Transport in Plants — Transpiration

This lesson covers the transpiration stream and the role of xylem in transporting water and mineral ions through a plant, as required by the Edexcel GCSE Combined Science specification (1SC0). You need to understand how water moves from roots to leaves and the factors that affect the rate of transpiration.

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Why Do Plants Need a Transport System?

Plants need to move substances around their bodies, but they do not have a circulatory system like animals. Instead, they have two types of specialised transport tissue:

Tissue	What it transports	Direction
Xylem	Water and dissolved mineral ions	Roots → leaves (upward only)
Phloem	Dissolved sugars (sucrose) and amino acids	Source → sink (up or down)

This lesson focuses on xylem and the transpiration stream. Phloem and translocation are covered in Lesson 4.

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Xylem Structure

Xylem vessels are dead, hollow tubes that form a continuous pipeline from roots to leaves.

Key structural features:

Feature	Purpose
Dead cells — no cytoplasm or cell contents	Hollow tube with no obstruction — water flows freely
End walls broken down	Creates a continuous column of water
Walls thickened with lignin	Provides strength and support; prevents collapse
Narrow lumen	Helps maintain a continuous water column by capillary action

graph TD
    A["Root hair cells absorb water by osmosis"] --> B["Water enters xylem in the root"]
    B --> C["Continuous water column in xylem"]
    C --> D["Water moves up the stem"]
    D --> E["Water reaches leaf mesophyll cells"]
    E --> F["Water evaporates into air spaces"]
    F --> G["Water vapour diffuses out through stomata"]
    G --> H["Transpiration pull draws more water up"]
    H --> C

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The Transpiration Stream

Transpiration is the loss of water vapour from the surface of a plant, mainly through the stomata on the underside of leaves.

The movement of water through a plant from roots to leaves is called the transpiration stream. The process works as follows:

1. Water vapour evaporates from the surfaces of mesophyll cells inside the leaf into the air spaces.
2. This water vapour diffuses out through the open stomata.
3. The loss of water from the leaf creates a slight "pull" (negative pressure) in the xylem — this is called the transpiration pull.
4. Water is drawn up the xylem vessels in a continuous column to replace what has been lost.
5. At the roots, water is absorbed from the soil by root hair cells through osmosis (movement of water from a dilute solution to a more concentrated solution across a partially permeable membrane).

Exam Tip: The transpiration stream is driven by evaporation from the leaves, not by the roots pushing water up. Water is "pulled" up the plant, not "pumped."

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Root Hair Cells

Root hair cells are adapted to absorb water efficiently:

Adaptation	How it helps
Long, thin hair-like extension	Increases the surface area in contact with soil water
Thin cell wall	Shorter diffusion pathway
Large permanent vacuole with dilute cell sap	Maintains a water potential gradient to draw water in by osmosis
Many mitochondria	Provide energy for active transport of mineral ions

Water enters root hair cells by osmosis: soil water is more dilute (higher water potential) than the cell sap (lower water potential), so water moves into the cell.

Mineral ions (e.g. nitrate, magnesium) are absorbed by active transport because they are present at a lower concentration in the soil water than in the root cells — they must move against the concentration gradient.

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Factors Affecting the Rate of Transpiration

Four main factors affect how quickly water is lost through the stomata:

Factor	Effect on transpiration rate
Temperature	Higher temperature → faster evaporation → faster transpiration
Humidity	Higher humidity → smaller diffusion gradient → slower transpiration
Wind speed	Higher wind speed → water vapour blown away from leaf surface → steeper diffusion gradient → faster transpiration
Light intensity	Higher light intensity → stomata open wider (to let CO₂ in for photosynthesis) → faster transpiration

graph TD
    A["Factors affecting transpiration rate"]
    A --> B["↑ Temperature → ↑ Rate"]
    A --> C["↑ Humidity → ↓ Rate"]
    A --> D["↑ Wind speed → ↑ Rate"]
    A --> E["↑ Light intensity → ↑ Rate"]

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Guard Cells and Stomata

Stomata (singular: stoma) are tiny pores found mainly on the lower surface of leaves. They are surrounded by two guard cells that control whether the stoma is open or closed.

Condition	Guard cells	Stoma
Light / plenty of water	Turgid (swollen with water); inner wall bows outward	Open — gas exchange and transpiration occur
Dark / water shortage	Flaccid (limp); inner walls close together	Closed — reduces water loss

Guard cells have an uneven cell wall — the inner wall (next to the stoma) is thicker. When the guard cells absorb water and become turgid, the thin outer wall stretches more than the thick inner wall, causing the cell to curve and the stoma to open.

Exam Tip: Stomata close at night and during drought to reduce water loss. If asked why stomata close, the answer is to conserve water when photosynthesis cannot occur (no light) or when the plant is at risk of wilting.

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Measuring Transpiration — The Potometer

A potometer measures the rate of water uptake by a plant shoot (which is closely related to the rate of transpiration). The apparatus consists of:

1. A leafy shoot sealed into a tube of water.
2. A capillary tube with a scale.
3. An air bubble is introduced; as the plant takes up water, the bubble moves along the scale.

The distance the bubble moves per unit time gives the rate of water uptake (and approximately the rate of transpiration).

Exam Tip: Strictly, a potometer measures water uptake, not transpiration directly. Some water is used in photosynthesis, not lost through the stomata — but the vast majority is lost by transpiration.

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Summary