in flowering plants the vast majority of water is transported through thick, hollow tubes = xylem vessels
1 of 13
WHAT IS TRANSPIRATION?
main force that pulls water through xylem vessels in stem of a plant is EVAPORATION of water from leaves.
energy= supplied by sun so it's passive.
2 of 13
MOVEMENT OF WATER THROUGHOUT STOMATA
(1)
- humidity of atmosphere is mostly less than that of the air spaces next to stomata so there's a w.p grad. from air spaces into surrounding air. water lost by diffusion from air spaces is replaced by water evaporating from cell walls of mesophyll cells su
3 of 13
(2)
- by change of size of stomatal pores, plants can control their rate of transpiration.
4 of 13
MOVEMENT OF WATER ACROSS CELLS OF A LEAF
water is lost from mesophyll cells by evaporation from cell walls to air spaces of leaf. replaced by water reaching mesophyll cells from xylem either via cell walls or cytoplasm:
5 of 13
(1)
- mesophyll cells lose water to air spaces by evaporation due to heat supplied by sun
- cells now have lower w.p. so water enters by osmosis from neighbouring cells
- loss of water from these cells lowers their w.p
- in turn they take in water from neighb
6 of 13
MOVEMENT OF WATER UP THE STEM IN THE XYLEM
main factor responsible for movement of H2O up xylem is cohesion-tension.
-water evaporates from m cells due to heat from sun -> transpiration
- H2O forms H bonds between one another and stick together: COHESION
- H2O forms continuous columns across m ce
7 of 13
(2)
- as H2O evaporates from m cells in leaf into air the spaces beneath stomata, more molecules of H2O = drawn up behind it as a result of cohesion
- column of H2O= pulled up xylem as a result of transpiration
- called transpiration pull
8 of 13
(3)
- transpiration pull puts xylem under tension= negative pressure within xylem hence cohesion-TENSION theory.
9 of 13
SUPPORT TOWARDS COHESION-TENSION THEORY
(1)
- change in diameter of tree trunks according to rate of T.
during day, more tension = walls of xylem inwards= shrinks diameter
at night T = at lowest & less tension so diameter increases.
10 of 13
(2)
- if xylem vessel is broken, air enters and tree can't draw up water; continuous column of water = broken so H2O the can't stick together
11 of 13
(3)
- when xylem is broken, water
doesn't leak out as would happen it
it were under pressure. instead air
is drawn in which is consistent with it being under tension
12 of 13
WHY IS TRANSPIRATION PULL PASSIVE?
as it doesn't require metabolic energy to take place
13 of 13
Other cards in this set
Card 2
Front
WHAT IS TRANSPIRATION?
Back
main force that pulls water through xylem vessels in stem of a plant is EVAPORATION of water from leaves.
energy= supplied by sun so it's passive.
Comments
No comments have yet been made