Xylem

Question 1

Define transpiration

Answer 1

evaporation of water from the leaves resulting in the movement of h2o up the xylem

Question 2

Describe the structure of the xylem

Answer 2

dead cells strengthen by rings of lignin
no end walls= continuous flow of water
found on the inside of the vascular bundle

Question 3

How does water move out of a leaf?

Answer 3

humidity of atmosphere < air spaces next to stomata
water vapour diffuses out down water potential gradient
h2o evaporates from cell mesophyll to replace lost vapour
h2o lost from mesophyll is replaced by water from xylem via cell walls or cytoplasm

Question 4

How is the rate of transpiration is controlled?

Answer 4

via size of stomatal pores

Question 5

Describe the cytoplasmic route

Answer 5

mesophyll lose h2o to air spaces by evaporation due to heat from the sun
cells decrease water potential so osmosis occurs and h2o enters from neighbouring cells
loss from h2o from neighbouring cells lowers their water potential
so uptake of water from their neighbours via osmosis
water potential gradient established that pulls water up from xylem

Question 6

Describe the movement of water up the stem

Answer 6

h2o evapourates from mesophyll cell sue to heat fro sun leading to transportation

h2o molecules form Hydrogen bonds between one another and stick together= cohesion

h2o forms a continuous, unbroken column across mesophyll and down xylem

water pulled up xylem = transpiration pull

transpiration puts xylem under negative pressure/ tension

COHESION-TENSION THEORY

Question 7

Is the transpiration active or passive?

Answer 7

passive as it uses heat from the sun

Question 8

Describe the evidence from this theory

Answer 8

changes in tree diameter according to the rate of T.
if x. is broken and air enters water can no longer be drawn up
xylem is broken water does not leak out

Question 9

Explain how changes in tree diameter show the cohesion tension theory

Answer 9

more transpiration = more tension = walls of xylem pulled inwards

Question 10

Explain how if x. is broken and air enters and water can no longer be drawn up show the cohesion tension theory

Answer 10

continuous column broken so molecules cannot stick together.

Question 11

Explain how when xylem is broken water does not leak out shows the cohesion tension theory

Answer 11

air is drawn in instead which fits being under tension

Question 12

Describe how a high pressure is produced in the leaves.

Answer 12

Water potential becomes lower as sugars enter pholem
Water enters phloem by osmosis;
Increased volume (of water) causes increased pressure.

Question 13

Describe the cohesion-tension theory of water transport in the xylem

Answer 13

Water lost from leaf because of transpiration
Lowers water potential of mesophyll / leaf
cells;
Water pulled up xylem (creating tension);
Water molecules cohere / ‘stick’ together by
hydrogen bonds;
(forming continuous) water column;
Adhesion of water (molecules) to walls of xylem