How a leaf
makes its lunch.
with a leaf and a lens nearby.
Plants do something no animal can.
You eat to get the materials your body is built from. A plant does not eat — it has no mouth, it goes nowhere. And yet an oak tree, over a century, builds tonnes of solid wood. Where does all that stuff come from? The honest answer surprised people for two hundred years: almost none of it comes from the soil. Most of a tree comes out of thin air — the carbon dioxide gas drifting past its leaves — stitched together using the energy in sunlight.
That trick is called photosynthesis — from the Greek photo (light) and synthesis (putting together). Putting-together, with light. A plant takes three cheap, everyday ingredients — carbon dioxide from the air, water from the soil, and light from the sun — and assembles them into glucose, a sugar. Glucose is food. It is also the raw material a plant builds the rest of itself from.
There is a second reaction, every bit as important, running the other way. Every living cell — a leaf cell, a root cell, one of your muscle cells right now — takes glucose and releases the energy locked inside it, to power everything the cell does. That one is called respiration. By the end of today the two will fit together like a key in a lock.
It feels back-to-front, but it is true. Take a dry log and weigh it. Most of that mass is carbon — and that carbon arrived as carbon dioxide gas, pulled out of the sky by leaves and built into wood. The soil mainly gives a plant water and a pinch of minerals. The bulk of a plant is captured air.
In the 1640s a Flemish scientist, Jan Baptista van Helmont, grew a willow in a pot for five years. The tree gained 74 kilograms; the soil lost only a few grams. He concluded — half-right — that the tree was made of water. He had no idea the air around it was doing most of the work.
Carbon dioxide and water, in. Glucose and oxygen, out.
Scientists write a reaction as a word equation: the ingredients on the left, an arrow, the products on the right. Anything written over the arrow is needed for the reaction but isn't used up — here, that's the light and the green pigment that catches it. This is the one equation to know by heart.
Read it slowly. Two cheap things go in — carbon dioxide (a gas from the air) and water (drawn up from the roots). Out comes glucose, the sugar, and oxygen, which the leaf lets go as a waste gas. That waste oxygen is the air you are breathing right now. The whole reaction only runs when light falls on chlorophyll, the green pigment that gives leaves their colour.
Chlorophyll soaks up red light and blue light and uses their energy. It can't use green light, so it bounces it straight back — and that reflected green is the colour your eye sees. A leaf is green because green is the one colour it has no use for.
A single large tree can release enough oxygen in a day to keep two people breathing. Across the planet, more than half of all the oxygen made each year doesn't come from forests at all — it comes from microscopic floating plants in the ocean called phytoplankton.
A leaf is a solar panel, built cell by cell.
Everything about a leaf is shaped to catch light and let gases in and out. It is broad and flat, to give a wide surface to the sun. It is thin, so light reaches the cells inside. Packed near the top are cells crammed with chloroplasts — the green factories — and on the underside are tiny holes that let carbon dioxide in and oxygen out.
So what does the plant do with the glucose once it has made it? Three main things. It can respire it straight away for energy. It can join glucose molecules into long chains to build cellulose — the tough stuff cell walls and wood are made of. Or it can store it for later: in a potato or an onion bulb as starch, ready for when light is scarce.
Tap each card — what the plant does with its glucose:
The pores on a leaf's underside, the stomata, can open and shut. A single leaf may have a hundred thousand of them on each square centimetre, and the plant closes them on a hot dry day to stop itself drying out — the same way you'd close a window in a draught.
Now run the recipe backwards.
Glucose is a store of energy, like a charged battery. But the energy is no use locked inside the sugar — a cell has to release it. That is what respiration does, and the equation is almost exactly photosynthesis in reverse.
Look at the two equations side by side and you'll see the beauty of it. What photosynthesis builds, respiration takes apart. What one breathes out, the other breathes in. The oxygen a leaf releases is the oxygen your cells need; the carbon dioxide your cells release is the carbon dioxide a leaf needs. The two reactions are mirror images, and together they keep the whole living world turning.
