What is Photosynthesis?

Photosynthesis is the biological process by which green plants, algae, and certain bacteria convert light energy into chemical energy stored in glucose. It is one of the most important biochemical processes on Earth.

The Two Stages

Photosynthesis occurs in two main stages:

1. •Light-Dependent Reactions - Occur in the thylakoid membranes
2. •Light-Independent Reactions (Calvin Cycle) - Occur in the stroma

A diagram showing the two stages of photosynthesis: light reactions in the thylakoid membrane producing ATP and NADPH, and the Calvin Cycle in the stroma using those products to fix CO2 into glucose.

Image description

The Chemical Equation

The overall equation for photosynthesis can be written as:

This tells us that carbon dioxide and water, in the presence of light, produce glucose and oxygen.

Light-Dependent Reactions

These reactions take place in the thylakoid membranes and require direct sunlight:

• •Photosystem II (PSII) absorbs light at 680 nm
• •Water molecules are split (photolysis): $2H_2O \rightarrow 4H^+ + 4e^- + O_2$
• •Photosystem I (PSI) absorbs light at 700 nm
• •NADP⁺ is reduced to NADPH

The Calvin Cycle

The Calvin Cycle uses the ATP and NADPH from the light reactions to fix carbon dioxide into organic molecules:

Step 1: Carbon Fixation
  CO₂ + RuBP → 2 molecules of 3-PGA
  (Catalyzed by RuBisCO)

Step 2: Reduction
  3-PGA → G3P (using ATP and NADPH)

Step 3: Regeneration
  G3P → RuBP (using ATP)

For every 3 molecules of CO₂ that enter the cycle, 1 molecule of G3P is produced. It takes 2 G3P molecules to make one glucose.

Why Photosynthesis Matters

"Photosynthesis is the most important chemical reaction on Earth - it provides the food and oxygen that sustain nearly all life."

• •Neil Campbell, Biology

Photosynthesis is foundational because it:

1. •Produces oxygen for aerobic organisms
2. •Fixes carbon from the atmosphere into organic molecules
3. •Provides energy that flows through food chains
4. •Regulates climate by removing CO₂ from the atmosphere