Photosynthesis Overview
Explain how photosynthesis converts light energy into chemical energy. Track reactants, products, chloroplast structures, and the role of pigments.
A lesson of Biology, in Energy Flow in Living Systems, about 10 minutes.
By the end of this lesson, you will be able to write and explain the overall photosynthesis equation, identify where photosynthesis occurs, describe the two stages, and explain how light energy is converted to chemical energy.
The Big Idea
Photosynthesis is the process by which plants, algae, and cyanobacteria convert light energy into chemical energy stored in glucose. It is the foundation of virtually all food chains — every organism that doesn't photosynthesize ultimately depends on organisms that do. It is also Earth's primary oxygen source.
The Overall Equation
$$6\text{CO}_2 + 6\text{H}_2\text{O} + \text{light energy} \to \text{C}6\text{H}{12}\text{O}_6 + 6\text{O}_2$$
Reactants: carbon dioxide (from air), water (from soil), light energy (from sun)
Products: glucose (stored chemical energy), oxygen (released to atmosphere)
Where Photosynthesis Occurs
In the chloroplast (of plant and algae cells):
- Thylakoid membranes (stacked into grana): site of light reactions
- Stroma (fluid surrounding thylakoids): site of the Calvin cycle
Chlorophyll (the main photosynthetic pigment) is embedded in thylakoid membranes.
Two Stages of Photosynthesis
Stage 1: Light Reactions (in thylakoid membranes)
- Input: light energy, water (H₂O), ADP, NADP⁺
- Output: ATP, NADPH, O₂ (released as byproduct)
- Process:
- Chlorophyll absorbs light → excites electrons
- Water is split (photolysis) → O₂ released, electrons replaced
- Electrons flow through the electron transport chain → ATP produced
- NADP⁺ accepts electrons and H⁺ → NADPH formed
Stage 2: Calvin Cycle (in stroma)
- Input: CO₂, ATP, NADPH
- Output: G3P (which becomes glucose), ADP, NADP⁺, Pi
- Process: CO₂ is "fixed" (incorporated into organic molecules); ATP and NADPH from light reactions power the reactions
The Role of Chlorophyll
Chlorophyll absorbs red and blue wavelengths most effectively; reflects green wavelengths (why plants look green).
Absorption spectrum: shows which wavelengths are absorbed.
Action spectrum: shows at which wavelengths photosynthesis proceeds most effectively.