Transport of water in the xylem

How does water move out of the stomata?

There’s a water potential gradient from the air space through the stomata to the air

If the stomata are open, water vapour diffuses out of the air spaces into the surrounding air air

How can plants control their rate of respiration?

Changing the size of their stomata pores

How does water move across the cells of a leaf?

Mesophyll cells lose water to the air spaces by evaporation

They now have a lower water potential so water enters by osmosis from neighbouring cells

Loss of water from neighbouring cells lowers their water potential

They take in water from neighbouring cells

How does water move up the stem in the xylem?

Water evaporated from mesophyll cells

Water forms a continuous unbroken column across the mesophyll cells and down the xylem

As water evaporates from the mesophyll cells in the leaf into the air spaces, more water molecules are brought up

Column of water is pulled up the xylem: transpiration stream

What is the cohesion-tension theory?

Transpiration pull puts the xylem under tension

Negative pressure within the xylem

What is cohesion?

Water molecules from hydrogen bonds between one another and stick together

What is apoplast pathway?

Water moves through the continuous system of adjacent cellulose cell walls: resulting from tension caused by transpiration

(Can be blocked by casparian strips)

What is symplast pathway?

Water moves though the cytoplasm for cell to cell: by osmosis

The xylem is made up of what kind of cells?

Dead cells

Xylem walls are impregnated with what?

Lignin: impermeable to water

What evidence is there to prove cohesion-tension theory?

If a xylem vessel is broke and air enters it, the tree can no longer draw up water: continuous column of water is broke

When a xylem vessel is broken, water doesn’t leak out: it would be if it was under pressure. Instead air is drawn in

The diameter of tree trunks changes: less transpiration at night - less tension - trunk expands

What happens to the tension in the xylem as light intensity increases?

as light intensity increases, tension increases

stomata open more - increase in cohesion

increases transpiration rate

water potential of leaf cells lowers

more water moves out the xylem to surrounding cells down water potential gradient