The big idea: Photosynthesis is how plants (and algae) make their own food using light.
The single most important idea is that photosynthesis is an energy conversion: it converts light energy into chemical energy.
The light energy from the Sun is captured and locked away in the bonds of glucose, a sugar the plant can later use for energy or building materials.
Photosynthesis converts light energy into chemical energy: carbon dioxide and water (raw materials) become glucose (chemical energy store) and oxygen.
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- Photosynthesis
- The process that converts light energy into chemical energy stored in glucose, using carbon dioxide and water as raw materials.
- Energy conversion (transformation)
- Changing energy from one form into another — here, light energy is changed into chemical energy.
- Chemical energy
- Energy stored in the bonds of a molecule (such as glucose); it can be released later by respiration.
- Chlorophyll
- The green pigment in chloroplasts that absorbs light energy to drive photosynthesis.
- Glucose
- The sugar made in photosynthesis; it stores the chemical energy that the light energy was converted into.
Why 'energy conversion' is the key phrase: Photosynthesis does not create energy — it converts it.
Light energy in → chemical energy (in glucose) out.
If an exam asks what photosynthesis does in terms of energy, this conversion is the answer.
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To build glucose, the plant needs raw materials and a source of energy.
The raw materials are carbon dioxide (taken from the air) and water (taken up by the roots). The energy source is light, absorbed by chlorophyll in the chloroplasts.
The word equation: Photosynthesis is summarised by a word equation:
carbon dioxide + water →(light, chlorophyll)→ glucose + oxygen
The arrow's label reminds you that light (absorbed by chlorophyll) is what drives the reaction — it is the energy input, not a raw material that ends up in the glucose.
Inputs on the left (light, CO₂, water), products on the right (glucose, O₂). The light energy is now stored as chemical energy in the glucose.
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- Raw materials
- The substances used up in photosynthesis: carbon dioxide and water.
- Products
- The substances made by photosynthesis: glucose and oxygen.
- Oxygen (O₂)
- A product of photosynthesis, released as a waste gas; it comes from the water that is split.
- Carbon dioxide (CO₂)
- A raw material of photosynthesis; its carbon is built into glucose.
Follow the energy: Trace the energy through the process:
Light energy arrives from the Sun → chlorophyll absorbs it → that energy is used to build glucose → the energy is now stored as chemical energy in glucose's bonds.
So the same energy changes form — from light to chemical — it is never made or destroyed, only converted.
| Stage | Form of energy | Where it is |
|---|---|---|
| Sunlight reaches the leaf | Light energy | In the sunlight / outside the cell |
| Chlorophyll absorbs the light | Light energy being captured | In the chloroplast |
| Glucose is built | Chemical energy | Stored in the bonds of glucose |
Goes IN
- Carbon dioxide (raw material, from the air)
- Water (raw material, from the roots)
- Light energy (the energy input, absorbed by chlorophyll)
Comes OUT
- Glucose (stores the chemical energy)
- Oxygen (released as a waste product)
- The light energy is now chemical energy in the glucose
A memory hook: 'CO₂ + water in, glucose + oxygen out.' And the energy: light in, chemical out.
The oxygen is the gas you see bubbling off a pondweed in the light — a sign that photosynthesis is happening.
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How this is tested: On Paper 2 an Outline or State question can ask what photosynthesis does in terms of energy — the marking point is the conversion of light energy into chemical energy (stored in glucose), with CO₂ and water as the raw materials and glucose and oxygen as the products.
On Paper 1B a short data question often shows an illuminated water plant releasing gas bubbles and asks you to deduce that photosynthesis is occurring and that the gas is oxygen.
A common trap is treating light as a raw material that ends up inside the glucose — it is the energy input, not a reactant that is built into the sugar.
IB-style question — describe photosynthesis as an energy conversion
Outline photosynthesis as a conversion of energy, naming the raw materials and products. [4]
How to score all four marks
- Name the energy conversion. Photosynthesis converts light energy into chemical energy.
- Say where the chemical energy ends up. The chemical energy is stored in glucose (in its bonds).
- Give the raw materials. The raw materials used are carbon dioxide and water.
- Give the products. The products made are glucose and oxygen. (Award 1 mark for each distinct point, up to 4.)
Final answer
Photosynthesis converts light energy into chemical energy stored in glucose; it uses carbon dioxide and water as raw materials and produces glucose and oxygen.
Why this scores full marks: Each sentence is a separate scoring point — the energy conversion, where the energy is stored, the raw materials, the products.
Writing only 'plants make food from sunlight' would earn far fewer marks because it misses the named raw materials, products and the light → chemical energy change.
| Goes IN (raw materials) | Comes OUT (products) | What happens to it |
|---|---|---|
| Carbon dioxide (CO₂) | — | Its carbon is built into glucose |
| Water (H₂O) | — | Split apart; supplies hydrogen and the oxygen released |
| Light energy | — | Absorbed by chlorophyll and converted to chemical energy |
| — | Glucose (C₆H₁₂O₆) | Stores the chemical energy in its bonds |
| — | Oxygen (O₂) | Released as a waste product (from the split water) |