ViewWiki
ExploreLife

The world, a little closer

Photosynthesis.

Plants build themselves out of air, water and light. Look inside a leaf to see the factory, then starve it of light or carbon and watch it slow.

Explore the scene Interactive article

01 / The experiment

Food from thin air.

Change the light and the carbon dioxide, then open the chloroplast to see where the light is caught.

Interactive scene

Preparing your experiment…

The explanation below is available while the scene loads.

One chloroplast, hugely enlarged. Glowing wave packets are light. The molecules use the usual chemistry colours (black carbon, red oxygen, white hydrogen) for carbon dioxide and water coming in, oxygen leaving, and the six-sided rings of sugar stored inside.A cartoon of the inputs and outputs. The molecules are drawn thousands of times too big for the chloroplast, and the real chemistry runs through dozens of steps and two linked cycles.

02 / The recipe

Materials, energy, products.

Air and water in, sugar and oxygen out, with light to pay for it.

Carbon dioxide and water01

Raw materials

Six molecules of carbon dioxide and six of water go in for every molecule of sugar that comes out.

Source
Air, through leaf pores; soil, through roots
Provides
Carbon, hydrogen, oxygen atoms
Without it
Nothing to build with
Sunlight02

Energy

Red and blue light are absorbed, green is reflected. The energy is used to pull water apart and drive the building steps.

Source
Caught by chlorophyll
Provides
The push to split water and fix carbon
Without it
No sugar, however much material
Sugar and oxygen03

Products

Sugar feeds the plant and becomes starch, cellulose and wood. Oxygen is the leftover, released for everything that breathes.

Source
The chloroplast
Provides
Food and building material; breathable air
Without it
No plant, and no animals either
The big idea

Plants build themselves from air and water, paying with light. The factory runs only as fast as its scarcest input.

6CO₂ + 6H₂O → C₆H₁₂O₆ + 6O₂

Building from air

A seed weighs almost nothing. The tree it becomes weighs tonnes. Where did the tonnes come from? Not from the soil, as people assumed for centuries; a tree grown in a pot leaves the soil almost as heavy as it found it. Nearly all of a tree’s substance is carbon, and the carbon came out of the air, one molecule of carbon dioxide at a time, stitched into sugar using the energy of sunlight. That process is photosynthesis, and every scrap of food you have ever eaten began with it.

The recipe

The ingredients are carbon dioxide from the air, taken in through tiny pores in the leaf, and water, drawn up from the roots. The energy is light. Inside the leaf’s cells, in green bodies called chloroplasts, the light is used to pull the water apart and to bolt its hydrogen onto carbon from the carbon dioxide, building sugar. The oxygen left over from the water is waste to the plant and is let out through the same pores. It is the oxygen you are breathing now.

Try it: set the light to Night and watch the sugar heap shrink. Then set it to Full sun but starve the leaf of carbon dioxide.

The green

Open the chloroplast and you find stacks of flattened discs. Their membranes are packed with chlorophyll, the pigment that gives leaves their colour. Chlorophyll absorbs red and blue light well and green poorly; the green is what bounces off into your eye. In autumn, when a tree stops making chlorophyll, the yellows and oranges that were there all along show through.

Whichever runs short

The factory can only go as fast as its slowest supply. In dim light there is not enough energy, however much carbon dioxide is on offer. In bright light with little carbon dioxide, the light is wasted. Growers of greenhouse tomatoes pump in extra carbon dioxide for exactly this reason, and a plant on a shady windowsill grows slowly however well it is fed. Temperature limits it too: the chemistry stalls when it is cold and breaks down when it is very hot.

The other half of the story

Photosynthesis stores energy in sugar. Every living thing, plants included, gets that energy back out by respiration: combining sugar with oxygen to release the energy and give off carbon dioxide and water. The two processes are mirror images, and between them they run the carbon of the whole living world round in a loop. The carbon in your breath was once in a leaf, and will be again.