Photosynthesis: how a leaf turns sunlight into food

How to use this simulation

1. Start with the Leaf view and change nothing: sunlight falls on the leaf, CO₂ drifts in through the stomata underneath, water climbs the stem in the xylem, O₂ leaves and the sugar meter on the right fills up.
2. Slide the light intensity up slowly and watch the “Limiting factor” reading: past a certain point more light does nothing, because now CO₂ is in short supply.
3. Raise the CO₂ concentration and the flat part of the top graph lifts. Then push the temperature towards 45 °C and watch the bottom graph collapse as the enzymes denature.
4. Switch to the Pondweed view: the readings panel shows how many oxygen bubbles rise per minute. Dim the lamp and count bubbles exactly as you would in the lab.
5. Switch to Inside a chloroplast: on the left the thylakoids split water and make ATP and NADPH; on the right the Calvin cycle in the stroma spends them to build glucose from CO₂.
6. Drag across either graph to set the light or temperature directly; the sliders do the same job.

From your lunch plate to a leaf

Think about what you ate for lunch. The rice, the bread, the potatoes, even the chicken, which ate grain. Trace any of it back far enough and you reach a green plant that made food out of almost nothing: water from the soil, carbon dioxide from the air and light from the sky. No shop, no kitchen. That piece of kitchen chemistry is photosynthesis.

Try a simple test at home. Leave a potted plant in a dark cupboard for a week and its leaves turn pale and yellow, and the plant droops. Put it back on a sunny windowsill and it greens up again. Without light, the plant’s kitchen is closed. And if you have ever looked closely at pondweed in a sunny pond or a fish tank, you may have seen tiny bubbles streaming from the leaves. Those bubbles are oxygen, made by photosynthesis.

The oxygen in the breath you just took was also released by a plant or by algae in the ocean. So photosynthesis is not just a plant topic: it is the starting point for the food and the air of almost every living thing on Earth. On this page we will see step by step how a leaf does it, what slows it down, and which questions examiners love to ask about it.

Starting from zero: what photosynthesis means

The word comes from Greek: “photo” means light and “synthesis” means putting together. So photosynthesis means building something using light. Plants put small, simple molecules (carbon dioxide and water) together into a larger, energy-rich one (glucose).

A definition you can write in an exam: photosynthesis is the process in which green plants, in the presence of sunlight and chlorophyll, make carbohydrates from carbon dioxide and water, releasing oxygen as a by-product. Light energy is converted into chemical energy and stored in the glucose.

Four things are needed: light (the energy source), chlorophyll (the green pigment that traps light), carbon dioxide (enters from the air through the stomata) and water (absorbed by the roots and carried up to the leaves in the xylem). Two things are made: glucose (food) and oxygen (released into the air). Most of the glucose is soon turned into starch for storage, which is why a leaf that has been photosynthesising turns blue-black with iodine.

Photosynthesis is an anabolic process, because it builds big molecules from small ones and stores energy. Respiration does the opposite: it breaks food down to release energy. You will find the two compared side by side further down.

Key terms in photosynthesis

These words come up in every question on this topic. Get them clear once and the rest is easy.

The photosynthesis equation and what it really says

In words: carbon dioxide + water → (light, chlorophyll) → glucose + oxygen. There are two common ways to write it with symbols. Many international syllabuses (GCSE, IGCSE) use the simplified 6CO₂ + 6H₂O form; the Indian NCERT and Bangladeshi NCTB textbooks use the full form with 12 water molecules on the left and 6 on the right.

Why keep water on both sides? Because all 12 water molecules on the left are really split, and every oxygen atom in the released O₂ comes from them. The 6 water molecules on the right are new ones, made during the Calvin cycle. In 1941 Ruben and Kamen fed plants water labelled with the heavy isotope ¹⁸O and found it in the oxygen given off; when they labelled the CO₂ instead, the released oxygen had none. So the oxygen of photosynthesis comes from water, not carbon dioxide.

The full equation balances by mass: on the left 264 g of CO₂ and 216 g of water make 480 g; on the right 180 g of glucose, 192 g of oxygen and 108 g of water also make 480 g. The table below does the sum molecule by molecule (taking C = 12, H = 1, O = 16).

Chlorophyll and chloroplasts: the leaf’s solar panels

Why are leaves green? Chlorophyll absorbs the red and blue parts of sunlight very strongly but reflects most of the green. That reflected green light is what reaches your eyes. Oddly, the colour we see is the one the plant uses least.

