Question 1
Paper 4 short-answer style1 markName the green pigment that absorbs light energy for photosynthesis. (1 mark)
Model answer
Chlorophyll.
Why this scores
One mark for 'chlorophyll'.
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Detailed notes on Plant Nutrition for Cambridge IGCSE Biology, covering key concepts, explanations, examples, and exam-focused revision points.
Plants use light energy to convert CO₂ and water into glucose and oxygen. Cambridge wants the equation, the role of chlorophyll, and the limiting factors that control rate.
Mapped to the Cambridge IGCSE 0610 syllabus (2026-2028).
Plants make their own food (glucose) using light, CO₂ and water. Equation must be balanced.
Definition. Photosynthesis is the process by which plants make GLUCOSE (a carbohydrate) from carbon dioxide and water, using LIGHT energy absorbed by CHLOROPHYLL.
Word equation:
carbon dioxide + water → glucose + oxygen (in the presence of light energy and chlorophyll)
Balanced symbol equation:
6CO2+6H2Olight, chlorophyllC6H12O6+6O2
Where it happens. Inside CHLOROPLASTS, found mainly in palisade mesophyll cells of the leaf. Chloroplasts contain CHLOROPHYLL — the green pigment that absorbs light energy.
Inputs and outputs:
| Input | Where from? |
|---|---|
| Carbon dioxide | Air (through stomata) |
| Water | Soil (taken up by roots, transported in xylem) |
| Light energy | The sun |
| Chlorophyll | Inside chloroplasts |
| Output | What happens to it? |
|---|---|
| Glucose | Used / stored / converted (see uses section) |
| Oxygen | Released through stomata (or used by the plant for respiration) |
Worked qualitative. Why is photosynthesis crucial for life on Earth?
Cambridge tip. Always include LIGHT and CHLOROPHYLL above the arrow in the equation. Mark schemes deduct for omitting either.
Broad and flat (large surface area), thin (short diffusion path), full of chloroplasts in the upper layer.
Why look at the leaf? The leaf is the main site of photosynthesis. Its structure is highly adapted to maximise the rate.
Adaptations and their reasons.
| Feature | How it helps |
|---|---|
| Broad, flat shape | Large surface area to absorb light and CO₂. |
| Thin | Short diffusion path for gases (CO₂ in, O₂ out). |
| PALISADE mesophyll at top | Tightly packed cells full of chloroplasts — maximum light absorption. |
| Spongy mesophyll | Air spaces let CO₂ diffuse to all cells. |
| STOMATA (mostly underside) | Pores for gas exchange; can be opened/closed by guard cells. |
| Veins (xylem + phloem) | Bring water + minerals; remove sugars. |
| Waxy cuticle | Reduces water loss without blocking light. |
| Chlorophyll | Absorbs RED + BLUE light; reflects green (why leaves look green). |
Worked qualitative. Why are most chloroplasts in the upper part of the leaf? Light comes from above. The palisade layer at the top has the most chloroplasts because it gets the most light → maximum photosynthesis. Lower layers (spongy mesophyll) have fewer chloroplasts but more air spaces — they specialise in CO₂ delivery.
When one factor runs short, it limits the rate. Three to know: light, CO₂, temperature.
A LIMITING FACTOR is the variable that, when in shortest supply, controls the rate of a process. As you increase a limiting factor, rate goes up — until something else becomes limiting.
1. Light intensity.
2. CO₂ concentration.
3. Temperature.
Reading limiting-factor graphs.
Worked qualitative. A graph shows photosynthesis rate vs CO₂ concentration at two light intensities (low + high). The HIGH light line plateaus at a higher rate. Why?
Worked qualitative. Why do greenhouse growers add EXTRA CO₂ to the air?
Cambridge tip. Always state which factor is limiting AT WHICH POINT on the graph. "At low light, light is limiting; at high light, CO₂ becomes limiting" earns more marks than "rate increases".
Respiration, stored as starch, cellulose for cell walls, combined with N for amino acids.
Photosynthesis MAKES the glucose; the plant then USES it.
