Answers · Xylem, phloem and water uptake

Xylem, phloem and water uptake — final check key

Check meaning, not exact wording. These are teaching points, not official Cambridge marks.

1. RK1 · Final check

Model answer. (a) Cut a fresh end on matched celery stalks with their leaves on, and stand them in a dye such as red food colouring or eosin solution. Take out a different stalk after each time, for example 30 minutes, 2 hours and 6 hours. Cut thin slices across each one at measured heights, look at them with a hand lens and under the microscope, and record where the colour is. (b) The control is a matched stalk in plain water, left as long as the longest and cut in the same way. It shows the slices with no dye, so any colour in the others came from the dye. (c) The colour will be in separate dots, the vascular bundles, and within them in the xylem. It will reach higher the longer a stalk stood in the dye, and in the end reach the leaf veins.

A full answer includes

(a) Matched celery stalks (or white flowers) with fresh cut ends, stood in a dye such as eosin; a different stalk taken out after each time; thin slices cut across at measured heights; looked at with a hand lens or microscope; where the colour is recorded.
(b) A matched stalk in plain water, treated the same: shows that any colour came from the dye.
(c) Coloured dots (the vascular bundles), with the colour in the xylem; higher the longer the time; in the end in the leaf veins.

Also accept

(a) stalks in dye, slices at measured heights after set times; (b) a stalk in plain water; (c) dye in the xylem of the bundles, higher with time, into the leaf veins

Watch for

“Leave every stalk in the dye for the same time.” — Different times, with a fresh stalk for each, show how far the dye gets.
“A control is not needed.” — Without a stalk in plain water you cannot tell that the colour came from the dye.

2. RK2 · Final check

Model answer. (a) P, the small cells on the outer side of a bundle, is phloem. R, the wide, thick-walled tubes on the inner side of a bundle, is xylem. (b) Phloem transports sucrose and amino acids. Xylem transports water and mineral ions, and helps to support the plant.

A full answer includes

(a) P: phloem; R: xylem.
(b) Phloem transports sucrose and amino acids; xylem transports water and mineral ions and helps to support the plant.

Also accept

(a) P phloem, R xylem; (b) phloem: sucrose and amino acids; xylem: water, mineral ions, support

Watch for

“P is xylem, because it is nearer the outside.” — In a stem bundle the phloem is on the outer side.
“R carries sucrose.” — R is xylem: it carries water and mineral ions.

3. RK3 · Final check

Model answer. Thick walls, strengthened with lignin: the vessel is strong and stays open, and it helps to support the plant. No cell contents: with no cytoplasm or nucleus, nothing blocks the water. Cells joined end to end with no cross walls: they form one long, continuous tube from the roots to the leaves, so water moves up it freely.

A full answer includes

Thick walls with lignin: strong, keep the tube open (and support the plant).
No cell contents: nothing blocks the water.
Cells joined end to end with no cross walls: a long, continuous tube.

Also accept

lignin walls; no cell contents; no cross walls, a continuous tube

Watch for

“Thin walls let the water in easily.” — Xylem vessel walls are thick, strengthened with lignin.
“Its living cytoplasm pumps the water.” — A xylem vessel has no cell contents.

4. HK1 · Final check

Model answer. (a) R is a root hair cell: it takes in water, by osmosis, and mineral ions from the soil. (b) The many root hairs give the root a large surface area in contact with the soil water, which increases the uptake of water and mineral ions.

A full answer includes

(a) R: takes in water and mineral ions from the soil.
(b) They give a large surface area, which increases the uptake of water and mineral ions.

Also accept

(a) R, water and mineral ions; (b) a large surface area increases uptake

Watch for

“S is a root hair cell.” — S is the cortex. Root hair cells are on the outside, with long, thin outgrowths.
“Root hairs make food by photosynthesis.” — They have no chloroplasts and get no light.

5. HK2 · Final check

Model answer. Water goes from the soil into the root hair cells, by osmosis. It crosses the root cortex cells to the xylem in the middle of the root. The xylem carries it up the root and the stem and into the veins of the leaf. It then leaves the xylem and moves into the mesophyll cells.

