Transport in Plants
Senior 2 · Biology
Transport in Plants
Lesson 2 of 5
20 min
Let's begin
When you transplant cabbage or tomato seedlings, they often wilt for a day or two, even in wet soil. Gardeners lift them with a ball of soil around the roots to reduce this. The reason lies in the tiny root hairs, which do almost all of a plant's water and mineral absorption and are easily torn off.
Just behind the growing tip of a root, many cells of the outer layer (the epidermis) grow long, thin extensions called root hairs. Each root hair is part of a single cell. Root hairs are adapted for absorption because:
Recall from the unit on transport across the cell membrane that osmosis is the net movement of water molecules from a dilute solution (high water potential) to a more concentrated solution (low water potential) through a partially permeable membrane.
Worked example
Soil water is a very dilute solution of mineral ions. The cell sap inside a root hair contains sugars and salts, so it is more concentrated. Which way does water move, and why? Water moves into the root hair by osmosis, from the dilute soil water (higher water potential) across the partially permeable cell membrane into the more concentrated sap (lower water potential). No energy is used.
Now suppose a farmer spreads far too much fertiliser, so the soil water becomes more concentrated than the cell sap. The gradient reverses, water leaves the root hairs by osmosis, and the plant wilts.
Water entering a root hair makes that cell more dilute than the cortex cell next to it, so water moves on by osmosis from cell to cell across the cortex. Much of the water also seeps along the cell walls between the cells.
Around the central vascular tissue is a ring of cells called the endodermis. Its walls contain a waterproof band called the Casparian strip, which stops water sliding along the walls. Water and ions must therefore pass through the membranes of the endodermis cells, which act as a checkpoint deciding what enters the xylem.
Path of water: soil → root hair → cortex → endodermis → xylem in the centre of the root.
Root hair cells often contain mineral ions such as nitrate and potassium at a higher concentration than the soil water. Ions cannot diffuse in against that gradient. Instead, carrier proteins in the cell membrane pump them in, using energy released by respiration. This is active transport.
Worked example
Barley roots were kept in a solution of potassium ions. In one tube air was bubbled through the solution; in the other, nitrogen gas (no oxygen). Example results: with air, the roots took up potassium steadily; with nitrogen, uptake almost stopped. Explanation: without oxygen, the root cells cannot carry out aerobic respiration, so there is too little energy for active transport. This is why crops grow poorly in waterlogged soil, where air cannot reach the roots.
Tip
Water in by osmosis (passive, no energy). Minerals in by active transport (against the gradient, energy from respiration).
If an exam question mentions oxygen, respiration, a respiratory poison or waterlogging, the answer almost always involves active transport.
Give two features of root hairs that help them absorb water.
Solution: They are long, thin and numerous, giving a large surface area; they have thin walls and no cuticle, so water passes through easily.
Explain why transplanted seedlings often wilt.
Solution: Many root hairs are torn off when the seedling is pulled up. The surface area for absorbing water falls, so the plant loses water from its leaves faster than it can take it in.
A root was treated with a poison that stops respiration. Predict the effect on the uptake of (a) water and (b) nitrate ions.
Solution: (a) Water uptake continues at first, because osmosis does not need energy from respiration. (b) Nitrate uptake falls sharply, because active transport needs energy from respiration.
What is the job of the Casparian strip in the endodermis?
Solution: It blocks water and ions from moving along the cell walls, forcing them to pass through the cell membranes of the endodermis. The membranes then control which substances reach the xylem.
Remember
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