# Transpiration

> Biology · CIE A-Level
> Source: https://www.owlsprep.com/study/cie-9700-u7-transpiration/

Transpiration is the loss of water vapour from plant leaves, and drives the movement of water and minerals from roots to shoots. This module covers the mechanism of water movement, controlling factors, experimental measurement, and plant adaptations.

**Prerequisites:** [Overview of transport in plants](https://www.owlsprep.com/study/cie-9700-u7-transport-overview/); [Structure of xylem tissue](https://www.owlsprep.com/study/cie-9700-u7-plant-tissues/)

## Learning objectives

- Define transpiration and outline its functional roles in plants
- Explain water movement from soil to leaf via the cohesion-tension theory
- Describe how biotic and abiotic factors affect transpiration rate
- Explain the use of a potometer to estimate transpiration rate
- Outline xerophyte adaptations to reduce transpiration

## Transpiration and the Cohesion-Tension Theory

**Transpiration** — The passive loss of water vapour from the aerial parts of a plant (mostly leaves) via diffusion through open stomata. It is an inevitable consequence of opening stomata for photosynthetic CO₂ uptake.

*Example:* A large oak tree can lose >100 litres of water per day via transpiration on a hot dry day.

The currently accepted model for water movement up xylem from roots to leaves is the **cohesion-tension theory**, which relies on two key properties of water.

**Worked example:** Outline the sequence of events that pulls water from root xylem to leaf mesophyll, according to cohesion-tension theory.

1. 1. Water evaporates from mesophyll cell surfaces into leaf air spaces, then diffuses out through open stomata. This removes water from the leaf mesophyll.
2. 2. Evaporation creates tension (a pulling force) on water in the mesophyll cell walls, pulling more water from the leaf xylem into mesophyll cells.
3. 3. Water molecules are cohesive (stick together via hydrogen bonding), so the entire column of water in xylem is pulled upwards as a continuous, unbroken chain.
4. 4. This pull lowers pressure in the root xylem, drawing more water into the root from the soil via osmosis, maintaining the continuous column.

> **tip**
>
> Evidence for cohesion-tension: Tree trunks shrink slightly during the day when transpiration is highest, as tension pulls xylem walls inwards.

> **Exam tip:** You will almost always lose marks if you mention cohesion but not tension, or vice versa – always link both terms in your answer.

## Factors Affecting Transpiration Rate

Transpiration rate depends on the steepness of the water potential gradient between the leaf air spaces and the external air, and the rate of diffusion. Any factor that changes either of these alters transpiration rate:

| Factor | Effect on Transpiration Rate | Reason |
| --- | --- | --- |
| Increased light intensity | Increases (to a plateau) | Stomata open in light for photosynthesis, increasing diffusion pathway |
| Increased temperature | Increases | Higher kinetic energy increases evaporation rate, lowers external relative humidity |
| Increased air humidity | Decreases | Reduces the water potential gradient between leaf and air |
| Increased wind speed | Increases | Removes stagnant humid air around stomata, maintains steep gradient |

**Worked example:** A student measures transpiration rate at 15°C and 25°C, with all other factors held constant. Predict and explain the difference in rate.

1. Prediction: Transpiration rate will be higher at 25°C than 15°C.
2. 1. Higher temperature increases the kinetic energy of water molecules in the leaf mesophyll.
3. 2. This increases the rate of evaporation from mesophyll cell surfaces into leaf air spaces.
4. 3. Higher temperature also reduces the relative humidity of external air, increasing the steepness of the water potential gradient between the leaf and the environment. Both factors increase water loss.

## Measuring Transpiration with a Potometer

**Potometer** — A piece of apparatus that estimates transpiration rate by measuring the rate of water uptake by a cut plant shoot. Water uptake is approximately equal to water loss via transpiration, so it gives a reliable estimate.

In a standard potometer, an air bubble is introduced to a capillary tube. As the plant takes up water to replace water lost via transpiration, the bubble moves along the capillary. The distance moved per unit time is used to calculate transpiration rate.

**Worked example:** A student sets up a potometer, but the air bubble does not move after 10 minutes in bright light. Suggest three possible reasons and their fixes.

1. 1. Reason: Air entered the xylem when cutting the shoot, breaking the water column, so no water can be pulled up. Fix: Cut the shoot under water to prevent air entry, then reassemble the potometer.
2. 2. Reason: Joints between the shoot and potometer are not sealed, so water leaks out instead of being taken up. Fix: Seal the joint with petroleum jelly to make it watertight and airtight.
3. 3. Reason: Stomata are all closed, so no transpiration occurs. Fix: Leave the set up in bright light for 10 more minutes to allow stomata to open before taking measurements.

