A sketchbook explainer · 7 min
Explained for ages 8–12
Your turn
Each lab is one part of the video. Tap, slide and see what happens.
Each lab isolates one part of the process. The models are simplified teaching sketches, not to scale.
Tap the buttons and slide the sliders.
Scrub through five years. The tree gains about 74 kg. How much does the soil lose?
Over five years the willow gained about 74 kg while the dried soil lost only 57 g. Where did the mass come from?
Slide the years. The tree gets big, but the soil hardly changes!
Switch off light, carbon dioxide or water. What happens to the sugar and oxygen?
Remove any one input (light, CO₂, water) and both outputs stop.
Turn off the sun, the air or the water. Does the leaf still make food?
Switch between the inside of the leaf and the inside of one chloroplast.
Stomata let CO₂ in; thylakoids host stage 1, the stroma hosts the Calvin cycle.
Slide through the rainbow. Which colours does the leaf keep, and which bounce into your eye?
Chlorophyll absorbs strongly in the blue and red and reflects much of the green. The curve is a simplified sketch.
Slide through the colours. Green bounces off the leaf into your eye!
Dim or brighten the light. Watch water split, oxygen drift away and the turbine spin.
Light drives water splitting at photosystem II, electron transport to photosystem I, proton pumping and ATP synthesis.
Make the sun brighter. The leaf makes more air and fills more batteries!
Cut the light or the carbon dioxide. Every 3 carbons in, 1 sugar block (G3P) comes out.
Fixation, reduction, regeneration: 3 CO₂ fixed per exported G3P. Without light-made ATP and NADPH the cycle stops.
Which slider actually speeds photosynthesis up? The scarcest ingredient sets the pace.
The rate is set by the scarcest input. Light saturates; closing stomata on a dry day cuts CO₂ supply.
A simplified model, not real measurements.
Plants make their own food. They do it with sunlight, air and water. This is called photosynthesis.
A leaf is like a little kitchen. It takes in a gas from the air called carbon dioxide. The roots bring up water from the soil. And sunlight gives the energy.
Out comes sugar. That's the plant's food, and it helps the plant grow. Out comes oxygen too. That's the air we breathe!
Leaves have a green helper called chlorophyll. It catches red and blue light. It bounces the green light away, into your eyes. So leaves look green.
Sunlight splits water apart inside the leaf. That's where the oxygen comes from.
Almost all the food we eat comes from plants, or from animals that ate plants.
Photosynthesis is the process plants, algae and cyanobacteria use to turn light energy into chemical energy: they take in carbon dioxide and water, capture energy from light, and make sugar while releasing oxygen.1
Written as a balanced chemical equation:
6 CO₂ + 6 H₂O + light energy → C₆H₁₂O₆ + 6 O₂
| Reactants (inputs) | Products (outputs) |
|---|---|
| Carbon dioxide (from the air, through the stomata) | Glucose and other sugars (the plant's food and building material) |
| Water (from the soil, through the roots) | Oxygen (released into the air) |
| Light energy (absorbed by chlorophyll) |
The equation is a summary. The real process takes dozens of steps, split into two stages.1
In plants, it happens mainly in the leaves, in a middle layer of cells called the mesophyll. Carbon dioxide enters, and oxygen leaves, through tiny pores called stomata, which sit mostly on the underside of the leaf. Each stoma is flanked by two guard cells that open and close it.1
Inside the mesophyll cells are chloroplasts. Each chloroplast holds stacks of disc-shaped thylakoids surrounded by a fluid called the stroma. Stage one happens in the thylakoid membranes; stage two happens in the stroma.1
The thylakoid membranes contain chlorophyll, a pigment that absorbs mostly blue and red light and reflects green light. The green light bounces back to our eyes, so leaves look green.3
The Calvin cycle doesn't use light directly, but it depends on the ATP and NADPH made in the light. Without light, it soon stops.1
The scarcest input sets the pace. More light speeds photosynthesis up only until the plant can't use it any faster. On hot, dry days, guard cells close the stomata to save water, which also cuts off carbon dioxide and slows photosynthesis.16
They are two halves of one cycle. Photosynthesis stores the Sun's energy in sugar and releases oxygen. Cellular respiration, which plants do too, uses oxygen to break sugar down and release that energy, giving off CO₂ and water.2
A plant uses the energy in light to turn carbon dioxide from the air and water from the soil into sugar, its food. Oxygen is released as a by-product.
The reactants are carbon dioxide and water, powered by light energy. The products are glucose (and other sugars) and oxygen.
In chloroplasts, mostly in the mesophyll cells of leaves. The light-dependent reactions happen in the thylakoid membranes and the Calvin cycle happens in the stroma.
The light-dependent reactions, which capture light energy, split water and make ATP and NADPH; and the Calvin cycle, which uses that ATP and NADPH to build sugar from carbon dioxide.
No. Humans and other animals are heterotrophs: we get our energy by eating plants, or animals that ate plants. Only plants, algae and some bacteria photosynthesize.
The light-dependent reactions need light, so they stop in the dark. The Calvin cycle doesn't use light directly, but it quickly runs out of the ATP and NADPH made in the light.
From water. Light energy is used to split water molecules, and the oxygen atoms are released as oxygen gas.