A full answer includes

Root hair cells, then root cortex cells, then xylem (up the root, stem and leaf), then mesophyll cells.

Also accept

root hair cells; root cortex cells; xylem; mesophyll cells

Watch for

“Root hair cells, xylem, root cortex cells, mesophyll cells.” — The root cortex cells come before the xylem.
“Water goes up in the phloem.” — Water goes up in the xylem.

6. HK3 · Final check

Model answer. (a) In a root the xylem is in the middle, shaped like a star or a cross, and the phloem is in groups between its arms. (b) In a leaf, in each vein, the xylem is towards the upper surface and the phloem towards the lower surface.

A full answer includes

(a) Root: xylem in the middle, like a star or a cross; phloem between the arms of the xylem.
(b) Leaf: in each vein, xylem towards the upper surface and phloem towards the lower surface.

Also accept

(a) xylem in the middle, phloem between its arms; (b) xylem towards the upper surface, phloem towards the lower

Watch for

“In a root, the xylem and phloem are in a ring near the outside.” — That is a stem. In a root the xylem is in the middle.
“In a leaf vein, the phloem is towards the upper surface.” — The xylem is towards the upper surface; the phloem towards the lower surface.

7. TK1 · Final check

Model answer. (a) Q, the full-grown leaf, is a source: it makes sugar by photosynthesis and releases sucrose and amino acids into the phloem. (b) Any two of: P, the young leaves at the top, use sucrose to grow and respire; R, the pod, uses it, and its growing seeds store it, for example as starch; S, the roots, use it for respiration and growth. (c) Translocation, in the phloem.

A full answer includes

(a) Q, the full-grown leaf: it makes sugar by photosynthesis and releases sucrose (and amino acids).
(b) Any two: P, the young leaves, use it to grow and respire; R, the pod, uses it, and its seeds store it (for example as starch); S, the roots, use it for respiration and growth.
(c) Translocation, in the phloem.

Also accept

(a) Q, releases sucrose; (b) two of P, R, S with use or store; (c) translocation, phloem

Watch for

“S is a source, because roots take things in.” — Roots take in water and mineral ions; for sucrose they are a sink.
“P is a source, because it is a leaf.” — P is still growing: it cannot yet make enough sugar for its growth, so it takes sucrose in. It is a sink.
“Sucrose moves in the xylem.” — Sucrose moves in the phloem.

8. TK2 · Final check

Model answer. A young leaf, still growing, cannot yet make enough sugar for its growth, so it uses sucrose and amino acids carried to it in the phloem: it is a sink. Once it is full-grown, in the light, it makes more sugar by photosynthesis than it uses and releases sucrose into the phloem: it is a source.

A full answer includes

Young, growing leaf: cannot yet make enough sugar for its growth, so it uses sucrose and amino acids from the phloem: a sink.
Full-grown leaf in the light: makes more sugar by photosynthesis than it uses, and releases sucrose: a source.

Also accept

young: uses more than it makes, a sink; full-grown: makes more than it uses, a source

Watch for

“Leaves are always sources.” — A young, growing leaf is a sink.
“A young leaf has no chloroplasts.” — It has chloroplasts, but it cannot yet make enough sugar for its own fast growth.

9. TK3 · Final check

Model answer. (a) Xylem carries water and mineral ions; phloem carries sucrose and amino acids. (b) Xylem carries water up only, from the roots to the leaves. Phloem carries sucrose and amino acids from sources to sinks, which can be up or down the plant.

A full answer includes

(a) Xylem: water and mineral ions; phloem: sucrose and amino acids.
(b) Xylem: up only, from the roots to the leaves; phloem: from sources to sinks, up or down.

Also accept

(a) xylem: water and mineral ions, phloem: sucrose and amino acids; (b) xylem up only, phloem source to sink

Watch for

“Both carry water.” — Water goes in the xylem; phloem carries sucrose and amino acids.
“Phloem carries things only down.” — Up or down: from source to sink.