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Topic 8 · 8.1–8.4

Transport in plants

Core and Extended · Papers 1–6

🎯What you need to be able to do

  • State the functions of xylem and phloem and identify their positions in roots, stems and leaves; relate xylem structure to function EXTENDED.
  • Identify root hair cells and outline the pathway of water through the plant; investigate it with a stain.
  • Describe transpiration and investigate the effects of temperature and wind speed on its rate.
  • Explain transpiration pull, the effects of temperature, wind and humidity, and wilting EXTENDED.
  • Describe translocation, sources and sinks EXTENDED.

📚The biology

Xylem and phloem

xylem: transports water and mineral ions, and gives support
phloem: transports sucrose and amino acids
Cross-sections of a root, a stem and a leaf midrib. Root: xylem is a star shape in the centre with phloem between its arms. Stem: vascular bundles in a ring near the edge, xylem on the inside of each and phloem on the outside. Leaf: xylem on top of the bundle, phloem below.
Positions in a non-woody dicotyledonous plant. In stems and leaves the xylem is always on the side nearer the centre of the stem.

EXTENDED Xylem vessels are adapted to carry water: thick walls strengthened with lignin support the plant and stop the vessel collapsing; there are no cell contents; and the cells are joined end to end with no cross walls, forming one long, continuous tube.

A length of xylem vessel made of cells joined end to end. The walls are thick and strengthened with lignin; there are no cross walls, so it is one continuous tube; the cells have no contents. An arrow shows water and mineral ions moving along it.
EXTENDED An empty, continuous tube offers little resistance to the water column.

Water uptake

Root hair cells absorb water (by osmosis) and mineral ions (by active transport, topic 3). Their long extensions give a large surface area, increasing uptake. Water then passes through the root cortex cells, into the xylem, up the stem and into the mesophyll cells of the leaves.

Water moves from the soil into a root hair cell, through a row of root cortex cells, into a xylem vessel, up the xylem, and into mesophyll cells in the leaf.
The pathway: root hair cells → root cortex cells → xylem → mesophyll cells.

To show the pathway, stand a leafy shoot in water containing a dye (a suitable stain) for a few hours, then cut sections of the stem and leaves: only the xylem is stained.

Transpiration

Transpiration is the loss of water vapour from leaves. Water evaporates from the surfaces of the mesophyll cells into the air spaces, then diffuses out of the leaf through the stomata as water vapour. Its rate is estimated with a potometer, which actually measures water uptake (a little water is used by the plant rather than lost).

A potometer. A leafy shoot, cut under water, is fitted with a bung into a water-filled tube with a reservoir and tap. A narrow horizontal capillary tube with a scale contains an air bubble, which moves towards the shoot as the shoot takes up water.
The shoot is cut and fitted under water so no air enters the xylem. The reservoir resets the bubble between readings.
Three small graphs of transpiration rate. It rises as temperature rises, rises with wind speed and then levels off, and falls as humidity rises.
Higher temperature and wind speed increase the rate; higher humidity decreases it. (Schematic shapes.)

EXTENDED The rate depends on:

  • EXTENDED Surface area: the interconnecting air spaces between mesophyll cells give a large internal surface for evaporation; more and larger stomata let more vapour out.
  • Temperature: water molecules have more kinetic energy, so evaporation and diffusion are faster.
  • Wind speed: moving air removes water vapour from around the leaf, keeping a steep concentration gradient.
  • Humidity: moist air lowers the concentration gradient between the air spaces and the air outside, so the rate falls.

EXTENDED Transpiration pull. As water evaporates from the leaves, water is drawn out of the xylem to replace it. This pulls up a column of water molecules, which are held together by forces of attraction between water molecules. If more water is lost than is taken up, the cells lose turgor, become flaccid, and the plant wilts.

Translocation EXTENDED

A plant. A blue arrow in the xylem runs up from the roots to the leaves. Purple arrows in the phloem run from a leaf, the source, both up to the growing tip and down to a fruit and to an underground storage organ, the sinks. Notes explain transpiration pull and sources and sinks.
EXTENDED Xylem carries water up only; phloem carries sucrose and amino acids in whichever direction a sink lies.

EXTENDED Translocation is the movement of sucrose and amino acids in the phloem from sources (parts that release sucrose or amino acids, such as photosynthesising leaves) to sinks (parts that use or store them, such as roots, fruits and growing shoots). The same organ can change role: a potato tuber is a sink while it stores starch in summer, and a source in spring, when its stores are broken down and sent to the new shoots.

✏️Worked example

Four potted plants of the same size were watered, their pots sealed in plastic bags, and weighed. The mass lost in 24 hours was: A, on a bench at 22 °C: 20 g; B, as A but with a clear plastic bag over the leaves: 6 g; C, in a warm room at 32 °C: 34 g; D, all leaves removed: 2 g. (a) Explain why the pots were sealed in plastic bags. [1] (b) Explain the difference between A and C. [2] (c) EXTENDED Explain why B lost less mass than A. [2] (d) Suggest why D still lost a little mass. [1]

(a) To stop water evaporating from the soil, so all the mass lost is water vapour lost from the plant.

(b) C lost more: at a higher temperature water molecules have more kinetic energy, so more water evaporates from the mesophyll cell surfaces and water vapour diffuses out of the stomata faster.

(c) The bag traps water vapour, raising the humidity around the leaves; the concentration gradient between the air spaces and the air outside is smaller, so less water vapour diffuses out.

(d) Some water is lost from the stem (and the cut surfaces) even without leaves.

Check it. Removing the leaves cut the loss from 20 g to 2 g, showing that about 90% of the water lost goes through the leaves.
“Water leaves through the stomata as water.” It evaporates inside the leaf and leaves as water vapour by diffusion.

📝Practise

In the style of the multiple-choice, theory and practical papers. EXTENDED marks Supplement content.

1. (Multiple choice.) What does the phloem transport? A: water and mineral ions. B: sucrose and amino acids. C: water and sucrose. D: mineral ions and amino acids.
B.
2. (Theory.) In a cross-section of a root, state where the xylem is found. [1]
In the centre, forming a star shape (with phloem between its arms).
3. (Theory.) Explain how root hairs are adapted for absorbing water. [2]
They are long, thin extensions, giving a large surface area in contact with the soil water, which increases the rate of uptake.
4. (Theory.) Describe how water leaves a leaf during transpiration. [3]
Water evaporates from the surfaces of the mesophyll cells into the air spaces, then diffuses out of the leaf through the stomata as water vapour.
5. (Practical.) Explain why a potometer only gives an estimate of the rate of transpiration. [1]
It measures water uptake, and some water taken up is used by the plant (e.g. in photosynthesis) rather than lost.
6. (Theory.) EXTENDED Explain how water moves up the xylem to the top of a tall tree. [3]
Water is lost from the leaves by transpiration; this creates a transpiration pull that draws water up the xylem; the column of water does not break because water molecules are attracted to each other.
7. (Theory.) EXTENDED Explain why a plant wilts on a hot, dry day. [3]
It loses water by transpiration faster than it can take it up; its cells lose water and turgor pressure, becoming flaccid, so the leaves and stem are no longer supported.
8. (Theory.) EXTENDED A developing seed and a mature leaf are joined by phloem. State which is the source and which the sink, and explain. [2]
The leaf is the source (it makes sucrose by photosynthesis and releases it); the seed is the sink (it uses and stores sucrose and amino acids).

🔗Go deeper — other people’s work

These are external resources, not mine. If one stops working, tell me and everything above it on this page still stands.

  • Science and Plants for Schools (SAPS) — making and using a simple potometer
  • BBC Bitesize — transport systems in plants