1. Respiration. The plant respires (day and night) — broken down to release energy for growth and other processes.
2. Stored as STARCH. Starch is insoluble, compact, doesn't affect water potential — perfect storage. Stored in:
3. Cellulose. Many glucose units join to make cellulose — the long, strong molecule that builds CELL WALLS. Cellulose can't be broken down by the plant for energy — it's structural.
4. Amino acids and proteins. Glucose + nitrate (from soil) → amino acids → proteins. Proteins are needed for growth, enzymes, etc.
5. Lipids (fats and oils). Some glucose is converted to lipids — energy stores in seeds (e.g. sunflower oil).
6. Sucrose (transport sugar). Glucose is converted to SUCROSE for transport through the phloem (sap). Sucrose is the sugar moved around the plant.
Worked qualitative. Why is starch (not glucose) used as a storage molecule?
Cambridge tip. When asked "uses of glucose", give FOUR specific uses, not three. The fourth (proteins) is often overlooked but worth a separate mark.
Variegated leaf, destarched plant in dark, pondweed bubbling — Cambridge favourites.
1. Test a leaf for STARCH (proves photosynthesis happened).
Steps:
2. Showing chlorophyll is needed (variegated leaf).
3. Showing CO₂ is needed.
4. Showing light is needed.
5. Counting bubbles from pondweed.
Why "destarch" first? A plant left in darkness for 24-48 hours uses up its starch (respiration continues). Without destarching, you can't tell whether starch came from your experiment or from before.
Cambridge tip. Always describe the CONTROL. "Compare a covered area to an uncovered area on the SAME leaf — same plant, same conditions, only variable is light."
Verbatim phrases and definitions Cambridge mark schemes credit.
Photosynthesis appears every Paper 4 (8-10 marks). Common formats: complete the equation; describe limiting factor experiment; interpret rate vs CO₂ graph; describe leaf adaptations. Examiner reports flag missing 'light + chlorophyll' on the equation arrow, students confusing chloroplast vs chlorophyll, and stating photosynthesis 'stops at night' (correct: it doesn't happen, but respiration still does).
Sources: Cambridge IGCSE Biology 0610 syllabus 2026-2028 (6.1); 0610/42 Oct/Nov 2024 — Q8 (photosynthesis, limiting factors); 0610 Examiner Reports 2022-2024. Last reviewed 2026-05-07.
Step-by-step solutions to past-paper-style questions on photosynthesis, written exactly the way a tutor would explain them at the board.
Question
Write the WORD equation and the BALANCED symbol equation for photosynthesis.
Step-by-step solution
Step 1
Word equation: carbon dioxide + water → glucose + oxygen (in the presence of light and chlorophyll).
Step 2
Symbol equation (balanced):
6CO2+6H2Olight, chlorophyllC6H12O6+6O2
Answer
Word: carbon dioxide+water→glucose+oxygen. Symbol: 6CO2+6H2O→C6H12O6+6O2.
Examiner tip
Show LIGHT and CHLOROPHYLL above the arrow (they are conditions, not reactants). Balance the symbol equation with the 6s.
Question
Give FOUR uses of the glucose made by photosynthesis in a plant.
Step-by-step solution
Step 1
Respiration: broken down to release energy.
Step 2
Stored as starch (in leaves, roots and seeds) for later use.
Step 3
Made into cellulose for building cell walls.
Step 4
Combined with nitrate to make amino acids (and then proteins).
Answer
Respiration (energy), storage as starch, cellulose for cell walls, and amino acids/proteins (with nitrogen from nitrates).
Question
List THREE limiting factors of photosynthesis and explain how each affects the rate.
Step-by-step solution
Step 1
Light intensity: more light provides more energy for photosynthesis, so the rate increases — until another factor becomes limiting.
Step 2
Carbon dioxide concentration: more CO₂ provides more raw material, so the rate increases — up to a saturation point.
Step 3
Temperature: higher temperature (up to about 35–40 °C) speeds up the enzyme-controlled reactions; above this the enzymes denature and the rate falls.