> **note**
>
> A common exam question asks for the limitation of potometers: remember that not all water taken up is lost via transpiration – some is used for photosynthesis and maintaining cell turgor, so it is only an estimate.

> **Exam tip:** Always state that potometers *estimate* transpiration rate, never directly measure it, to get full marks.

## Xerophyte Adaptations to Reduce Transpiration

Xerophytes are plants adapted to survive in dry habitats with limited water, so they have evolved a range of structural and physiological adaptations to reduce transpiration rate and avoid dehydration.

- Thick waxy cuticle: reduces non-stomatal water loss through the leaf epidermis
- Sunken stomata: trap humid air in pits around stomata, reducing the water potential gradient
- Reduced leaf surface area (e.g. cactus spines): lowers total surface area for water loss
- Leaf hairs: trap stagnant humid air around stomata, reducing diffusion out
- Stomatal closure during the day: reduces water loss when temperatures are highest

**Worked example:** Explain how sunken stomata reduce transpiration rate.

1. 1. Sunken stomata are located in deep pits below the surface of the leaf.
2. 2. Water vapour that diffuses out of the stomata gets trapped inside the pit, rather than being blown away by wind.
3. 3. This increases the humidity of the air immediately outside the stomatal pore, reducing the water potential gradient between the leaf interior and external air.
4. 4. A smaller gradient reduces the rate of diffusion of water vapour out of the leaf, lowering overall transpiration rate.

## Common pitfalls

- **Wrong:** Stating that tension pulls water up xylem but omitting mention of cohesion between water molecules
  - Why it fails: Cohesion is required to maintain a continuous water column; without it the column would break and no movement can occur
  - Correct: Always mention both cohesion (water molecules stick together) and tension (pull from evaporation) when explaining water movement up xylem
- **Wrong:** Claiming that potometers directly measure transpiration rate
  - Why it fails: Potometers measure water uptake, and some water taken up is used for processes other than transpiration
  - Correct: Always state that potometers *estimate* transpiration rate, as water uptake is approximately equal to water loss
- **Wrong:** Saying increased humidity increases transpiration rate because there is more water in air
  - Why it fails: Transpiration relies on diffusion down a water potential gradient; more water in external air reduces the gradient
  - Correct: Increased external humidity reduces the water potential gradient between the leaf and air, so transpiration rate decreases
- **Wrong:** Cutting the plant shoot in air when setting up a potometer
  - Why it fails: Cutting in air allows air bubbles to enter the xylem, breaking the water column and stopping water movement
  - Correct: Always cut the shoot under water to prevent air from entering the xylem during potometer setup
- **Wrong:** Stating that transpiration is only harmful to plants
  - Why it fails: Transpiration has key functional roles for plant survival
  - Correct: Recognise that transpiration supplies water for photosynthesis, transports mineral ions up the plant, and cools leaves via evaporation

## Cheatsheet

| Concept | Key Exam Point |
| --- | --- |
| Transpiration | Loss of water vapour via stomata, inevitable for gas exchange |
| Cohesion-tension | Cohesion = H bonds between water; Tension = pull from evaporation |
| Potometer | Estimates rate = measures water uptake, not direct measurement |
| Increased light → | Increased rate (more open stomata) |
| Increased temperature → | Increased rate (faster evaporation) |
| Increased humidity → | Decreased rate (lower water potential gradient) |
| Increased wind speed → | Increased rate (removes humid air) |
| Xerophyte adaptations | All act to reduce transpiration rate in dry habitats |

## What's next

Transpiration is a core process that links to almost all areas of plant physiology in CIE A-Level Biology. Understanding water movement helps explain how plants obtain and distribute mineral ions required for growth and photosynthesis, and how plants adapt to different environments. Potometer experiments are a common topic in both written and practical CIE papers, so mastering its setup, sources of error and limitations is critical for exam success. Transpiration also connects to abiotic stress responses, such as stomatal closure controlled by plant hormones. Next, you can explore the transport of organic solutes in phloem, and deepen your understanding of A-Level practical biology.

- [Translocation in Phloem](https://www.owlsprep.com/study/cie-9700-u7-translocation/)
- [Xerophyte adaptations](https://www.owlsprep.com/study/cie-9700-u7-xerophyte-adaptations/)
- [Transport in Mammals](https://www.owlsprep.com/study/cie-9700-u8-overview/)

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