What is transpiration in plants and why is it important?

Transpiration is the process by which water is absorbed by plant roots from the soil, transported through the plant, and then evaporated from the leaves into the atmosphere. This process is crucial as it helps in the transport of nutrients from the soil to different parts of the plant, maintains plant temperature, and facilitates photosynthesis by ensuring a continuous flow of water and minerals.

How does transpiration occur in plants?

Transpiration occurs primarily through small openings on the leaves called stomata. When these stomata open, water vapor escapes into the atmosphere. This loss of water creates a negative pressure within the leaf, pulling more water up from the roots through the xylem vessels, thus maintaining a steady flow of water and nutrients throughout the plant.

What factors affect the rate of transpiration in plants?

Several factors influence the rate of transpiration, including temperature, humidity, wind speed, and light intensity. Higher temperatures and wind speeds increase transpiration rates by causing more rapid evaporation of water. Conversely, high humidity reduces transpiration as the air is already saturated with moisture. Light intensity affects the opening of stomata, with more light generally increasing transpiration.

How is transpiration measured in a laboratory setting?

Transpiration can be measured using a potometer, a device that measures the rate of water uptake by a plant. By observing the movement of a bubble in a capillary tube connected to the plant, students can calculate the rate of transpiration. This method is commonly used in Cambridge IGCSE Biology practicals to demonstrate the concept of water transport in plants.

What are the adaptations of plants to reduce excessive transpiration?

Plants have developed several adaptations to minimize water loss through transpiration. These include having a waxy cuticle on leaves to reduce evaporation, stomata that close during the hottest parts of the day, and leaves that are reduced in size or modified into spines. Some plants, like succulents, store water in their tissues, while others, like cacti, have deep root systems to access water from deeper soil layers.

Why is "Saying transpiration is 'bad' — water loss should be avoided." a common mistake in Transpiration?

Why it happens: It's a loss of water; sounds wasteful. How to avoid it: Transpiration has BENEFITS: pulls water up the plant (transpiration stream), delivers minerals, COOLS the leaf. It's not waste — it's what powers water transport.

Why is "Saying transpiration STOPS at night." a common mistake in Transpiration?

Why it happens: Stomata are mostly closed at night. How to avoid it: Transpiration SLOWS at night (most stomata closed) but doesn't stop completely — some water escapes through the cuticle.

Why is "Saying high humidity INCREASES transpiration." a common mistake in Transpiration?

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 → smaller gradient → less diffusion.

Transport in PlantsCambridge IGCSE Biology (0610)

Transpiration

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Transpiration — Cambridge IGCSE 0610 Biology Extended (2026)

Loss of water from leaves drives the upward flow of water through the entire plant — the 'transpiration stream'.

What you’ll learn

Mapped to the Cambridge IGCSE 0610 syllabus (2026-2028).

8.3 — Define transpiration.
8.3 — Describe how environmental factors (light, temperature, humidity, wind) affect transpiration rate.
8.3 — Explain how transpiration drives water transport in xylem (transpiration pull, cohesion-tension).

What is transpiration?

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.

90%+ through STOMATA.
A small amount through the cuticle on leaves and stems (called cuticular transpiration).
Roots and stems lose negligible water.

Worked qualitative. A typical large tree (e.g. an oak) loses ~400 litres of water per day in summer. Where does it go?

Photosynthesis uses TINY fraction (a few mL).
Cell maintenance uses tiny fraction.
Transpiration accounts for ~99% of water uptake — most of it lost as water vapour.

Cambridge tip. Definition: "loss of water VAPOUR from a plant". Saying "loss of water" is too vague.

Evaporation inside the leaf.
Diffusion out via stomata.
Down a concentration gradient.
Most water taken up is lost this way.

Factors affecting transpiration rate

Anything that changes evaporation rate or the water-vapour gradient changes the rate.

Four main factors:

1. Light intensity.

More light → STOMATA OPEN (for photosynthesis).
Open stomata → more water vapour can diffuse out.
Effect: light → transpiration ↑.

2. Temperature.

Higher T → water molecules have more kinetic energy → evaporate faster.
Air can hold more water vapour at higher T → larger gradient.
Effect: T → transpiration ↑.

