The root as the uptake organ
The root is the plant organ responsible for absorbing water and mineral salts from the soil. The absorbing surface is greatly increased by thousands of root hair cells found just behind the root tip, in the region of maturation. Each root hair is a long, thin extension of a single epidermal cell.
Adaptations of the root hair cell
- A long, narrow projection that pushes between soil particles — thousands of them together give an enormous surface area for absorption.
- A thin cell wall and membrane, giving a short distance for substances to cross.
- A large permanent vacuole containing cell sap, which helps set up an osmotic gradient.
- Many mitochondria that release the ATP needed for active transport of mineral ions.
- No waxy cuticle, so water can pass in freely.
Key idea
Water enters the root hair by osmosis (down a water potential gradient), while mineral salts are usually taken in by active transport — moving ions against their concentration gradient using energy from respiration.
How water and salts are absorbed
Soil water usually has a higher water potential than the cell sap inside the root hair, so water moves in by osmosis across the partially permeable membrane. Mineral ions are often more concentrated inside the root than in the soil, so they must be pumped in by active transport, which requires ATP. Where the soil concentration is higher, some ions may also enter by diffusion or facilitated diffusion.
Example
If a root is placed in a solution with the same water potential as its cell sap, no net osmosis occurs and the root neither gains nor loses water — showing that water uptake depends on a water potential gradient.
Remember
Root hairs are delicate and short-lived. As the root grows, old root hairs die and new ones form nearer the tip, so the absorbing zone constantly moves forward through the soil.