Photosynthesis Lecture Notes

Introduction

• Transitioning from cellular respiration and fermentation to how organisms make food.
• Focus on photosynthesis, divided into two stages: Photosynthesis I and Photosynthesis II.

Types of Organisms

• Autotrophs
  • Self-feeders, do not consume other organisms.
  • Produce organic molecules from CO2.
  • Examples: Plants, algae, protists, prokaryotes, some cyanobacteria.
  • Use light as a source of energy.
• Heterotrophs
  • Consume other organisms to sustain themselves.

Photosynthesis in Plants

• Occurs in the chloroplasts found in green parts of plants.
• Chloroplasts located mainly in the mesophyll.
• Structures involved:
  • Stomata: Openings for CO2 and O2 exchange.
  • Veins: Transport water and sugars.
  • Mesophyll Cells: Contain chloroplasts (30-40 per cell).
  • Chloroplast Membranes: Outer, inner, and thylakoid system.
  • Chlorophyll: Pigments that capture light energy.

Photosynthesis Overview

• A redox reaction converting light energy, CO2, and water into glucose and oxygen.
• Opposite electron flow compared to cellular respiration.

Stages of Photosynthesis

Light Reactions

• Convert solar energy into chemical energy.
• Occur in the thylakoid membranes.
• Split water molecules, release O2 as a waste product.
• Electrons transferred to NADP+ to form NADPH.
• ATP is generated, but no sugars are made.

Calvin Cycle (Dark Cycle)

• Occurs in the stroma.
• Incorporates CO2 into organic molecules (carbon fixation).
• Reduces carbon into carbohydrates using NADPH and ATP.
• Does not require light directly.

Light Absorption by Chlorophyll

• Pigments: Absorb light of specific wavelengths, mainly chlorophyll a.
• Absorption Spectrum: Indicates violet-blue and red wavelengths are best for photosynthesis.
• Action Spectrum: Confirms effectiveness of absorption spectrum.

Photosystems

• Photosystem II (PS2) and Photosystem I (PS1): Capture light energy to drive electron transport.
• Process:
  • Light excites electrons, transferred through pigment molecules.
  • Electron transport chain (ETC) produces ATP.
  • Electrons eventually reduce NADP+ to NADPH.

Linear vs Cyclic Electron Flow

• Linear Electron Flow: Involves both PS2 and PS1, producing ATP and NADPH.
• Cyclic Electron Flow: Involves only PS1, produces ATP but no NADPH.

Chemiosmosis in Photosynthesis

• Similar to cellular respiration, but protons are pumped into the thylakoid space.
• Proton gradient drives ATP synthesis.

Summary

• Light reactions capture energy from light to produce ATP and NADPH.
• Calvin cycle uses these molecules to fix carbon into sugars.
• Photosynthesis involves complex interactions between light absorption, electron transport, and chemical synthesis.