Photosynthesis Diagram: Interactive 2D and 3D

What is photosynthesis?

Photosynthesis is how plants, algae and some bacteria make their own food using the energy in light. The word means "putting together with light." A plant takes in carbon dioxide from the air and water from the soil, and inside its leaves it uses light energy to rearrange them into glucose, a sugar. Oxygen is left over and released into the air.

That one process feeds nearly all life on Earth. OpenStax Biology points out that the energy in almost every food chain traces back to photosynthesis, and the oxygen we breathe was put into the atmosphere by photosynthesizing organisms. When you eat an apple, a slice of bread or even a hamburger, you are eating energy that a plant once captured from sunlight.

Photosynthesis for kids: the short version

Think of a leaf as a tiny kitchen powered by the Sun:

• The ingredients: water (drunk up through the roots) and carbon dioxide (breathed in through tiny holes in the leaf).
• The power: sunlight, caught by a green substance called chlorophyll.
• The kitchen: chloroplasts, little green parts inside leaf cells.
• The food: glucose, a sugar the plant uses to live and grow.
• The leftover: oxygen, which goes back into the air for us to breathe.

Set the depth toggle above the diagram to Kid and every explanation uses words like these. Switch to Middle school or High school for more detail.

The photosynthesis equation

Scientists write the overall reaction like this:

The numbers balance the equation: six carbon atoms go in as carbon dioxide and come out in one glucose molecule. Notice that light is an input but not a substance; it supplies the energy that ends up stored in the sugar's chemical bonds. The equation also hides one surprise: the oxygen given off comes from the water, not from the carbon dioxide.

Inputs and outputs: the whole plant

The first view of the diagram shows where each ingredient comes from and where each product goes.

Chlorophyll absorbs red and blue light best and reflects green, which is why most leaves look green. In autumn many trees stop making chlorophyll, and the yellow and orange pigments that were there all along show through.

Inside a leaf: the leaf cross-section

A leaf is built for photosynthesis, and the second view slices one open. From top to bottom, OpenStax describes these layers:

1. Cuticle. A clear, waxy coat that keeps water in and lets light through.
2. Upper epidermis. One layer of see-through cells that protect the leaf.
3. Palisade mesophyll. Tall, tightly packed cells full of chloroplasts, right where the light is strongest. Most photosynthesis happens here.
4. Spongy mesophyll. Loose, rounded cells with air spaces between them, so carbon dioxide can reach every cell.
5. Veins. Bundles of xylem, which bring water in, and phloem, which carry sugar out.
6. Lower epidermis with stomata. Pores, each opened and closed by two guard cells, let carbon dioxide in and oxygen and water vapor out.

Stomata are a balancing act. Open, they let in carbon dioxide; but water escapes through them too. On hot, dry days many plants close their stomata to save water, which also slows photosynthesis.

Inside a chloroplast: light reactions and the Calvin cycle

The third view zooms into a single chloroplast. It has two outer membranes, stacks of flattened sacs called thylakoids (each stack is a granum), and a fluid around them called the stroma. Photosynthesis happens in two linked stages, which the Order it quiz walks through as five steps.

The light-dependent reactions (in the thylakoids)

1. Light is absorbed. Chlorophyll in the thylakoid membrane absorbs light, and the energy boosts electrons to a higher energy level.
2. Water is split. To replace those electrons, the chloroplast splits water molecules. Their oxygen atoms pair up and leave as O₂.
3. ATP and NADPH are made. As the electrons pass along a chain of proteins, their energy is used to make two energy carriers, ATP and NADPH.

At the high school level: light drives photosystem II, which splits water and passes electrons through an electron transport chain to photosystem I. Protons pumped into the thylakoid space flow back out through ATP synthase, which makes ATP. Photosystem I's electrons reduce NADP⁺ to NADPH.

The Calvin cycle (in the stroma)

4. Carbon dioxide is fixed. The enzyme RuBisCO attaches carbon dioxide to a five-carbon molecule called RuBP. ATP and NADPH then turn the products into a three-carbon sugar, G3P. More ATP rebuilds RuBP so the cycle can keep turning.
5. Sugar is made. G3P leaves the cycle and becomes glucose, sucrose, starch and cellulose.

The Calvin cycle doesn't use light directly, which is why it is sometimes called the light-independent reactions. It does depend on the ATP and NADPH from the light reactions, so in practice it runs during the day. Making the carbon for one glucose molecule takes six turns of the cycle, 18 ATP and 12 NADPH. The cycle is named for Melvin Calvin, who mapped it using radioactive carbon and won the 1961 Nobel Prize in Chemistry.

Photosynthesis vs cellular respiration

Plants make sugar by photosynthesis, then break it down for energy by cellular respiration in their mitochondria, the same way animals do. The table under the diagram compares them side by side. The key idea: photosynthesis stores energy and respiration releases it. Together they cycle carbon and oxygen between living things and the air. For the cell parts involved, see the plant cell diagram.

Why photosynthesis matters

• Food. Plants and algae are the producers at the base of almost every food chain. Explore how that energy moves from plants to animals in the food chain game.
• Oxygen. Photosynthesis by ocean phytoplankton and land plants keeps oxygen in the air.
• Carbon. Plants pull carbon dioxide out of the atmosphere and lock its carbon into wood, roots and soil. A tree's trunk is built mostly from carbon it took from the air, not from the soil.
• Water. Water drawn up through plants and released from leaves is part of the water cycle.

Easy photosynthesis experiments

• Bubbling leaves. Put a fresh spinach leaf or a sprig of the water plant Elodea in a clear glass of water and set it in bright sun. After a while, tiny bubbles of oxygen collect on the leaves. Move it into shade and the bubbling slows.
• Leaf in the dark. Cover part of a potted plant's leaf with foil for a few days. The covered patch stops making sugar and often turns paler. (Older students can test both halves for starch with iodine, with an adult's help.)
• Which color of light? Grow seedlings under different colored films. Plants under green film grow worst, because chlorophyll reflects most green light instead of using it.

Using this diagram at home or in class

• Grades 3 to 5: use the Inputs and outputs view at Kid depth. Have children say the word equation out loud, then label the photosynthesis worksheet.
• Grades 6 to 8: add the Leaf cross-section view and the vocabulary quiz. The labeled leaf cross-section diagram makes a good notebook page.
• High school biology: use the chloroplast view at High school depth and the Order it quiz, then print the labeled chloroplast diagram to annotate with photosystems and the Calvin cycle's three stages.

To compare full science programs by grade, see our best homeschool science curriculum guide.