Cambridge O Level Biology 5090: Plant nutrition (5090)

This guide teaches Topic 6, Plant nutrition, for Cambridge O Level Biology 5090. It follows the Cambridge O Level Biology 5090 syllabus for examination in 2026, 2027 and 2028 (version 4), and covers every learning outcome in sections 6.1 Photosynthesis, 6.2 Leaf structure and 6.3 Mineral nutrition. The syllabus is not tiered: every candidate studies all of this content.

Use it with the plant nutrition revision notes and the plant nutrition practice questions. The course hub is at Cambridge O Level Biology, and you can tick off outcomes on the printable checklist. To find your weak spots first, try a free 10-minute diagnostic.

What this topic covers

Section	What you must be able to do
6.1.1	Understand photosynthesis as the process by which plants make carbohydrates from raw materials using energy from light
6.1.2–6.1.3	State that chlorophyll is a green pigment in chloroplasts that transfers light energy into chemical energy
6.1.4	Outline how the carbohydrates made are used and stored: starch, cellulose, glucose, sucrose
6.1.5	State the word equation and balanced chemical equation
6.1.6	Investigate the need for chlorophyll, light and carbon dioxide, using controls
6.1.7–6.1.8	Describe, explain and investigate the effects of light intensity, carbon dioxide concentration and temperature on rate
6.1.9	Identify and explain limiting factors in different conditions
6.2.1–6.2.2	Explain leaf adaptations, and identify and explain leaf tissues in diagrams and photomicrographs
6.3.1–6.3.2	Explain why plants need nitrate ions and magnesium ions

6.1 Photosynthesis

What photosynthesis is

Photosynthesis is the process by which plants make carbohydrates from raw materials using energy from light. The raw materials are carbon dioxide (from the air, via stomata) and water (from the soil, via the xylem).

Chlorophyll is a green pigment found in chloroplasts. It transfers light energy into chemical energy, which is used to form glucose and other carbohydrates. The energy ends up stored in the bonds of those carbohydrate molecules.

The equations

Word equation:

carbon dioxide + water  →  glucose + oxygen
       (light energy, chlorophyll)

Balanced chemical equation:

6CO₂ + 6H₂O  →  C₆H₁₂O₆ + 6O₂
   (light energy, chlorophyll)

Light and chlorophyll go by the arrow: they are conditions, not reactants. Each side has 6 carbon, 18 oxygen and 12 hydrogen atoms.

What happens to the carbohydrate

Glucose is converted to other carbohydrates. The syllabus limits this to four uses:

Carbohydrate	Use in the plant
Starch	Energy store (insoluble, so it does not affect water movement by osmosis)
Cellulose	Builds cell walls
Glucose	Used in respiration to provide energy
Sucrose	Transported through the plant (in the phloem)

Starch, cellulose and glucose are covered as molecules in Biological molecules, including the iodine test you will use below.

Investigating the need for chlorophyll, light and carbon dioxide

All three experiments test a leaf for starch, because starch builds up when photosynthesis has happened. Two steps come first every time.

1. Destarch the plant. Keep it in the dark for a day or two so the leaves use up their starch; any starch found later was made during the experiment.
2. Test the leaf for starch at the end, using the method below.

Testing a leaf for starch

1. Dip the leaf in boiling water for about a minute. This kills the cells and breaks down membranes so iodine can get in.
2. Put the leaf in a tube of ethanol, standing in a hot water bath, until the leaf goes pale. Ethanol removes the chlorophyll so the colour change can be seen. Turn off any flame first, because ethanol is flammable.
3. Dip the leaf in warm water to soften it, and spread it on a white tile.
4. Add iodine solution. Blue-black shows starch. Orange-brown (no change) shows no starch.

The three tests

Need tested	Set-up	Control	Result
Chlorophyll	Variegated leaf (green and white parts) in light	The green parts of the same leaf	Only green parts go blue-black
Light	Part of a leaf covered on both sides with foil or black paper	The uncovered part of the same leaf	Covered part stays orange-brown
Carbon dioxide	Leaf or plant enclosed with soda lime or sodium hydroxide, which absorbs carbon dioxide	Identical set-up with water in place of the absorber	Leaf with no carbon dioxide stays orange-brown

A control is identical except for the factor being tested, so it shows the result is caused by that factor alone.

Worked example: planning the light experiment

Question: Describe how you would show that light is needed for photosynthesis.

Answer, step by step:

1. Keep a potted plant in the dark for a day or two to destarch it.
2. Cover part of one leaf on both sides with aluminium foil, held with paper clips.
3. Place the plant in bright light for several hours.
4. Remove the leaf, take off the foil and test the whole leaf for starch (boiling water, hot ethanol, iodine).
5. Expected result: the uncovered part turns blue-black; the covered part stays orange-brown.
6. Conclusion: starch was only made where light reached the leaf, so light is needed for photosynthesis. The uncovered part is the control.

Destarching is the step most often left out.

Factors that affect the rate

Light intensity. More light means more energy for chlorophyll, so the rate rises, then levels off when another factor limits it.

Carbon dioxide concentration. Carbon dioxide is a raw material. More of it raises the rate until another factor becomes limiting.

Temperature. Photosynthesis is controlled by enzymes. As temperature rises towards the optimum, molecules have more kinetic energy and there are more effective collisions, so the rate increases. Above the optimum the enzymes are denatured: their active sites change shape, the substrate no longer fits and the rate falls sharply. The model is the same as in Enzymes.

Limiting factors

A limiting factor is the factor in shortest supply, holding the rate down. Increasing it raises the rate; increasing any other factor has no effect.

How to read a rate graph:

• Where the curve is rising, the factor on the x-axis is limiting.
• Where the curve is flat, the x-axis factor is no longer limiting. Something else is.
• If a second curve (for example, at higher carbon dioxide) sits above the first on its flat part, that second factor was what limited the first curve.

