Every time you eat a vegetable, fruit, or grain, you’re consuming energy that was captured from sunlight. Photosynthesis is the remarkable process that makes this possible — and understanding it unlocks a window into chemistry, biology, and ecology all at once.
The Big Picture
Photosynthesis converts carbon dioxide and water into glucose and oxygen, using light energy:
6CO₂ + 6H₂O + light energy → C₆H₁₂O₆ + 6O₂
This single reaction sustains nearly all life on Earth. Plants, algae, and some bacteria perform it; everything else — including us — depends on them for food and oxygen.
Where Does It Happen?
Photosynthesis takes place in chloroplasts, organelles found mainly in leaf cells. Chloroplasts contain a green pigment called chlorophyll, which absorbs red and blue light while reflecting green — which is why plants look green to our eyes.
Inside the chloroplast, there are two main structures:
- Thylakoids: stacked membrane discs where the light reactions occur
- Stroma: the fluid surrounding thylakoids where the Calvin cycle runs
Stage 1: The Light Reactions
In the thylakoid membranes, chlorophyll absorbs photons of light. This energy is used to:
- Split water molecules (releasing O₂ as a byproduct)
- Produce ATP (the cell’s energy currency)
- Produce NADPH (an electron carrier)
The oxygen released in this stage is what fills our atmosphere. Every breath you take owes its oxygen content to these light reactions.
Stage 2: The Calvin Cycle
Using the ATP and NADPH from the light reactions, the Calvin cycle in the stroma “fixes” carbon dioxide — incorporating it into organic molecules. After three turns of the cycle, one molecule of G3P (glyceraldehyde-3-phosphate) is produced, which plants use to build glucose and other organic compounds.
Why It Matters for Climate
Plants don’t just feed us — they’re the planet’s primary carbon sink. Forests absorb roughly 2.6 billion tonnes of CO₂ per year through photosynthesis. Deforestation disrupts this balance, contributing to rising atmospheric CO₂ levels.
Understanding photosynthesis is foundational to understanding climate science. Learn more with our Climate Science Explorer and Energy Sources tools.
Quick Reference
| Stage | Location | Inputs | Outputs |
|---|---|---|---|
| Light reactions | Thylakoid membrane | H₂O, light | O₂, ATP, NADPH |
| Calvin cycle | Stroma | CO₂, ATP, NADPH | G3P (→ glucose) |
Explore more chemistry with our Chemistry Reactions Lab and review the elements involved with the Periodic Table.