mass transport in plants

Question 1

describe the mass flow hypothesis for the mechanism of translocation in plants

Answer 1

-sugars in source cell actively transported into the phloem by companion cells.
-increase in sugars in sieve tube elements, lowers water potential in the sieve tube elements and water moves into it via osmosis.
-increase in pressure causes mass movement towards sink cell
-sugars in sink cell are then used in respiration or stored as starch

Question 2

describe how water moves from the root hairs into the centre of root

Answer 2

-water moves through endodermis through the apoplastic pathway until it reaches a casparian strip (waterproof) prevents the movement of water.
-water(with ions) is forced to move through the cytoplasm.
-endodermal cells actively transport ions into xylem by carrier proteins -lowering water potential in the xylem.
- water moves into the xylem via osmosis, down the concentration gradient
-this enables the movement of water from the root hair to the centre of root

Question 3

what are the adaptations of the xylem

Answer 3

-dead hollow tubes:
no cytoplasm
contains lignin which makes it waterproof and rigid
-thick walls:
to withstand negative hydrostatic water pressure
narrow tubes so water can only travel up by capillary action
-pits:
so water can move laterally between xylem vessels

Question 4

what is the basic meaning of the movement in the xylem and phloem

Answer 4

xylem:
-only movement of water upwards,
-thick walls stiffened by lignin

phloem
-movement of photosynthesis up and down plants