Introduction to Photosynthesis
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.
Fun Fact
The Two Stages
Photosynthesis occurs in two main stages:
- •Light-Dependent Reactions - Occur in the thylakoid membranes
- •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.
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.
Chlorophyll
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):
- •Photosystem I (PSI) absorbs light at 700 nm
- •NADP⁺ is reduced to NADPH
Key Insight
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.
Common Misconception
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:
- •Produces oxygen for aerobic organisms
- •Fixes carbon from the atmosphere into organic molecules
- •Provides energy that flows through food chains
- •Regulates climate by removing CO₂ from the atmosphere
