O-Level Biology: Transpiration & Plant Transport

Q: What is the exact definition of transpiration?

Transpiration is the 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.

Q: What are the structural differences between Xylem and Phloem?

Xylem vessels transport water and minerals UPWARDS only; they are made of dead cells with thick walls strengthened by lignin. Phloem vessels transport sucrose and amino acids UP and DOWN; they consist of living cells with sieve plates and companion cells.

Q: What environmental factors increase the rate of transpiration?

High temperature, low humidity, high wind speed, and high light intensity (which opens the stomata) all increase the rate of transpiration.

Q: What is a potometer used for?

A potometer is used to measure the rate of water UPTAKE by a plant shoot. We assume that the rate of water uptake is roughly equal to the rate of transpiration.

A diagram showing water moving up the xylem of a plant and evaporating from the leaves.

How does water travel up a 100-meter tall tree against gravity?

Through the Transpiration Stream. When water evaporates and diffuses out of the leaf stomata, it creates negative pressure (tension) at the top of the plant. Because water molecules stick together (cohesion) in the xylem vessels, the loss of one water molecule at the top literally 'pulls' the entire unbroken chain of water molecules all the way up from the roots. It works exactly like sucking on a straw.

1. The CAIE Definition of Transpiration
2. Xylem vs Phloem (Structure & Function)
3. The 4 Factors Affecting Transpiration
4. Frequently Asked Questions

Plants don't have hearts to pump blood. Their entire transport system relies on structural pipes and the physics of evaporation. Paper 2 examiners love testing your ability to distinguish between the two pipe systems (xylem and phloem). This guide from our Ultimate O-Level Biology Guide provides the exact comparisons you need.

1. The CAIE Definition of Transpiration

Do not just write "plants sweating." You must memorise this specific sentence to get the 3 marks:

Transpiration is:

the 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.

This constant loss of water creates a suction effect (transpiration pull) that draws water and dissolved mineral ions up through the plant from the roots.

2. Xylem vs Phloem (Structure & Function)

Feature	Xylem	Phloem
Transports	Water & Mineral Ions	Sucrose & Amino Acids
Direction	UPWARDS only (Roots to Leaves)	UP and DOWN (Leaves to Storage/Roots)
Living Status	Dead cells (hollow tubes)	Living cells (with companion cells)
Structure	Thick walls strengthened by tough Lignin	Thin walls, separated by porous sieve plates
Process Name	Transpiration Stream	Translocation

💡 Tutor's Tip

The "Sugar" Trap: Never write that phloem transports "glucose." Plants make glucose during photosynthesis, but they convert it immediately into Sucrose for transport, or Starch for storage. Always write "sucrose."

3. The 4 Factors Affecting Transpiration

Transpiration is just evaporation + diffusion. Any environmental condition that increases either of those will increase transpiration.

Temperature: Higher temps give water molecules more kinetic energy. They evaporate faster. Rate INCREASES.
Humidity: High humidity means the air outside the leaf is already full of water. This decreases the concentration gradient. Rate DECREASES.
Wind Speed: Wind blows away the water vapour sitting outside the leaf. This maintains a steep concentration gradient. Rate INCREASES.
Light Intensity: Plants open their stomata wider in bright light to let in CO2 for photosynthesis. Wider holes mean more water can escape. Rate INCREASES.

📋 From the Desk of Sarah Mitchell

If a question asks you to design a potometer experiment to test the effect of WIND, state you will use a fan. If they ask to test TEMPERATURE, use a heater or a warm room. If they ask to test HUMIDITY, place a clear plastic bag over the plant to trap the transpiring moisture.

Frequently Asked Questions

What is the exact definition of transpiration?▼

The loss of water vapour from plant leaves by evaporation at mesophyll surfaces, followed by diffusion through the stomata.

What are the structural differences between Xylem and Phloem?▼

Xylem are dead, lignified tubes that move water up. Phloem are living tubes with sieve plates that translocate sucrose up and down.

What environmental factors increase the rate of transpiration?▼

High temperatures (more kinetic energy), high wind speed (steeper gradient), and high light intensity (wider stomata). High humidity DECREASES the rate.

What is a potometer used for?▼

It measures the rate of water uptake by a plant shoot. We assume uptake is roughly equal to transpiration loss, so it's a proxy for transpiration rate.

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The Transpiration Stream: Xylem, Phloem, and Water Loss