CH9 water transport in multicellular plants

Question 1

Why is water important in both the structure and metabolism of plants

Answer 1

• turgor pressure as a result of osmosis provides a hydrostatic skeleton to support stems and leaves
• turgor drives cell expansion, enabling plants to force their way through concrete
• loss of water by evaporation helps keep plants cool
• mineral ions and products of photosynthesis are transported in aqueous solutions
• water is a raw material for photosynthesis

Question 2

Movement of water into the root

Answer 2

• root hair cells are the exchange surface
• soil water has a high water potential, and cytoplasm of root hair cell has a lower water potential
• water moves in by osmosis

Question 3

Adaptation of root hair cells

Answer 3

• microscopic size means they can penetrate easily between soil particles
• each microscopic hair has a large sa:v ratio and there are thousands on each growing root tip
• each hair has a thin surface layer
• the concentration of solutes in cytoplasm of cells maintains water potential gradient

Question 4

Movement of water across root

Answer 4

• occurs in either symplast pathway or apoplast pathway

Question 5

Symplast pathway

Answer 5

• water moves through symplast which is the continuous cytoplasm of the living plant cell, connected through plasmodesmata
• root hair has a higher water potential than next cell along, so water moves from it to the next by osmosis
• process continues until xylem reached

Question 6

Apoplast pathway

Answer 6

• movement of water through apoplast which is the cell walls and intracellular spaces
• water fills spaces between loose, open network of fibres in cellulose cell wall
• as water molecules move into xylem, more water molecules are pulled through the apoplast behind due to cohesion
• pull from water into xylem and up plant creates a tension meaning there is a continuous flow of water

Question 7

Moevment of water into xylem

Answer 7

• reaches endodermis
• water in apoplast pathway can go no further and is forced into cytoplasm of cell, joining water in symplast pathway
• solute concentration in cytoplasm of endodermal cells is dilute compared to cells in xylem, so water moves in by osmosis
• once in vascular bundle, water returns to apoplast pathway to enter xylem itself

Question 8

What is casparian strip

Answer 8

• band of waxy material called suberin that runs around each of the endodermal cells forming a waterproof layer

Question 9

Why is diversion of apoplast pathway into cytoplasm significant

Answer 9

• water must pass through selectively permeable cell surface membranes
• excludes any potentially toxic solutes in soil water as membranes have no carrier proteins

Question 10

What is root pressure

Answer 10

• result of active pumping of minerals into the xylem to produce movement of water by osmosis
• gives water push up xylem

Question 11

Evidence for role of active transport in root pressure

Answer 11

• cyanide affects mitochondria and when applied to root cells, root pressure disappears
• root pressure increases with temperature rise, suggests chemical reaction involved
• lower level of oxygen causes root pressure to fall