Question 1
Paper 4 short-answer style1 markName the structures in a leaf through which most water vapour is lost. (1 mark)
Model answer
The stomata.
Why this scores
One mark for 'stomata'.
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Detailed notes on Transport in Plants for Cambridge IGCSE Biology, covering key concepts, explanations, examples, and exam-focused revision points.
Loss of water from leaves drives the upward flow of water through the entire plant — the 'transpiration stream'.
Mapped to the Cambridge IGCSE 0610 syllabus (2026-2028).
Water evaporates from leaf cells, then diffuses out through stomata.
Step 1. Water from the xylem enters cells in the leaf.
Step 2. Water EVAPORATES from the surfaces of cells (especially in the spongy mesophyll) into the air spaces inside the leaf.
Step 3. Water vapour DIFFUSES out through the STOMATA (mostly on the lower surface) to the atmosphere.
Why it happens. The air inside the leaf is saturated; outside is usually drier. Water vapour moves DOWN the concentration gradient → out of the leaf.
Where most water is lost.
Worked qualitative. A typical large tree (e.g. an oak) loses ~400 litres of water per day in summer. Where does it go?
Cambridge tip. Definition: "loss of water VAPOUR from a plant". Saying "loss of water" is too vague.
Anything that changes evaporation rate or the water-vapour gradient changes the rate.
Four main factors:
1. Light intensity.
2. Temperature.
3. Humidity (water vapour in the surrounding air).
4. Wind speed.
Other factors.
Worked qualitative. A wet, foggy morning vs a hot, windy noon — when is transpiration higher?
Cambridge tip. Memorise the directions. The trap: humidity is the ONLY factor that DECREASES transpiration as it increases. The other three increase it.
Cohesion + adhesion of water molecules let transpiration suck water all the way up from roots.
The transpiration stream is the continuous flow of water from roots → up xylem → into leaves → out as vapour.
The mechanism — cohesion-tension.
This whole chain is the TRANSPIRATION STREAM — sometimes called the "transpiration pull".
Why it can lift water 100+ metres in a tall tree.
Worked qualitative. Why does cutting a stem and putting it in water often "save" a wilted flower?
Cambridge tip. When asked HOW water rises in xylem, mention BOTH cohesion (water-water) and adhesion (water-wall). Cambridge marks both.
A potometer measures water UPTAKE — used as a proxy for transpiration rate.
Setup.
What it measures. Rate of water UPTAKE by the shoot, in mm/min or cm³/min.
Why it approximates transpiration. Most water taken up is lost via transpiration — only a few % goes to photosynthesis or growth.
Investigating factors. Vary one condition at a time:
Cambridge tip. Always identify CONTROL VARIABLES. "Same plant, same time of day, only one factor changing".
Verbatim phrases and definitions Cambridge mark schemes credit.
Transpiration appears every Paper 4 (8-10 marks: define, factor effects, mechanism). Examiner reports flag students saying humidity raises transpiration, calling transpiration 'wasted water', and forgetting cohesion + adhesion when explaining the stream.
Sources: Cambridge IGCSE Biology 0610 syllabus 2026-2028 (8.3); 0610/42 May/Jun 2024 — Q8 (transpiration factors); 0610 Examiner Reports 2022-2024. Last reviewed 2026-05-07.
Step-by-step solutions to past-paper-style questions on transpiration, written exactly the way a tutor would explain them at the board.
Question
Define transpiration and state where in the leaf it happens.
Step-by-step solution
Step 1
Transpiration is the loss of water vapour from a plant — mainly through the stomata of the leaves.
Step 2
Water first EVAPORATES from the surfaces of the mesophyll cells into the air spaces of the leaf.
Step 3
The water vapour then DIFFUSES out through the open stomata into the drier air outside.
Answer
Transpiration is the loss of water vapour from a plant through the stomata: water evaporates from leaf cells into the air spaces, then diffuses out.
Examiner tip
Cambridge credits 'evaporation' followed by 'diffusion'. Saying just 'water leaves' loses both halves.
Question
State how each affects the rate of transpiration: (a) higher light intensity, (b) higher temperature, (c) higher humidity, (d) faster air movement (wind).
Step-by-step solution
Step 1
Light: brighter light makes the stomata open wider → more transpiration.
Step 2
Temperature: higher temperature → faster evaporation and faster diffusion → more transpiration.
Step 3
Humidity: higher humidity → smaller concentration gradient between the leaf and the air → less transpiration.
