Transport in Plants

Plants do not have a heart or a circulatory system, but they still need to transport substances from one part of the organism to another. For AQA GCSE Biology, you need to understand the two main transport processes in plants: the movement of water and minerals through the xylem (the transpiration stream) and the movement of dissolved sugars through the phloem (translocation). This lesson covers both processes in detail, including the factors affecting the rate of transpiration.

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Water Uptake by Roots

Water enters the plant through the root hair cells, which are found on the surface of young roots. Root hair cells are specialised for absorption:

Adaptation	How It Helps
Long, thin hair-like extension	Increases the surface area in contact with soil water
Thin cell wall	Reduces the distance for water to travel into the cell
Large permanent vacuole	Maintains a low water potential inside the cell (more concentrated solution), creating a steep concentration gradient

How Water Enters the Root

Water enters root hair cells by osmosis — the movement of water molecules from a dilute solution (the soil water) to a more concentrated solution (inside the root hair cell) across a partially permeable membrane.

Once inside the root hair cell, the water moves from cell to cell across the root cortex by osmosis (each successive cell has a slightly more concentrated solution) until it reaches the xylem in the centre of the root.

Mineral ions (such as nitrates, phosphates and potassium) are absorbed from the soil by active transport. This requires energy (from respiration) because the minerals are often at a higher concentration inside the root cells than in the soil — they must be moved against the concentration gradient.

Exam Tip: Do not confuse the transport of water (osmosis — passive, down a concentration gradient) with the transport of mineral ions (active transport — requires energy, against the concentration gradient). This distinction is frequently tested.

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The Transpiration Stream

Once water has entered the xylem vessels in the root, it is transported upward through the stem to the leaves in a continuous column. This movement of water through the plant is called the transpiration stream.

graph TD
    A[Water absorbed from soil by root hair cells — osmosis] --> B[Water moves across root cortex by osmosis]
    B --> C[Water enters xylem vessels in the root]
    C --> D[Water moves up through xylem in the stem]
    D --> E[Water reaches the leaves]
    E --> F[Water evaporates from spongy mesophyll cells]
    F --> G[Water vapour diffuses out through stomata]
    G --> H[This is TRANSPIRATION]
    H -->|Creates a pull| D

What Drives the Transpiration Stream?

The transpiration stream is driven by the evaporation and loss of water from the leaves:

1. Water vapour inside the leaf (in the air spaces of the spongy mesophyll) diffuses out through the open stomata — this is transpiration
2. This water loss from the leaf cells causes water to be pulled up from the xylem vessels in the leaf veins to replace it
3. This creates a continuous pull (tension) on the column of water in the xylem, drawing water up from the roots
4. As water leaves the root xylem, more water enters from the root hair cells by osmosis

This entire process — from water absorption in the roots to water loss from the leaves — is the transpiration stream.

Exam Tip: Transpiration itself is the loss of water vapour from the leaves through the stomata. The transpiration stream is the entire movement of water through the plant from roots to leaves. These are related but different concepts — make sure you can distinguish them.

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Factors Affecting the Rate of Transpiration

Several environmental factors affect how quickly water is lost from the leaves:

Factor	Effect on Transpiration Rate	Explanation
Temperature	Increasing temperature increases the rate	Water molecules have more kinetic energy and evaporate more quickly from the mesophyll cells; diffusion of water vapour out of the stomata is faster
Humidity	Increasing humidity decreases the rate	A higher concentration of water vapour in the air outside the leaf reduces the concentration gradient between the inside and outside of the leaf, so diffusion slows
Wind speed	Increasing wind speed increases the rate	Wind blows away water vapour from around the leaf surface, maintaining a steep concentration gradient and increasing the rate of diffusion
Light intensity	Increasing light intensity increases the rate	Stomata open wider in bright light (to allow more CO2 in for photosynthesis), so more water vapour can escape
Number of stomata	More stomata increase the rate	More exit points for water vapour to escape
Waxy cuticle	A thicker cuticle decreases the rate	Reduces evaporation from the leaf surface

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). It works by:

1. A leafy shoot is attached to a sealed tube filled with water
2. An air bubble is introduced into the tube
3. As the plant takes up water (due to transpiration), the air bubble moves along the tube
4. The distance moved by the bubble in a given time indicates the rate of water uptake

By changing environmental conditions (temperature, light, wind, humidity) and measuring the rate of bubble movement, you can investigate the effect of each factor on transpiration.

Exam Tip: The potometer actually measures the rate of water uptake, not transpiration directly. However, since the vast majority of water taken up by a plant is lost through transpiration, the two values are very closely related. Be precise in your language if asked about this.

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Translocation

Translocation is the movement of dissolved sugars (mainly sucrose) and other organic substances through the phloem from where they are produced (the source) to where they are needed (the sink).

Term	Definition	Examples
Source	The part of the plant where sugars are produced or released	Leaves (photosynthesis), storage organs releasing stored food
Sink	The part of the plant where sugars are used or stored	Growing roots, developing fruits, flowers, storage organs (e.g. potato tubers)

graph TD
    A[Source: Leaves — sugars made by photosynthesis] --> B[Phloem — sieve tubes and companion cells]
    B --> C[Sink: Growing roots — sugars used for respiration and growth]
    B --> D[Sink: Developing fruits — sugars stored or used]
    B --> E[Sink: Storage organs — sugars converted to starch for storage]

How Translocation Works

• Sucrose is loaded into the phloem sieve tubes at the source (using energy from companion cells)
• The sucrose solution moves through the phloem to the sink
• At the sink, sucrose is unloaded from the phloem and used for respiration, growth or converted to starch for storage
• Translocation requires energy — it is an active process driven by the companion cells

Xylem vs Phloem: A Full Comparison

Feature	Xylem	Phloem
Substance transported	Water and dissolved mineral ions	Dissolved sugars (sucrose) and amino acids
Direction	Upward only (roots to leaves)	Both directions (source to sink)
Cell type	Dead, hollow cells	Living cells (sieve tube elements + companion cells)
Cell walls	Thick, lignified	Thin, not lignified
End walls	No end walls (continuous tube)	Sieve plates with pores
Energy required?	No (passive — driven by transpiration pull)	Yes (active — energy from companion cells)
Process	Transpiration stream	Translocation

Exam Tip: Translocation is an active process that requires energy. The energy comes from the companion cells next to the sieve tubes. If asked how you could prove that translocation is active, describe an experiment where you poison companion cells (e.g. with a metabolic inhibitor) — if translocation stops, it must require energy from respiration.

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Transpiration and Water Loss — Xerophyte Adaptations

Some plants live in very dry environments and have special adaptations to reduce water loss. These plants are called xerophytes. Examples include cacti and marram grass.