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Cambridge IGCSE Biology 0610: Photosynthesis Notes (Plant Nutrition)
Photosynthesis is the process plants use to make glucose from carbon dioxide and water, using energy from light. These notes cover plant nutrition for the Cambridge IGCSE Biology (0610) syllabus: the photosynthesis equation, chlorophyll, leaf structure, uses of glucose, mineral ions, the starch test, investigations and limiting factors, with a leaf labelling exercise, a pondweed simulation, a quiz, worked examples and exam tips.
Try the pondweed experimentBook a 1-to-1 lessonWritten by Ms. Humaira Asim, a science teacher with 15+ years of teaching experience. Written for the Cambridge IGCSE Biology 0610 syllabus.
What is photosynthesis?
Photosynthesis is the process by which plants make carbohydrates (glucose) from raw materials (carbon dioxide and water), using energy from light. It happens in the chloroplasts, mainly in the leaves.
Chlorophyll is the green pigment inside chloroplasts. It absorbs light energy and transfers it into chemical energy, which is stored in glucose.
The photosynthesis equation
Plants get carbon dioxide from the air, through tiny pores called stomata. They get water from the soil, through their roots and up the xylem. The oxygen is a waste product and leaves through the stomata.
Leaf structure
| Part of the leaf | How it helps photosynthesis |
|---|---|
| Large surface area and thin | Absorbs lots of light; short distance for carbon dioxide to diffuse in |
| Waxy cuticle | Waterproof layer that reduces water loss; it is transparent so light passes through |
| Upper epidermis | Thin and transparent, so light reaches the cells below |
| Palisade mesophyll | Tall cells packed with chloroplasts near the top of the leaf; most photosynthesis happens here |
| Spongy mesophyll and air spaces | Air spaces let carbon dioxide diffuse to the cells, and let oxygen diffuse out |
| Stomata and guard cells | Pores that let carbon dioxide in and oxygen out; guard cells open and close them |
| Xylem | Brings water and mineral ions to the leaf |
| Phloem | Carries sugars (sucrose) made in the leaf to the rest of the plant |
Label the leaf
Your turn: name the 10 parts
Choose the correct label for each number, then press Check my answers.
What plants do with glucose
- Respiration: to release energy for the plant’s cells.
- Starch: glucose is changed into starch for storage, because starch is insoluble.
- Cellulose: to build cell walls.
- Sucrose: for transport around the plant in the phloem.
- Nectar: to attract insects for pollination.
- Proteins and fats: glucose is combined with nitrate ions to make amino acids, and is also used to make fats and oils for storage in seeds.
Mineral ions: nitrate and magnesium
| Mineral ion | Why the plant needs it | What happens if it is missing |
|---|---|---|
| Nitrate ions | To make amino acids, and so proteins | Poor, weak growth and yellow older leaves |
| Magnesium ions | To make chlorophyll | Leaves turn yellow (chlorosis) |
Plants take in mineral ions from the soil through their root hair cells. Farmers add fertilisers to give crops more of these ions.
Testing a leaf for starch
Plants store the glucose they make as starch, so if a leaf contains starch, it has been photosynthesising.
- Dip the leaf in boiling water for about 30 seconds. This kills the leaf and stops chemical reactions.
- Put the leaf in a tube of ethanol and stand it in a beaker of hot water. This removes the chlorophyll, so the leaf goes white. Turn off any flames first, because ethanol is flammable.
- Dip the leaf in warm water to soften it.
- Spread the leaf on a white tile and add iodine solution.
- Blue-black means starch is present. Orange-brown means no starch.
Investigations: what photosynthesis needs
First, destarch the plant by keeping it in the dark for 48 hours, so that any starch already in its leaves is used up. Then test a leaf after the experiment.
| To show the need for… | What you do | Result after the starch test |
|---|---|---|
| Light | Cover part of a leaf with black paper or foil, then leave the plant in the light | Only the uncovered part turns blue-black |
| Chlorophyll | Use a variegated leaf (green and white parts) | Only the green parts turn blue-black |
| Carbon dioxide | Seal a leaf in a bag with soda lime or sodium hydroxide, which absorbs carbon dioxide | That leaf stays orange-brown; a control leaf turns blue-black |
Rate of photosynthesis
You can measure the rate of photosynthesis by counting the oxygen bubbles given off by pondweed (for example, Elodea) in a set time, or by collecting the gas in a measuring cylinder. The rate goes up when there is more light, more carbon dioxide, or a higher temperature (up to about 35–40 °C, when enzymes start to denature).
To change light intensity, move a lamp closer or further away. Keep the temperature the same, for example with a beaker of water between the lamp and the plant (a heat shield).
Interactive: pondweed and light
Try the pondweed experiment
Move the lamp closer or further away and watch how fast oxygen bubbles are given off. Then change the carbon dioxide level.
These numbers come from a simple model to show the pattern, not real results.
Limiting factors (Extended)
A limiting factor is the factor that is in the shortest supply, so it stops the rate of photosynthesis going any higher. Light intensity, carbon dioxide concentration and temperature can all be limiting factors.
- Where the graph is rising, light intensity is limiting: more light means a faster rate.
- Where the graph is flat, more light makes no difference, because something else (carbon dioxide or temperature) is limiting.
- Raising the carbon dioxide concentration makes the curve level off higher.
- Growers use greenhouses with heaters, lamps and extra carbon dioxide to keep crops photosynthesising quickly.
Light intensity is inversely proportional to the square of the distance from the lamp: light intensity ∝ 1/d². If you halve the distance, the light intensity becomes four times greater.
