Photosynthesis Overview and Stages

Photosynthesis Overview

• Photosynthesis is the process by which plants use sunlight, carbon dioxide, and water to create sugars (glucose), and release oxygen.
• The overall reaction is: CO₂ + H₂O + Light Energy → C₆H₁₂O₆ + O₂

Stages of Photosynthesis

• Photosynthesis occurs in two main stages: Light-dependent reactions and Light-independent reactions (Calvin cycle).

Light-Dependent Reactions

• Location: Thylakoid membranes within chloroplasts.
• Reactants: Water (H₂O), light energy, and chlorophyll.
• Products: Oxygen (O₂), ATP (adenosine triphosphate), and NADPH (nicotinamide adenine dinucleotide phosphate).
• Process: Light energy is absorbed by chlorophyll, splitting water molecules to release oxygen, generating energy carriers ATP and NADPH. During this, electrons move along an electron transport chain.
• Key Steps:
  • Light energy is absorbed by chlorophyll.
  • Water molecules are split, releasing oxygen.
  • ATP and NADPH are produced to carry energy.

Light-Independent Reactions (Calvin Cycle)

• Location: Stroma of chloroplasts.
• Reactants: CO₂, ATP, and NADPH.
• Products: Glucose (C₆H₁₂O₆), ADP, and NADP⁺
• Process: The energy carriers ATP and NADPH from the light-dependent reactions supply the energy to convert carbon dioxide into sugars.
• Key point: This stage is also known as the carbon fixation stage. It does not directly require light.
• Key Process: CO₂ is fixed into an organic molecule, and with input of ATP and NADPH, glucose is created.

Products of Light-Dependent Reactions

• ATP: Used in the Calvin cycle to drive the synthesis of glucose.
• NADPH: Also used in the Calvin cycle to contribute to the creation of glucose.
• Oxygen: Released as a byproduct.

Factors Affecting Photosynthesis

• Carbon Dioxide Concentration: Higher CO₂ levels usually increase the rate of photosynthesis until a certain point.
• Light Intensity: Increasing light intensity increases the rate of photosynthesis until a certain point.
• Temperature: Optimal temperature range for maximum photosynthesis rates exists. Above or below this range, the rate decreases.

CAM Plants

• Adapt to hot, dry environments.
• Keep their stomata closed during the day to prevent water loss, and open them at night to take in CO₂.
• Their Calvin Cycle is modified to store CO₂ during the night and use it during the day for photosynthesis.

Limiting Factors in Photosynthesis

• If any necessary factor (e.g., light intensity, CO₂ concentration, temperature) is below its optimal level, it can limit the rate of photosynthesis.
• The factor that is closest to its minimum is often called the limiting factor.