Why plants need a transport system
Roots take in water and minerals from the soil but leaves are where food is made. Leaves need water from the roots; roots need food from the leaves. In a small plant diffusion is enough, but in a tall tree the distance is many metres, so plants have two dedicated pipe systems: xylem and phloem, together called vascular tissue.
Plants have low energy needs (they do not move and many cells are dead support cells), so their transport is slower than in animals.
Transport of water and minerals: xylem
Xylem is made of vessels and tracheids (dead, hollow, joined cells) in roots, stems and leaves, forming one unbroken water column.
Root pressure
Root cells actively take in ions from soil, so the root has a higher ion concentration than soil. Water moves in by osmosis and builds up a push that moves water up the xylem. This matters most at night, when stomata are closed.
Transpiration pull
In the day, water evaporates from leaf cells and escapes through stomata. This creates a suction that pulls water up the xylem column, because water molecules cling to each other. This is the main force for lifting water in tall trees.
Transpiration and its role
Transpiration is the loss of water as vapour from the aerial parts of a plant, mostly through stomata.
- Creates the suction (transpiration pull) that moves water and minerals up.
- Cools the leaves, like sweating cools us.
- Helps carry minerals dissolved in water to leaves.
Transpiration is faster in bright sun, wind, heat and dry air; slower in humid, still, cool air and at night.
Transport of food: phloem and translocation
The movement of soluble food made by photosynthesis from leaves to other parts is called translocation. It happens in phloem, made of sieve tubes with companion cells. Phloem also carries amino acids and other substances.
Unlike xylem, phloem transport needs energy. Sugar like sucrose is loaded into sieve tubes using ATP. This makes the tube contents concentrated, water enters by osmosis, and the pressure pushes food towards places that need it, such as roots, fruits, seeds and growing buds. In spring, stored sugar in roots or stems is sent upward to buds; so phloem flow can go up or down.
Xylem vs phloem and exam focus
| Xylem | Phloem |
|---|---|
| Carries water and minerals | Carries food (sugar, amino acids) |
| One way: roots → leaves | Both ways, to where food is needed |
| Mostly dead cells: vessels, tracheids | Living cells: sieve tubes, companion cells |
| Driven by transpiration pull and root pressure; no ATP used directly | Driven by pressure made using ATP |
Exam tip: the xylem vs phloem table, 'how water reaches the top of a tall tree', and the role of transpiration are common 3-mark questions.
Key formulas and definitions
- Xylem: water + minerals, roots → leaves (one way)
- Phloem: food, both directions (translocation, needs ATP)
- Transpiration: loss of water vapour through stomata
- Transpiration pull: main force lifting water in the day
- Root pressure: push from roots by osmosis; important at night
- Vascular tissue = xylem + phloem
Worked examples
1. How does water reach the top leaves of a 30 m tall tree?
Leaves lose water vapour through stomata. The water lost is replaced from the xylem, and because water molecules hold on to each other, the whole column in the xylem is pulled up like liquid in a straw. Root pressure gives a small extra push from below.
2. A potted plant is covered with a plastic bag. After some hours water drops appear inside the bag. Why?
The plant lost water vapour by transpiration. The vapour could not escape the bag, so it cooled and condensed as droplets on the inside.
3. Why does a plant wilt on a hot, windy day even if the soil is wet?
Heat and wind make transpiration very fast. If roots cannot take in water as fast as leaves lose it, cells lose water, become limp and the plant wilts.
4. Why is transport in phloem said to be 'active' but in xylem 'passive'?
Phloem loading uses ATP energy to push sugar into sieve tubes, creating pressure. Xylem movement is driven by evaporation (transpiration pull) and osmosis in roots, so the xylem cells themselves spend no energy.
5. A ring of bark (containing phloem) is removed from a tree trunk. What happens to the roots over time?
Food from leaves cannot pass the ring down to the roots, so the roots starve and slowly die, and then the whole tree dies. Water still rises for a while because xylem is deeper inside.
Common mistakes
- Saying phloem carries water up. Xylem carries water and minerals; phloem carries food.
- Writing that food moves only downward. Phloem carries food both up and down, wherever it is needed.
- Thinking transpiration is just waste. It pulls water up, carries minerals and cools the leaves.
- Forgetting that root pressure matters mainly at night; in the day transpiration pull is the main force.