Answer
Light intensity, carbon dioxide concentration and temperature. Each raises the rate up to a point; temperature also has a maximum beyond which enzymes denature.
Question
Describe how to test a leaf to find out whether it has been photosynthesising and making starch.
Step-by-step solution
Step 1
Boil the leaf in water for about a minute to kill it and stop reactions / break down the cell membranes.
Step 2
Boil the leaf in ethanol (with the flame off, using a water bath) to remove the chlorophyll, so the colour change is easy to see. Then dip it in warm water to soften it.
Step 3
Spread the leaf out and add iodine solution.
Step 4
If starch is present the leaf turns blue-black; if not, it stays orange-brown. So a blue-black colour shows the leaf has been photosynthesising.
Answer
Boil in water (kill), boil in ethanol (remove chlorophyll), soften in warm water, then add iodine: blue-black = starch present (photosynthesis occurred).
Examiner tip
Ethanol is flammable, so it must be heated using a water bath with the flame off. The order — water, then ethanol, then iodine — is marked.
Question
On a graph of rate of photosynthesis against light intensity, the rate rises steeply at first and then levels off to a plateau. Explain the shape of the graph and identify what is limiting the rate in each region.
Step-by-step solution
Step 1
On the steep, rising part, increasing the light increases the rate — so light intensity is the limiting factor here: it is in short supply and controls the rate.
Step 2
On the flat plateau, increasing the light no longer increases the rate, so light is no longer limiting.
Step 3
Something else must now be limiting the rate — usually the carbon dioxide concentration or the temperature.
Step 4
To raise the plateau, you would increase the factor that is now limiting (e.g. provide more CO₂ or raise the temperature).
Answer
On the rising part, light intensity limits the rate (more light → faster). At the plateau, light is no longer limiting; another factor — CO₂ concentration or temperature — now limits the rate, so adding light has no further effect.
Examiner tip
The key idea is that the limiting factor is whichever is in shortest supply. Naming CO₂ or temperature as the new limiting factor at the plateau is essential.
Question
Explain how a destarched variegated leaf (green in parts, white in others) can be used to show that chlorophyll is needed for photosynthesis.
Step-by-step solution
Step 1
First destarch the plant by leaving it in the dark for 1–2 days, so any blue-black result later is from new photosynthesis, not stored starch (this is the control step).
Step 2
Record which parts of the leaf are green (have chlorophyll) and which are white (no chlorophyll) — for example by drawing the leaf.
Step 3
Leave the plant in bright light for several hours, then test the leaf for starch (boil in water, boil in ethanol, add iodine).
Step 4
Only the parts that were green turn blue-black; the white parts stay orange-brown. This shows starch — and therefore photosynthesis — only happened where chlorophyll was present.
Answer
Destarch the plant first (control), record green vs white areas, expose to light, then test for starch. Only the green (chlorophyll-containing) parts turn blue-black, showing chlorophyll is needed for photosynthesis.
Examiner tip
The destarching step is the control that makes the conclusion valid. The variegation provides a built-in comparison (chlorophyll vs none) on the same leaf.
High-scoring sample answers for photosynthesis on the Cambridge IGCSE 0610 paper, with examiner-style notes mapping each response to the mark scheme and assessment objectives.
Name the green pigment that absorbs light energy for photosynthesis. (1 mark)
Model answer
Chlorophyll.
Why this scores
One mark for 'chlorophyll'.
State the word equation for photosynthesis. (2 marks)
Model answer
carbon dioxide + water → glucose + oxygen (in the presence of light and chlorophyll).
Why this scores
One mark for the correct reactants (carbon dioxide + water) and one for the correct products (glucose + oxygen). Naming light/chlorophyll as conditions is good practice.
State three uses of the glucose made by a plant during photosynthesis. (3 marks)
Model answer
The glucose can be used in respiration to release energy; stored as starch for later use; converted to cellulose to build cell walls; or combined with nitrate to make amino acids for proteins. (Any three.)
Why this scores
One mark each for any three correct uses. 'For energy' should be linked to respiration; 'storage' should specify starch.
