Lecture 9: Photosynthesis

General Biology - Laramie County Community College

Overview

• Purpose: Capture energy from the sun and convert it into chemical energy (sugar).
• Importance: Powers terrestrial life and the first few feet of ocean life.
• Photosynthesis produces glucose which is essential for cellular respiration to produce ATP for cellular work.

Introduction to Photosynthesis

• Inputs: Water, Carbon Dioxide, and Light.
• Outputs: Oxygen and Glucose.
• Process
  • Absorption of water by roots and carbon dioxide by leaves.
  • Chloroplasts containing chlorophyll convert CO2 and H2O into oxygen and glucose using sunlight.

Energy from the Sun

• Types of Energy: Potential, Kinetic, Radiation (travels in waves, measured by wavelength).
• Wavelength: Distance between wave crests or troughs.
  • Radio waves (long wavelength, low energy) to gamma rays (short wavelength, high energy).
• Radiation Interactions: Reflected, Transmitted, Absorbed.

Chlorophyll and Light Absorption

• Chlorophyll: Main pigment in plants, green in color, absorbs all light except green which is reflected.
• Structure: Hydrophilic head, hydrophobic tail; embedded in thylakoid membrane.
• Spectrophotometer: Used to determine light absorption spectrum; chlorophyll absorbs blue and red light.
• Other Pigments: Carotenoids and xanthophylls provide additional light absorption or protection from solar damage.

Light and Electrons

• Atoms & Electrons: Electrons in varying energy shells around the nucleus.
• Photosynthesis Process: Light energy excites and ejects electrons from chlorophyll, used to drive photosynthesis reactions.
• Fluorescence: If energy isn’t used by the plant, it can be emitted as heat or different light.
• Key Reaction: Light energizes electrons which leave chlorophyll to participate in the photosynthesis process.

Organisms Conducting Photosynthesis

• Land Plants: Use chloroplasts, exceptions are parasitic plants.
• Algae: Different pigments in addition to chlorophyll for photosynthesis.
• Cyanobacteria: Conduct photosynthesis using their entire cell, no chloroplasts.

Photosynthesis Equation

• Direction: Opposite of cellular respiration.
  • CO2 + H2O + Light → C6H12O6 + O2
• Types: C3 Photosynthesis (produces 3-carbon molecules).

Photosynthesis Stages

• Stages: Split into light reactions (energy) and carbon fixation reactions (sugar).
  • Light Reactions: Occur in thylakoid membrane.
  • Carbon Fixation: Occurs in stroma of chloroplast.

Light Reactions

• Purpose: Capture light energy to make ATP and NADPH.
• Photosystems: Proteins and chlorophyll (Photosystem II and I).
  • Electron Flow: Light excites electrons, travel through Electron Transport Chain (ETC).
  • Water Splitting: Provides replacement electrons, releases O2.

Carbon Fixation (Calvin Cycle)

• Purpose: Use ATP and NADPH to fix carbon and produce sugar.
• Process:
  1. RuBP + CO2 (via Rubisco) → 3-PGA → G3P
  2. Use ATP and NADPH.
  3. Regenerate RuBP.
• Output: G3P used to form glucose.
• Enzyme: Rubisco - most abundant enzyme, fixes CO2.

Variations in Photosynthesis

• Photorespiration Problem: Rubisco binding with O2 instead of CO2, wasting ATP and NADPH.
• Adaptations: Alternative mechanisms to avoid photorespiration.
  • C4 Photosynthesis: Separates initial CO2 fixation and Calvin cycle in space (different cells).
  • CAM Photosynthesis: Separates these processes in time (night vs day).

Summary

• Light Reactions: Convert light energy into chemical energy (ATP, NADPH).
• Calvin Cycle: Use chemical energy to build sugars.
• Overall Flow: Starts with sunlight, ends with production of ATP via photosynthesis and cellular respiration.
• Diagrams: Important tools to visualize processes; study recommended.