Photosynthesis
- Only in cells with chloroplasts (green plant cells).
- Needs light — runs in the day.
- Stores energy, builds glucose.
- Takes in CO₂ and water; lets out oxygen.
Respiration
- In every living cell — plant, animal, fungus.
- Needs no light — runs all the time, day and night.
- Releases energy, breaks glucose down.
- Takes in oxygen; lets out CO₂ and water.
One thing to keep clear in your head: respiration is not breathing. Breathing is the muscle work that moves air in and out of your lungs. Respiration is the chemistry happening inside every single cell, releasing energy from food. You could hold your breath, but your cells never stop respiring — not for a moment.
Aerobic respiration — the kind that uses oxygen — takes place inside the mitochondria, the tiny powerhouses in your cells. A busy cell, like a muscle cell, is packed with them. A plant cell has them too: plants respire as you do, around the clock.
When you sprint and run out of breath, your muscles switch to anaerobic respiration — releasing a little energy from glucose without oxygen. It's a rushed, wasteful version that leaves behind lactic acid, the burn you feel. Yeast does the anaerobic trick too, but its leftover is alcohol — which is how bread rises and beer is brewed.
Photosynthesis, in motion.
You've met the word equation and the green chloroplasts on paper. Now watch it animated. As it plays, listen for the two ingredients going in — carbon dioxide and water — and the two products coming out. See if you can catch where the light energy actually does its job.
Name the four arrows.
Here is the cycle that links the two reactions — a plant on the left, an animal on the right, with gases passing between them. Four arrows are numbered. Pick a label, then drop it on its matching pin. Two of the chips name things that don't belong on this diagram at all — read before you place.
Match each label to its arrow
The missing product.
Where does the work happen?
Which cells respire?
A plant in a dark cupboard.
What can slow a leaf down.
A leaf can only photosynthesise as fast as its slowest ingredient allows. Give it bright light but no carbon dioxide and it stalls. The thing in shortest supply is called the limiting factor. For photosynthesis there are three to know:
The three factors
- Light — more light, faster reaction, up to a point.
- Carbon dioxide — usually scarce in air, so often the limit.
- Temperature — warmth speeds it up, but too hot and it falls off a cliff.
How they behave
- Raise the limiting one and the rate climbs.
- Keep raising it and a different factor becomes the limit.
- Temperature is two-sided — too cold is slow, too hot is harmful.
Temperature is the odd one out. With light or carbon dioxide, more is always better (until something else runs short). But photosynthesis is run by proteins called enzymes, and enzymes are fragile. Warm them and they work faster — until, past about 40 °C, the heat damages them and the whole reaction drops away sharply. That's why a heatwave can wilt a plant even in bright sun.
Name the limit.
Fresh one. A plant on a warm, bright windowsill is photosynthesising fast. Night falls and the rate drops almost to nothing. Which factor became the limit? One word.
Explain how photosynthesis and respiration fit together.
This is a "compare and link" question. The marker wants both reactions described, and then the connection drawn out — how the products of one are the ingredients of the other. Aim for a short paragraph. Use both word equations if it helps. Try to use the phrase "the products of one are the reactants of the other" somewhere, in your own way.
What goes in and out of each reaction · where each one happens · and the link between them. A few sentences is plenty, Florence. No rush.
A few thoughts on your answer, Florence
strong You wrote both equations and put each in the right place — photosynthesis in the chloroplasts, respiration in the mitochondria. That's the backbone of the answer, and you have it. Saying photosynthesis "stores" energy and respiration "releases" it is exactly the contrast the marker is after.
try this You described the two reactions clearly, then stopped right before the link. That's the doom-spiral move — getting the safe parts down and leaving the hard sentence out. The hard sentence is the one that scores most: the oxygen and glucose from photosynthesis are the very things respiration takes in. Write that line.
to add One sentence on timing would lift it further. Photosynthesis needs light, so it pauses at night; respiration carries on around the clock. Noticing that the two run on different schedules shows you understand them as living processes, not two equations to memorise.
