The big idea
Plants, algae and some bacteria turn light energy into chemical energy stored in glucose.
The overall equation is 6CO2 + 6H2O + light -> C6H12O6 + 6O2.
Numbers to remember
2
stages: light-dependent reactions and the Calvin cycle
70%
of the oxygen we breathe comes from ocean phytoplankton
100-1,000
stomata per square millimetre on a typical leaf
Key facts
- Happens in the chloroplasts, which contain the green pigment chlorophyll.
- Chlorophyll absorbs mostly red and blue light and reflects green.
- Ocean phytoplankton supply most of the oxygen we breathe.
- Leaves exchange gases through stomata on their surface.
The two stages
| Light-dependent reactions | Calvin cycle |
|---|---|
| Where | Thylakoid membranes |
| Needs light | Yes |
| Inputs | Water, light, NADP+, ADP |
| Outputs | Oxygen, ATP, NADPH |
Step by step
- 1Light excites electrons
Light excites electrons in chlorophyll.
- 2Water is split
Water is split, releasing oxygen.
- 3Energy carriers are made
ATP and NADPH are made.
- 4Carbon is fixed
The Calvin cycle uses them to fix carbon dioxide into sugar.
Exam tip: always say where each stage happens and what it produces.