Transport of Water and Mineral Salts in Plants

The pathway from root hair to leaf

Water enters the root hair by osmosis and crosses the root cortex from cell to cell before entering the xylem at the centre of the root. Once inside the xylem vessels, water and dissolved mineral ions travel upward through the stem and into the veins of the leaves, where the water eventually evaporates and diffuses out through the stomata as transpiration.

Transpiration pull, the main driving force

As water evaporates from the mesophyll cells inside a leaf, it creates a slight suction that pulls more water up through the xylem to replace it. Because water molecules stick to each other (cohesion) and to the walls of the xylem vessel (adhesion), this pull is transmitted as a continuous, unbroken column all the way down to the roots, drawing water upward against gravity.

Supporting forces

• Root pressure, active transport of mineral ions into the xylem lowers the water potential there, drawing water in from the root cells and pushing the column upward slightly.
• Capillary action, the narrow diameter of xylem vessels allows water to rise a short distance on its own due to adhesion with the vessel walls.
• These two forces are minor compared with transpiration pull and cannot alone explain water rising to the top of a tall tree.

How it is examined

Questions often ask you to trace and name the pathway of water from soil to leaf, to explain why transpiration pull is the main force moving water in tall plants, or to interpret an experiment (such as using a stained solution in a plant stem) that shows the position of the xylem.

Worked exam-style question

Question. A leafy shoot is set up with its cut end in water containing a blue dye, and left in bright, windy conditions for one hour. When the stem is later cut across, a ring of blue-stained tissue is seen near the outer edge of the stem, and the dye is also visible in the veins of the leaves.

(a) Name the tissue stained blue. (b) Explain why the dye reaches the leaves within one hour.

(c) State and explain what would happen to the rate of dye uptake if the shoot were instead placed in still, humid air.

Model answer. (a) Xylem (vessels). (b) The dye is carried upward dissolved in water as part of the transpiration stream; water evaporates from the mesophyll cells and diffuses out through the stomata, creating a suction (transpiration pull) that draws the dye-stained water up through the continuous column of water in the xylem to replace it.

(c) The rate of dye uptake would decrease, because still, humid air reduces the concentration gradient for water vapour between the leaf's air spaces and the surrounding air, slowing the rate of transpiration and therefore the transpiration pull that draws water (and the dye) upward.

Practice question

Try this. A student compares two similar leafy shoots, one placed in front of a fan and one in a closed, humid box, using a potometer to measure water uptake. (a) Predict which shoot would take up water faster, and name the factor being tested.

(b) Name one other environmental factor that affects the rate of transpiration. (c) Suggest a reason why root pressure alone could not explain the results observed.

Exam tip

Key terms

• Transpiration, the loss of water vapour from a plant, mainly through the stomata of the leaves, which generates transpiration pull.
• Transpiration stream, the continuous movement of water from the roots through the xylem to the leaves, driven by transpiration.
• Root hair cell, a root cell with a long extension that increases surface area for absorbing water and minerals from the soil.
• Xylem, the dead, tube-like plant tissue through which water and mineral salts are transported.

Source:SRC-DSKP-EN

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