Pulled up the xylem, and wilting — answer key

1. TQ1 · Practice

Model answer. The transpiration pull. Water evaporates from the mesophyll cells in the leaves and diffuses out; the mesophyll cells take in water from the xylem, and this pulls up the column of water molecules, which hold together.

A full answer includes

• The transpiration pull: water evaporating from the leaves pulls up the column of water in the xylem.

Also accept

• the transpiration pull from the leaves

Watch for

• “Root pressure pushes it up.” — This shoot has no roots. The pull comes from the leaves.
• “Living cells in the xylem squeeze it up.” — Xylem vessels have no cell contents; they cannot squeeze.

2. TQ2 · Practice

Options as numbered on the pupil sheet: 1 — The mesophyll cells take in water from the xylem; 2 — More water enters the roots from the soil; 3 — Water evaporates from the mesophyll cells and the vapour diffuses out of the leaf; 4 — The whole column of water in the xylem is drawn up.

Model answer. Water evaporates from the surfaces of the mesophyll cells and the vapour diffuses out through the stomata. The mesophyll cells take in water from the xylem to replace it. Because water molecules attract each other, the whole column of water in the xylem is drawn up. At the bottom, more water enters the roots from the soil.

A full answer includes

• Order: water evaporates from the mesophyll cells and the vapour diffuses out; the mesophyll cells take in water from the xylem; the whole column of water in the xylem is drawn up; more water enters the roots from the soil.

Also accept

• evaporation from leaf; water from xylem into mesophyll; column drawn up; water enters roots

Watch for

• “Water enters the roots first and pushes the column up.” — The pull starts at the top, where water is lost from the leaves.
• “The column moves up before any water evaporates.” — Evaporation from the leaves is what pulls the column.

3. TQ3 · Practice

Model answer. Water molecules attract one another. These forces of attraction between water molecules hold the water in the xylem together as a continuous column, so when water is pulled from the top, the whole column moves up.

A full answer includes

• There are forces of attraction between water molecules, which hold the column together.

Also accept

• forces of attraction between water molecules hold the column together

Watch for

• “The xylem walls are sticky and pull the water up.” — The column holds together because the water molecules attract each other.
• “The column is held up by air pressure from the bubble.” — The bubble is in the glass tube; it only marks the movement.

4. TQ4 · Practice

Model answer. A plant wilts when it loses water by transpiration faster than its roots can take water up. Its cells lose water and become flaccid, and the leaves and stems droop.

A full answer includes

• When it loses water by transpiration faster than its roots take water up.

Also accept

• loses water faster than it takes it up

Watch for

• “A plant wilts when it runs out of food.” — Wilting is about water, not food.
• “A plant wilts when its cells take in too much water.” — Cells that take in water become turgid and firm. Wilting is caused by water loss.

5. TQ5 · Practice

Model answer. Water evaporates from the mesophyll cells in the leaves. This causes a transpiration pull that draws up a column of water molecules in the xylem. The molecules are held together by forces of attraction between them.

A full answer includes

• evaporates; transpiration; column; attraction

Also accept

• evaporates, transpiration, column, attraction

Watch for

• “The molecules are held together by the xylem walls.” — They are held together by forces of attraction between the water molecules.
• “Root pressure draws up the column.” — The transpiration pull from the leaves draws it up.

6. TQ6 · Practice

Model answer. Cutting and fitting the shoot under water stops air getting into the cut ends of the xylem vessels. An air bubble in a vessel breaks the continuous column of water: the forces of attraction act between water molecules, not across a gap of air, so the column could not be pulled up past it, and the shoot would take up water slowly or not at all.

A full answer includes

• So that no air gets into the xylem vessels.
• An air bubble would break the column of water (the attraction between water molecules cannot act across air), so the shoot could not take up water properly.

Also accept

• no air in the xylem; an air bubble breaks the column

Watch for

• “To keep the shoot cool while cutting.” — The point is to keep air out of the xylem.
• “To wash the dirt off the stem.” — The point is to keep air out, so the water column is not broken.

7. TQ7 · Practice

Options as numbered on the pupil sheet: 1 — The cells lose water and their vacuoles shrink; 2 — The cells become flaccid; 3 — The leaves and stems droop; 4 — Water is lost by transpiration faster than the roots take it up.

Model answer. First the plant loses water by transpiration faster than its roots take it up. Its cells lose water and their vacuoles shrink, so the turgor pressure falls and the cells become flaccid. Flaccid cells no longer hold the leaves and stems up, so they droop.

A full answer includes

• Order: water is lost by transpiration faster than it is taken up; the cells lose water and their vacuoles shrink; the cells become flaccid; the leaves and stems droop.

Also accept

• loss faster than uptake; cells lose water; flaccid; droop

Watch for

• “The leaves droop first, then the cells lose water.” — The drooping is caused by the cells becoming flaccid.
• “The cells burst, then the plant droops.” — Plant cells do not burst when they lose water; they become flaccid.

