7.7 Transport of Water in the Xylem Flashcards
Where in plants is water absorbed
By the roots through extensions called root hair
Where is the majority of water transported through in plants
Hollow tubes called xylem vessels
What is the main force that pulls water through the xylem vessels called
The evaporation of water from the leaves. This is called transpiration
Describe the movement of water through the stomata
The humidity of the atmosphere is usually less than that of the air spaces next to the stomata.
As a result, there is a water potential gradient from the air spaces through the stomata to the end
Provided the stomata are open, water vapour molecules diffuse out of the air spaces into the surrounding air.
Water lost by evaporation out of air spaces is replaced by water evaporating from the cell walls of mesophyll cells
By changing the size of stomatal pores, plants can control transpiration rate
Describe the cytoplasmic route of water across the cells of a leaf
Mesophyll cells lose water to the air spaces by evaporation due to heat from the sun
These cells have lower water potential and so water enters by osmosis from neighbouring cells
The loss of water from these neighbouring cells lowers their water potential.
They, in turn, take in water from their neighbours via osmosis
In this way, a water potential gradient is established that pulls water from the xylem, across the leaf mesophyll, and finally out into the atomosphere
Describe the process by which water moves up the stem in the xylem
Water evaporates from mesophyll cells due to heat from the sun, leading to transpiration
Water molecules form hydrogen bonds with one another and tend to stick together. This is cohesion
Water forms a continuous, unbroken column across the mesophyll cells and down the xylem
As water evaporates from the mesophyll cells in the leaf into air spaces beneath the stomata, more molecules are drawn up behind it due to cohesion
A column of water is therefore pulled up the xylem as as result of transpiration. This is known as transpiration pull
Transpiration pull puts the xylem under tension, there is a negative pressure within. This is why the process is called cohesion-tension theory
Why is it called the cohesion-tension theory
Because the transpiration pull puts the xylem under tension, there is a negative pressure within. This is why the process is called cohesion-tension theory
What evidence supports the cohesion-tension theory
- Change in tree trunk diameter according to the rate of transpiration
- Xylem vessel broken & air enters = no water drawn up
- Xylem vessel broken = Water doesn’t leak out, which would be the case if the vessel was under pressure
Why does a change in trunk diameter support the C-T theory
Because during the day, when transpiration is at its greatest, there is more tension in the xylem. This pulls the xylem walls inwards and causes the trunk to shrink in diameter
Why does the tree no longer drawing up water when the vessel is broken support the C-T theory
Because the continuous column of water is broken and so water molecules dont stick together
Why does the tree not leaking water when broken support the C-T theory
Because if the vessel was under pressure then water would leak out - instead, air is drawn in, which is consistent with it being under tension
What type of process is transpiration pull
A passive process
What energy does the transpiration pull require
No metabolic energy, only energy from the sun to evaporate the water
How does the structure of xylem vessels make them suitable for the cohesion tension theory
They have no end walls which means they form a series of continuous unbroken tubes from roots to leaves
