OxfordAQA International GCSE Biology: Photosynthesis, limiting factors and transport in plants (9201) – Revision Notes

Condensed recall notes for OxfordAQA International GCSE Biology (9201), Version 5.2, sections 3.2.1 and 3.2.2. For full explanations and worked examples, read the study guide. When you can answer the self-test below, move on to the practice questions. Related pages: the Bioenergetics revision notes, the course hub, the printable checklist and the free 10-minute diagnostics.

3.2.1 Photosynthesis at a glance

Item	Recall
Word equation	carbon dioxide + water → glucose + oxygen (light energy)
Symbol equation	6CO₂ + 6H₂O → C₆H₁₂O₆ + 6O₂
Light absorbed by	Chlorophyll, in chloroplasts (some plant cells and algae)
Carbon dioxide from	The air
Water from	The soil
By-product	Oxygen

Atom count: the same three totals (carbon, hydrogen, oxygen) appear on both sides.

Uses of glucose (3.2.1 d, e)

Product or use	Purpose
Respiration	Energy release
Starch	Insoluble storage
Fat or oil	Storage, for example in seeds
Cellulose	Strengthens cell walls
Protein	Growth and repair; needs nitrate ions from the soil

Memory hook: starch is the store you can carry without water problems; cellulose is the wall; protein needs nitrogen.

Limiting factors (3.2.1 c)

• Three factors: low temperature, shortage of carbon dioxide, shortage of light.
• They interact. The one in shortest supply is the limiting factor.
• Rising part of a graph: the x-axis factor is limiting. Flat part: another factor is limiting.
• To find the new limiting factor, name something that was constant and could now be the cause, then support it with data.

Method in steps for a limiting-factor question:

1. Look at the section of the line the question points to.
2. Is the rate still changing with the x-axis factor? If yes, that factor limits.
3. If flat, compare a second line, or name the factor held constant.
4. Quote two numbers from the data.

Greenhouse economics

Factor	How a grower raises it	Pays off when
Temperature	Heating	Cold limits the rate and the extra crop is worth more than the fuel
Carbon dioxide	Burning fuel or releasing the gas	Carbon dioxide is limiting
Light intensity	Lamps	Light is limiting, especially in short winter days

Work it through: extra yield × price per kg − extra cost = net gain. Negative means not worth it.

Small reminder: a greenhouse yield rises from 48 kg to 60 kg per bed. Percentage increase = (60 − 48) ÷ 48 × 100 = 25%.

Required practical: rate of photosynthesis

Part	Notes
Independent variable	One of lamp distance, carbon dioxide supply (hydrogencarbonate) or temperature
Dependent variable	Bubbles per minute, or volume of oxygen per minute
Control variables	The other two factors, same piece of pondweed, time allowed per setting
Reliability	Repeat three or more times, calculate a mean, leave out anomalies
Safety	Keep mains lamps away from water; take care with hot water

Worked reminder: bubbles in 4 minutes were 60, 64 and 62. Mean = 62. Rate = 62 ÷ 4 = 15.5 bubbles per minute.

Common improvements: heat shield between lamp and plant, longer counting time, measuring gas volume rather than counting bubbles.

Key terms

Term	Meaning
Limiting factor	The factor in shortest supply, which sets the maximum rate at that moment
Stoma (plural stomata)	A pore in the leaf surface, through which gases diffuse
Guard cells	A pair of cells around each stoma that change shape to open or close it
Wilting	Loss of firmness when water leaves cells faster than it is replaced
Transpiration stream	Movement of water from roots, through the xylem, out of the leaves
Translocation	Movement of dissolved sugars in the phloem from leaves to the rest of the plant
Root hair	A tiny extension of a root cell that increases surface area

Worked reminder: using data to name the limiting factor

A grower records the rate of photosynthesis (arbitrary units) as light increases, at 15 °C and at 25 °C, with the same carbon dioxide supply.

Light (units)	1	2	3	4	5
Rate at 15 °C	4	8	10	10	10
Rate at 25 °C	4	8	12	16	17

At light 1 and 2 the rates are identical at both temperatures, so light limits. At light 4, the rate is 10 at 15 °C but 16 at 25 °C, so temperature limits at 15 °C. Percentage increase from warming at light 4 = (16 − 10) ÷ 10 × 100 = 60%. Model sentence: “At 15 °C the rate is flat from 3 to 5 units because temperature, not light, is limiting; warming to 25 °C raises it from 10 to 16.”

