7.7- TRANSPORT OF WATER IN THE XYLEM

Question 1

How is water absorbed in plants?

Answer 1

by roots through extensions called hair roots

Question 2

How is the vast majority of water transported in flowering plants?

Answer 2

through hollow, thick-walled tubes called xylem vessels

Question 3

What is the main force that pulls water through the xylem vessels in the stem of a plant?

Answer 3

evaporation of water from leaves- process called transpiration

Question 4

What is the energy for transpiration supplied by?

Answer 4

sun

Question 5

What type of process is transpiration as the energy for transpiration is supplied by the sun?

Answer 5

passive

Question 6

What is the humidity of the atmosphere usually like in comparison to the air spaces next to the stomata? (movement of water out through stomata)

Answer 6

humidity of atmosphere usually less than that of air spaces next to stomata

Question 7

What does it mean as the humidity of atmosphere usually less than that of air spaces next to stomata? (movement of water out through stomata)

Answer 7

there’s water potential gradient from air spaces through stomata to air

Question 8

Provided the stomata are open, what happens? (movement of water out through stomata)

Answer 8

water molecules diffuse out of air spaces into surrounding air

Question 9

What is water lost by diffusion from air spaces replaced by? (movement of water out through stomata)

Answer 9

water evaporating from cell walls of surrounding mesophyll cells

Question 10

What can plants do by changing the size of the stomatal pores? (movement of water out through stomata)

Answer 10

plants can control their rate of transpiration

Question 11

How is water lost from mesophyll cells? (movement of water across cells of leaf)

Answer 11

by evaporation from their cells walls to air spaces of leaf

Question 12

What is the water lost from the mesophyll cells replaced by? (movement of water out through stomata)

Answer 12

water reaching mesophyll cells from xylem either via cell walls or via cytoplasm

Question 13

In the case of water reaching mesophyll cells from xylem via cytoplasm- water movement occurs as: lose water #1?

Answer 13

mesophyll cells lose water to air spaces by evaporation due to heat supplied by sun

Question 14

In the case of water reaching mesophyll cells from xylem via cytoplasm- water movement occurs as: water potential #2

Answer 14

mesophyll cells now have lower water potential + so water enters by osmosis from neighbouring cells

Question 15

In the case of water reaching mesophyll cells from xylem via cytoplasm- water movement occurs as: neighbouring cells #3

Answer 15

loss of water from neighbouring cells lowers their water potential

Question 16

In the case of water reaching mesophyll cells from xylem via cytoplasm- water movement occurs as: take in water #4

Answer 16

mesophyll cells take in water from their neighbours by osmosis

Question 17

What is the main factor that is responsible for the movement of water up the xylem? (movement of water up stem in xylem)

Answer 17

cohesion-tension

Question 18

movement of water up stem: water evaporate #1

Answer 18

water evaporates from mesophyll cells due to heat from sun leading to transpiration

Question 19

movement of water up stem: hydrogen bonds #2

Answer 19

water molecules have hydrogen bonds between one another + hence tend to stick together
this known as cohesion

Question 20

movement of water up stem: what does water form #3

Answer 20

water forms continuous, unbroken column across mesophyll cells + down xylem

Question 21

movement of water up stem: what happens as water evaporates from mesophyll cells? #4

Answer 21

as water evaporates from mesophyll cells in leaf into air spaces beneath stomata, more molecules of water drawn up behind it as a result of this cohesion

Question 22

movement of water up stem: column of water #5

Answer 22

column of water pulled up xylem due to transpiration

this called transpiration pull

Question 23

movement of water up stem: what does transpiration pull do #6

Answer 23

transpiration pull puts xylem under tension, there’s negative pressure within xylem, hence name cohesion-tension theory

Question 24

Evidence to support cohesion-tension theory- change in diameter of tree trunks?

Answer 24

change in diameter of tree trunks according to rate of transpiration

Question 25

Evidence to support cohesion-tension theory- change in diameter of tree trunks?
time of day

Answer 25

during day. when transpiration at its greatest, there’s more tension (more negative pressure) + causes trunk to shrink in diameter
at night, when transpiration at its lowest, there’s less tension in xylem + so diameter of trunk increases

Question 26

Evidence to support cohesion-tension theory- if xylem vessel broken?

Answer 26

if xylem vessel broken + air enter s it, tree can no longer draw up air

Question 27

Evidence to support cohesion-tension theory- if xylem vessel broken?
why can the tree no longer draw up air when

Answer 27

as continuous column of water broken + so water molecules can no longer stick together

Question 28

Evidence to support cohesion-tension theory- what happens when xylem vessel broken?

Answer 28

when xylem vessel broken, water doesn’t leak out, as would be the case if it were under pressure
instead air drawn in, which is consistent with it being under tension

Question 29

What sort of process is transpiration pull?

Answer 29

passive process

Question 30

As transpiration pull is a passive process what does it not require?

Answer 30

does not require metabolic energy to take place

Question 31

What are the xylem vessels through which water passes describes as and so what?

Answer 31

dead + so cannot actively move water

Question 32

What do xylem vessels not have?

Answer 32

have no end walls

Question 33

What does xylem vessels having no end walls mean?

Answer 33

xylem forms series of continuous, unbroken tubes from root to leaves, which is essential to cohesion-tension theory of water flow up stem

Question 34

What is energy needed to drive the process of transpiration in the form of?

Answer 34

in the form of heat that evaporates water from leaves + it ultimately comes from sun