Photosynthesis Notes

13.1 Photosynthesis as an Energy Transfer Process

Light-Dependent Stage

• Energy Capture: Involves capturing light energy.
• Chemiosmosis: Generation of ATP.

Light-Independent Stage (Calvin Cycle)

• Carbon Fixation: Uses ATP from light reactions to fix carbon dioxide into sugars.

Pigment Regeneration

• Electron Replacement: From splitting of H2O in oxygenic photosynthesis.

Reaction Sites

• Light-Dependent: Occurs in the thylakoid membrane.
• Light-Independent: Takes place in the stroma, which is a colorless matrix surrounding the grana.

Chloroplast Pigments and Light Absorption

• Pigments: Absorb light; color is from reflected, not absorbed, light.
• Types of Pigments:
  • Chlorophylls: Chlorophyll a (green) - absorbs red, blue-violet.
  • Carotenoids: Carotene (orange) - absorbs blue-violet.
  • Xanthophyll (yellow).

Absorption and Action Spectra

• Absorption Spectrum: Graph showing light absorbance by pigments.
• Action Spectrum: Graph showing photosynthesis rate at different wavelengths.
• Comparison: More absorption and photosynthesis with blue-violet and red light, less with green-yellow.

Chromatography

• Purpose: Separates and identifies chloroplast pigments.
• Procedure Overview:
  • Spot sample on base line.
  • Use solvent for separation.
  • Calculate Rf value to identify samples.

Light-Dependent Stage of Photosynthesis

• Photolysis of Water: Happens in Photosystem II.
• Photophosphorylation:
  • Cyclic: Involves only PSI, no NADP reduction.
  • Non-Cyclic: Involves both PSI and PSII, NADP is reduced.

Light-Independent Stage of Photosynthesis (Calvin Cycle)

• Stages:
  1. Carbon fixation with RuBP and CO2, catalyzed by rubisco.
  2. GP reduced to TP using reduced NADP and ATP.
  3. Regeneration of RuBP from TP.
• Conversion: TP to glucose, amino acids, and lipids.

Investigation of Limiting Factors

• Limiting Factor: Greatest effect in reducing photosynthesis rate.
• Factors:
  • Light intensity and wavelength.
  • Temperature.
  • Carbon dioxide concentration.
• Glasshouse Enhancements:
  • Artificial light and heating.
  • Additional CO2 release.

Investigating the Effect of Light on Photosynthesis

• Hill Reaction: Investigates light-dependent reactions.
• Methodology:
  • Use of DCPIP as a redox indicator.
  • Measurement of rate by color change.

Adaptations for Photosynthesis

Structure and Function of Chloroplasts

• Chloroplasts: Biconvex discs, site of light-dependent reactions.
• Thylakoids: Stacks called grana, large surface area for light absorption.

C4 Plants

• Carbon Fixation: First stable product is a 4-carbon compound (oxaloacetate).
• Adaptations: Maximize carbon fixation, avoid photorespiration, suited for high light and temperature.

Summary

Photosynthesis involves capturing light energy and converting it into chemical energy in the form of ATP and reduced NADP, which is then used in the Calvin cycle to produce organic molecules. Understanding the structure of chloroplasts, the role of pigments, and the environmental factors affecting photosynthesis helps in optimizing conditions for plant growth, especially in controlled environments like glasshouses.