Cambridge O Level Biology 5090: Plant nutrition (5090) – Revision Notes

For full explanations and worked examples, use the plant nutrition study guide. These notes condense Topic 6, Plant nutrition, of the Cambridge O Level Biology 5090 syllabus for examination in 2026, 2027 and 2028 (version 4). They cover sections 6.1 Photosynthesis, 6.2 Leaf structure and 6.3 Mineral nutrition. The syllabus is not tiered, so all of this applies to every candidate.

When you are ready, test yourself with the plant nutrition practice questions. The full course list is on the Cambridge O Level Biology hub, and the printable checklist lets you track every outcome. A free 10-minute diagnostic will show which topics need most work.

Key definitions (6.1.1–6.1.3)

• Photosynthesis: the process by which plants make carbohydrates from raw materials using energy from light.
• Chlorophyll: a green pigment found in chloroplasts. It transfers light energy into chemical energy for the formation of glucose and other carbohydrates.
• Limiting factor: the factor in shortest supply that keeps the rate of photosynthesis down. Only increasing this factor raises the rate.
• Control: a set-up identical to the test except for the one factor being investigated.
• Destarching: keeping a plant in the dark so its leaves use up stored starch before an experiment.

Equations (6.1.5)

Type	Equation
Word	carbon dioxide + water → glucose + oxygen
Balanced	6CO₂ + 6H₂O → C₆H₁₂O₆ + 6O₂
Conditions (by the arrow)	light energy, chlorophyll

Atom check: C 6 = 6; H 12 = 12; O 18 = 18.

What the plant does with the carbohydrate (6.1.4)

Carbohydrate	Use
Starch	Energy store
Cellulose	Builds cell walls
Glucose	Respiration, to provide energy
Sucrose	Transport through the plant

Memory hook: Store, Cell walls, Respiration, Transport.

Showing what photosynthesis needs (6.1.6)

Method in steps: testing a leaf for starch

1. Boiling water, about 1 minute: kills cells, breaks membranes.
2. Hot ethanol in a water bath (no naked flame, ethanol is flammable): removes chlorophyll.
3. Warm water: softens the brittle leaf.
4. Iodine solution on a white tile: blue-black = starch; orange-brown = no starch.

The three experiments at a glance

Testing for	Treatment	Control	Starch result
Chlorophyll	Variegated leaf in light	Green areas of the same leaf	Green areas only
Light	Foil strip over part of a leaf	Uncovered part	Uncovered part only
Carbon dioxide	Soda lime or sodium hydroxide absorbs carbon dioxide around the leaf	Same set-up with water instead	Control leaf only

Every one starts with a destarched plant and ends with the starch test.

Rate of photosynthesis (6.1.7–6.1.9)

Effect of each factor

Factor	Shape of graph	Explanation
Light intensity	Rises, then levels off	More light energy transferred by chlorophyll; plateau when another factor limits
Carbon dioxide concentration	Rises, then levels off	More raw material; plateau when another factor limits
Temperature	Rises to an optimum, then falls sharply	Up to the optimum: more kinetic energy, more effective collisions between enzymes and substrates. Above it: enzymes denature, active site changes shape, substrate no longer fits

The enzyme model is the same one used in the Enzymes revision notes.

Method in steps: naming the limiting factor from a graph

1. Find the part of the curve you are asked about.
2. Rising? The factor on the x-axis is limiting.
3. Flat? The x-axis factor is not limiting; name another factor from the conditions given.
4. Two curves? The factor that differs between them was limiting the lower curve’s plateau.

Typical conditions

Situation	Usually limiting
Early morning, late evening, deep shade	Light intensity
Cold but bright winter day	Temperature
Bright, warm day	Carbon dioxide concentration
Sealed greenhouse in sunshine	Carbon dioxide concentration

Investigating the rate (6.1.8)

Pondweed (submerged aquatic plant)

• Measure: bubbles counted in a set time, or volume of gas collected (more accurate, as bubbles vary in size).
• Rate = amount ÷ time, for example cm³ per minute or bubbles per minute.
• Vary light by lamp distance, carbon dioxide by concentration of sodium hydrogencarbonate solution, temperature by water bath.
• Control variables: same plant piece, same solution volume, same temperature (use a heat shield of water), acclimatisation time at each condition, repeats with a mean.

Hydrogencarbonate indicator

Indicator colour	Carbon dioxide compared with air	Meaning with a plant
Yellow	Higher	Respiration greater than photosynthesis (dark)
Red	Same	Photosynthesis and respiration balance
Magenta to purple	Lower	Photosynthesis greater than respiration (bright light)

Always include a tube with no plant as a control.

Worked reminder: turning gas volumes into rates

A syringe collects 2.4 cm³ of gas from pondweed in 8 minutes at one temperature, and 1.2 cm³ in 8 minutes at a lower temperature.

• Rate at the higher temperature = 2.4 ÷ 8 = 0.3 cm³ per minute.
• Rate at the lower temperature = 1.2 ÷ 8 = 0.15 cm³ per minute.
• Units matter: give cm³ per minute (or cm³ min⁻¹), not just cm³.

