Water Transport In Multicellular Plants

Question 1

why is water important to plants?

Answer 1

• hydrostatic pressure as a result of osmosis provides a hydrostatic skeleton to support stems and leaves.
• turgor drives cell expansion which enables plant roots to force their way through tarmac and concrete.
• the loss of water through evaporation helps keep plants cool.
• mineral ions and products of photosyntheis are transported in aqeuous solutions.
• water is a raw material for photosynthesis.

Question 2

what is the exchange surface in plants?

Answer 2

root hair cells

Question 3

how are root hair cells adapted for its function?

Answer 3

• microscopic size allows them to penetrate between soil particles.
• each microscopic hair has large SA:V ratio and there are thsousands of them on each tip.
• each hair has a thin surface layer to reduce diffusion distance.
• concentration of solutes in root hair cell cytoplasm maintains a water potential gradient between soil water and the cell.

Question 4

how does water move into the root hair cell?

Answer 4

osmosis. the root hair cell contains many different solvents so water potential is lower in the cell than the soil so water moves in.

Question 5

what are the two different pathways for the movement to the xylem from the root?

Answer 5

• symplast pathway
• apoplast pathway.

Question 6

what is the symplast pathway?

Answer 6

water moves through the continuous cytoplasm of living cells connected by plasmodesmata.

Question 7

what is the apoplast pathway?

Answer 7

water moves through the cell walls and intercellular spaces.

Question 8

how does the apoplast pathway work?

Answer 8

water molecules move into xylem. moer water molecules are pulled through the apoplast behind them due to the cohesive forces between water molecules creates a tension that means there is a continuous flow through the open structure of the cellulose wall.

Question 9

how does water move into the xylem?

Answer 9

water moves across the root until it reaches the endodermis of the root. water in the apoplast pathway cannot move further and must enter the cytoplasm. water must pass through selectively permeable cell membranes.

Question 10

what is the casparian strip?

Answer 10

a band of waxy material called suberin that creates a waterproof layer and runs around endodermal cells.

Question 11

is the water potential of xylem cells higher or lower than endodermal cells?

Answer 11

much lower.

Question 12

what happens once water is inside the vascular bundle?

Answer 12

water returns to the apoplastic pathway to enter the xylem itself and move up the plant.

Question 13

how is root pressure created?

Answer 13

active pumping of minerals into the xylem to produce movement of water by osmosis.

Question 14

what is the evidence for the role of active transport in root pressure?

Answer 14

• some poisons (such as cyanide) affect the mitochondria and prevent the production of ATP. If cyanide is applied to root cells so there is no energy supply. the root pressure therefore disappears.
• root pressure increases with rise in temperature and falls with a fall in temperature, suggesting chemical reactions are involved.
• if levels of oxygen or respiratory substrates fall, root pressure falls.
• xylem sap may excude from the cut end of stems at certain times.