Transpiration and Translocation

This lesson covers the two main transport processes in plants: transpiration (the movement of water) and translocation (the movement of dissolved sugars). Understanding how plants move substances is essential for the AQA GCSE Combined Science Trilogy specification (8464).

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Transpiration

What Is Transpiration?

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

The transpiration stream is the continuous flow of water through the plant:

graph TD
    A["Water absorbed from soil by root hair cells (by osmosis)"] --> B["Water moves through root cortex cells (by osmosis)"]
    B --> C["Water enters xylem vessels in the root"]
    C --> D["Water transported up through xylem in the stem"]
    D --> E["Water reaches the leaves via xylem in leaf veins"]
    E --> F["Water evaporates from cell surfaces inside the leaf"]
    F --> G["Water vapour diffuses out through stomata"]
    G --> H["Water vapour lost to the atmosphere (transpiration)"]

How Does the Transpiration Stream Work?

1. Root hair cells absorb water from the soil by osmosis (water moves from a dilute solution in the soil to a more concentrated solution inside the root hair cell)
2. Water passes from cell to cell across the root cortex by osmosis
3. Water enters the xylem vessels in the centre of the root
4. Water is pulled up through the xylem in the stem by the transpiration pull — as water evaporates from the leaves, it creates a continuous "pull" that draws water upward from the roots
5. In the leaves, water evaporates from the surfaces of mesophyll cells into the air spaces
6. Water vapour diffuses out through the stomata into the surrounding air

Exam Tip: The transpiration stream is driven by evaporation from the leaves, not by the roots pushing water up. The loss of water at the top creates a "pull" that draws water upward through the xylem. This is called the transpiration pull or cohesion-tension mechanism.

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Factors Affecting the Rate of Transpiration

Several environmental factors affect how fast a plant loses water by transpiration:

Factor	Effect on Transpiration Rate	Explanation
Temperature	Increases ↑	Higher temperature gives water molecules more kinetic energy, so evaporation is faster; the concentration gradient for water vapour diffusion is steeper
Wind speed	Increases ↑	Wind blows humid air away from the leaf surface, maintaining a steep concentration gradient for water vapour diffusion
Light intensity	Increases ↑	Light causes stomata to open (guard cells photosynthesise and become turgid), allowing more water vapour to escape
Humidity	Decreases ↓	High humidity means the air already contains a lot of water vapour, reducing the concentration gradient and slowing diffusion of water out of the leaf

Measuring Transpiration — The Potometer

A potometer is a piece of apparatus used to measure the rate of water uptake by a plant (which is closely related to the rate of transpiration).

How it works:

1. A leafy shoot is placed in a sealed tube connected to a capillary tube with a scale
2. As the plant transpires, it draws water from the capillary tube
3. The distance an air bubble moves along the capillary tube in a given time is measured
4. This indicates the rate of water uptake

Important: A potometer measures the rate of water uptake, not transpiration directly. However, since most of the water taken up is lost through transpiration, it gives a very close approximation.

Exam Tip: If asked to describe an experiment using a potometer, always mention: (1) how you would set it up, (2) what you would measure (distance the air bubble moves in a set time), (3) how you would change the independent variable (e.g. use a fan for wind speed), and (4) what you would keep constant (control variables).

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Worked Example: Transpiration Investigation

Question: A student uses a potometer to investigate the effect of wind on the rate of transpiration. Describe how the student should set up the investigation and suggest expected results.

Answer:

Method:

1. Set up the potometer with a leafy shoot, ensuring there are no air bubbles and all connections are sealed
2. Record the starting position of the air bubble in the capillary tube
3. Leave the plant in still air (no wind) and measure the distance the bubble moves in 10 minutes — this is the control
4. Reset the bubble and repeat with a fan at a set distance from the plant (e.g. 30 cm)
5. Repeat each condition at least 3 times and calculate the mean distance moved
6. Keep all other variables constant: temperature, light intensity, humidity, same plant, same volume of water

Expected results:

• With wind, the bubble moves a greater distance in 10 minutes
• This is because wind removes humid air from around the stomata, maintaining a steeper concentration gradient for water vapour, which increases the rate of diffusion and therefore transpiration

$$\text{Rate of water uptake} = \frac{\text{distance bubble moves (mm)}}{\text{time (minutes)}}$$Rate of water uptake=time (minutes)distance bubble moves (mm)​

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Translocation

What Is Translocation?

Translocation is the transport of dissolved sugars (mainly sucrose) and other organic molecules (such as amino acids) through the phloem from where they are produced (sources) to where they are needed (sinks).

Term	Definition	Examples
Source	Where sugars are produced or released	Leaves (photosynthesis), storage organs releasing stored food
Sink	Where sugars are used or stored	Roots (for storage or respiration), growing tips, developing fruits, flowers

graph TD
    S1["Source: Leaves (produce sugars by photosynthesis)"] -->|"Phloem — translocation"| SK1["Sink: Root (storage as starch)"]
    S1 -->|"Phloem — translocation"| SK2["Sink: Growing shoot tip (growth)"]
    S1 -->|"Phloem — translocation"| SK3["Sink: Developing fruit (growth and sugar storage)"]
    S2["Source: Storage organ releasing food (e.g. potato tuber in spring)"] -->|"Phloem — translocation"| SK4["Sink: New shoots and leaves"]

Key Facts About Translocation

• Translocation occurs in the phloem
• The phloem is made of living cells — sieve tube elements and companion cells
• Translocation is an active process — it requires energy from respiration (supplied by companion cells)
• Transport can occur in both directions (up and down) depending on where the source and sink are
• Sucrose is the main sugar transported because it is soluble and less reactive than glucose

Comparing Transpiration and Translocation

Feature	Transpiration (via Xylem)	Translocation (via Phloem)
What is transported	Water and dissolved minerals	Dissolved sugars (sucrose) and amino acids
Direction	One-way: roots → leaves (upward)	Bidirectional: source → sink
Tissue	Xylem	Phloem
Cell type	Dead, hollow, lignified	Living sieve tubes + companion cells
Energy required	No (passive — driven by evaporation/transpiration pull)	Yes (active — energy from companion cells)
Mechanism	Evaporation and transpiration pull	Pressure flow (active loading of sucrose)

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The Role of Root Hair Cells in Water Absorption

Root hair cells are found on the surface of roots and are specifically adapted for absorbing water and mineral ions from the soil.

Adaptation	How It Helps
Long, thin extension (root hair)	Massively increases the surface area for absorption
Thin cell wall	Short diffusion distance for water
Large permanent vacuole	Maintains a low water potential inside the cell, supporting osmosis
Many mitochondria	Provide energy for active transport of mineral ions
No chloroplasts	Underground — no light available for photosynthesis