3. Humidity (water vapour in the surrounding air).

High humidity → little gradient between leaf interior and outside.
Less diffusion of water vapour out.
Effect: humidity ↑ → transpiration ↓.

4. Wind speed.

Wind blows away the humid air right next to the leaf surface.
Replaces it with drier air → maintains a steep gradient.
Effect: wind → transpiration ↑.

Other factors.

Soil water availability — too little → plant can't keep up → wilts.
Stomatal density and pore size (varies by species).
Surface area of leaves (a small cactus has way less than a maple tree).

Worked qualitative. A wet, foggy morning vs a hot, windy noon — when is transpiration higher?

Foggy morning: high humidity + low temp + little wind → LOW transpiration.
Hot windy noon: low humidity + high T + wind → HIGH transpiration.
Hence wilting risk is at noon.

Cambridge tip. Memorise the directions. The trap: humidity is the ONLY factor that DECREASES transpiration as it increases. The other three increase it.

Light: opens stomata; more transpiration.
T: faster evaporation.
Humidity: smaller gradient → less.
Wind: refreshes outside air → more.

Transpiration drives water transport

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.

Water lost from leaves (transpiration). Pull of water out of mesophyll cells.
Mesophyll cells PULL water from the xylem in the leaf veins.
The water COLUMN in the xylem is continuous — pulling at the top creates TENSION (suction) all the way down.
COHESION: water molecules cling to each other (hydrogen bonds). Pulling one drags the next.
ADHESION: water molecules cling to the xylem walls — helps the column climb against gravity.
Water at the bottom is pulled INTO the xylem from the root cells → osmosis pulls more water from the soil.

This whole chain is the TRANSPIRATION STREAM — sometimes called the "transpiration pull".

Why it can lift water 100+ metres in a tall tree.

The cohesion of water is incredibly strong (hydrogen bonding).
The lignified xylem walls don't collapse under tension.
Together: a redwood pulls water from roots to canopy ~100 m up — no pump needed.

Worked qualitative. Why does cutting a stem and putting it in water often "save" a wilted flower?

Wilting = water column broken (air bubble in the xylem).
Cutting under water gives a fresh end without bubbles.
The intact column can resume the transpiration pull.
Always cut flowers underwater.

Cambridge tip. When asked HOW water rises in xylem, mention BOTH cohesion (water-water) and adhesion (water-wall). Cambridge marks both.

Transpiration creates suction at top.
Cohesion: water molecules stick.
Adhesion: water sticks to xylem.
Continuous column pulled up.
Lifts water 100 m in tall trees.

Measuring transpiration — the potometer

A potometer measures water UPTAKE — used as a proxy for transpiration rate.

Setup.

Cut a leafy shoot UNDER WATER (avoid air bubbles).
Quickly insert into a water-filled tube connected to a horizontal capillary tube with markings.
Seal joints to prevent leaks.
Allow shoot to equilibrate, then introduce a small AIR BUBBLE in the capillary.
Time how far the bubble moves over a fixed period.

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:

Light: turn lamp on/off or change distance.
Temperature: place in warmer/cooler room.
Humidity: put plant in plastic bag (high humidity).
Wind: use a fan at varied speeds.

Cambridge tip. Always identify CONTROL VARIABLES. "Same plant, same time of day, only one factor changing".

Potometer: measures water uptake.
Approximates transpiration.
Vary one factor; control others.
Cut shoots underwater to avoid air bubbles.

How it’s examined

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.

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Step-by-step worked examples — Transpiration

Step-by-step solutions to past-paper-style questions on transpiration, written exactly the way a tutor would explain them at the board.

