Root hairs: water and ion uptake

Explanation

A root hair is a long, narrow extension of a root epidermal cell. It increases the cell surface area in contact with water films around soil particles. A large contact area allows more absorption at the same time than a compact cell of similar volume.

The cell wall is thin, giving a short route to the membrane. The membrane controls movement into the living cell. A large vacuole contains cell sap; when soil water has higher water potential than the cell contents, water enters through the membrane by osmosis.

Mineral ions dissolved in soil water are also absorbed across the root-hair surface. They supply materials needed for growth: nitrate ions, for example, provide nitrogen used in making amino acids and proteins. Mineral ions are not glucose or ready-made plant food.

Separate adaptation from process. The long extension benefits both water and ion uptake by increasing surface area. Osmosis specifically names water movement, so do not call the movement of mineral ions osmosis. Detailed uphill ion-transport mechanisms are not needed to explain this structural adaptation.

Water does not stop at the root hair. It moves from the root hair cell across the cortex cells to the xylem by osmosis. Each cell along the route has a slightly lower water potential than the cell before it, because water keeps being drawn away into the xylem. This water potential gradient from the root hair cell to the xylem keeps water moving inwards, cell to cell, until it enters the xylem vessels and is carried up the stem.