Limiting factors in different conditions

Condition	Likely limiting factor	Why
Dawn or dusk; shaded forest floor	Light intensity	Too little light energy
Bright summer midday	Carbon dioxide concentration (sometimes temperature)	Plenty of light, but carbon dioxide in air is low
Cold winter day	Temperature	Enzymes work slowly
Closed greenhouse in bright sun	Carbon dioxide concentration	Plants use up the carbon dioxide in the enclosed air

Worked example: identifying the limiting factor

Data: A plant’s rate rises steeply as light intensity increases from zero, then levels off. When the experiment is repeated at a higher carbon dioxide concentration, the curve rises further before levelling off at a higher rate. Both runs are at 20 °C.

Answer:

• On the rising part of each curve, light intensity is the limiting factor, because increasing it increases the rate.
• On the flat part of the lower curve, carbon dioxide concentration was limiting. Raising it gave a higher rate.
• On the flat part of the higher curve, temperature or carbon dioxide could be limiting; this data cannot tell you which.

Investigating the rate (practical)

Method 1: counting bubbles or collecting gas from pondweed

• Put a cut piece of pondweed upside down in a test-tube of water containing sodium hydrogencarbonate (a source of carbon dioxide).
• Vary light intensity by moving a lamp to different distances, carbon dioxide by changing the concentration of sodium hydrogencarbonate, or temperature by standing the tube in water baths.
• Count bubbles released in a set time, or collect the gas in a syringe or measuring cylinder and read its volume.
• Keep the other factors constant (same plant, same solution, same temperature); a beaker of water between lamp and tube absorbs heat. Let the plant adjust for a few minutes at each condition, and repeat readings.

Bubbles vary in size, so gas volume is more accurate than a count.

Worked example: processing results

A student counted bubbles from pondweed in 2 minutes, three times at each lamp distance.

Lamp distance / cm	Count 1	Count 2	Count 3	Mean bubbles in 2 min	Rate / bubbles per min
10	46	50	48	48	24.0
20	33	30	36	33	16.5
40	17	15	16	16	8.0
80	9	9	9	9	4.5

Working for 20 cm: mean = (33 + 30 + 36) ÷ 3 = 99 ÷ 3 = 33 bubbles in 2 minutes. Rate = 33 ÷ 2 = 16.5 bubbles per minute.

Conclusion: the closer the lamp, the higher the light intensity and the faster the rate. Light intensity is limiting over this range.

If gas is collected instead, divide volume by time: 1.8 cm³ in 6 minutes gives 1.8 ÷ 6 = 0.3 cm³ per minute.

Method 2: hydrogencarbonate indicator

Hydrogencarbonate indicator shows carbon dioxide concentration by colour:

Carbon dioxide concentration	Colour
Higher than in air	Yellow
Same as air	Red
Lower than in air	Magenta, then purple

Put equal pieces of aquatic plant in sealed tubes of indicator at different light intensities, with a no-plant tube as the control. In bright light, photosynthesis removes carbon dioxide faster than respiration releases it: purple. In darkness only respiration happens: yellow. In dim light the two can balance: red. The no-plant tube stays red, showing the plant caused the changes.

6.2 Leaf structure

Most leaves have a large surface area, which absorbs more light and gives more area for carbon dioxide to enter. They are thin, so carbon dioxide has a short distance to diffuse to the mesophyll cells and light reaches all the photosynthesising cells.

From the top surface down, a dicotyledonous leaf shows these structures:

Structure	Feature	How it helps photosynthesis or gas exchange
Cuticle	Waxy, transparent layer	Reduces water loss; lets light through
Upper epidermis	One layer of transparent cells	Lets light pass to the mesophyll; protects
Palisade mesophyll	Tall cells packed tightly, near the top	Most chloroplasts here, where light is strongest
Spongy mesophyll	Irregular, loosely packed cells	Fewer chloroplasts; large surface for gas exchange
Air spaces	Between spongy cells	Let carbon dioxide diffuse to cells and oxygen diffuse away
Vascular bundle	Xylem and phloem in the veins	Xylem brings water and mineral ions; phloem carries sucrose and amino acids away
Lower epidermis	Contains most stomata	Gas exchange surface
Stomata and guard cells	Pores between pairs of guard cells	Guard cells open stomata so carbon dioxide can diffuse in and oxygen out

Distribution of chloroplasts: most are in the palisade mesophyll, fewer in the spongy mesophyll. Epidermal cells have none, apart from guard cells. Specialised palisade cells were met in Cell structure and organisation.

In a photomicrograph, find the tightly packed palisade layer first: epidermis is above it, spongy mesophyll and air spaces below.

6.3 Mineral nutrition

• Nitrate ions supply nitrogen. Plants use it to make amino acids, which are joined to make proteins, needed for growth. Glucose from photosynthesis contains no nitrogen, so it cannot make amino acids alone.
• Magnesium ions are needed to make chlorophyll. Without enough magnesium the plant makes less chlorophyll, absorbs less light and photosynthesises less.

The effect of mineral ions on plant growth is a listed practical context: grow identical seedlings in solutions each lacking one ion, with a complete solution as the control.

Common errors

• Writing that plants “get food from the soil”. The soil supplies only water and mineral ions.
• Putting light or chlorophyll in the equation as reactants.
• Saying chlorophyll “makes” or “stores” energy. It transfers light energy into chemical energy.
• Leaving destarching out of the chlorophyll, light or carbon dioxide experiments.
• Calling a flat part of a graph “photosynthesis has stopped”. The rate is constant and another factor is limiting.
• Writing that high temperature “kills” enzymes. They are denatured.
• Saying magnesium is “for growth” instead of linking it to 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.