1Define transpiration
Core• definition
Question
Define transpiration and state where it happens.
Step-by-step solution
1. Step 1
  Transpiration = the loss of water vapour from a plant — mainly through STOMATA in the leaves.
2. Step 2
  Water EVAPORATES from the surfaces of cells inside the leaf into the air spaces.
3. Step 3
  Then DIFFUSES out through the stomata to the atmosphere (because the air outside is drier).
Answer
Transpiration is loss of water vapour from a plant via stomata. Water evaporates from leaf cells, then diffuses out.
Examiner tip
Cambridge mark schemes credit 'evaporation followed by diffusion'. Saying just 'water comes out' loses both halves.
2Factors affecting transpiration rate
Core• Adapted from 0610/42 May/Jun 2024 Q8• transpiration factors
Question
State how each affects transpiration rate: (a) higher light intensity, (b) higher temperature, (c) higher humidity, (d) faster wind.
Step-by-step solution
1. Step 1
  Light: more light → stomata OPEN → more transpiration (also linked to photosynthesis).
2. Step 2
  Temperature: higher → water EVAPORATES faster + air can hold more water → more transpiration.
3. Step 3
  Humidity: higher → less concentration gradient between leaf air and outside → LESS transpiration.
4. Step 4
  Wind: faster → moves humid air AWAY from leaf, maintains gradient → MORE transpiration.
Answer
↑ light, ↑ temperature, ↓ humidity, ↑ wind — all increase transpiration. (Humidity is the only one that DECREASES it.)
3How transpiration pulls water UP the plant
Extended• transpiration pull
Question
Explain how transpiration helps move water from roots to leaves (the 'transpiration stream').
Step-by-step solution
1. Step 1
  Water EVAPORATES from leaf cells into air spaces; lost via stomata.
2. Step 2
  This pulls more water from the xylem in the leaf — water exits the xylem, replacing what's lost.
3. Step 3
  Water molecules are COHESIVE (stick to each other by hydrogen bonds) — pulling one drags the next.
4. Step 4
  A continuous WATER COLUMN is pulled up the xylem from roots to leaves — the TRANSPIRATION STREAM.
5. Step 5
  Water molecules are also ADHESIVE — stick to xylem walls, helping climb against gravity.
Answer
Water lost from leaves → suction → cohesion + adhesion → continuous column pulled up xylem (transpiration stream).
Examiner tip
Cambridge wants COHESION (water-water) and ADHESION (water-xylem) terms — both worth a mark.

Key Definitions and Keywords — Transpiration

Definitions to memorise and the exact keywords mark schemes credit for transpiration answers — sharpened from recent examiner reports for the 2026 0610 sitting.

Transpiration
Examiner keyword
Loss of water vapour from a plant — mainly via stomata in the leaves. Water evaporates from cell surfaces in the leaf and diffuses out.
Transpiration stream
Examiner keyword
Continuous flow of water from roots, up the xylem, through the stem, into the leaves — driven by transpiration.
Cohesion
Water molecules sticking to each other (by hydrogen bonds). Allows a continuous column of water to be pulled up the xylem.
Adhesion
Water molecules sticking to the xylem walls. Helps water climb against gravity.
Potometer
Instrument used to measure the rate of water uptake by a plant cutting (an indirect measure of transpiration).

Common Mistakes and Misconceptions — 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.

✕Saying transpiration is 'bad' — water loss should be avoided.
Why it happens
It's a loss of water; sounds wasteful.
How to avoid it
Transpiration has BENEFITS: pulls water up the plant (transpiration stream), delivers minerals, COOLS the leaf. It's not waste — it's what powers water transport.
✕Saying transpiration STOPS at night.
Why it happens
Stomata are mostly closed at night.
How to avoid it
Transpiration SLOWS at night (most stomata closed) but doesn't stop completely — some water escapes through the cuticle.
✕Saying high humidity INCREASES transpiration.
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 → smaller gradient → less diffusion.

Practice questions

Exam-style questions with step-by-step worked solutions. Try one before checking the method.

Past paper style quiz

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Transpiration — frequently asked questions

The things students keep getting wrong in this sub-topic, answered.

Transpiration

Short Study Notes in the form of Slides

Detailed Study Notes

What you’ll learn

How it’s examined

1Define transpiration

2Factors affecting transpiration rate

3How transpiration pulls water UP the plant

Transpiration

Transpiration stream

Cohesion

Adhesion

Potometer

✕Saying transpiration is 'bad' — water loss should be avoided.

✕Saying transpiration STOPS at night.

✕Saying high humidity INCREASES transpiration.

Practice questions

Past paper style quiz

Assess your understanding

Video lesson

Transpiration — frequently asked questions