Photosynthesis

Science

Definition

Photosynthesis is the process by which plants, algae, and some bacteria convert light energy, water, and carbon dioxide into glucose and oxygen. It occurs in chloroplasts and consists of the light-dependent reactions (in the thylakoid membranes) and the Calvin cycle (in the stroma).

How It Works

  1. Light energy is absorbed by chlorophyll in thylakoid membranes.
  2. Water molecules are split (photolysis), releasing oxygen and providing electrons.
  3. Light-dependent reactions produce ATP and NADPH using the electron transport chain.
  4. The Calvin cycle uses ATP and NADPH to fix CO₂ into G3P (glyceraldehyde-3-phosphate).
  5. G3P molecules are used to build glucose and other organic compounds.

Examples

  • Trees absorbing sunlight and CO₂ to produce wood, leaves, and oxygen
  • Algae in the ocean producing roughly half of Earth's oxygen through photosynthesis
  • Crop plants converting solar energy into food energy stored in grain
Key Fact

6CO₂ + 6H₂O + light energy → C₆H₁₂O₆ + 6O₂

Related Terms

Cell respiration Energy flow Plant structure Biochemistry

Study This Concept

Practice photosynthesis with free review games in these units:

AP Biology Unit 3: Cellular Energetics Biology Unit 9: Plant Biology