7.7 - transport of water in the xylem

Question 1

What cells is water absorbed from, where are they located?

Answer 1

• root hair cells
• in the roots

Question 2

Where is water transported in the plants

Answer 2

hollow, thick-walled tubes called xylem vessels

Question 3

What is the main force that pulls water through the xylem vessels in the stem of a plant

Answer 3

• the evaporation of water from leaves — a process called transpiration.
• The energy for this is supplied by the sun and the process is therefore passive.

Question 4

Describe movement through the stomata

Answer 4

• 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 air.
• Provided the stomata are open, water vapour molecules diffuse out of the air spaces into the surrounding air.
• Water lost by diffusion from the air spaces is replaced by water evaporating from the cell walls of the surrounding mesophyll cells.
• By changing the size of the stomatal pores, plants can control their rate of transpiration.

Question 5

Describe movement of water across the cells of a leaf

Answer 5

• Water is lost from mesophyll cells by evaporation from their cell walls to the air spaces of the leaf.
• This is replaced by water reaching the mesophyll cells from the xylem either via cell walls or via the cytoplasm.
• In the case of the cytoplasmic route the water movement occurs because:
  1) mesophyll cells lose water to the air spaces by evaporation due to heat supplied by the sun
  2) these cells now have a lower water potential and so water enters by osmosis from neighbouring cells
  3) the loss of water from these neighbouring cells lowers their water potential
  4) they, in turn, take in water from their neighbours by 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 atmosphere.

Question 6

What is the main factor that is responsible for the movement of water up the xylem, from the roots to the leaves,

Answer 6

cohesion-tension

Question 7

Describe the movement of water up the xylem

Answer 7

• Water evaporates from mesophyll cells due to heat from the sun leading to transpiration.
• Water molecules form hydrogen bonds between one another and hence tend to stick together. This is known as 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 the air spaces beneath the stomata, more molecules of water are drawn up behind it as a result of this cohesion.
• A column of water is therefore pulled up the xylem as a result of transpiration. This is called the transpiration pull.
• Transpiration pull puts the xylem under tension, that is, there is a negative pressure within the xylem, hence the name cohesion— tension theory

Question 8

What is some of the supporting evidence for cohesion-tension theory

Answer 8

• Change in the diameter of tree trunks according to the rate of transpiration. During the day, when transpiration is at its greatest, there is more tension (more negative pressure) in the xylem. This pulls the walls of the xylem vessels inwards and causes the trunk to shrink in diameter. At night, when transpiration is at its lowest, there is less tension in the xylem and so the diameter of the trunk increases.
• If a xylem vessel is broken and air enters it, the tree can no longer draw up water. This is because the continuous column of water is broken and so the water molecules can no longer stick together.
• When a xylem vessel is broken, water does not leak out, as would be the case if it were under pressure. Instead air is drawn in, which is consistent with it being under tension.

Question 9

Is transpiration pull a passive process

Answer 9

Yes
- therefore it doesn’t use metabolic energy

Question 10

Describe the features of the xylem vessel

Answer 10

• The vessels through which the water passes are dead and so cannot actively move the water.
• Xylem vessels have no end walls which means that xylem forms a series of continuous, unbroken tubes from root to leaves, which is essential to the cohesion-tension theory of water flow up the stem.
• Energy is nevertheless needed to drive the process of transpiration. This energy is in the form of heat that evaporates water from the leaves and it ultimately comes from the sun.

Question 11

At what time of day is transpiration rate greatest? Explain your answer using information in Figure 5.

Answer 11

at 12.00 hours because this is when water flow is at its maximum. As transpiration creates most of the water flow they are both at a maximum at the same time.

Question 12

Describe the changes in the rate of flow of water during the 24-hour period.

Answer 12

Rate of flow increases from a minimum at 00.00 hours to a maximum at 12.00 hours and then decreases to a minimum again at 24.00 hours.

Question 13

Explain in terms of the cohesion—tension theory the changes in the rate of flow of water during the 24-hour period.

Answer 13

• As evaporation / transpiration from leaves increases during the morning (due to higher temperature / higher light intensity)
• it pulls water molecules through the xylem because water molecules are cohesive / stick together.
• This transpiration pull creates a negative pressure / tension.
• The greater the rate of transpiration, the greater the water flow.
• The reverse occurs as transpiration rate decreases during the afternoon and evening.

Question 14

Explain the changes in the diameter of the tree trunk over the 24-hour period

Answer 14

• As transpiration increases up to 12.00 hours, so there is a higher tension (negative pressure) in the xylem.
• This reduces the diameter of the trunk.
• As transpiration rate decreases, from 12.00 hours to 24.00 hours, the tension in the xylem reduces and the trunk diameter increases again.

Question 15

If the tree was sprayed with ammonium sulfamate, a herbicide that kills living cells, the rate of water flow would be unchanged. Explain why.

Answer 15

• Transpiration pull is a passive process / does not require energy.
• Xylem is non-living and so cannot provide energy.
• Although root cortex and leaf mesophyll cells are living — the movement of water across them uses passive processes, e.g. osmosis, and so continues at least for a while, even though the cells have been killed.

