Cohesion-tension Flashcards
What vessel transports water in a plant?
The xylem tissue
How is water moved up a plant against gravity?
A combined action of cohesion and negative pressure (known as tension)
Xylem structure summary
Long tube like vessels formed from dead cells joined end to end with no end walls
Contains no cytoplasm or organelles
Containing lignin in the walls
Why is the xylem lignified?
To thicken it to provide support to withstand tension in the water column caused by transpiration
Why is the xylem vessel made up of long cell tubes with no end walls?
To allow for continuous columns of water
Why do xylem cells not have cytoplasm or organelles?
To allow for uninterrupted water flow in a column of water
Why do xylem vessels have pits in the walls?
So it can allow for lateral movement of water, so water can move into the next vessel if one vessel is blocked
What is the job of xylem?
To facilitate the transpiration stream of water: the mass flow of water column from roots to leaves then into air down a water potential gradient
Stages of how water is moved through a plant according to cohesion-tension theory
Water evaporates from open stomata in leaves via transpiration down a water potential gradient
Reduces the water potential in a cell causing an osmotic gradient across cells
So water is drawn out of xylem into leaves down water potential gradient
Creates tension so more water is pulled up the leaf
Due to cohesive forces between H2O (H bonds), water is pulled up as a column (+ thanks to adhesion to xylem walls)
What is transpiration, and what are the stages of transpiration?
When water evaporates from a plants surface eg the leaves
Because water has accumulated in air spaces in leaves due to not being taken in to plant cells
And once the stomata opens, water moves out the leaf into the air down a water potential gradient (higher water potential in leaf than air)
What is transpiration a side effect of?
Photosynthesis, as when the stomata opens to let CO2 in, water leaves down its water potential gradient
Why does the diameter of a trunk shrink during transpiration?
Due to adhesion of the water columns to the walls of the xylem, pulling xylem in
Why is more water lost in transpiration on the lower side of leaf than upper side?
Because there are more stomata on the lower surface
And also thicker waxy cuticle on the upper surface: which acts as waterproofing
What affects the rate of transpiration?
Light intensity
Temperature
Humidity
Wind
Correlation between light intensity and rate of transpiration
Positive
