What Is Photosynthesis?
How plants turn sunlight into food — and why every living thing on Earth depends on it.
Plants Make Their Own Food
Animals eat food to get energy. But plants can't walk to a grocery store or hunt for dinner. Instead, they make their own food using something incredible: sunlight. This process is called photosynthesis, which literally means "putting together with light" (photo = light, synthesis = putting together).
Photosynthesis is one of the most important chemical reactions on Earth. It feeds the plants, which feed the animals, which feed other animals — including us. It also produces the oxygen we breathe. Without photosynthesis, there would be no food, no oxygen, and no life as we know it.
The Recipe for Photosynthesis
Photosynthesis needs three ingredients:
- Sunlight — the energy source that powers the whole reaction
- Water (H₂O) — absorbed by the roots and transported up through the stem
- Carbon dioxide (CO₂) — absorbed from the air through tiny pores on leaves called stomata
The plant combines these three ingredients to produce two outputs:
- Glucose (C₆H₁₂O₆) — a sugar the plant uses as food for energy and growth
- Oxygen (O₂) — released into the air as a byproduct (lucky for us!)
Where Does It Happen?
Photosynthesis happens inside chloroplasts — tiny structures found in plant cells, especially in the leaves. Chloroplasts contain a green pigment called chlorophyll, which is what makes leaves green. Chlorophyll's job is to capture sunlight energy — it absorbs red and blue light and reflects green light, which is why we see leaves as green.
Think of chlorophyll as a tiny solar panel inside every leaf cell. Millions of these solar panels work together to capture enough light energy to power the chemical reaction that turns water and CO₂ into sugar and oxygen.
Why Leaves Change Color in Fall
In autumn, many trees stop producing chlorophyll because there isn't enough sunlight to make photosynthesis worthwhile. As the green chlorophyll breaks down, other pigments that were hidden underneath become visible: yellow (xanthophyll), orange (carotene), and red (anthocyanin). The leaves were always these colors — you just couldn't see them behind all that green.
Why Photosynthesis Matters to You
Every time you eat — a sandwich, an apple, a piece of chicken — you're eating energy that originally came from photosynthesis. Plants captured sunlight and stored it as chemical energy in glucose. When you eat plants (or eat animals that ate plants), you're using that stored solar energy to power your body. Photosynthesis is the foundation of almost every food chain on Earth.
And then there's the oxygen. About 70% of the oxygen in Earth's atmosphere comes from photosynthesis by ocean phytoplankton (tiny plant-like organisms), and the rest comes from trees and land plants. Every breath you take exists because of photosynthesis happening right now, all around the world.
A single large tree can produce enough oxygen for 2–4 people to breathe for an entire year. It does this by performing photosynthesis in roughly 200,000 leaves, each one a tiny food factory powered by sunlight.
Last reviewed: April 2026