Photosynthesis and respiration side by side

	Photosynthesis	Respiration
Where	Cells with chloroplasts	All living cells
Light needed	Yes	No
Reactants	Carbon dioxide and water	Glucose and oxygen
Energy	Absorbed from light	Released
When	When light is available	All the time

Command words for this unit

• State: a short fact, for example “chlorophyll”.
• Describe: say what happens, using the data if there is any.
• Explain: give the reason, using “because” or “so that”.
• Suggest: apply what you know to an unfamiliar situation, such as a new crop or a windy day.
• Evaluate: give points for and against and reach a decision.

3.2.2 Exchange in plants

Feature	Function
Stomata	Let carbon dioxide in and oxygen out by diffusion; help control water loss
Root hairs	Increase surface area for water and mineral ion uptake
Flat, thin leaf	Large surface area to volume ratio
Internal air spaces	Let gases diffuse to cells inside the leaf
Guard cells	Surround each stoma and control its size

Surface area to volume ratio, worked: leaf 6 cm × 2 cm × 0.1 cm has surface area 2(12 + 0.6 + 0.2) = 25.6 cm² and volume 1.2 cm³, so the ratio is 21.3 : 1. Thin shapes give high ratios.

Water loss and wilting

• Water vapour is lost mainly from leaves, mostly through stomata.
• Faster in hot, dry and windy conditions.
• If water is lost faster than roots replace it, the stomata can close. This prevents wilting but reduces carbon dioxide uptake.
• Water entering the guard cells opens the pore; water leaving them closes it.

Small reminder: a shoot loses 5.0 g of water in a hot room and 3.5 g in a cooler room. The cooler room loses (5.0 − 3.5) ÷ 5.0 × 100 = 30% less.

Xylem and phloem

	Xylem	Phloem
Carries	Water, mineral ions	Dissolved sugars
Direction	Roots to stem and leaves	From leaves to growing regions and storage organs
Named process	Transpiration stream	Translocation
Structure	Hollow dead tubes, strong walls with lignin	Living sieve tubes with pores, companion cells
Why it suits the job	Open tube for flow; walls resist collapse	Pores allow flow; companion cells supply energy

Must-know distinction: the transpiration stream carries water upwards to be lost at the leaves; translocation carries sugar away from the leaves. Both are in separate tissues.

Quick self-test

1. Give the symbol equation for photosynthesis, balanced.
2. Where is chlorophyll found?
3. Name two substances a plant makes from glucose for storage.
4. Why does a plant need nitrate ions?
5. A graph of rate against light intensity flattens. What does that tell you?
6. A greenhouse yield rises by 90 kg, selling at £1.50 per kg, and the extra cost is £110. Is it worth it?
7. Give two ways of making the pondweed practical more reliable.
8. How do root hairs help a plant?
9. Which cells control the size of a stoma?
10. Name the tissue that carries dissolved sugars, and the process.

Answers

1. 6CO₂ + 6H₂O → C₆H₁₂O₆ + 6O₂ (light above the arrow).
2. In chloroplasts, found in some plant cells and in algae.
3. Any two of starch, fat or oil.
4. To make proteins (with glucose-derived molecules).
5. Another factor is now limiting, not light.
6. Income 90 × 1.50 = £135; net = 135 − 110 = £25 gain, so yes, marginally.
7. Repeat and take a mean; use a heat shield; measure gas volume.
8. They increase the surface area for uptake of water and mineral ions.
9. Guard cells.
10. Phloem; translocation.

Where marks are usually lost

• A symbol equation that does not balance, or a missing energy source.
• Saying “plants take in oxygen for photosynthesis”.
• Explaining a flat graph with “the plant is full” rather than naming another limiting factor.
• Giving an economics answer with no numbers when the question supplies costs and prices.
• Describing the practical without naming the variable that is kept constant.
• Writing “stomata open and close” without saying that guard cells change shape.
• Saying water moves up the xylem “because the roots push it” and forgetting the pull from evaporation in leaves.
• Naming xylem as the sugar carrier.

Official syllabus

OxfordAQA International GCSE Biology (9201), Version 5.2, for exams May/June 2018 onwards, published by OxfordAQA. Sections 3.2.1 and 3.2.2.