TRANSPORT IN THE XYLEM

Question 1

Define the apoplast pathway. (1)

Answer 1

the route of water transport across the plant cells that passes through the fibrous cell walls.

Question 2

define the symplast pathway. (1)

Answer 2

the route of water transport across the plant cells that passes from cell to cell through plasmoddesmata.

Question 3

what are the main routes of water movement through plant cells and tissues? (3)

Answer 3

-mass flow
-diffusion
-osmosis

Question 4

what is the mass flow route? (2)

Answer 4

through the interconnected free spaces between the cellulose fibres of the plant cell walls. pathway does not pass through membranes or the living contents of the cell, is known as the apoplast pathway.

Question 5

what is the apoplast? (1)

Answer 5

the apoplast includes water-filled spaces of dead cells such as xylem vessels and is the major route of water transport.

Question 6

what is the diffusion route? (2)

Answer 6

through the cytoplasm of the cells using plasmodesmata. this is called the symplast pathway. the cytoplasm is packed with organelles that slow diffusion so it is a much more restricted pathway.

Question 7

what is the osmosis route? (3)

Answer 7

from vacuole to vacuole of plant cells using their partially permeable membranes known as the vascular pathway. driven by the gradient of water potential. is a limited pathway for movement of large quantities of water it is the way in which individual cells take in water.

Question 8

what is transpiration? (1)

Answer 8

the loss of water vapour from the aerial parts of plants.

Question 9

what is cohesion? (1)

Answer 9

a property of water molecules by which hydrogen bonds give columns of water great tensile strength.

Question 10

what evidence is there that supports the cohesion-tension model? (4)

Answer 10

-when xylem vessels are punctured air enters, demonstrating water under tension, not pressure.
-xylem vessels have thickened lignin walls to prevent them collapsing under tension.
-rates of water uptake are linked to factors affecting transpiration from leaves.
-fine columns of water placed under tension show sufficient tensile strength to account for transport up the highest trees.

Question 11

what factors affect the rate of water movement? (4)

Answer 11

temperature
light
humidity
air movement

Question 12

how does temperature affect the rate of water movement? (3)

Answer 12

in leaves water vapour evaporates from the apoplast of the spongy mesophyll cells into the air spaces, it then diffuses into the atmosphere through the stomata. an increase in temperature means the water molecules have more kinetic energy, with more energy more water molecules will evaporate into the air spaces and they will diffuse out faster increasing the rate of transpiration.

Question 13

how does light affect the rate of water movement? (2)

Answer 13

to escape into the atmosphere water molecules must pass through the stomatal pores, as light intensity decreases the guard cells of the stomata loose their turgidity causing them to flatten against each other, closing the stomatal pores. diffusion of water vapour is severely restricted.

Question 14

how does humidity affect the rate of water movement? (1)

Answer 14

as the number of water molecules in the air increases, the diffusion gradient compared with the inside of the leaf is reduced slowing diffusion.

Question 15

what is humidity? (1)

Answer 15

a measure of the amount of water vapour in the surrounding atmosphere.

Question 16

how does air movement affect the rate of water movement? (2)

Answer 16

in still air, water vapour diffusing out of the stomata tends to build up close to the surface of the leaf. this reduces the diffusion gradient and slows transpiration.