What you'll learn
Plants move substances around their bodies using two separate transport tissues: xylem and phloem. This guide explains transpiration, the loss of water vapour from the leaves that pulls water up through the xylem, and translocation, the movement of dissolved sugars through the phloem. For AQA GCSE Biology you need to understand the structure of each tissue, the direction and driving force of each process, the factors that change the rate of transpiration, and how a potometer is used to investigate them. By the end you should be able to compare the two processes, explain how leaf structure supports water loss, and predict how environmental conditions affect the transpiration rate.
Key terms and definitions
Xylem — Plant tissue made of dead, hollow cells that transports water and dissolved mineral ions from the roots to the rest of the plant; transport is one-directional (upwards).
Phloem — Living plant tissue that transports dissolved sugars (mainly sucrose) around the plant in any direction; this movement is called translocation.
Transpiration — The loss of water vapour from the surfaces of a plant, mainly through the stomata in the leaves.
Translocation — The transport of dissolved sugars (and amino acids) through the phloem from sources (where they are made) to sinks (where they are used or stored).
Stomata — Small pores, usually on the underside of leaves, through which water vapour is lost and gases are exchanged.
Guard cells — Pairs of cells that surround each stoma and open or close it to control water loss.
Transpiration stream — The continuous flow of water from the roots, up the xylem, and out of the leaves.
Potometer — A piece of apparatus used to measure the rate of water uptake by a plant shoot, used as an estimate of transpiration rate.
Core concepts
Xylem and phloem compared
Xylem and phloem run alongside each other in vascular bundles. Xylem is made of dead cells with no end walls, forming continuous hollow tubes strengthened by a substance called lignin. It carries water and mineral ions upwards only. Phloem is made of living cells — sieve tubes with perforated end walls, supported by companion cells. It carries dissolved sugars in both directions, up or down the plant, depending on where the sugar is needed.
| Feature | Xylem | Phloem |
|---|---|---|
| Cells | Dead, hollow, lignified | Living sieve tubes with companion cells |
| Transports | Water and mineral ions | Dissolved sugars and amino acids |
| Direction | Upwards only | Both directions |
| Process | Transpiration stream | Translocation |
How transpiration works
Water evaporates from the surface of the mesophyll cells inside the leaf and diffuses out through the stomata as water vapour. This loss of water lowers the pressure at the top of the xylem, so more water is pulled up from below in a continuous column — the transpiration stream. Water is drawn all the way from the roots, replacing what is lost. Transpiration therefore keeps water and dissolved minerals moving up the plant, keeps cells turgid, and cools the plant as water evaporates.
Factors affecting the rate of transpiration
Four environmental factors change how fast a plant transpires:
- Light intensity — In brighter light, stomata open wider to allow carbon dioxide in for photosynthesis, so more water vapour escapes and transpiration increases.
- Temperature — Higher temperatures give water molecules more energy, so evaporation and diffusion are faster and transpiration increases.
- Air movement (wind) — Moving air carries away water vapour from around the leaf, maintaining a steep concentration gradient, so transpiration increases.
- Humidity — In humid air there is already a lot of water vapour outside the leaf, so the concentration gradient is shallow and transpiration decreases.
Leaf structure and water loss
Most stomata are on the lower surface of the leaf, where they are shaded and cooler, which reduces water loss. Guard cells swell and open the stomata in the light and shrink to close them in the dark or when water is short. The waxy cuticle on the upper surface is waterproof and reduces evaporation from the top of the leaf. These features let the plant balance the need to take in carbon dioxide against the need to conserve water.
How translocation is different
Translocation moves sugars made in the leaves (the source) to parts of the plant that need them (the sinks), such as growing shoots, roots, or storage organs. Because the same plant can have sources and sinks in different places at different times of year, phloem transport can go up or down. This two-way movement of dissolved food is the key difference from the one-way, water-carrying xylem.
Worked examples
Example 1: Using a potometer
A student sets up a potometer and records that an air bubble moves 60 mm in 10 minutes. What is the rate of water uptake? Rate = distance ÷ time = 60 mm ÷ 10 min = 6 mm per minute. The potometer measures water uptake, which is used as an estimate of transpiration rate because most of the water taken up is lost by transpiration.
Example 2: Predicting the effect of wind
A plant is moved from still air to in front of a fan. Predict and explain what happens to the transpiration rate. The transpiration rate increases. Moving air blows away the water vapour that collects around the stomata, keeping a steep concentration gradient between the inside of the leaf and the air outside, so water vapour diffuses out faster.
Example 3: Explaining the transpiration stream
Explain how water gets from the roots to the top of a tall tree. Water evaporates from the leaf mesophyll and diffuses out through the stomata. This lowers the water pressure at the top of the xylem, pulling the continuous column of water upwards. Water is drawn up the xylem from the roots, replacing what is lost, so a steady stream flows from roots to leaves.
Example 4: Comparing readings
In one hour a plant transpires more in the morning than at midday on a very hot, dry day. Suggest why. On a very hot, dry day the plant may close its stomata around midday to prevent excessive water loss and wilting. With the stomata closed, less water vapour escapes, so the transpiration rate falls even though it is hotter — the plant is protecting itself from drying out.
Common mistakes and how to avoid them
A frequent error is mixing up xylem and phloem. Remember: xylem carries water up (think "xylem = water"), phloem carries food (sugars) in both directions. Getting the substance or the direction wrong loses marks.
Students often say transpiration "pushes" water up the plant. It does not push — evaporation from the leaves creates a pull, drawing the water column upwards. Use the word "pull" or "draw", not "push".
Another mistake is claiming humidity increases transpiration. It does the opposite: humid air reduces the concentration gradient, so less water diffuses out. High humidity lowers the transpiration rate.
Be careful with the potometer. It measures water uptake, not transpiration directly. In the exam, say it gives an estimate of transpiration rate, because a small amount of the water taken up is used in photosynthesis and to keep cells turgid.
Finally, do not confuse translocation with transpiration. Translocation is the phloem moving sugars; transpiration is water loss from the leaves. They are different processes in different tissues.
Exam technique for "Plant transport: transpiration and translocation"
Many questions ask you to explain how a named factor changes the transpiration rate. Always link the factor to the concentration gradient or the behaviour of the stomata: for example, "wind removes water vapour, keeping a steep gradient, so transpiration increases". A cause-and-effect chain earns full marks.
When comparing xylem and phloem, use a clear point-by-point structure covering the cells, the substance transported, and the direction. Tables are a good way to organise this in your revision, but in the exam write full comparative sentences.
For potometer questions, be ready to describe how to change one factor while keeping the others constant, and to calculate a rate from a distance and a time. State that the potometer measures uptake as an estimate of transpiration. Practise explaining the transpiration stream in three clear steps: evaporation from the leaf, lowered pressure, water pulled up from the roots.
Quick revision summary
- Xylem carries water and minerals upwards through dead, hollow, lignified cells; phloem carries dissolved sugars in both directions through living sieve tubes.
- Transpiration is water vapour loss from the leaves through the stomata; evaporation pulls water up the xylem in the transpiration stream.
- Transpiration increases with higher light, temperature and air movement, and decreases with higher humidity.
- Stomata on the lower leaf surface and a waxy cuticle help balance carbon dioxide uptake against water loss.
- Translocation moves sugars from sources to sinks through the phloem in either direction.
- A potometer measures water uptake as an estimate of transpiration rate; change one factor at a time.