Photosynthesis is the process by which plants, algae and some bacteria use light energy to turn carbon dioxide and water into glucose, releasing oxygen as a by-product. The overall equation is 6CO₂ + 6H₂O → C₆H₁₂O₆ + 6O₂, driven by light absorbed by chlorophyll. It happens in chloroplasts, in two linked stages. The light-dependent reactions, on the thylakoid membranes, use light to split water, release oxygen and make ATP and NADPH. The light-independent reactions, the Calvin cycle in the stroma, use that ATP and NADPH to fix carbon dioxide into sugar. The rate is limited by light intensity, carbon dioxide concentration and temperature.

This guide goes from the GCSE and middle-school version to the A level, IB and AP Biology version in the same order you would draw it. Stop where your course stops.

The equation, and what it does not tell you

The word equation is the version every course starts with:

carbon dioxide + water → glucose + oxygen (in the presence of light and chlorophyll)

And balanced:

6CO₂ + 6H₂O → C₆H₁₂O₆ + 6O₂

It is endothermic: energy goes in, from light, and is stored in the chemical bonds of glucose. Respiration is roughly the same equation run backwards, releasing that energy.

The equation is correct but misleading in one way. It looks as if the oxygen released comes from carbon dioxide. It does not. It comes from water. Experiments in the 1940s using water labelled with the heavy isotope oxygen-18 showed that the labelled oxygen came out as oxygen gas. The carbon dioxide's oxygen ends up in the sugar and in new water. Written more precisely, the equation is 6CO₂ + 12H₂O → C₆H₁₂O₆ + 6O₂ + 6H₂O: twelve water molecules supply all twelve oxygen atoms in the six O₂, and six new water molecules are made. Most exams accept the simple version; the precise one shows why. That one fact is the key to the two stages below, and it is a favourite exam question.

Where it happens

Draw this as a zoom, from the leaf to the chloroplast.

Chlorophyll absorbs mostly red and blue light and reflects green, which is why leaves look green. Other pigments, such as carotenoids, absorb some of the wavelengths chlorophyll misses.

Stage one: the light-dependent reactions

These happen on the thylakoid membranes and need light directly. Think of this stage as charging batteries.

1. Light is absorbed. Chlorophyll in two groups of pigments, photosystem II and photosystem I, absorbs light. The energy excites electrons in chlorophyll to a higher energy level, and they leave.
2. Water is split. To replace those electrons, water is split at photosystem II. This is photolysis: 2H₂O → 4H⁺ + 4e⁻ + O₂. The oxygen diffuses out of the leaf. This is where all of the oxygen comes from.
3. Electrons move down a transport chain. As they pass along carrier proteins in the membrane, their energy pumps hydrogen ions into the thylakoid space.
4. ATP is made. Hydrogen ions flow back out through the enzyme ATP synthase, which uses that flow to make ATP from ADP and phosphate. This is chemiosmosis, the same principle as in respiration.
5. NADPH is made. At photosystem I the electrons are excited again by light and finally passed, with hydrogen ions, to NADP⁺, reducing it to NADPH.

Outputs: oxygen (released), ATP and NADPH (passed to stage two). No sugar has been made yet.

Stage two: the Calvin cycle

These reactions happen in the stroma. They do not use light directly, which is why they are called light-independent, but they stop within moments in the dark because they run out of ATP and NADPH. Calling them the "dark reactions" is old and misleading; most courses now avoid it.

The cycle has three steps:

1. Fixation. Carbon dioxide combines with a five-carbon sugar, ribulose bisphosphate (RuBP). The enzyme that does this is RuBisCO, often said to be the most abundant protein on Earth. The six-carbon product is unstable and immediately splits into two molecules of a three-carbon compound, glycerate 3-phosphate (GP, also called 3-PGA).
2. Reduction. GP is converted into triose phosphate (TP, also called G3P or GALP) using energy from ATP and hydrogen from NADPH. This is where stage one's batteries are spent.
3. Regeneration. Most of the triose phosphate is used, with more ATP, to rebuild RuBP so the cycle can continue. The rest leaves the cycle.

For every three carbon dioxide molecules fixed, one triose phosphate leaves the cycle, at a cost of nine ATP and six NADPH. Two triose phosphates make one glucose, so it takes six turns of the cycle, fixing six carbon dioxide, to make one glucose molecule. That is the 6CO₂ in the equation.

What the plant does with the glucose

Glucose is not the end point. Exam questions about "uses of glucose" expect these:

• Respiration, to release energy for the plant's own cells, day and night.
• Starch, insoluble, for storage in leaves, seeds and tubers. This is why the iodine test turns a photosynthesising leaf blue-black.
• Sucrose, for transport around the plant in the phloem.
• Cellulose, to build cell walls.
• Amino acids and proteins, combined with nitrate taken up from the soil.
• Fats and oils, for storage, especially in seeds.

