Water uptake and the transpiration stream
From soil, into roots, up the xylem.
Water uptake by root hair cells.
- Root hair cells are epidermal cells with a long, thin extension into the soil.
- Their value is surface area — many root hairs hugely increase the area for absorption.
- Water enters by osmosis: soil water has a higher (less negative) water potential than the cell cytoplasm, so water moves down the water-potential gradient into the cell.
- Mineral ions (e.g. nitrate) are taken up by active transport, lowering the cell's water potential and keeping osmosis going.
Xylem structure — built for transport.
- Xylem vessels are dead at maturity: no cytoplasm or end walls, so they form continuous hollow tubes with low resistance to flow.
- Walls are thickened with lignin — waterproof and strong, preventing the vessel collapsing under tension (negative pressure).
- Pits (gaps in the lignified wall) let water move sideways between vessels and into surrounding cells.
The transpiration stream is the continuous flow of water from roots → stem → leaves, replacing water lost by transpiration.
- Root hairs: large SA → osmosis down water-potential gradient.
- Xylem: dead, hollow, lignified vessels = low-resistance tubes.
- Lignin waterproofs and prevents collapse under tension.
- Transpiration stream = continuous flow root → leaf.