Photosynthesis Lecture Notes

Introduction

Photosynthesis is the process by which plants produce their own energy.
Unlike animals, plants use sunlight, water, and carbon dioxide to produce energy.
Photosynthesis means 'building with light' - photo (light) + synthesis (to build).
Plants are crucial as they form the foundation of the food chain.

Light Reactions
- Location: Occur in the thylakoid membranes within chloroplasts.
- Function: Convert solar energy into chemical energy.
- Pigment: Chlorophyll absorbs sunlight. Exists in two forms, A and B.
  - Structure: Porphyrin ring helps in unique absorption of sunlight.
  - Function: Excites electrons to an excited state.
- Photosystems:
  - Photosystem II: Starts the electron transport chain, produces ATP via proton gradient.
  - Photosystem I: Converts NADP+ into NADPH.
- Overall Reaction: Requires sunlight and water, produces oxygen, ATP, and NADPH.
Calvin Cycle
- Location: Occurs in the stroma of chloroplasts.
- Nature: Anabolic process (building up), unlike the catabolic citric acid cycle.
- Phases:
  - Phase 1: Carbon Fixation
    - Enzyme: Rubisco captures CO2.
    - Reaction: CO2 is attached to ribulose bisphosphate (RuBP), forming an unstable 6-carbon compound, splitting into two 3-phosphoglycerate.
  - Phase 2: Reduction
    - Process: 3-phosphoglycerate receives a phosphate from ATP, is reduced by NADPH to become glyceraldehyde 3-phosphate (G3P).
  - Phase 3: Regeneration
    - Some G3P goes on to form glucose; some regenerates RuBP.
- Requirements: 9 ATPs and 6 NADPH for net 1 G3P output.

Light Reactions: Light + water → Oxygen + ATP + NADPH.
Calvin Cycle: CO2 + ATP + NADPH → Organic compounds (sugars).
Photosynthesis is essentially the reverse of cellular respiration, involving redox reactions where electrons flow oppositely.

Photosynthesis allows plants to produce their own food, supporting entire ecosystems by forming the base of the food chain.