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

This guide follows the OxfordAQA International GCSE Biology (9201) specification, Version 5.2, for exams May/June 2018 onwards. It covers sections 3.2.1 (Photosynthesis) and 3.2.2 (Exchange and transport in plants) of the Bioenergetics topic. The qualification is untiered, so there is one set of content for everyone. For the condensed version use the revision notes, then test yourself with the practice questions. The wider topic is in the Bioenergetics study guide. Course page: OxfordAQA IGCSE Biology hub. Printable list: checklist. If you are unsure where you stand, take a free 10-minute diagnostic first.

What this unit covers

Section	What you must be able to do
3.2.1 a	Give the word and balanced symbol equations for photosynthesis
3.2.1 b	Describe what happens: chlorophyll in chloroplasts absorbs light; carbon dioxide and water become glucose; oxygen is a by-product
3.2.1 c	Explain how low temperature, shortage of carbon dioxide and shortage of light limit the rate; relate this to the economics of enhancing greenhouse conditions
3.2.1 c (required practical)	Investigate how variables affect the rate of photosynthesis
3.2.1 d, e	List the uses of glucose; explain why nitrate ions are needed for protein
3.2.2 a-c	Explain gas and water uptake, and how roots and leaves are adapted for exchange
3.2.2 d, e	Explain water loss, wilting and how guard cells control stomata
3.2.2 f	Describe xylem, phloem, the transpiration stream and translocation, and link structure to function

3.2.1 a-b: The photosynthesis equation

Photosynthesis is how a plant (or alga) makes its own food using light.

Word equation: carbon dioxide + water → glucose + oxygen, with light as the energy source (written over the arrow).

Symbol equation: 6CO₂ + 6H₂O → C₆H₁₂O₆ + 6O₂

Count the atoms to confirm the balance. Left: 6 carbon, 12 hydrogen, 18 oxygen (12 from the carbon dioxide, 6 from the water). Right: the same totals (6 oxygen atoms in glucose, 12 in the oxygen gas).

What happens, in order:

• Light is absorbed by chlorophyll, the green substance found in chloroplasts. Chloroplasts are present in some plant cells (mainly in the leaf, for example palisade cells) and in algae.
• Light energy is used to convert carbon dioxide from the air and water from the soil into glucose.
• Oxygen is released as a by-product.

Roots and the inside of a potato contain no chloroplasts, so they do not photosynthesise. Say “some plant cells”, not “all plant cells”.

3.2.1 d-e: What the plant does with glucose

Glucose is a source of chemical energy and a building block. It can be:

Use	What it is for
Respiration	Releases energy for growth, active transport and other life processes
Converted to starch	Insoluble, so it stores glucose without affecting water movement by osmosis
Made into fat or oil	Stored, for example in seeds, as an energy-rich reserve
Made into cellulose	Strengthens the cell wall
Made into protein	For growth and repair; this also needs nitrate ions

Plants absorb nitrate ions from the soil through their roots. Nitrate supplies the nitrogen that glucose lacks, so the plant can build amino acids and then proteins. With too little nitrate, a plant makes less protein and grows poorly.

3.2.1 c: Limiting factors

The rate of photosynthesis may be limited by low temperature, a shortage of carbon dioxide or a shortage of light. These factors interact: whichever is in shortest supply sets the maximum rate. Supplying more of that factor lifts the rate until a different factor takes over.

• Light: rate rises as light intensity rises, then levels off.
• Carbon dioxide: the same pattern as concentration rises.
• Temperature: at low temperatures the reactions are slow, so warming increases the rate. Very high temperatures damage the enzymes involved, and the rate falls (enzymes are covered in section 3.2.4).

How to read a graph: on the rising part of the line, the factor on the x-axis is limiting. On the flat part, that factor is no longer limiting; something else is.

Worked example 1: reading limiting factors from data

A student measures the rate of photosynthesis (arbitrary units) at two carbon dioxide concentrations. Temperature is constant and warm.

Light (units)	0	2	4	6	8	10
Rate at 0.04% CO₂	0	8	16	22	22	22
Rate at 0.12% CO₂	0	8	16	24	32	36

(a) What limits the rate at 0.04% CO₂ and light 2? Light: the rate is the same at both CO₂ levels, and it rises with light.

(b) What limits the rate at 0.04% CO₂ and light 8? Carbon dioxide: more light does not raise the rate (22 stays 22), but raising CO₂ to 0.12% increases it from 22 to 32.

(c) Why is the answer in (b) not temperature? Temperature was constant and warm, and changing CO₂ alone produced a change in rate, so CO₂ is the factor that is limiting.

3.2.1 c: Economics of greenhouse conditions

Growers can raise temperature (heaters), carbon dioxide concentration (burning fuel or releasing the gas) and light intensity (lamps). Each raises costs. It only pays to change a factor that is currently limiting, and only if the extra crop is worth more than the extra running cost. Adding light to a plant that is short of carbon dioxide wastes money.

Worked example 2: is it worth it?

A greenhouse produces 450 kg of tomatoes per season, sold at £1.20 per kg. Carbon dioxide enrichment costs £360 per season and raises the yield to 540 kg.