8. TQ8 · Practice

Model answer. A plant wilts when it loses water faster than it takes it up. Hot air, wind and dry air all make transpiration faster; dry soil means the roots cannot take up enough water. Cool, still, humid air slows transpiration, and wet soil lets the roots take up plenty of water.

A full answer includes

• More likely: hot air; a strong wind; dry soil; dry air.
• Less likely: cool, still, humid air; wet soil.

Also accept

• hot, windy, dry soil, dry air

Watch for

• “Humid air makes wilting more likely.” — Humid air slows transpiration, so wilting is less likely.
• “Wet soil makes wilting more likely.” — Wet soil lets the roots take up water, so wilting is less likely.

9. TQ9 · Practice

Model answer. (a) In the hot, windy afternoon, transpiration was so fast that the roots could not take up water as fast as it was lost. The cells of the leaves lost water and became flaccid, so the leaves drooped. (b) At night it is cooler and the air is often stiller and more humid, so transpiration slows down. The roots, in moist soil, take up water faster than it is lost; water enters the cells by osmosis, they become turgid again, and the leaves are firm.

A full answer includes

• (a) In hot, windy conditions transpiration was faster than the roots could take water up; the cells lost water, became flaccid and the leaves drooped.
• (b) At night it is cooler, the air is often stiller and more humid, so transpiration slows; the roots take up water faster than it is lost, water enters the cells by osmosis and they become turgid again.

Also accept

• (a) transpiration faster than uptake, cells flaccid; (b) slower transpiration at night, uptake catches up, cells turgid

Watch for

• “It wilted because the soil was dry.” — The soil was moist. The problem was the rate of transpiration.
• “It recovered because it made water by photosynthesis overnight.” — Photosynthesis uses water, and needs light. The roots took up water from the soil.

10. TQ10 · Practice

Model answer. The movement of water up the stem slows almost to a stop. Water is drawn up the xylem by the transpiration pull, caused by water evaporating from the leaves. With no leaves, there is almost no transpiration, so almost nothing pulls the column of water up.

A full answer includes

• It slows almost to a stop.
• With no leaves there is (almost) no transpiration, so (almost) no transpiration pull to draw the column of water up.

Also accept

• slows almost to a stop: no leaves, no transpiration pull

Watch for

• “It speeds up, because the water no longer has to go into the leaves.” — The leaves are where the pull comes from. Without them the water hardly moves.
• “No change, because the roots keep pushing.” — Most of the movement is caused by the pull from the leaves.

11. TT1 · Apply

Model answer. (a) Air gets into the cut ends of the xylem vessels. An air bubble breaks the column of water, because the attraction between water molecules cannot act across air, so the transpiration pull cannot draw water up to the flowers and leaves. They keep losing water by transpiration, so they droop. (b) Cutting again under water removes the end that has air in it, and no new air can get in, so the xylem holds unbroken columns of water that the transpiration pull can draw up.

A full answer includes

• (a) Air gets into the cut ends of the xylem vessels; air bubbles break the columns of water, so water cannot be pulled up to the flowers and leaves; they lose water and droop.
• (b) Cutting under water removes the part with air in it, and no new air gets in, so unbroken columns of water can be pulled up again.

Also accept

• (a) air in the xylem breaks the water column; (b) recut under water: air removed and none gets in

Watch for

• “The flowers droop because the vase water is dirty.” — The problem is air in the xylem breaking the column of water.
• “Cutting under water lets the flowers drink faster through the cut.” — The point is a fresh cut with no air in it, so the column is unbroken.

12. TT2 · Apply

Model answer. Root hairs give the roots a large surface area for taking up water. With many torn off, the seedlings take up water more slowly, so they lose water by transpiration faster than they take it up, their cells become flaccid and they wilt. In the evening, or on a cloudy day, it is cooler and often more humid, so transpiration is slower. The damaged roots can then keep up with the loss while new root hairs grow.

A full answer includes

• They wilt because, with fewer root hairs, they take up water more slowly, so they lose water by transpiration faster than they take it up.
• In the evening (or on a cloudy day) it is cooler and often more humid, so transpiration is slower, and the damaged roots can keep up.

Also accept

• fewer root hairs, slower uptake, loss faster than uptake; cooler, more humid evening slows transpiration

Watch for

• “They wilt because moving them uses up their food.” — Wilting is about water: loss faster than uptake.
• “In the evening the seedlings drink more.” — Uptake does not rise much; the point is that transpiration slows.

13. TT3 · Apply

Model answer. The bubble almost stops. No water vapour can diffuse out of the coated leaves, so there is almost no transpiration, and almost no transpiration pull to draw water up the xylem and out of the potometer.

A full answer includes

• The bubble almost stops: no water vapour can leave the leaves, so there is almost no transpiration pull.

Also accept

• almost stops: no transpiration, no pull

Watch for

• “The bubble moves faster, because water builds up in the leaves.” — With no loss of water vapour, nothing pulls more water in.
• “The bubble moves backwards, out of the shoot.” — The shoot does not push water out. The bubble simply stops.