Plant Nutrition
Photosynthesis is the process that supplies energy to almost every food chain on Earth. It is also, along with respiration, one of the two equations you must be able to write without hesitation.
1. Photosynthesis
Photosynthesis is the process by which plants synthesise carbohydrates from raw materials using energy from light.
Word equation:
carbon dioxide + water → glucose + oxygen
(in the presence of light and chlorophyll)
Balanced chemical equation:
6CO₂ + 6H₂O → C₆H₁₂O₆ + 6O₂
The four requirements:
| Requirement | Source |
|---|---|
| Carbon dioxide | From the air, entering through the stomata |
| Water | From the soil, absorbed by root hair cells and carried in the xylem |
| Light energy | Usually the Sun |
| Chlorophyll | The green pigment inside chloroplasts |
Light and chlorophyll are written above and below the arrow, not as reactants. Chlorophyll is not used up — it absorbs light energy and transfers it to chemical energy in glucose. Writing “chlorophyll” on the left-hand side as though it were consumed is wrong.
Oxygen is a product of photosynthesis, not a requirement. Students asked what a plant needs for photosynthesis very often answer “oxygen”, importing it from respiration. Oxygen is needed for respiration; carbon dioxide is needed for photosynthesis.
Where it happens
Photosynthesis takes place in the chloroplasts, which contain chlorophyll. This is why the green parts of a leaf photosynthesise and the white parts of a variegated leaf do not — the white areas have no chlorophyll.
What happens to the glucose
Glucose made in photosynthesis is used for:
- Respiration, to release energy
- Converting to starch for storage (starch is insoluble, so it does not affect water potential)
- Converting to cellulose to build cell walls
- Converting to sucrose for transport in the phloem
- Making amino acids and then proteins, when combined with nitrate ions
- Making nectar and stored in seeds as fats and oils
2. Limiting factors
A limiting factor is something present in the environment in such short supply that it restricts life processes.
The three limiting factors for photosynthesis:
| Factor | Effect |
|---|---|
| Light intensity | Rate increases as intensity increases, then levels off when another factor becomes limiting |
| Carbon dioxide concentration | Rate increases as concentration increases, then levels off |
| Temperature | Rate increases up to an optimum, then falls sharply as enzymes denature |
Reading the graphs: where a curve is rising, the factor on the x-axis is limiting. Where it has levelled off (plateaued), something else has become limiting.
Temperature behaves differently from the other two. Light and carbon dioxide graphs level off and stay level. The temperature graph rises to a peak and then drops, because photosynthesis is controlled by enzymes, which are denatured above the optimum. Questions that ask you to explain the shape of the temperature curve are testing this.
Glasshouses apply this directly: growers increase light, temperature and carbon dioxide concentration to raise yields.
3. Leaf structure
| Structure | Adaptation and function |
|---|---|
| Waxy cuticle | Transparent so light passes through; waterproof, reducing water loss |
| Upper epidermis | Thin and transparent, so light reaches the mesophyll |
| Palisade mesophyll | Tall, column-shaped cells packed with chloroplasts, near the top surface — maximum light absorption |
| Spongy mesophyll | Air spaces between cells allow gases to diffuse rapidly |
| Stomata | Pores allowing carbon dioxide in and oxygen and water vapour out |
| Guard cells | Open and close the stomata, controlling gas exchange and water loss |
| Vascular bundle (veins) | Xylem brings water and mineral ions; phloem carries sucrose away |
Overall leaf adaptations: leaves are broad and flat to give a large surface area for absorbing light, and thin to give a short diffusion distance for gases.
Stomata are usually on the lower surface. This reduces water loss, because the lower surface is shaded and cooler. A diagram question that puts most stomata on the upper surface is describing an unusual plant — such as a floating water plant.
4. Mineral requirements
Plants need mineral ions from the soil in addition to the products of photosynthesis.
| Mineral ion | Needed for | Deficiency symptom |
|---|---|---|
| Nitrate | Making amino acids and proteins | Stunted growth; older leaves yellow |
| Magnesium | Making chlorophyll | Yellowing of leaves (chlorosis) |
A pairing that is often mixed up: magnesium → chlorophyll; nitrate → protein. The link for nitrate is that proteins contain nitrogen — so a nitrogen-containing ion is needed to build them.
