fixation:

  • CO₂ is attached to ribulose bisphosphate (RuBP) by the enzyme ribulose bisphosphate carboxylase/oxygenase (RuBisCO).

  • Forms an unstable 6-carbon compound that splits into two 3-carbon molecules (3-phosphoglycerate).

  • Phase II - Reduction:

  • ATP and NADPH are used to convert 3-PGA into glyceraldehyde-3-phosphate (G3P).

  • For every three cycles, one G3P molecule leaves the cycle to form glucose and other carbohydrates.

  • Phase III - Regeneration:

  • The remaining G3P molecules are used to regenerate RuBP, allowing the cycle to continue.

  • Requires additional ATP (3 ATPs per regeneration of 3 RuBP).

5. Photosynthetic Pigments

• Function: Capture light energy.
• Types of Pigments:
  • Chlorophyll a: The primary pigment involved in photosynthesis.
  • Chlorophyll b: Accessory pigment that helps capture additional light energy.
  • Carotenoids: Accessory pigments that provide photoprotection by absorbing excessive light energy.
• Light Absorption: Pigments differentially absorb light based on their structure, reflecting green wavelengths and absorbing red and blue wavelengths.

6. Alternative Mechanisms of Carbon Fixation

6.1 C4 and CAM Pathways

• C4 Plants:

  • Fix carbon dioxide into a 4-carbon intermediate before entering the Calvin cycle.
  • Adapted to high temperatures and low CO₂ conditions.
  • Example: Sugarcane and corn.

• CAM Plants:

  • Open stomata at night to fix CO₂ into organic acids, which are stored until daylight.
  • Stomata close during the day to minimize water loss.
  • Example: Cacti.

7. Life Dependence on Photosynthesis

• Photosynthesis is critical for the survival of most life forms on Earth as it provides the oxygen necessary for cellular respiration and forms the basis of the food web by producing energy-rich organic molecules.