Transport in Plants
Senior 2 · Biology
Transport in Plants
Lesson 1 of 5
20 min
Let's begin
A eucalyptus tree on a Rwandan hillside can grow taller than a ten-storey building. Every day, water must travel from its roots to its highest leaves, and sugar made in those leaves must travel down to feed the roots. A cassava plant does the same thing when it fills its roots with starch. How does a plant move materials over such distances without a heart or blood?
In a single cell, or a tiny organism, substances can move by diffusion alone. A large plant is different: its roots have water but no light, and its leaves have light but no water. Diffusion over metres would take far too long, so plants have two special transport tissues, together called vascular tissue:
Xylem is mostly made of vessels and tracheids. A vessel forms from a column of cells joined end to end. As they mature, the cells die, their contents disappear and the end walls between them break down, leaving a long, hollow tube. Their walls are thickened with lignin, a hard, waterproof substance. Small unthickened areas called pits let water pass sideways between vessels.
Phloem is made of sieve tubes with companion cells beside them. A sieve tube is a column of living cells (sieve-tube elements) joined end to end. The end walls have pores and are called sieve plates. A mature sieve-tube element has no nucleus and very little cytoplasm, which leaves room for the sap to flow. The companion cell next to it has a nucleus and many mitochondria, and supplies it with energy.
| Xylem | Phloem | |
|---|---|---|
| Carries | water and mineral ions | sucrose and amino acids |
| Direction | upwards only | up and down |
| Cells | dead, hollow vessels | living sieve tubes with companion cells |
| End walls | absent | sieve plates with pores |
| Walls | thickened with lignin | no lignin |
Xylem and phloem run side by side in strands called vascular bundles.
Worked example
A student stands a leafy stalk of celery in water coloured with red dye for three hours, then cuts across the stalk. Result: small red dots appear in a ring near the outside of the stalk, and the leaf veins turn red. Explanation: the dye travelled up with the water, so it stains only the tissue that carries water. The red dots are the xylem in the vascular bundles, and their ring arrangement shows celery is a dicot.
Tip
PHloem carries PHotosynthesis products. Xylem carries water, one way, up.
In a stem bundle, xylem is on the inside, phloem on the outside. When asked for a difference, compare the same feature in both tissues in one sentence: "Xylem cells are dead, but phloem sieve tubes are living."
State two ways in which xylem vessels are adapted for carrying water.
Solution: They are hollow with no end walls or cell contents, so water flows easily. Their walls are thickened with lignin, so they are strong and do not collapse.
Why does a sieve-tube element need a companion cell?
Solution: A mature sieve-tube element has no nucleus and few organelles. The companion cell carries out its life processes and supplies the energy needed to load sugar into the sieve tube.
A white flower is placed in blue ink. After a day its petals are streaked with blue. Which tissue carried the ink?
Solution: Xylem. The ink moved up with the water, and water travels in the xylem.
Give two differences between xylem and phloem.
Solution: Xylem carries water and minerals, but phloem carries sucrose and amino acids. Xylem vessels are dead with lignified walls, but sieve tubes are living with sieve plates. (Direction is another answer: xylem flows up only, phloem both ways.)
Remember
Sources and credits
This lesson adapts CC BY-SA material and is shared under CC BY-SA 4.0.
