Photosynthesis Lecture Notes

Introduction to Photosynthesis

• Cellular respiration: Converts food into ATP.
• Plants: Get energy directly from the sun instead of eating.
• Photosynthesis: Process by which plants produce their own nourishment using sunlight, water, and carbon dioxide.

Definition of Photosynthesis

• Etymology:
  • Photo: Greek for light.
  • Synthesis: To build.
• Plants use light to build larger molecules (e.g., glucose) for energy metabolism.
• Plants serve as the foundation of the food chain.

Stages of Photosynthesis

1. Light Reactions
2. Calvin Cycle

Light Reactions

• Location: Occur in chloroplasts (30-40 per plant cell).
• Structure of Chloroplasts:
  • Two membranes surround a fluid called stroma.
  • Thylakoids: Sacs suspended in stroma, often stacked in columns called grana.
  • Chlorophyll: Pigment molecule in thylakoid membranes.
    • Two forms: Chlorophyll A and Chlorophyll B (differ in functional groups).
    • Porphyrin ring: Absorbs sunlight; electrons become excited when absorbing photons.
• Excitation Transfer:
  • Electron excitation is passed from molecule to molecule like a pinball machine.
  • When excited, an electron is transferred to the primary electron acceptor, resulting in the production of Oxygen from water.

Electron Transport in Light Reactions

• Photosystem II:
  • Electrons flow through a series of compounds, generating ATP via a proton gradient and ATP synthase.
• Photosystem I:
  • Converts NADP+ into NADPH (not producing ATP).
• Overall reaction for light reactions:
  • Require sunlight and water; produce oxygen, ATP, and NADPH.

Calvin Cycle

• Location: Occurs in the stroma.
• Type of Reaction: Anabolic (builds larger organic molecules).
• Three Phases:
  1. Carbon Fixation:
     • Enzyme: Rubisco captures CO2 and attaches to ribulose bisphosphate (5 carbon sugar).
     • Forms an unstable 6-carbon molecule that splits into two 3-phosphoglycerate (3-PGA).
  2. Reduction:
     • Each 3-PGA receives a phosphate from ATP, reduced by NADPH to form glyceraldehyde 3-phosphate (G3P).
  3. Regeneration of RuBP:
     • Some G3P is used to regenerate ribulose bisphosphate (RuBP) to start the cycle again.
• Energy Requirements:
  • To produce 1 G3P, the cycle requires 9 ATP and 6 NADPH.

Summary: Photosynthesis Overview

• Light Reactions:

  • Inputs: Light and water.
  • Outputs: Oxygen, ATP, and NADPH.

• Calvin Cycle:

  • Inputs: Carbon Dioxide.
  • Outputs: Organic products (e.g., sugars).

• Comparison to Cellular Respiration:

  • Photosynthesis and cellular respiration are almost reverse processes, involving opposite electron flows and building vs. degrading sugars.