Explain how light intensity, carbon dioxide concentration and temperature each affect the rate of photosynthesis. (4 marks)
Model answer
Increasing the light intensity increases the rate, because more light energy is available for photosynthesis, until another factor becomes limiting. Increasing the carbon dioxide concentration increases the rate, because CO₂ is a raw material, again up to a saturation point. Increasing the temperature increases the rate up to an optimum, because the enzymes controlling photosynthesis work faster; but above the optimum the rate falls because the enzymes are denatured.
Why this scores
Four marks: light (more energy); CO₂ (raw material); temperature increases rate via enzymes; temperature above optimum denatures enzymes / rate falls.
Describe how you would use a destarched plant to show that light is needed for photosynthesis. (5 marks)
Model answer
First destarch the plant by leaving it in the dark for one to two days, so that any starch found later must have been made during the experiment. Then partly cover one leaf with a strip of foil (or card) so that part of the leaf gets light and part is in darkness, and leave the plant in bright light for several hours. Remove the leaf and test it for starch: boil it in water, then in ethanol to remove the chlorophyll, soften it in warm water, and add iodine solution. The part of the leaf that received light turns blue-black (starch present), while the covered part stays orange-brown (no starch). This shows that starch, and therefore photosynthesis, only occurred where light reached the leaf, proving light is needed.
Why this scores
Five marks: destarch first (control); partly cover the leaf (light vs dark); expose to light; test for starch correctly; valid conclusion (lit part has starch, covered part does not).
A graph shows the rate of photosynthesis against light intensity at two carbon dioxide concentrations: 0.04% (normal air) and 0.4%. Both curves rise then level off, but the 0.4% curve levels off at a higher rate. Explain the shape of the curves and why the 0.4% curve reaches a higher plateau. (6 marks)
Model answer
On the steeply rising part of each curve, increasing the light intensity increases the rate, so light intensity is the limiting factor here — it is in short supply and controls the rate. As light intensity continues to rise, each curve levels off to a plateau, where increasing the light no longer increases the rate, so light is no longer limiting; instead, another factor — the carbon dioxide concentration (or temperature) — has become limiting. The curve at 0.4% CO₂ levels off at a higher rate than the one at 0.04% because there is more carbon dioxide available as a raw material, so photosynthesis can continue faster before CO₂ becomes limiting. This shows that the rate is always controlled by whichever factor is in shortest supply at the time.
Why this scores
Up to 6 marks: rising part — light is limiting; plateau — light no longer limiting; another factor (CO₂/temperature) now limits; higher CO₂ → higher plateau because more raw material; the general principle of the limiting factor being whatever is in shortest supply.
Definitions to memorise and the exact keywords mark schemes credit for photosynthesis answers — sharpened from recent examiner reports for the 2026 0610 sitting.
The process by which plants make glucose (carbohydrate) from carbon dioxide and water, using light energy absorbed by chlorophyll.
The green pigment in chloroplasts that absorbs (transfers) light energy for photosynthesis.
The factor that is in shortest supply and so limits the rate of a process such as photosynthesis.
Example
On a cloudy day, light intensity is the limiting factor of photosynthesis.
The traps other students keep falling into on photosynthesis questions — taken from recent Cambridge IGCSE 0610 examiner reports and mark schemes — and how to avoid them.
Why it happens
Plants are always alive.
How to avoid it
Photosynthesis needs LIGHT. At night, plants only RESPIRE. They photosynthesise only when light is available.
Why it happens
The word equation looks balanced; the symbol equation needs the 6s.
How to avoid it
Include the 6 in front of CO₂, H₂O and O₂. One glucose has 6 carbons, so 6 CO₂ are needed.
Why it happens
Chlorophyll absorbs the light.
How to avoid it
Photosynthesis happens in CHLOROPLASTS. Chlorophyll is the GREEN PIGMENT inside chloroplasts that absorbs the light.
Why it happens
Students describe the shape but not the cause.
How to avoid it
At a plateau, state that the changed factor is no longer limiting and name the new limiting factor (usually CO₂ or temperature).
The things students keep getting wrong in this sub-topic, answered.