Water transport into the xylem

Question 1

What does water enter before it reaches the xylem?

Answer 1

the endodermis - a layer of cells surrounding the vascular tissue of the roots

Question 2

What is the casparian strip?

Answer 2

present in the endodermis

band of impermeable material that runs around each of the endodermal cells forming a waterproof layer

Question 3

What is the function of the casparian strip?

Answer 3

block the apoplast pathway for water so it is forced into the cytoplasm

water must pass through the selectively permeable cell surface membranes to exclude potentially-toxic solutes in the water from reaching tissues, as the carrier proteins can’t admit them

water join the symplast pathway

Question 4

How does water get from the endodermal cells to the xylem?

Answer 4

solute concentration in the cytoplasm is dilute compared to the cells in the xylem.

Endodermal cells move mineral ions into the xylem by active transport to lower the water potential

this increases the water moving into the xylem by osmosis down a water potential gradient

Question 5

What happens when water reaches the vascular bundle?

Answer 5

returns to the symplast pathway to enter the xylem itself and move up the plant

the active pumping of minerals, results in root pressure, this pushes water up the xylem

Question 6

What is the evidence for active transport in root pressure?

Answer 6

the posion cyanide affect mitochondria, prevent ATP production, when applied to root cells, root pressure disappears

root pressure increases with a temperature rise, suggesting chemical reactions are involved

if levels of oxygen or respiratory substrates fall, root pressure falls

xylem sap exude from the cut end of stems, in the natural world, xylem sap is forced out of special pores in some condition, e.g. overnight