IGCSE Biology: Transport in Plants Practice Questions
Xylem carries water and mineral ions upwards from the roots to the leaves. Phloem carries sucrose and amino acids from sources such as leaves to sinks such as roots and growing points, a process called translocation.
Topic 8 of Cambridge IGCSE Biology 0610 turns on two things: knowing which vessel carries what in which direction, and being able to explain how an environmental factor changes the transpiration rate. The questions below cover the definitions, the factors and the potometer practical.
What you need to know for Transport in Plants
- XylemDead, hollow tubes with lignified walls. They carry water and dissolved mineral ions from the roots to the stem and leaves, in one direction only, and also support the plant.
- PhloemLiving tubes that carry sucrose and amino acids from sources to sinks. Transport can occur in either direction depending on where the sink is.
- TranspirationThe loss of water vapour from plant leaves by evaporation of water at the surfaces of the mesophyll cells, followed by diffusion of water vapour through the stomata.
- Transpiration pullWater lost from the leaf is replaced by water pulled up the xylem in a continuous column, which is what draws water from the roots to the top of the plant.
- Factors affecting transpiration rateHigher temperature, lower humidity, more wind and higher light intensity all increase the rate. Light intensity acts by opening the stomata.
- WiltingWhen water loss exceeds uptake, cells lose turgor, become flaccid and no longer support the leaf and stem, so the plant wilts.
IGCSE Biology Transport in Plants questions and answers
4 exam-style questions written to the 0610 syllabus. Try each one on paper first, then open the worked answer to check your method against the marks.
Define transpiration.
Show the worked answer
- State that it is the loss of water vapour, not liquid water, from the plant.
- State the first stage: water evaporates from the surfaces of the mesophyll cells inside the leaf into the air spaces.
- State the second stage: the water vapour diffuses out through the stomata down a concentration gradient.
- Together: transpiration is the loss of water vapour from a plant, by evaporation of water at the mesophyll cell surfaces followed by diffusion of water vapour out through the stomata.
Explain why the rate of transpiration increases on a windy day but decreases when the humidity is high.
Show the worked answer
- Diffusion of water vapour out of the leaf depends on the concentration gradient between the air spaces inside the leaf and the air outside.
- Wind blows away the water vapour that has accumulated just outside the stomata, so the concentration of water vapour outside stays low.
- This keeps the concentration gradient steep, so water vapour diffuses out faster and transpiration increases.
- In humid air the concentration of water vapour outside the leaf is already high, so the gradient is shallow, diffusion is slower and transpiration decreases.
State two differences between xylem and phloem in terms of what they transport and the direction of transport.
Show the worked answer
- Xylem transports water and dissolved mineral ions. Phloem transports sucrose and amino acids.
- Xylem transports in one direction only, upwards from the roots to the leaves.
- Phloem transports from source to sink, which can be upwards or downwards depending on where the sink is.
- Additional differences: xylem cells are dead with lignified walls, phloem cells are living.
A student uses a potometer to compare the rate of water uptake by a leafy shoot at two temperatures. Describe how she should set it up and state one variable she must keep constant.
Show the worked answer
- Cut the shoot under water and assemble the potometer under water, so that no air enters the xylem and breaks the water column.
- Dry the leaves, seal the joints with petroleum jelly to make the apparatus airtight, and introduce a single air bubble into the capillary tube.
- Record the distance the bubble moves along the scale in a fixed time, then repeat and calculate a mean. Rate is distance divided by time.
- Light intensity, humidity and wind speed must be kept constant, since each affects the transpiration rate independently of temperature.
Common mistakes in this topic
- Saying xylem carries food. Xylem carries water and mineral ions. Phloem carries dissolved food.
- Describing transpiration as loss of water rather than water vapour.
- Explaining wind or humidity effects without referring to the concentration gradient.
- Confusing translocation with transpiration.
- Forgetting that stomata are mainly on the lower surface of the leaf, which reduces water loss.
Exam tips
- Learn a two column table: xylem versus phloem, covering substance, direction, living or dead, and wall structure.
- Every factor affecting transpiration should be explained through the concentration gradient or the opening of stomata.
- In potometer questions, always mention cutting under water. It is a standard mark.
- Rate equals distance moved divided by time. Include the unit, usually millimetres per minute.
- Link wilting to loss of turgor in cells rather than to the plant being dry.
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Transport in Plants FAQs
What is the difference between xylem and phloem?
Xylem consists of dead, hollow, lignified tubes that carry water and mineral ions in one direction only, upwards from the roots. Phloem consists of living tubes that carry sucrose and amino acids from sources to sinks, which can mean upwards or downwards. Xylem also provides support to the plant.
What factors increase the rate of transpiration?
Higher temperature, lower humidity, greater wind speed and higher light intensity all increase transpiration. Temperature, humidity and wind act by changing the water vapour concentration gradient between the leaf air spaces and the surrounding air. Light acts by causing the stomata to open.
Does a potometer measure transpiration?
Strictly it measures the rate of water uptake by the shoot. Most of that water is lost by transpiration, so it is used as an estimate, but a small proportion is used in photosynthesis and to keep cells turgid. Cambridge accepts the estimate as long as you describe the measurement as water uptake.
Why do plants wilt?
A plant wilts when water loss by transpiration exceeds water uptake by the roots. Cells lose water, become flaccid and lose the turgor pressure that normally pushes against the cell wall. Without that pressure the leaves and non-woody stems can no longer be held up.
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Written to the published Cambridge IGCSE Biology (0610) syllabus. Check your school entry code and syllabus year, because Core and Extended candidates are assessed on different content. Last reviewed 2026-08-12.