Limiting factors

The rate of photosynthesis is set by whichever factor is in shortest supply. Raise that one, and the rate goes up until something else becomes the limit.

The classic practical counts bubbles of oxygen from pondweed (Elodea or Cabomba) as a lamp is moved closer. Light intensity follows an inverse square law, so halving the distance quadruples the intensity. Commercial greenhouses use this knowledge directly, adding heat, lighting and extra carbon dioxide until the cost outweighs the extra growth.

The mistakes that lose marks

• Saying oxygen comes from carbon dioxide. It comes from water.
• Saying plants photosynthesise instead of respiring. Plants do both. They respire all the time; in bright light photosynthesis is faster, so there is a net release of oxygen.
• Putting the stages in the wrong place. Light-dependent on the thylakoids, Calvin cycle in the stroma.
• Saying the Calvin cycle happens at night. It happens in the light, using the products of the light-dependent reactions.
• Saying chlorophyll absorbs green light. It absorbs red and blue and reflects green.
• Writing "light energy is turned into glucose". Light energy is converted to chemical energy stored in glucose.

A scene plan for a four minute video

Photosynthesis is a process, so the video should follow something through it. Two good choices: a carbon atom from the air into a sugar, or a water molecule from the root to the oxygen bubble. Here is the carbon version, about 600 words:

1. A tree and the question: where does all this wood come from? (Mostly from the air.)
2. The precise equation, with twelve waters, drawn large, with the water's oxygen and the carbon dioxide's oxygen in different colours.
3. Zoom: leaf, cross-section, palisade cell, chloroplast, thylakoids and stroma labelled.
4. Light hits photosystem II; a water molecule splits; an oxygen molecule floats out through a stoma.
5. Electrons down the chain; hydrogen ions through ATP synthase; ATP and NADPH drawn as charged batteries.
6. A carbon dioxide molecule enters the stroma and meets RuBP and RuBisCO.
7. The Calvin cycle as a wheel: fixation, reduction (batteries spent), regeneration.
8. Six turns, one glucose; then starch, sucrose and cellulose branching off.
9. Back to the tree, now visibly built from carbon dioxide and light.

Scene 2's colour trick is the part students remember, and it heads off the most common wrong answer. For other ways to build a biology video around one travelling thing, see how to make a biology explainer video for a school assignment.

Make your own version

1. 1

   Decide your level

   GCSE and middle school stop at the equation, the leaf, uses of glucose and limiting factors. A level, IB and AP go on to photosystems, ATP synthase and the Calvin cycle. Check your specification first.

2. 2

   Pick the thing that travels

   A carbon atom from air to sugar, or a water molecule from root to oxygen bubble. Every scene should move it one step on.

3. 3

   Write one paragraph per scene

   Follow the scene plan above. Use the exact terms your course uses, such as GP or 3-PGA. About 600 words is four minutes at 150 spoken words a minute.

4. 4

   Paste the script into openCanviz

   Choose Keep my wording so every term is said as written, set a target length, and pick the whiteboard style so the chloroplast diagram builds as you explain it.

5. 5

   Check every label and arrow

   Pause on each scene. Drafted diagrams can put the Calvin cycle on the thylakoid or draw oxygen leaving from carbon dioxide. Fix any wrong label in the editor.

If you already have notes or a mind map on this topic, how to turn a mind map into a video works through photosynthesis from the map side, and how to turn class notes into a revision video covers using a video like this before an exam. Watching it is not revising it: pause before each scene and say what comes next.

Common questions

What are the two stages of photosynthesis? The light-dependent reactions on the thylakoid membranes, which split water, release oxygen and make ATP and NADPH, and the light-independent reactions (the Calvin cycle) in the stroma, which use ATP and NADPH to fix carbon dioxide into sugar.

Why is photosynthesis important? It is the source of almost all the chemical energy in food chains and of the oxygen in the atmosphere, and it removes carbon dioxide from the air. Nearly every food you eat traces back to it.

Do plants photosynthesise at night? The light-dependent reactions cannot run without light, and the Calvin cycle stops soon after because it runs out of ATP and NADPH. Plants keep respiring at night. Some desert plants (CAM plants, such as cacti) take in carbon dioxide at night and fix it into sugar during the day.

What is the difference between photosynthesis and respiration? Photosynthesis stores energy: carbon dioxide and water into glucose and oxygen, using light. Respiration releases it: glucose and oxygen into carbon dioxide and water. In plants, photosynthesis happens only in cells with chloroplasts and only in light; respiration happens in all living cells all the time.