Worked reminder: explaining an indicator result

Three sealed tubes of hydrogencarbonate indicator each contain pondweed. After two hours tube P (dark) is yellow, tube Q (near a lamp) is purple, and tube R (no plant, near the lamp) is still red.

• P: no light, so no photosynthesis; respiration releases carbon dioxide, so its concentration rises above air.
• Q: photosynthesis uses carbon dioxide faster than respiration releases it, so its concentration falls below air.
• R: stays red, showing that the lamp alone does not change the indicator and the plant caused the changes in P and Q.

A full answer names the process and the change in carbon dioxide concentration for each tube.

Leaf structure (6.2.1–6.2.2)

Large surface area: absorbs more light; more area for carbon dioxide to enter. Thin: short diffusion distance for gases; light reaches all mesophyll cells.

Structure (top to bottom)	Adaptation
Cuticle	Waxy, transparent: cuts water loss, lets light through
Upper epidermis	Transparent, no chloroplasts: light passes to mesophyll
Palisade mesophyll	Tightly packed, most chloroplasts, near the light
Spongy mesophyll	Loosely packed, fewer chloroplasts, large surface for gas exchange
Air spaces	Carbon dioxide diffuses in to cells, oxygen out
Vascular bundle	Xylem supplies water and mineral ions; phloem removes sucrose and amino acids
Lower epidermis	Most stomata found here
Stomata and guard cells	Guard cells open and close the pore for gas exchange

Distribution of chloroplasts: most in palisade, fewer in spongy, none in epidermal cells except guard cells.

Mineral nutrition (6.3.1–6.3.2)

Ion	Needed for	Why it matters
Nitrate	Making amino acids, then proteins	Glucose has no nitrogen; proteins are needed for growth
Magnesium	Making chlorophyll	Less chlorophyll means less light absorbed, less photosynthesis

Must-know distinctions

• Raw materials vs conditions. Carbon dioxide and water are raw materials. Light and chlorophyll are conditions.
• Starch vs sucrose. Starch is stored; sucrose is transported.
• Glucose vs starch in tests. You test leaves for starch, not glucose, because glucose is quickly converted.
• Rate levels off vs rate falls. Levelling off means another factor is limiting. A fall with temperature means enzymes are denaturing.
• Palisade vs spongy. Palisade: photosynthesis (most chloroplasts). Spongy: gas exchange (air spaces).
• Nitrate vs magnesium. Nitrate: amino acids and proteins. Magnesium: chlorophyll.

Quick self-test

1. State the balanced chemical equation for photosynthesis.
2. What does chlorophyll do with light energy?
3. Name the carbohydrate a plant uses to build cell walls.
4. Why is a plant kept in the dark before a starch experiment?
5. Why is the leaf placed in hot ethanol during the starch test?
6. What colour does hydrogencarbonate indicator turn in the dark with pondweed inside?
7. A pondweed releases 54 bubbles in 3 minutes. Calculate the rate.
8. Name the limiting factor on a sunny morning at 3 °C.
9. Give two reasons why a leaf is thin.
10. Where are most chloroplasts in a leaf?
11. Explain why a plant short of nitrate ions grows poorly.
12. Which ion is needed to make chlorophyll?

Answers

1. 6CO₂ + 6H₂O → C₆H₁₂O₆ + 6O₂, with light and chlorophyll by the arrow.
2. It transfers light energy into chemical energy, used to form glucose and other carbohydrates.
3. Cellulose.
4. To destarch it, so any starch found later was made during the experiment.
5. To remove the chlorophyll, so the iodine colour change can be seen.
6. Yellow: only respiration happens, so carbon dioxide concentration rises.
7. 54 ÷ 3 = 18 bubbles per minute.
8. Temperature: enzymes work slowly in the cold.
9. Short diffusion distance for carbon dioxide to reach mesophyll cells; light can reach all the photosynthesising cells.
10. Palisade mesophyll.
11. Nitrate supplies nitrogen to make amino acids; without amino acids it cannot make proteins needed for growth.
12. Magnesium.

Where marks are usually lost

• Using “energy is made” or “chlorophyll produces energy”. Energy is transferred.
• Writing glucose as C₆H₁₂O₆ but forgetting to balance with 6 on carbon dioxide, water and oxygen.
• Describing the starch test without saying why each step is done; “explain” questions need the reason.
• Forgetting the safety point about ethanol and flames.
• Naming the factor on the x-axis as limiting when the curve is flat.
• Describing a hydrogencarbonate colour without linking it to carbon dioxide concentration.
• Saying palisade cells have chloroplasts “for photosynthesis” without their position near the upper surface, where light is strongest.
• Stating that stomata “let air in”: name the gases (carbon dioxide in, oxygen out).
• Writing that magnesium “makes the leaf green” without naming chlorophyll.

Official syllabus

Cambridge O Level Biology 5090 syllabus for examination in 2026, 2027 and 2028, version 4 (Cambridge University Press & Assessment), section 6 Plant nutrition.