Carbon dioxide to glucose at Higher Level: The same statement as SL. At HL, 3.3.11 shows water being split in photosystem II and 3.3.15-3.3.16 show carbon dioxide being fixed and reduced in the Calvin cycle.
Practise this as you read
- Write the word equation.
- State where the oxygen comes from.
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To build glucose, the plant needs raw materials and a source of energy.
What goes in
- Carbon dioxide, taken from the air (raw material)
- Water, taken up by the roots (raw material)
- Light, the energy source, 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
An easy way to remember: '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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The big idea: Photosynthesis splits water to get hydrogen, and uses the hydrogen to turn carbon dioxide into glucose. The oxygen from the water is released.
The points to remember
- Water is split using light energy: hydrogen + oxygen.
- The hydrogen converts carbon dioxide into glucose (carbon dioxide is reduced).
- The oxygen is released as a by-product.
- carbon dioxide + water → glucose + oxygen.
Remember it as: Carbon from the air, hydrogen from water.
Real example: tracing the oxygen. In the 1940s, scientists gave algae water labelled with a heavy isotope of oxygen. The labelled oxygen appeared in the gas released, showing that the oxygen comes from water, not carbon dioxide.
Trap: The oxygen released comes from water, not from carbon dioxide.
How it is asked: Expect the word equation, the roles of carbon dioxide and water, and where the oxygen comes from.
Outline the roles of water and carbon dioxide in photosynthesis.
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