• Extra yield: 540 − 450 = 90 kg
• Extra income: 90 × 1.20 = £108
• Net change: 108 − 360 = −£252

The enrichment loses money (each extra kilogram costs £4.00 to produce but sells for £1.20). The grower should not do it at these prices. A cheaper method, or a higher price per kilogram, could change the answer.

Required practical: rate of photosynthesis

The specification asks you to investigate how variables affect the rate of photosynthesis. A common method uses a water plant such as pondweed (Elodea).

1. Place a piece of pondweed in water in a beaker or boiling tube, with a lamp at a measured distance.
2. Record the oxygen given off: either count bubbles each minute, or trap the gas and read its volume after a fixed time.
3. Change one variable (lamp distance, hydrogencarbonate concentration to supply carbon dioxide, or water temperature in a water bath).
4. Keep the others constant. Repeat at least three times and calculate a mean.

Control points: a heat shield or water bath stops the lamp changing the temperature; waiting a few minutes lets the plant adjust to each new setting.

Worked example 3: mean and rate

Oxygen collected in 5 minutes (mm³), three repeats at each lamp distance:

Distance (cm)	Repeat 1	Repeat 2	Repeat 3	Mean	Rate (mm³ per min)
10	52	49	55	52	10.4
20	30	33	27	30	6.0
30	18	21	15	18	3.6

Mean at 10 cm = (52 + 49 + 55) ÷ 3 = 52. Rate = 52 ÷ 5 = 10.4 mm³ per minute. Conclusion: the rate falls as the lamp moves away, because light intensity at the plant falls. Evaluation: counting bubbles is subjective when the bubbles differ in size, and a measured gas volume is more dependable; light from the room is a possible source of error.

Anomalous results should be identified, explained if possible, and left out of the mean.

3.2.2 a-c: Getting materials in and out

• Carbon dioxide enters leaves by diffusion through the stomata (pores, often most numerous on the lower surface).
• Most water and mineral ions are absorbed by the roots.
• Stomata let oxygen made in photosynthesis leave, and help to control water loss.

The surface area of roots is increased by root hairs: tiny extensions of root cells. A larger surface area means faster uptake of water and ions. Leaves have a large surface area to volume ratio because they are flat and thin, and because of the air spaces inside, which let carbon dioxide diffuse to the cells that need it.

Worked example 4: surface area to volume ratio

A leaf is modelled as a cuboid 5 cm × 4 cm × 0.1 cm.

• Surface area = 2(5×4 + 5×0.1 + 4×0.1) = 41.8 cm²
• Volume = 5 × 4 × 0.1 = 2.0 cm³
• Ratio = 41.8 ÷ 2.0 = 20.9 : 1

A cube of the same volume has sides of 1.26 cm, a surface area of 9.52 cm² and a ratio of 4.76 : 1. The thin leaf has about four times the ratio, so gases reach its cells much faster.

3.2.2 d-e: Water loss, wilting and guard cells

Plants mainly lose water vapour from their leaves, and most of this loss is through the stomata. Evaporation is more rapid in hot, dry and windy conditions.

If a plant loses water faster than the roots can replace it, the cells lose their firmness and the plant wilts. To prevent this, the stomata can close.

Each stoma is surrounded by two guard cells, which control its size. Guard cells that fill with water swell and curve apart, which opens the pore. Guard cells that lose water sag, and the pore shuts. Closing saves water but also stops carbon dioxide entering, so photosynthesis slows.

Worked example 5: water loss data

A leafy shoot loses mass as water evaporates. Mass lost in 2 hours:

Condition	Cool, still air	Warm, still air	Warm, windy air
Mass lost (g)	1.2	2.4	4.6

Percentage increase from cool still to warm windy = (4.6 − 1.2) ÷ 1.2 × 100 = 283%. Per hour, warm windy air gives 2.3 g. Explanation: warmth speeds evaporation and wind removes the water vapour near the stomata, which keeps the concentration gradient steep.

3.2.2 f: Transport systems

Flowering plants have two separate transport tissues.

	Xylem	Phloem
Carries	Water and mineral ions	Dissolved sugars
Direction	Roots to stem and leaves	From leaves to growing regions and storage organs
Process	Transpiration stream	Translocation
Structure	Hollow tubes of dead cells, walls strengthened with lignin	Living sieve tubes with end walls containing pores, plus companion cells
Link to function	Hollow, continuous tubes let water flow; strong walls stop collapse	Pores let sugar solution pass between cells; companion cells provide energy

The transpiration stream is the flow of water that enters at the roots, rises through the xylem and leaves the plant at the leaves. As water evaporates there, more is drawn up behind it. Translocation can move sugars both up and down, which is why phloem delivers to roots, fruits and buds alike.

Common errors

• Writing “oxygen is needed for photosynthesis” instead of “oxygen is a product”.
• Thinking respiration happens only at night: plants respire continuously, including in daylight.
• Calling the flat part of a graph “the factor has stopped working” rather than naming another limiting factor.
• Mixing up xylem (water, one direction) and phloem (sugars, translocation).
• Saying stomata “breathe”; they allow diffusion.

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.