How water passes How water passes up the stemup the stem

Movement of water and mineral ions Movement of water and mineral ions through the xylemthrough the xylem

How water passes up the How water passes up the stemstem

The tallest trees are The tallest trees are over 60m high and over 60m high and may need as much may need as much as 250 dmas 250 dm33 of water of water an hour to be an hour to be transported upwards.transported upwards.

How water passes up the How water passes up the stemstem

In leavesIn leaves Water evaporates Water evaporates

from spongy from spongy mesophyll cells so mesophyll cells so the air spaces the air spaces become saturated become saturated with water.with water.

How water passes up the How water passes up the stemstem

The air outside the The air outside the stoma is not stoma is not saturated so water saturated so water diffuses out from an diffuses out from an area of high water area of high water potential to region of potential to region of lower water potential.lower water potential.

How water passes up the How water passes up the stemstem

As water evaporates As water evaporates from mesophyll walls from mesophyll walls more is drawn into more is drawn into cells to replace it.cells to replace it.

Water can pass from Water can pass from the xylem to the leaf the xylem to the leaf cells by both the cells by both the apoplastapoplast and and symplastsymplast pathway. pathway.

Water

Water Water

is drawn up the xylem

Water Water

is drawn up the xylem

cytoplasm

Water Water

is drawn up the xylem

cytoplasm cell wall

Water Water

is drawn up the xylem

cytoplasm cell wall plasmodesmata

Water Water

is drawn up the xylem

cytoplasm cell wall plasmodesmata

stoma

Water Water

is drawn up the xylem

cytoplasm cell wall plasmodesmata

stoma

vacuole

Water Water

is drawn up the xylem

cytoplasm cell wall plasmodesmata

stoma

vacuole

epidermal cell

Water Water

is drawn up the xylem

cytoplasm cell wall plasmodesmata

stoma

vacuole

epidermal cell

xylem vessel

Water Water

is drawn up the xylem

cytoplasm cell wall plasmodesmata

stoma

vacuole

epidermal cell

xylem vessel

air space

Water Water

is drawn up the xylem

cytoplasm cell wall plasmodesmata

stoma

vacuole

epidermal cell

xylem vessel

air space

Water Water

is drawn up the xylem

cytoplasm cell wall plasmodesmata

stoma

vacuole

epidermal cell

xylem vessel

air space

Water Water

is drawn up the xylem

cytoplasm cell wall plasmodesmata

stoma

vacuole

epidermal cell

xylem vessel

air space

Apoplast pathway (cell wall)

Water Water

is drawn up the xylem

cytoplasm cell wall plasmodesmata

stoma

vacuole

epidermal cell

xylem vessel

air space

Apoplast pathway (cell wall)

Water Water

is drawn up the xylem

cytoplasm cell wall plasmodesmata

stoma

vacuole

epidermal cell

xylem vessel

air space

Apoplast pathway (cell wall)

Water Water

is drawn up the xylem

cytoplasm cell wall plasmodesmata

stoma

vacuole

epidermal cell

xylem vessel

air space

Apoplast pathway (cell wall)

Water Water

is drawn up the xylem

cytoplasm cell wall plasmodesmata

stoma

vacuole

epidermal cell

xylem vessel

air space

Apoplast pathway (cell wall)

Symplast pathway (cytoplasm)

Water Water

is drawn up the xylem

cytoplasm cell wall plasmodesmata

stoma

vacuole

epidermal cell

xylem vessel

air space

Apoplast pathway (cell wall)

Symplast pathway (cytoplasm)

Evaporation of water vapour

How water passes up the How water passes up the stemstem

Cohesion-tension theoryCohesion-tension theory As water is removed As water is removed

from xylem more water from xylem more water molecules are ‘pulled up’ molecules are ‘pulled up’ to replace them – this to replace them – this pulling force is known as pulling force is known as transpiration pulltranspiration pull (a bit (a bit like sucking on a straw).like sucking on a straw).

How water passes up the How water passes up the stemstem

This mass flow of water This mass flow of water relies upon two relies upon two properties of water.properties of water.

cohesioncohesion – water – water molecules tend to molecules tend to stick togetherstick together

adhesionadhesion – the – the molecules also stick molecules also stick to the inside of the to the inside of the xylem vessels.xylem vessels.

How water passes up the How water passes up the stemstem

Drawing up a column of water through Drawing up a column of water through the xylem in this way is called the the xylem in this way is called the cohesion-tension theorycohesion-tension theory. .

How water passes up the How water passes up the stemstem

It requires a large force It requires a large force to break such a water to break such a water column – the xylem column – the xylem vessels would collapse vessels would collapse under such tension if under such tension if they were not thickenedthey were not thickened

The cohesion-tension The cohesion-tension theory is considered the theory is considered the main way water reaches main way water reaches the leaves from the the leaves from the roots.roots.

How water passes up the How water passes up the stemstem

Root pressure Root pressure also also contributes to flow of contributes to flow of water up the plant – the water up the plant – the roots push water into the roots push water into the xylem and help to push xylem and help to push up the water column.up the water column.

(Evidence: if you cut a (Evidence: if you cut a plants stem at soil level plants stem at soil level water will continue to water will continue to come out of the cut stem come out of the cut stem for some time)for some time)

How water passes up the How water passes up the stemstem

CapillarityCapillarity is a third is a third force contributing to the force contributing to the movement of water. movement of water. Water will move up Water will move up narrow tubes by capillary narrow tubes by capillary action as water action as water molecules will stick to molecules will stick to the walls of the xylem the walls of the xylem vessels. vessels.

This force is important in This force is important in small plants but of little small plants but of little significance in large significance in large trees.trees.