There are several pigments. Chlorophyll a is the main one and takes part directly in the light reactions. Chlorophyll b and the carotenoids (orange carotene, yellow xanthophyll) are accessory pigments: they catch extra light and pass the energy on to chlorophyll a. In autumn, when chlorophyll breaks down, the hidden yellow and orange pigments show through. Because magnesium sits at the heart of chlorophyll, plants in magnesium-poor soil get yellow leaves (chlorosis).

Each mesophyll cell, especially in the palisade layer just under the upper surface, holds dozens of chloroplasts. A chloroplast has a double membrane. Inside is the stroma, a fluid in which stacks of thylakoids (grana) sit, linked by stroma lamellae. Chlorophyll is embedded in the thylakoid membranes. The Inside a chloroplast view of the simulation draws exactly this.

The two stages: light-dependent and light-independent

Think of the chloroplast as a factory with two departments. The first uses light to “charge batteries”, making ATP and NADPH. The second spends those charged batteries to build sugar from CO₂. The first is the light-dependent stage; the second is the light-independent stage, or Calvin cycle.

Factors affecting the rate of photosynthesis

Anything that speeds photosynthesis up or slows it down is a factor. External factors come from the surroundings: light intensity, carbon dioxide concentration, temperature, and water supply. Internal factors belong to the plant: the amount of chlorophyll, the age and structure of the leaf, the number of stomata, and how much sugar has built up in the leaf.

Light: more light gives a faster rate, up to a point. Colour matters too: red light drives photosynthesis best, then blue, with green the least useful. Carbon dioxide: the air holds only about 0.04% CO₂, around 420 ppm today (older books say 0.03%). That is so little that on a bright day CO₂ is often the factor holding plants back.

Temperature: every step of the Calvin cycle is run by enzymes, and enzymes are very sensitive to temperature. In the cold they work slowly; warming speeds them up. Most plants photosynthesise fastest somewhere between about 20 and 35 °C, depending on the species. Much hotter than that and the enzymes lose their shape (denature), and the rate falls steeply. Water: a plant short of water closes its stomata to save it, which also shuts CO₂ out, so the rate drops.

Among internal factors, no chlorophyll means no photosynthesis at all. Young and very old leaves photosynthesise more slowly than mature ones. And if sugar is not moved away from the leaf fast enough, it builds up and slows the process down.

Limiting factors and Blackman’s law

In 1905 the British plant physiologist F. F. Blackman put it this way: when a process depends on several separate factors, its rate is limited by the factor in shortest supply. Imagine three friends carrying a sofa and one of them is slow. However quick the other two are, the sofa moves at the slow friend’s pace. Help that one and the whole job speeds up; helping the fast two changes nothing.

The simulation’s model follows that law exactly. Each factor sets a ceiling: the light ceiling is 0.30 × the light percentage, the CO₂ ceiling is 0.05 × the concentration in ppm, and the temperature ceiling is 35 × the enzymes’ efficiency. The rate is the lowest of the three ceilings, measured in µmol of CO₂ fixed per square metre of leaf per second, the unit plant physiologists use (a sunlit crop leaf peaks at roughly 20 to 30).

The table shows the rate as light increases at 25 °C, in normal air and with extra CO₂. In normal air the rate stops rising, because the limiting factor has switched from light to CO₂. With more CO₂ the plateau lifts higher. That is the classic limiting-factor graph examiners ask you to sketch and explain.

Temperature and enzymes: why there is an optimum

Below the optimum, every 10 °C rise roughly doubles the rate of an enzyme reaction (a temperature coefficient, Q₁₀, of 2). In the model the optimum is 30 °C. Above it the enzyme proteins start to lose their shape, and at 45 °C the model’s rate falls to zero.

The table shows only the enzyme ceiling: the fastest the temperature allows when light and CO₂ are plentiful. Notice how gently the rate climbs and how steeply it falls. In real plants, enzymes damaged by heat do not fully recover when things cool down again.

Experiments to try in the simulation

Press Reset before each experiment so that the glucose and bubble counts start from zero.

Classic practicals: starch, light, chlorophyll and oxygen

These practicals turn up in almost every exam. They all rest on one idea: where photosynthesis has happened, starch has been stored, and iodine solution turns starch blue-black. The table, computed from the simulation’s model, shows how the pondweed’s bubble count changes with lamp distance (25 °C, normal air).

C3 and C4 plants (advanced)

In most plants, including rice, wheat, potatoes and soya, the first stable product after CO₂ is fixed is the three-carbon 3-PGA, so they are called C3 plants. Some plants from hot, sunny places, such as maize, sugarcane and sorghum, first fix CO₂ into the four-carbon oxaloacetate (OAA): they are C4 plants, and the pathway is called the Hatch–Slack pathway.