Find the chloroplasts in a real leaf.
You've seen the moss-leaf photograph. Now find the green factories for yourself. The thinnest leaf you can get hold of will work — a moss leaf from the garden wall is ideal, because it can be only one cell thick, but a thin sliver from the edge of any soft leaf will do. You have a microscope. Use it.
Leaf cells under your microscope — what you need
Equipment — your microscope, a glass slide, a coverslip, tweezers, a drop of water, and a thin leaf. Moss growing on a damp wall or pot is the easiest, because its leaves are often a single layer of cells. No stain is needed — the chloroplasts are already green.
Method (short version)
- Pick a tiny leaf, or peel a thin sliver from the edge of a larger one. Smaller and thinner is better — light has to pass through it.
- Lay it flat on the slide and add one drop of water on top.
- Lower the coverslip gently from one edge, like closing a book, so no air bubbles get trapped.
- Place the slide on the stage, clip it, and start on the ×4 objective.
- Use the coarse focus to find the cells, then the fine focus for a sharp image. Rotate up to ×10, then ×40 if you can.
- Look inside a single cell. The small green dots scattered through the jelly are the chloroplasts — sometimes you can even watch them drift slowly round the cell.
Sketch one cell and mark the green dots. Count them roughly — how many chloroplasts in one cell? Note whether they sit still or move. And ask yourself the real question: why are there so many of them, all crowded near where the light comes in?
Aim · what you saw (drawings welcome on paper, words here) · what you think it means · one question you'd want to answer next. 150–200 words is plenty, Florence.
A few thoughts on your write-up, Florence
strong Your aim is clear and your observations are concrete — saying how many green dots you counted, or whether they moved, is exactly the kind of specific detail a write-up lives on. And you tied the green dots back to photosynthesis, which is the whole point of looking.
try this You wrote "the cells were green" and then "they had green bits in" — two goes at the same thing. That's the doom-spiral: unsure you've said enough, you say it twice. Cross one out, and spend the space on something new — are the chloroplasts spread evenly, or clumped to one side of the cell?
to add In your conclusion, link what you saw to why. The chloroplasts crowd near the lit surface because that's where the light is — structure following function. One sentence like that turns a description into a real conclusion.
Films and series for the green world.
Sit down with Dad for any of these. They make the quiet machinery of plants and life feel vast. Heavier titles flagged for a chat first.
The words from today.
- Photosynthesis
- How a plant builds glucose from carbon dioxide and water, using light: carbon dioxide + water → glucose + oxygen.
- Respiration
- How a cell releases energy from glucose: glucose + oxygen → carbon dioxide + water + energy. In every living cell, all the time.
- Chlorophyll
- The green pigment that absorbs light energy and starts photosynthesis. Sits inside chloroplasts.
- Chloroplast
- The green organelle where photosynthesis happens. Found in plant cells that meet the light.
- Mitochondria
- The cell's powerhouses, where aerobic respiration releases energy from glucose. In plant and animal cells alike.
- Limiting factor
- Whatever is in shortest supply and so holds back the rate of photosynthesis: light, carbon dioxide or temperature.
Two reactions, one living world.
You learned how a plant builds sugar out of air, water and light, and where it does it. You learned that every living cell runs the recipe backwards to release the energy again. You can name the three things that slow a leaf down. And you know the quiet truth underneath it all — the oxygen you breathe and the carbon dioxide a leaf needs are the same atoms, passed back and forth forever. Now go and find a chloroplast. Florence, this is biology.
Video · Khan Academy, "Photosynthesis" (high-school biology), embedded from YouTube under their standard terms.
Film recommendations are factual reference only — see each title's own copyright owner.