Hydrogencarbonate indicator
Hydrogencarbonate indicator shows the carbon dioxide level in water around an aquatic plant.
| Colour | Carbon dioxide level | When you see it |
|---|---|---|
| Purple | Lower than in air | Plant in bright light: photosynthesis is faster than respiration |
| Red (orange-red) | Same as in air | Dim light: photosynthesis and respiration are balanced |
| Yellow | Higher than in air | Plant in the dark: only respiration takes place |
Worked examples
Example 1: finding the rate
A student counts 36 bubbles from pondweed in 3 minutes. Calculate the rate of photosynthesis.
- rate = bubbles ÷ time = 36 ÷ 3
- = 12 bubbles per minute
Example 2: light intensity (Extended)
A lamp is moved from 20 cm to 10 cm away from a plant. How does the light intensity change?
- Light intensity ∝ 1/d²
- Distance is halved, so intensity changes by (20 ÷ 10)² = 2²
- 4 times greater
Example 3: reading a limiting factor graph (Extended)
On the graph above, the low-CO₂ curve is flat at high light intensity. Explain why, and say what would increase the rate.
- The rate does not rise with more light, so light is not the limiting factor
- Carbon dioxide concentration is limiting
- Increasing the carbon dioxide concentration (or temperature, up to the optimum) would increase the rate
Example 4: checking the balanced equation (Extended)
Show that 6CO₂ + 6H₂O → C₆H₁₂O₆ + 6O₂ is balanced.
- Left: C = 6, H = 12, O = 12 + 6 = 18
- Right: C = 6, H = 12, O = 6 + 12 = 18
- Same number of each atom on both sides, so it is balanced
Try it yourself: photosynthesis calculators
Light intensity and distance
Light intensity ∝ 1 ÷ distance² (Extended)
Rate from bubble counts
rate = bubbles ÷ time
Quick quiz
Quick quiz: photosynthesis
Choose an answer for each question. You get feedback straight away.
1. What is the word equation for photosynthesis?
2. Which pigment absorbs light for photosynthesis?
3. Which layer of the leaf does most photosynthesis?
4. Carbon dioxide enters the leaf through the…
5. What colour does iodine turn if starch is present?
6. Why is a leaf boiled in ethanol in the starch test?
7. A plant is short of magnesium ions. What will you see?
8. A lamp is moved from 20 cm to 10 cm from a plant. The light intensity becomes…
Common mistakes
- Saying plants get their food from the soil. Plants make their own glucose; from the soil they take only water and mineral ions.
- Saying chlorophyll “makes” energy. Chlorophyll absorbs light energy and transfers it into chemical energy.
- Forgetting that plants respire all the time. Plants respire day and night; they photosynthesise only in the light.
- Mixing up the starch test steps. Boiling water first, then ethanol (to remove chlorophyll), then iodine.
- Saying the rate keeps rising with temperature. Above about 35–40 °C, enzymes denature and the rate falls.
Practice questions
Try each question first, then open it to check your answer.
Q1. State the two raw materials needed for photosynthesis.
Carbon dioxide and water
Q2. Name the gas given off during photosynthesis.
Oxygen
Q3. Why are palisade cells found near the top of the leaf?
So they receive the most light; they contain many chloroplasts to absorb it.
Q4. Why is a plant destarched before an investigation?
So that any starch found must have been made during the experiment.
Q5. A plant has yellow leaves. Which mineral ion might be missing, and why?
Magnesium, because it is needed to make chlorophyll.
Q6. Pondweed gives off 50 bubbles in 5 minutes. Calculate the rate.
50 ÷ 5 = 10 bubbles per minute
Q7. Why does a greenhouse grower add carbon dioxide on a bright, warm day? (Extended)
On a bright, warm day, carbon dioxide is the limiting factor, so adding it increases the rate of photosynthesis and the crop yield.
Q8. An aquatic plant in hydrogencarbonate indicator is kept in the dark. What colour will the indicator turn, and why?
Yellow. In the dark the plant only respires, so carbon dioxide builds up in the water.
Exam tips
- Learn the word equation exactly, and write “light” and “chlorophyll” above the arrow.
- For investigations, always mention a control and destarching the plant first.
- When describing a graph, quote values from the axes and use the words “limiting factor”.
- Link each leaf adaptation to its job: “many chloroplasts to absorb more light”.
Frequently asked questions
What is the word equation for photosynthesis?
Carbon dioxide + water → glucose + oxygen, using light energy absorbed by chlorophyll.
Where does photosynthesis take place?
In the chloroplasts of plant cells, mainly in the palisade mesophyll cells of the leaves.
What are the limiting factors of photosynthesis?
Light intensity, carbon dioxide concentration and temperature. Whichever is in the shortest supply limits the rate.
Do plants respire as well as photosynthesise?
Yes. Plants respire all the time, day and night, to release energy. They photosynthesise only when there is light.
Are these notes suitable for KS3 or Year 7 science?
Yes, the word equation, the parts of a leaf and the starch test suit Year 7 to 9 (KS3) science. The balanced equation and limiting factors are GCSE and IGCSE content.
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Book a 1-to-1 lessonTry the Biology calculatorRelated notes: Next topic: respiration · Cells and cell structure · Diffusion, osmosis and active transport · Biology notes topic list. This topic is also part of GCSE and IGCSE Combined Science: see all Combined Science notes. These notes follow the Cambridge IGCSE Biology 0610 syllabus. Always check the current syllabus on the Cambridge International website.