C4 leaves have Kranz anatomy: a ring of bundle sheath cells around each vein. In the mesophyll the enzyme PEP carboxylase grabs CO₂ and carries it into the bundle sheath, where it is released at high concentration for RuBisCO and the Calvin cycle. This almost eliminates photorespiration, so C4 plants do better in heat and bright light. The price is energy: 18 ATP per glucose in C3 plants, 30 in C4 plants.

Solved problems

All problems use C = 12, H = 1, O = 16. The simulated leaf has an area of 50 cm².

Photosynthesis vs respiration

The two processes are mirror images: the products of one are the raw materials of the other. Plants respire all the time, day and night, but photosynthesise only in the light. In daylight photosynthesis runs much faster than respiration, so overall a plant gives out oxygen.

Why photosynthesis matters

Without photosynthesis, almost every food chain on Earth would collapse within a few years. Here are the main points.

• It starts almost every food chain: plants are the producers, herbivores eat plants and carnivores eat herbivores. Every grain of rice, lentil and apple is stored sunlight.
• It supplies the oxygen in the atmosphere and keeps the balance of oxygen and carbon dioxide.
• It removes CO₂ from the air, slowing global warming; forests and ocean algae are huge carbon stores.
• Coal, oil and natural gas are the remains of plants and plankton: sunlight captured by photosynthesis millions of years ago.
• It provides wood, cotton, jute, rubber and the raw materials for many medicines.
• It is practically the only way sunlight enters the living world as usable chemical energy.

Common mistakes

Check this list before you write your answer.

• Writing that the dark reaction happens at night. It happens in the day; it just does not use light directly.
• Saying the oxygen comes from carbon dioxide. It comes from water, split by light.
• Mixing the two equations, for example writing 12H₂O on the left without the 6H₂O on the right.
• Saying plants only respire at night. They respire all the time; in daylight photosynthesis simply outpaces it.
• Assuming any extra factor raises the rate. Only raising the limiting factor does.
• Describing “temperature” as limiting because it kills the plant. At low temperatures enzymes are simply slow; only high temperatures denature them.
• Saying leaves are green because they absorb green light. They reflect it.
• Heating ethanol directly over a Bunsen flame in the starch test. Always use a hot water bath.

Photosynthesis in real life

Tomato and cucumber growers in cold countries use greenhouses with lamps for extra light, heaters for warmth and burners or tanks that raise the CO₂ inside to around 1000 ppm. They apply Blackman’s law directly: work out which factor is limiting and spend money only on that one.

Pondweed in a fish tank adds oxygen for the fish during the day. At night, though, the plants use oxygen too, so a tank packed with plants and fish can run short by dawn, and the fish gasp at the surface. The same thing can kill fish in a pond overgrown with algae.

Farmers watch water closely: in a hot, dry spell plants close their stomata to save water, which also blocks CO₂, so photosynthesis and growth slow down. Irrigation therefore feeds the plant twice, with water and, indirectly, with carbon dioxide.

Scientists are working on artificial photosynthesis, using sunlight to split water into hydrogen fuel, and on engineering C4 traits into rice so that the same field can feed more people.

Exam corner

Examiners return to the same few ideas. Learn to draw and explain the limiting-factor graph: label the rising part “light is limiting” and the plateau “another factor (CO₂ or temperature) is limiting”, and explain that raising CO₂ lifts the plateau. For the pondweed practical, name the independent variable (light intensity or lamp distance), the dependent variable (bubbles per minute or volume of gas) and the control variables (temperature, CO₂ supply, same plant, time allowed to adjust).

In longer answers, use the precise words: chlorophyll absorbs light; water is split (photolysis); ATP and NADPH are made; CO₂ is fixed by RuBisCO; glucose is converted to starch, cellulose, sucrose, fats and proteins. Always give the balanced equation with state conditions (light and chlorophyll) written over the arrow.

One-screen revision summary

Read this the night before the exam.

• Definition: using light and chlorophyll to make glucose from CO₂ and water, releasing O₂.
• Full equation: 6CO₂ + 12H₂O → C₆H₁₂O₆ + 6O₂ + 6H₂O; simplified: 6CO₂ + 6H₂O → C₆H₁₂O₆ + 6O₂.
• Light-dependent stage (thylakoids): photolysis, O₂ released, ATP and NADPH made.
• Light-independent stage (stroma): Calvin cycle, CO₂ fixed, glucose made.
• One glucose costs 18 ATP and 12 NADPH (C3).
• Factors: light, CO₂, temperature, water; internal: chlorophyll, leaf age, stomata, sugar build-up.
• The factor in shortest supply sets the rate (Blackman).
• Practicals: iodine turns starch blue-black; foil tests light, variegated leaf tests chlorophyll, KOH tests CO₂, pondweed measures rate.

Frequently asked questions