Question 16

Water leaves from the air spaces in a plant by a process called __a__. This takes place mainly through pores called __b__ in the epidermis of the leaf. Water evaporates into the air spaces from mesophyll cells. As a result these cells have a __c__ water potential and so draw water by __d__ from neighbouring cells. In this way, a water potential gradient is set up that draws water from the xylem. Water is pulled up
the xylem because water molecules stick together — a phenomenon called __e__. During the night the diameter of a tree trunk __f__.

Answer 16

A) transpiration
B) stomata
C) lower/reduced/more negative
D) osmosis
E) cohesion
F) increases

Question 17

The rate of water loss in a plant can be measured using a…

Answer 17

Potometer

Question 18

How do you set up a potometer experiment

Answer 18

• A leafy shoot is cut under water. Care is taken not to get water on the leaves.
• The potometer is filled completely with water, making sure there are no air bubbles.
• An air bubble is introduced into the capillary tube.
• The distance moved by the air bubble in a given time is measured a number of times and the mean is calculated.
• Using this mean value, the volume of water lost is calculated.
• The volume of water lost against the time in minutes can be plotted on a graph.
• Once the air bubble nears the junction of the reservoir tube and the capillary tube, the tap on the reservoir is opened and the syringe is pushed down until the bubble is pushed back to the start of the scale on the capillary tube. Measurements then continue as before
• The experiment can be repeated to compare the rates of water uptake under different conditions, for example at different temperatures, humidity, light intensity, or the differences in water uptake between different species under the same condition

Question 19

From your knowledge of how water moves up the stem, suggest a reason why each of the following procedures is carried out:
A) The leafy shoot is cut under water rather than in the air.
B) All joints are sealed with waterproof jelly.

Answer 19

A) As xylem is under tension, cutting the shoot in air would lead to air being drawn into the stem, which would stop transport of water up the shoot. Cutting under water means water, rather than air, is drawn in and a continuous column of water is maintained.

B) Sealing prevents air being drawn into the xylem and stopping water flow up it / Sealing prevents water leaking out which would produce an inaccurate result.

Question 20

State what assumptions must be made if a potometer is used to measure the rate of transpiration

Answer 20

that all water taken up is transpired

Question 21

The volume of water taken up in a given time can be calculated using the formula pie r squared x the distance moved by the air bubble. In an experiment the mean distance moved by the air bubble in a capillary tube of a radius of 0.5mm during 1 min was 15.28mm. Calculate the rate of water uptake in cm^3 h^-1. Show your workings

Answer 21

Volume of water taken up in one minute:
3.142 x (0.5 x 0.5) x 15.28 = 12.00mm?’. Volume of water taken up in | hour:
12.00 x 60 = 720mm. Volume in cm? =
720 + 1000 = 0.72cm’.
Answer = 0.72cm^3h^-1

Question 22

If a potometer is used to compare the transpiration rates of 2 different species of plant, suggest one feature of both plant shoots that should, as far as possible, be kept the same

Answer 22

their surface area / surface area of the leaves

Question 23

Suggest reasons why the results obtained from a laboratory experiment may not be representative of the transpiration rate of the same plant in the wild

Answer 23

1) an isolated shoot is much smaller than the whole plant / may not be representative of the whole plant / may be damaged when cut.

2) Conditions in the lab may be different from those in the wild, e.g. less air movement / greater humidity / more light (artificial lighting when dark)

Question 24

State two features shown in Figure 7 that suit a root hair cell to its function of absorbing water and mineral ions.

Answer 24

thin cell wall, large surface area / long, hair-like extension

Question 25

Define osmosis

Answer 25

Osmosis is the passage of water from a region where it has a higher water potential to a region where it has a lower water potential, through a selectively permeable membrane.

Question 26

Explain in terms of water potential how water might be absorbed into a root hair cell,

Answer 26

• The water potential of the soil solution is higher than that in the vacuole / cytoplasm of the root hair cell.
• Water therefore moves along a water potential gradient.

Question 27

Suggest the name of an organelle that you might expect to occur in large numbers in a root hair cell,
giving a reason for your answer.

Answer 27

mitochondria because they release energy / make ATP during respiration and this energy / ATP is essential for active transport

Question 28

Xylem vessels vary in appearance, depending on the type and amount of thickening of their cell walls. As they mature, their walls incorporate a substance called….

Answer 28

Lignin

Question 29

The process of transporting water in plants in the transpiration stream involves water being pulled up the plant, which causes a negative pressure in the xylem vessels. Explain how xylem vessels are adapted to cope with this.

Answer 29

They have thick walls to prevent the vessels collapsing.

Question 30

Name two other features shown in Figure 8 that suit xylem vessels to their function of transporting water up a plant.

Answer 30

• hollow
• elongated

Question 31

Suggest one advantage of xylem vessels being dead cells in order to carry out their function effectively.

Answer 31

Living cells have a cell-surface membrane and cytoplasm, and water movement would be slowed as it crossed this membrane / cytoplasm

Question 32

Suggest another possible feature of lignin, other than its mechanical strength, that would be useful in ensuring that water was carried up the plant.

Answer 32

Waterproofing

Question 33

The thickening of the cell wall in xylem vessels is often spiral. Suggest three advantages to the plant having this arrangement rather than continuous thickening.

Answer 33

1) allows the vessel to elongate as the plant grows
2) uses less material and therefore is less wasteful
3) uses less material and therefore the plant has lower mass
4) allows stems to be flexible