Lecture on Photosynthesis

Overview

• Photosynthesis may seem simple: sunlight, water, carbon dioxide = oxygen + glucose.
• The actual process is complex.

Sunlight and Energy

• Sun emits energy that travels as waves.
• Visible light (white light) is crucial for photosynthesis.
  • White light is a mix of ROYGBIV (red, orange, yellow, green, blue, indigo, violet).
  • A prism can separate white light into a rainbow.

Light Absorption

• Objects reflect or absorb light; we see reflected light.
  • Green leaves reflect green light.
  • White objects reflect all colors; black objects absorb all light.
• Photosynthesis mainly uses blue (~400nm) and red (~700nm) light.
  • Green light (~550nm) is least effective as it's mostly reflected.

Photosynthesis Definition

• Converts solar energy into glucose.
  • Reactants: Carbon dioxide, water, sunlight.
  • Products: Glucose, oxygen.

Organisms Performing Photosynthesis (Autotrophs)

• Not just plants; includes algae, phytoplankton, kelp, cyanobacteria.

Chemical Nature

• Endergonic reaction: requires energy input (sunlight).

Leaf Structure and Function

• Stomata: Pores for gas exchange (CO2 in, O2 out).

Chloroplast Structure

• Photosynthesis occurs in chloroplasts.
• Thylakoid Membranes: Site of light-dependent reactions.
  • Granum: Stack of thylakoids.
• Stroma: Fluid-filled area for light-independent reactions.

Photosynthesis Stages

Light-Dependent Reactions

• Occur in thylakoid membranes.

• Photosystems (PS1 & PS2): Contain chlorophyll.

• Steps:

  1. Chlorophyll absorbs sunlight, electrons get excited.
  2. Electron Transport Chain (ETC): Electrons move through thylakoid membrane, creating a charge.
  3. Hydrogen Ions (H+): Pulled into thylakoids, creating high concentration.
  4. Water Splitting: Replenishes electrons in PS2, produces O2.
  5. NADPH Formation: H+ and electrons form NADPH (hydrogen carrier).
  6. ATP Formation: H+ flow through ATP synthase, ATP is formed.

• Outputs: Oxygen, NADPH, ATP.

• No glucose is created yet.

Light-Independent Reactions (Calvin Cycle)

• Occur in the stroma.

• Ribulose Biphosphate (RuBP): Bonds with CO2 to form short-lived 6-carbon molecule.

• Steps:

  1. RuBP and CO2 bond to create unstable 6-carbon compound.
  2. Energy from ATP and NADPH breaks it into 3-carbon phosphoglycerate (PGA).
  3. Glucose Formation: Some PGAs bond to form glucose.
  4. Cycle Regeneration: Remaining PGAs help regenerate RuBP.

• Inputs: CO2, ATP, NADPH.

• Output: Glucose.

Summary

• Photosynthesis is a complex process involving multiple stages and structures within chloroplasts.
• Key molecules: ATP, NADPH, Glucose.
• Understand the roles of light-dependent and light-independent reactions.