Xylem Flashcards
Define transpiration
evaporation of water from the leaves resulting in the movement of h2o up the xylem
Describe the structure of the xylem
dead cells strengthen by rings of lignin
no end walls= continuous flow of water
found on the inside of the vascular bundle
How does water move out of a leaf?
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
How is the rate of transpiration is controlled?
via size of stomatal pores
Describe the cytoplasmic route
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
Describe the movement of water up the stem
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
Is the transpiration active or passive?
passive as it uses heat from the sun
Describe the evidence from this theory
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
Explain how changes in tree diameter show the cohesion tension theory
more transpiration = more tension = walls of xylem pulled inwards
Explain how if x. is broken and air enters and water can no longer be drawn up show the cohesion tension theory
continuous column broken so molecules cannot stick together.
Explain how when xylem is broken water does not leak out shows the cohesion tension theory
air is drawn in instead which fits being under tension
Describe how a high pressure is produced in the leaves.
Water potential becomes lower as sugars enter pholem
Water enters phloem by osmosis;
Increased volume (of water) causes increased pressure.
Describe the cohesion-tension theory of water transport in the xylem
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
