Chapter 8: Photosynthesis

Pre-requisites

• Understanding of Chapter 7: Cellular Respiration is essential.
• Photosynthesis is the reverse of aerobic cellular respiration.

Anatomy of a Leaf

• Chloroplasts are prevalent in the top layer of leaves.
• Watch the animated video on leaf anatomy for better understanding.

Autotrophs vs. Heterotrophs

• Autotrophs: Organisms that make their own food.
  • Photoautotrophs: Use sunlight to make sugars (e.g., cyanobacteria, algae, plants).
  • Chemoautotrophs: Use chemical energy (e.g., bacteria around hydrothermal vents).
• Heterotrophs: Obtain food from the environment. Includes fungi, animals, and some protists.

Photosynthesis Overview

• Focuses on photoautotrophs.
• Occurs in chloroplasts, specifically in the thylakoid membranes and stroma.
• Converts solar energy into chemical energy in the form of glucose.
• Reactants: Carbon dioxide (CO2) and water (H2O).
• Products: Oxygen (O2) and glucose (C6H12O6).
• Consists of light-dependent reactions and the Calvin cycle.

Light-Dependent Reactions

• Occur in the thylakoid membranes.
• Use solar energy to split water, releasing O2, and generate ATP and NADPH.
• Main components:
  • Photosystem II: Splits water, releases O2, and excites electrons.
  • Electron Transport Chain: Uses excited electrons to pump hydrogen ions into the thylakoid lumen.
  • Photosystem I: Re-excites electrons for NADPH formation.
  • ATP Synthase: Uses hydrogen ion gradient to generate ATP.

Calvin Cycle (Light-Independent Reactions)

• Occurs in the stroma.
• Does not require light but uses ATP and NADPH from light reactions.
• Steps:
  • Carbon Fixation: CO2 is fixed to form a 6-carbon molecule.
  • Reduction: Produces glyceraldehyde-3-phosphate (G3P), a precursor to glucose.
  • Regeneration: Reforms molecules to enable the cycle to continue.

Pigments in Photosynthesis

• Chlorophyll: Main pigment, absorbs light primarily in blue and red regions.
• Accessory Pigments: Include carotenoids, which help absorb additional wavelengths.
• In fall, chlorophyll breaks down, and accessory pigments are visible.

Variations in Photosynthesis

• C3 Photosynthesis: Typical process, works well in moderate environments.
• C4 and CAM Plants: Adaptations for hot and dry environments.
  • CAM plants open stomata at night to conserve water.

Important Concepts

• Photosynthesis pre-dates aerobic respiration.
• Chloroplast-generated ATP is used exclusively in photosynthesis.
• G3P is a central intermediate for glucose and other carbohydrates.

Study Tips

• Watch recommended videos for visual understanding.
• Revisit Chapter 7 on cellular respiration for context.

Questions & Clarifications

• Bring questions to lab or contact the instructor for more help.

Note: Additional resources and animations are highly recommended for deeper understanding.