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 TESION in the water column, otherwise it would collapse
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 out of open stomata from mesophyll cells via transpiration down a water potential gradient
Reduces the water potential in a cell causing a water gradient across cells = osmosis of water into mesophyll cells from xylem to replace water lost in mesophyll cells
Due to cohesive forces between H2O (H bonds), water is pulled up as a column from roots to leaf . This is known as tension
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
Explanation of affect of light intensity on transpiration rate
When the light intensity is highest, more stomata are open (area or stomatal openings increase) due to letting in more CO2 for increased rate of photosynthesis.
Thus more water will be evaporated from the leaf air spaces down water potential gradient
Correlation between temperature and rate of transpiration
Positive
Explanation for correlation between temperature and transpiration
Because water molecules have more kinetic energy at higher temperatures
So as a result, will evaporate from cells’s air spaces faster
Thus water potential difference inside cell and outside is higher so faster rate of transpiration
Correlation between humidity and rate of transpiration
Negative
Explanation for correlation between humidity and rate of transpiration
Lower the water potential in the air, the faster the rate of transpiration
Because this means that there is a larger water potential gradient: (lower in air, compared to inside the leaf)
So faster rate of transpiration from stomata in leaf
Correlation between rate of transpiration and wind levels
Positive
Explanation for correlation between wind and rate of transpiration
Because more winds will move H2O in air near the stomata away from the stomata
So this increases the water potential gradient as it is lowered near the stomata opening in air compared to in the cell
Thus increased rate of transpiration
