Photosynthesis is the process by which green plants transform light energy into chemical energy. It occurs in two main stages:

• Light-Dependent Reactions: These occur in the thylakoid membranes of chloroplasts. They involve the absorption of light and the production of energy-rich molecules, ATP and NADPH.
• Dark Reactions (Light-Independent): These occur in the stroma of chloroplasts. They utilize the ATP and NADPH produced in the light reactions to reduce Carbon Dioxide (CO2) into carbohydrates.

The light reaction utilizes two photosystems to capture light energy:

• Photosystem II (PS II): P680 reaction center; absorbs light energy to split water (photolysis), releasing oxygen and electrons.
• Photosystem I (PS I): P700 reaction center; receives electrons through an electron transport chain and reduces NADP+ to NADPH.
• Electron Transport: Can be Cyclic (only PS I involved, producing only ATP) or Non-Cyclic (both PS I and PS II involved, producing ATP, NADPH, and O2).
• Photophosphorylation: The process of ATP synthesis during the light reactions, driven by a proton gradient.

Plants have evolved different mechanisms for carbon fixation based on their environment:

The rate of photosynthesis is influenced by several external and internal factors:

• External Factors: Light intensity, CO2 concentration, and temperature.
• Kranz Anatomy: A specialized leaf structure found in C4 plants. It features two types of photosynthetic cells: Mesophyll cells and large Bundle Sheath cells arranged in a "wreath" like manner.

Respiration is the metabolic process by which cells break down organic molecules to release energy.

• Aerobic Respiration: Occurs in the presence of oxygen, completely oxidizing glucose into CO2, H2O, and a large amount of ATP.
• Anaerobic Respiration: Occurs in the absence of oxygen, resulting in partial breakdown of glucose (fermentation) and less energy.
• Respiratory Quotient (RQ): The ratio of CO2 evolved to O2 consumed during respiration.

The breakdown of glucose in aerobic respiration involves multiple sequential steps:

• Glycolysis: Occurs in the cytosol; breaks glucose into two molecules of pyruvate.
• Krebs (TCA) Cycle: Occurs in the mitochondrial matrix; oxidizes acetyl-CoA into CO2, producing NADH and FADH2.
• Electron Transport System (ETS): Occurs on the inner mitochondrial membrane; uses high-energy electrons from NADH/FADH2 to create a proton gradient.
• Oxidative Phosphorylation: The final stage where the energy from the proton gradient is used to synthesize ATP.

• Photorespiration: A seemingly wasteful process where RuBisCO binds with Oxygen instead of CO2, leading to CO2 loss; common in C3 plants under high light/heat.
• Source-Sink Relationship: The movement of carbohydrates from photosynthetic parts (sources like leaves) to non-photosynthetic or storage parts (sinks like roots and fruits).

Frequently Asked Questions

• Q: Why do C4 plants have Kranz anatomy?
  A: To physically separate the initial carbon fixation (mesophyll) from the Calvin Cycle (bundle sheath), allowing them to concentrate CO2 and avoid photorespiration.
• Q: What is the primary function of photosystems?
  A: To absorb light energy and convert it into chemical energy (ATP/NADPH) through electron transport.