Cohesion-tension

Question 1

What vessel transports water in a plant?

Answer 1

The xylem tissue

Question 2

How is water moved up a plant against gravity?

Answer 2

A combined action of cohesion and negative pressure (known as tension)

Question 3

Xylem structure summary

Answer 3

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

Question 4

Why is the xylem lignified?

Answer 4

To thicken it to provide support to withstand tension in the water column caused by transpiration

Question 5

Why is the xylem vessel made up of long cell tubes with no end walls?

Answer 5

To allow for continuous columns of water

Question 6

Why do xylem cells not have cytoplasm or organelles?

Answer 6

To allow for uninterrupted water flow in a column of water

Question 7

Why do xylem vessels have pits in the walls?

Answer 7

So it can allow for lateral movement of water, so water can move into the next vessel if one vessel is blocked

Question 8

What is the job of xylem?

Answer 8

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

Question 9

Stages of how water is moved through a plant according to cohesion-tension theory

Answer 9

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)

Question 10

What is transpiration, and what are the stages of transpiration?

Answer 10

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)

Question 11

What is transpiration a side effect of?

Answer 11

Photosynthesis, as when the stomata opens to let CO2 in, water leaves down its water potential gradient

Question 12

Why does the diameter of a trunk shrink during transpiration?

Answer 12

Due to adhesion of the water columns to the walls of the xylem, pulling xylem in

Question 13

Why is more water lost in transpiration on the lower side of leaf than upper side?

Answer 13

Because there are more stomata on the lower surface
And also thicker waxy cuticle on the upper surface: which acts as waterproofing

Question 14

What affects the rate of transpiration?

Answer 14

Light intensity
Temperature
Humidity
Wind

Question 15

Correlation between light intensity and rate of transpiration

Answer 15

Positive

Question 16

Explanation of affect of light intensity on transpiration rate

Answer 16

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

Question 17

Correlation between temperature and rate of transpiration

Answer 17

Positive

Question 18

Explanation for correlation between temperature and transpiration

Answer 18

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

Question 19

Correlation between humidity and rate of transpiration

Answer 19

Negative

Question 20

Explanation for correlation between humidity and rate of transpiration

Answer 20

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

Question 21

Correlation between rate of transpiration and wind levels

Answer 21

Positive

Question 22

Explanation for correlation between wind and rate of transpiration

Answer 22

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