the cohesion-tension model of water transport in xylem

Question 1

What does cohesion mean (not the definition)?

Answer 1

Cohesion means similar molecules sticking together (water has a high cohesion).

Question 2

What does adhesion mean (not the definition)?

Answer 2

Adhesion means different molecules sticking together (water and xylem have a lower adhesion).

Question 3

Why is water pulled out of xylem?

Answer 3

Water is pulled out of xylem (and leaves) because it evaporates into the air (i.e. transpiration).

Question 4

When water is pulled out of xylem, what happens with the water molecules? What does this create?

Answer 4

Each water molecule pulls the next one behind it, creating tension in the xylem.

Question 5

Each water molecule pulls the next one behind it, creating tension in the xylem. What is this tension caused by?

Answer 5

transpiration

Question 6

As a result of cohesion, what happens to the tension created by the water molecules pulling the next out of the xylem?

Answer 6

The tension is transmitted down to the root xylem as a result of cohesion between water molecules.

Question 7

What does the tension in the water column do to xylem?

Answer 7

Tension in the water column causes xylem to become narrow during
transpiration.

Question 8

When transpiration stops, what happens to the xylem? What is this a result of in the xylem?

Answer 8

When transpiration stops, xylem cells return to their normal wider shape, because of the elasticity of their walls.

Question 9

What are the two ways that water is moved up through a flowering plant?

Answer 9

Water moves up through a flowering plant:
Partly by being pushed up by root pressure
Mostly by being pulled up by transpiration.

Question 10

What does the cohesion-tension model explain?

Answer 10

The cohesion-tension model explains how water is transported in plants to great heights against the force of gravity.

Question 11

Who put forward the cohesion-tension model (2 irish scientists)?NB

Answer 11

Henry Dixon and John Joly

Question 12

Does water have a low or high cohesion? Explain your answer.

Answer 12

Water has a high cohesion, i.e. water molecules tend to stick together. Water adheres to the walls of xylem, but this force is not as great as the cohesive forces of water.

Question 13

How does cohesion begin? (step 1)

Answer 13

Water evaporates from the xylem into the air spaces of the leaf (and eventually out of the stomata into the air by transpiration). As transpiration pulls each water molecule out of the xylem, the next water molecule is pulled with it,

Question 14

Water evaporates from the xylem to where? (2)

Answer 14

into the air spaces of the leaf and eventually out of the stomata into the air by transpiration

Question 15

The ability of water to be pulled upwards, like in this model, only works in what environment?

Answer 15

The ability of water to be pulled upwards in this way only works in narrow tubes such as xylem.

Question 16

Transpiration and cohesion combine to generate what?

Answer 16

Transpiration and cohesion combine to generate a ‘pulling force’ from the top of the plant all the way down to the roots at the bottom of the plant.

Question 17

Why is the water in the xylem said to be under tension? (step 2)

Answer 17

When water molecules are pulled in this way the entire column of water in the xylem is stretched (like a piece of elastic).

Question 18

How strong are the cohesive forces in the water column?

Answer 18

The cohesive forces between the water molecules are great enough to hold water molecules in a column without breaking, even when tension is applied.

Question 19

The tension in the xylem due to transpiration is how strong/great? (what can it do)

Answer 19

The tension in the xylem due to transpiration is great enough to pull water to a height of approximately 150 m.

Question 20

When do stomata open, what occurs then?

Answer 20

Stomata open in daylight and transpiration occurs.

Question 21

What causes xylem to become narrower?

Answer 21

The tension produced in the water column causes xylem to become narrower.

Question 22

The tension produced in the water column causes xylem to become narrower. What does this cause to happen to the plant?

Answer 22

This in turn causes stems to become slightly narrower by day

Question 23

What prevents xylem cells from collapsing inwards as a result of the tension produced in the water column (which narrowes the xylem cells)?

Answer 23

To prevent xylem cells collapsing inwards, each xylem cell is strengthened with lignin.

Question 24

When do the xylem cells return to their normal shape?

Answer 24

When transpiration stops (i.e. at night when the stomata close), the lack of tension allows the xylem to return to its original wider shape.

Question 25

When does transpiration stop?

Answer 25

at night when the stomata close

Question 26

When do stomata close?

Answer 26

at night

Question 27

the xylem return to its original wider shape at night (when transpiration stops and stomata close). What allows the xylem to return to its og shape?

Answer 27

This happens as a result of the elasticity of the xylem walls