Step 4
Wind: faster air movement carries away the water vapour, keeping a steep gradient → more transpiration.
Answer
↑ light, ↑ temperature and ↑ wind all increase transpiration; ↑ humidity decreases it.
Question
Explain how transpiration moves water from the roots to the leaves (the transpiration stream).
Step-by-step solution
Step 1
Water evaporates from the mesophyll cells and is lost through the stomata, so these cells lose water.
Step 2
This creates a pull (suction) that draws water out of the xylem in the leaf to replace what is lost.
Step 3
Water molecules are cohesive (they stick to each other), so pulling one drags up the next, keeping a continuous column.
Step 4
Water is therefore pulled up the xylem all the way from the roots — the transpiration stream — and the molecules also adhere to the xylem walls, helping the column rise.
Answer
Water lost from the leaves creates a pull; because water molecules are cohesive (and adhesive to the walls), a continuous column is pulled up the xylem from the roots — the transpiration stream.
Examiner tip
Use the terms cohesion (water–water) and adhesion (water–xylem). Both can earn marks.
Question
Explain, in terms of diffusion, why transpiration is faster in moving air but slower in humid air.
Step-by-step solution
Step 1
Transpiration depends on water vapour diffusing out of the leaf, which needs a concentration gradient between the air spaces (high water vapour) and the air outside (lower).
Step 2
Moving air (wind) blows away the humid air next to the leaf, so the air just outside stays dry — keeping the gradient steep and speeding up diffusion.
Step 3
Humid air already contains a lot of water vapour, so the difference between inside and outside is small — a shallow gradient.
Step 4
A shallow gradient means water vapour diffuses out slowly, so transpiration is reduced.
Answer
Transpiration is diffusion of water vapour down a gradient. Wind removes humid air and keeps the gradient steep (faster); humid air lowers the gradient (slower).
Examiner tip
Always link these two factors to the concentration gradient for diffusion — that is the mechanism examiners want.
Question
Describe how a potometer is used to compare the rate of transpiration in still air and in moving air.
Step-by-step solution
Step 1
Cut a leafy shoot under water (to avoid air entering the xylem) and fit it into the potometer, which is filled with water with no air bubbles, except one introduced air bubble.
Step 2
As the shoot transpires, it draws up water, and the air bubble moves along the scale; the distance the bubble moves in a set time gives the rate of water uptake (≈ transpiration).
Step 3
Record the bubble's movement in still air first; then place a fan near the shoot (moving air) and record again, keeping temperature, light and the shoot the same.
Step 4
The bubble moves faster in moving air, showing transpiration is faster, because wind keeps the concentration gradient steep.
Answer
A potometer measures water uptake from how far an air bubble moves in a set time. Comparing still and moving air (with other factors constant) shows transpiration is faster in moving air.
Examiner tip
Cut the shoot under water and exclude air bubbles. The potometer measures uptake, which is taken as a measure of transpiration.
Question
A potometer's air bubble moved these distances in 10 minutes under different conditions: still air in the shade → 20 mm; still air in bright light → 35 mm; bright light with a fan → 60 mm. Explain the results.
Step-by-step solution
Step 1
Moving from shade to bright light increased the distance (20 → 35 mm) because brighter light makes the stomata open wider, so more water vapour is lost.
Step 2
Adding a fan increased it further (35 → 60 mm) because moving air removes the humid air next to the leaf, keeping a steep concentration gradient.
Step 3
Each change increases the rate of diffusion of water vapour out of the leaf, so the shoot takes up water faster and the bubble moves further.
Step 4
The fastest rate is in bright, windy conditions; the slowest is in still shade — matching the factors that affect transpiration.
Answer
Bright light opens the stomata (more loss); a fan keeps the gradient steep (more loss). So the rate, shown by how far the bubble moves, is highest in bright, windy conditions and lowest in still shade.
Examiner tip
Use the data values and link each increase to a named factor and its effect on stomata or the concentration gradient.
High-scoring sample answers for transpiration on the Cambridge IGCSE 0610 paper, with examiner-style notes mapping each response to the mark scheme and assessment objectives.
Name the structures in a leaf through which most water vapour is lost. (1 mark)
Model answer
The stomata.
Why this scores
One mark for 'stomata'.
State how (a) an increase in temperature and (b) an increase in humidity affect the rate of transpiration. (2 marks)
Model answer
(a) An increase in temperature increases the rate of transpiration. (b) An increase in humidity decreases the rate of transpiration.
Why this scores
One mark each, with the correct direction. Humidity is the factor that decreases the rate.
