Why Roots Absorb Water

What you do

• A four-month-old rye plant was found to have roots totalling about 600 km in length, with over 14 billion root hairs. Surface area is the whole point.

The parts you label

• Root hair — A long, thin extension of a single epidermal cell. Cheap to build, and it multiplies the absorbing surface enormously.
• Cell wall — Thin and freely permeable, so it presents no barrier to water.
• Cell membrane — Selectively permeable — this is the layer that actually makes osmosis possible.
• Cytoplasm — The living layer lining the cell.
• Epidermal cell — The root's outer layer. Water passes from cell to cell from here across to the xylem.
• Cortex — The wide band of packed cells between the epidermis and the stele. Water crosses it cell by cell, and also seeps through the walls.
• Endodermis — The single ring of cells wrapping the stele. Every drop of water reaching the xylem has to pass through it.
• Casparian strip — A waterproof band in the endodermal wall. It blocks the route through the walls, forcing water through a living cell instead — which is how the root controls what gets in.
• Xylem — Carries the water and dissolved minerals up to the rest of the plant.
• Phloem — Carries food made in the leaves back down. It runs alongside the xylem, in the opposite direction.

Challenges to clear

• Tap the layer that is selectively permeable — the one osmosis actually depends on.
• Label the whole root hair and its surroundings.
• Tap the waterproof band that forces water out of the walls and through a living cell.
• Tap the tissue the water is finally handed to, for the journey up the plant.

Hint

Water crosses the cortex two ways at once — through the walls, and through the living cells. The Casparian strip in the endodermis blocks the wall route, so everything reaching the xylem has to pass through a living membrane first.