Unit-II: Photosynthesis and Respiration
1. Photosynthesis: Light and Dark Reactions
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.
2. Photosystem I and II: Electron Transport
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.
[Image of non-cyclic photophosphorylation Z-scheme]
3. C3, C4, and CAM Pathways
Plants have evolved different mechanisms for carbon fixation based on their environment:
4. Factors Affecting Photosynthesis and Kranz Anatomy
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.
5. Respiration: Aerobic and Anaerobic
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.
6. Glycolysis, Krebs Cycle, and ETS
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.
7. Photorespiration and Source-Sink Relationship
- 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).
Exam Tip: For respiration, be ready to calculate RQ. If glucose is oxidized, RQ = 1.0. If fats are oxidized, RQ < 1.0. If organic acids are oxidized, RQ > 1.0.
Common Mistake: Confusing the locations of metabolic cycles. Remember: Glycolysis is in the cytoplasm, but Krebs Cycle and ETS are strictly in the mitochondria.
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.