What Is Photosynthesis?

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.

Where Kids Get Stuck

The biggest misconception about photosynthesis is that plants get their food from the soil. Most of a plant's mass actually comes from carbon dioxide in the air, not nutrients in the ground. Soil provides water and small amounts of minerals, but the carbon, hydrogen, and oxygen that make up the plant's sugars and cellulose come from air and water. This is hard for children to believe — how can something solid come from a gas? — but it's true, and understanding it is a breakthrough moment.

Another common confusion is thinking that plants only do photosynthesis and animals only do respiration. In reality, plants do both. During the day, photosynthesis dominates — plants absorb CO₂ and release O₂. But at night (and even during the day), plants also perform cellular respiration, using oxygen to break down glucose for energy, just like animals do. The difference is that plants can produce their own glucose; animals cannot.

Students also sometimes confuse chlorophyll with chloroplast. Chloroplasts are the structures (organelles) inside plant cells. Chlorophyll is the green pigment inside chloroplasts that captures light. Think of the chloroplast as the factory and chlorophyll as the solar panel on its roof.

The Photosynthesis Equation

The full chemical equation for photosynthesis is:

6CO₂ + 6H₂O + sunlight → C₆H₁₂O₆ + 6O₂

In plain language: six molecules of carbon dioxide plus six molecules of water, powered by sunlight, produce one molecule of glucose and six molecules of oxygen. The equation is perfectly balanced — every atom on the left appears on the right, just rearranged. This is a beautiful example of conservation of matter, a core principle in chemistry.

What makes this reaction remarkable is its scale. A single leaf performs this reaction millions of times per second during daylight hours. Across the entire planet, photosynthesis converts roughly 100 billion tons of carbon into organic matter every year — the foundation of nearly all life on Earth.

Try This at Home

• Oxygen bubble experiment — Place a water plant (like elodea, available at pet stores) in a glass of water in sunlight. Watch tiny oxygen bubbles form on the leaves — photosynthesis happening in real time.
• Light deprivation test — Cover one leaf of a houseplant with aluminum foil for a week. Compare it to an uncovered leaf. The covered leaf will pale because it couldn't photosynthesize.
• Starch test — After the foil experiment, test both leaves with iodine solution. The uncovered leaf turns dark blue-black (starch present); the covered leaf won't.
• Leaf chromatography — Crush green leaves into rubbing alcohol and use a coffee filter strip to separate the hidden pigments. You'll see greens, yellows, and sometimes oranges.

For classroom ideas, see: Free Science Tools for the Classroom.

🌿 Explore the Photosynthesis Tool

Last reviewed: May 2026