Mineral ions are absorbed by root hair cells by active transport, because they are usually in lower concentration in the soil than in the cell.
5. Investigating photosynthesis
Destarching
Before any starch test, the plant must be destarched — kept in the dark for 24–48 hours so that existing starch is used up. Without this step, a positive result proves nothing, because the starch might have been there beforehand.
Testing a leaf for starch
- Boil the leaf in water — kills the tissue and breaks down cell membranes
- Boil in ethanol — removes the chlorophyll so the colour change is visible. Use a water bath, not a naked flame, because ethanol is flammable
- Rinse in water — softens the brittle leaf
- Add iodine solution
Blue-black means starch is present, and therefore photosynthesis has occurred.
The three classic experiments
| To show ___ is needed | Method | Result |
|---|---|---|
| Light | Cover part of a leaf with foil | Covered part: no starch. Uncovered: starch |
| Chlorophyll | Use a variegated leaf | Green parts: starch. White parts: no starch |
| Carbon dioxide | Enclose a plant with soda lime (absorbs CO₂) | No starch |
Measuring the rate
Using pondweed, the rate is measured by counting oxygen bubbles produced per minute, or by collecting the gas.
Varying the distance of a lamp changes the light intensity — and the closer the lamp, the higher the intensity and the more bubbles.
Controlled variables: temperature (use a heat shield or water bath, since a lamp also warms the water), carbon dioxide concentration, and the same piece of pondweed.
The lamp is a heat source as well as a light source. A well-designed answer includes a beaker of water or a glass screen between the lamp and the plant to absorb the heat, so that temperature stays constant and only light intensity varies.
6. Mistakes that cost marks
Saying plants need oxygen for photosynthesis. They need carbon dioxide.
Writing chlorophyll as a reactant. It is not used up; it goes above the arrow.
Saying plants respire only at night. Plants respire all the time. During the day photosynthesis is usually faster, so there is a net release of oxygen.
Forgetting to destarch in an experiment description.
Forgetting the safety point about ethanol being flammable.
Explaining a levelling-off graph as “the plant is full.” It means another factor has become limiting.
Confusing magnesium with nitrate.
Saying stomata “let air in” without naming the gases and directions.
Frequently asked questions
What is the word equation for photosynthesis? Carbon dioxide + water → glucose + oxygen, in the presence of light and chlorophyll.
Where does photosynthesis take place? In the chloroplasts, which contain chlorophyll.
What are the limiting factors? Light intensity, carbon dioxide concentration and temperature.
Why does the temperature graph drop but the light graph doesn’t? Photosynthesis is controlled by enzymes, which denature above the optimum temperature. Light and carbon dioxide graphs simply level off when another factor becomes limiting.
Why must a plant be destarched first? So any starch found afterwards must have been made during the experiment.
Why is a leaf boiled in ethanol? To remove the chlorophyll, so the iodine colour change can be seen.
What do plants need magnesium and nitrate for? Magnesium for chlorophyll; nitrate for amino acids and proteins.
Do plants respire? Yes — continuously, day and night.
Quick revision checklist
- I can write the word and balanced equations for photosynthesis
- I know light and chlorophyll go above the arrow, not as reactants
- I know oxygen is a product, not a requirement
- I can list what glucose is converted into and why
- I can define a limiting factor and name all three
- I can explain the shape of light, carbon dioxide and temperature graphs
- I can label a leaf and give the adaptation of each part
- I know why leaves are broad, flat and thin
- I know magnesium → chlorophyll and nitrate → protein
- I know mineral ions are absorbed by active transport
- I can describe destarching and the starch test, with the ethanol safety point
- I can describe the light, chlorophyll and carbon dioxide experiments
- I can describe the pondweed experiment and control the heat from the lamp
These notes cover topic 6 of the Cambridge IGCSE Biology (0610) syllabus and are written for Grade 9–11 / Year 10–11 students. They are based on teaching patterns observed across many one-to-one IGCSE Biology lessons, with particular attention to the errors students make most often and the wording examiners reward.
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