A column of water, pulled from the top.

Two routes across the root.

Choose apoplast, then symplast, then both. Watch where the blue particles travel, where the green ones travel, and what happens to the blue route when it reaches the endodermis.

• Dissolved cargo: water carries mineral ions such as nitrate, phosphate and magnesium ions up the xylem, and organic compounds such as sucrose and amino acids travel dissolved in water in the phloem.
• Into the root hair: the cytoplasm and vacuole of the root hair cell have a lower water potential than soil water, so water enters by osmosis.
• Apoplast pathway: water moves through the network of cellulose cell walls and the spaces between cells. The cellulose microfibrils are loosely packed, so water passes easily without crossing any membrane. This route carries most of the water and is fast.
• Symplast pathway: water moves through the cytoplasm of one cell into the next through plasmodesmata, down a water potential gradient, crossing a partially permeable membrane only as it enters the first cell.
• Endodermis: the Casparian strip is a band of waterproof suberin in the walls of endodermal cells. It blocks the apoplast, so water and ions must cross the endodermal cell membranes into the cytoplasm. Membrane proteins select which ions pass, so the plant controls what enters the xylem.
• Into the xylem: xylem walls are waterproofed with lignin, so water enters the vessels through pits, where only the cellulose wall remains.

Holding the column together.

Press Transpiration on and watch the water molecules rise. The blue dashes are hydrogen bonds between water molecules; the yellow lines are bonds to the wall. Then let in an air bubble and see what happens to the column.

• Hydrogen bonds: water molecules are polar (slightly negative oxygen, slightly positive hydrogens), so they form hydrogen bonds with each other.
• Cohesion: these hydrogen bonds make water molecules stick together, so the water in a xylem vessel behaves as a continuous column.
• Adhesion: water molecules also form hydrogen bonds with the cellulose in the vessel walls, which helps hold the column in place and draws water up narrow vessels.
• Tension (transpiration pull): water lost by transpiration from the leaves is replaced from the xylem. Because the molecules are bonded together, removing water at the top pulls the whole column up from the roots, putting it under tension, like a rope being pulled.
• Lignin: the tension pulls the walls inward; lignified walls resist this and the vessel does not collapse.
• Air bubble: a gap breaks the column, the tension below it is lost and water stops moving up that vessel. Pits let water move sideways into a neighbouring vessel and bypass the bubble.

Evidence for tension.

Exam questions often give you data and ask you to link it to cohesion-tension. Learn what each observation shows.