7.7- TRANSPORT OF WATER IN THE XYLEM Flashcards
How is water absorbed in plants?
by roots through extensions called hair roots
How is the vast majority of water transported in flowering plants?
through hollow, thick-walled tubes called xylem vessels
What is the main force that pulls water through the xylem vessels in the stem of a plant?
evaporation of water from leaves- process called transpiration
What is the energy for transpiration supplied by?
sun
What type of process is transpiration as the energy for transpiration is supplied by the sun?
passive
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)
humidity of atmosphere usually less than that of air spaces next to stomata
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)
there’s water potential gradient from air spaces through stomata to air
Provided the stomata are open, what happens? (movement of water out through stomata)
water molecules diffuse out of air spaces into surrounding air
What is water lost by diffusion from air spaces replaced by? (movement of water out through stomata)
water evaporating from cell walls of surrounding mesophyll cells
What can plants do by changing the size of the stomatal pores? (movement of water out through stomata)
plants can control their rate of transpiration
How is water lost from mesophyll cells? (movement of water across cells of leaf)
by evaporation from their cells walls to air spaces of leaf
What is the water lost from the mesophyll cells replaced by? (movement of water out through stomata)
water reaching mesophyll cells from xylem either via cell walls or via cytoplasm
In the case of water reaching mesophyll cells from xylem via cytoplasm- water movement occurs as: lose water #1?
mesophyll cells lose water to air spaces by evaporation due to heat supplied by sun
In the case of water reaching mesophyll cells from xylem via cytoplasm- water movement occurs as: water potential #2
mesophyll cells now have lower water potential + so water enters by osmosis from neighbouring cells
In the case of water reaching mesophyll cells from xylem via cytoplasm- water movement occurs as: neighbouring cells #3
loss of water from neighbouring cells lowers their water potential
In the case of water reaching mesophyll cells from xylem via cytoplasm- water movement occurs as: take in water #4
mesophyll cells take in water from their neighbours by osmosis
What is the main factor that is responsible for the movement of water up the xylem? (movement of water up stem in xylem)
cohesion-tension
movement of water up stem: water evaporate #1
water evaporates from mesophyll cells due to heat from sun leading to transpiration
movement of water up stem: hydrogen bonds #2
water molecules have hydrogen bonds between one another + hence tend to stick together
this known as cohesion
movement of water up stem: what does water form #3
water forms continuous, unbroken column across mesophyll cells + down xylem
movement of water up stem: what happens as water evaporates from mesophyll cells? #4
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
movement of water up stem: column of water #5
column of water pulled up xylem due to transpiration
this called transpiration pull
movement of water up stem: what does transpiration pull do #6
transpiration pull puts xylem under tension, there’s negative pressure within xylem, hence name cohesion-tension theory
Evidence to support cohesion-tension theory- change in diameter of tree trunks?
change in diameter of tree trunks according to rate of transpiration
Evidence to support cohesion-tension theory- change in diameter of tree trunks?
time of day
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
Evidence to support cohesion-tension theory- if xylem vessel broken?
if xylem vessel broken + air enter s it, tree can no longer draw up air
Evidence to support cohesion-tension theory- if xylem vessel broken?
why can the tree no longer draw up air when
as continuous column of water broken + so water molecules can no longer stick together
Evidence to support cohesion-tension theory- what happens when xylem vessel broken?
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
What sort of process is transpiration pull?
passive process
As transpiration pull is a passive process what does it not require?
does not require metabolic energy to take place
What are the xylem vessels through which water passes describes as and so what?
dead + so cannot actively move water
What do xylem vessels not have?
have no end walls
What does xylem vessels having no end walls mean?
xylem forms series of continuous, unbroken tubes from root to leaves, which is essential to cohesion-tension theory of water flow up stem
What is energy needed to drive the process of transpiration in the form of?
in the form of heat that evaporates water from leaves + it ultimately comes from sun
