In short: Plants move materials in two kinds of tubes. Xylem carries water and minerals up from the roots, pulled by transpiration (water evaporating from the leaves), and phloem carries sugars from where they are made to where they are used, by translocation.
Starting point: what you already know
- Water moves by osmosis. Minerals can be taken in by active transport (see The Cell Membrane and Transport).
- Leaves make glucose by photosynthesis. Stomata let gases in and water vapour out (see Nutrient and Gas Exchange).
- Plants have roots, stems and leaves.
Two transport tissues
Plants have vascular tissue made of two kinds of tubes, usually found together in vascular bundles.
| Xylem | Phloem | |
|---|---|---|
| Carries | Water and mineral ions | Sugars (mainly sucrose) and amino acids |
| Direction | Up only, from roots to leaves | Up and down, from where sugar is made or stored to where it is needed |
| Cells | Dead, hollow tubes with no end walls | Living sieve tubes with companion cells |
| Walls | Thick and strengthened with lignin | Thin |
| Process | Transpiration | Translocation |
Lignin in xylem walls also makes wood strong. A tree trunk is mostly old xylem.
Transpiration: how water moves up
Transpiration is the loss of water vapour from the leaves, mostly through the stomata. It powers the movement of water up the plant.
- Water evaporates from cells inside the leaf and diffuses out through the stomata.
- This loss pulls water out of the xylem in the leaf.
- Water molecules stick to each other (cohesion) and to the xylem walls (adhesion), so a continuous column of water is pulled up the stem.
- Water enters the root hairs from the soil by osmosis to replace it.
This is called the transpiration stream, and the idea is the cohesion-tension theory.
Factors that change the rate of transpiration
| Factor | Effect | Why |
|---|---|---|
| Higher temperature | Faster | Water evaporates faster |
| Stronger wind | Faster | Moist air is blown away from the leaf |
| Higher humidity | Slower | Air outside is already moist, so the gradient is smaller |
| Brighter light | Faster | Stomata open for photosynthesis |
Transpiration can be measured with a potometer: a cut shoot sealed into a water-filled tube. As the shoot takes up water, a bubble in the tube moves, and its speed shows the rate.
Translocation: how sugar moves
Sugar moves in the phloem from a source (where it is made or released, like a leaf) to a sink (where it is used or stored, like a growing root, a fruit or a potato tuber). In spring, stored starch in roots can become a source and send sugar up to new buds.
- Water and minerals
- Upward only
- Dead lignified tubes
- Evaporation from leaves
- Cohesion and adhesion
- Pulls water up
- Heat and wind: faster
- Humidity: slower
- Potometer measures it
- Sugars and amino acids
- Living sieve tubes
- Source to sink
Quick summary
- Xylem carries water and minerals up from the roots. Phloem carries sugars from sources to sinks.
- Transpiration is the loss of water vapour from leaves. It pulls water up through the xylem.
- Cohesion and adhesion keep the water column unbroken.
- Heat, wind and light speed up transpiration. Humidity slows it down.
- Translocation moves sugar in the phloem from source to sink.
Quiz
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Which tissue carries water up a plant?
- A Xylem
- B Phloem
- C Epidermis
- D Stomata
Show answer
A: Xylem
Xylem carries water and minerals upward.
Which tissue carries sugars?
- A Xylem
- B Phloem
- C Root hairs
- D Cuticle
Show answer
B: Phloem
Phloem carries sugars from sources to sinks.
Transpiration is
- A the movement of sugar in phloem
- B the loss of water vapour from leaves
- C the uptake of minerals by roots
- D the making of glucose
Show answer
B: the loss of water vapour from leaves
Water evaporates and leaves through stomata.