Explain why the rate of transpiration is higher on a windy day. (3 marks)
Model answer
Transpiration depends on water vapour diffusing out of the leaf down a concentration gradient. On a windy day, the moving air blows away the humid air next to the leaf surface, so the air just outside stays drier. This keeps a steep concentration gradient between the inside of the leaf and the air, so water vapour diffuses out faster and the rate of transpiration increases.
Why this scores
Three marks: wind removes humid air at the surface; keeps a steep concentration gradient; so faster diffusion/transpiration.
Explain how water is moved from the roots up to the leaves in the transpiration stream. (4 marks)
Model answer
Water evaporates from the mesophyll cells and is lost through the stomata, so these cells draw water from the xylem in the leaf. This creates a pull (tension) on the column of water in the xylem. Because water molecules are cohesive (they stick to one another), the whole column is pulled up together, so water is drawn up the xylem from the roots. The molecules also adhere to the xylem walls, which helps hold the column up against gravity.
Why this scores
Four marks: water lost from leaves creates a pull; transmitted through the xylem; cohesion of water molecules keeps a continuous column; adhesion to walls (or steep gradient maintained).
Describe how a potometer is used to investigate the effect of one factor on the rate of transpiration. (5 marks)
Model answer
Cut a leafy shoot under water and fit it tightly into the potometer, which is filled with water and has no air bubbles in the tube. Introduce a single air bubble into the capillary tube. As the shoot transpires it draws up water, and the air bubble moves along a scale; measure the distance the bubble moves in a set time to find the rate of water uptake, which is taken as the rate of transpiration. Change one factor — for example increase the temperature, or add a fan — while keeping all other conditions the same, and measure the bubble's movement again. Repeat and find a mean. Comparing the rates shows how that factor affects transpiration.
Why this scores
Five marks: cut shoot under water/fit airtight; introduce an air bubble; measure distance moved per time; change only one factor (control the rest); repeat for reliability.
Explain why the rate of transpiration is highest in hot, dry, windy conditions and lowest in cool, humid, still conditions. (6 marks)
Model answer
Transpiration is the evaporation of water from the leaf cells followed by diffusion of water vapour out through the stomata, so anything that speeds up evaporation or steepens the concentration gradient increases the rate. In hot conditions, the higher temperature gives the water molecules more energy, so water evaporates faster from the cell surfaces and diffuses out faster. In dry air, there is little water vapour outside the leaf, so the concentration gradient between the leaf and the air is steep, speeding up diffusion. In windy conditions, the moving air removes the humid air next to the leaf, keeping the gradient steep. All three effects add together, so the rate is highest. In cool, humid, still conditions the opposite is true: evaporation is slow, the air outside is already moist so the gradient is shallow, and the still air lets humid air build up next to the leaf — so diffusion is slow and the rate is lowest.
Why this scores
Up to 6 marks: heat → faster evaporation; dry air → steep gradient; wind → removes humid air/keeps gradient steep; and the reverse for cool/humid/still. Reward linking each condition to evaporation or the diffusion gradient.
Definitions to memorise and the exact keywords mark schemes credit for transpiration answers — sharpened from recent examiner reports for the 2026 0610 sitting.
Loss of water vapour from a plant — mainly via the stomata. Water evaporates from cell surfaces inside the leaf and diffuses out.
The continuous flow of water from the roots, up the xylem, through the stem and into the leaves — driven by transpiration.
Water molecules sticking to each other. Allows a continuous column of water to be pulled up the xylem.
Water molecules sticking to the xylem walls. Helps water rise against gravity.
Instrument used to measure the rate of water uptake by a leafy shoot — taken as an indirect measure of transpiration.
The traps other students keep falling into on transpiration questions — taken from recent Cambridge IGCSE 0610 examiner reports and mark schemes — and how to avoid them.
Why it happens
It's a loss of water, which sounds wasteful.
How to avoid it
Transpiration drives the transpiration stream (water and mineral transport) and cools the leaf. It is a consequence of the stomata being open for photosynthesis.
Why it happens
Most stomata close at night.
How to avoid it
Transpiration SLOWS at night as the stomata close, but some water still escapes through the cuticle.
Why it happens
Reasoning that 'humid air is wet, so more water moves'.
How to avoid it
Humidity REDUCES transpiration. Diffusion needs a concentration gradient — humid air outside has lots of water vapour, so the gradient is small and less water diffuses out.
The things students keep getting wrong in this sub-topic, answered.