What Is Photosynthesis? How Plants Make Food from Light
✦ Key takeaways
- Photosynthesis turns light, carbon dioxide, and water into glucose and oxygen.
- The simplified equation is: 6CO₂ + 6H₂O + light → C6H12O6 + 6O₂.
- It happens in chloroplasts, helped by the green pigment chlorophyll.
- This process is the source of oxygen and food for most living things.
Every breath you take and every bite you eat owes itself to a silent process happening inside green leaves under the sunlight. It is photosynthesis: the process by which plants, algae, and some bacteria make their own food from simple ingredients—light, air, and water. Without it there would be no oxygen to breathe, no food on our tables, and no fossil fuels beneath the ground.
The simple equation
Photosynthesis can be summed up in one elegant equation: a plant takes six molecules of carbon dioxide and six molecules of water, and with the help of light energy, turns them into one molecule of sugar (glucose) and six molecules of oxygen. It is written like this:
Personal Budget Planner
Plan income & expenses, see your savings rate & 50/30/20 — live charts.
6CO₂ + 6H₂O + light energy → C6H12O6 + 6O₂
In other words, the plant swallows a gas we treat as waste (carbon dioxide) and releases a gas we depend on for life (oxygen), while storing the Sun's energy in chemical bonds inside the sugar to use later as fuel for its growth.
Inputs and outputs
The table below shows what goes into the process, what comes out, and where each part comes from:
| Component | Type | Source or destination |
|---|---|---|
| Carbon dioxide (CO₂) | Input | Absorbed from the air through leaf pores (stomata) |
| Water (H₂O) | Input | Taken up by roots from the soil |
| Sunlight | Input (energy) | Captured by green chlorophyll |
| Glucose (C6H12O6) | Output | Sugar that stores energy as the plant's food |
| Oxygen (O₂) | Output | Released into the air for us to breathe |
How does it happen?
This chemistry takes place inside tiny structures in plant cells called chloroplasts, which contain the pigment chlorophyll that gives leaves their green color. Chlorophyll absorbs red and blue light efficiently and reflects green—which is why plants look green. The process has two stages: light-dependent reactions that capture light energy and split water to release oxygen, and the Calvin cycle, which uses that energy to build sugar from carbon dioxide.
Why does photosynthesis matter?
Photosynthesis is the foundation of most food chains on Earth. Plants are producers that make their own food; herbivores eat the plants, and predators eat the herbivores—so all that energy traces back to the Sun. Moreover, about half of the oxygen we breathe comes from microscopic phytoplankton in the oceans, not from land forests alone.
Consider a massive oak tree: most of its woody mass did not come from the soil, as you might assume, but was built mainly from carbon pulled out of thin air through millions of photosynthetic reactions over decades. It is literally a tree woven from air and light—the greatest silent chemical factory the planet has ever known.