Xylem, phloem and water uptake — return questions key

1. RR1 · Return

Model answer. (a) Xylem transports water and mineral ions from the roots to the rest of the plant, and helps to support the plant. (b) Phloem transports sucrose and amino acids from the parts that release them to the parts that use or store them.

A full answer includes

• (a) Xylem: transports water and mineral ions; helps to support the plant.
• (b) Phloem: transports sucrose and amino acids.

Also accept

• (a) water and mineral ions, and support; (b) sucrose and amino acids

Watch for

• “Phloem transports water.” — Water goes up in the xylem; phloem carries sucrose and amino acids.
• “Xylem only transports water.” — Xylem also transports mineral ions and helps to support the plant.

2. RR2 · Return

Options as numbered on the pupil sheet: 1 — Cut a fresh end on four matched stalks.; 2 — Stand three stalks in red dye and one in plain water.; 3 — Take out a different stalk at each time.; 4 — Cut thin slices across each stalk at measured heights.; 5 — Look at the slices with a hand lens and record where the red is..

Model answer. First cut a fresh end on four matched stalks. Then stand three of them in red dye and the control in plain water. Take out a different stalk at each time. Cut thin slices across each one at measured heights. Last, look at the slices with a hand lens and record where the red is.

A full answer includes

• Order: cut a fresh end on matched stalks; stand three in red dye and one in plain water; take out a different stalk at each time; cut thin slices at measured heights; look with a hand lens and record where the red is.

Also accept

• fresh ends; dye and control; one stalk out at each time; slices at measured heights; look and record

Watch for

• “Cut the slices first, then stand them in the dye.” — The dye has to travel up the whole stalk first; then you cut it to see how far the dye got.
• “Take all the stalks out at the end.” — A different stalk at each time shows how far the dye had got by each time.

3. RR3 · Return

Model answer. The vascular bundles are arranged in a ring near the outside of the stem. In each bundle the phloem is towards the outside of the stem, and the xylem is towards the inside.

A full answer includes

• g1: ring.
• g2: outside (outer side).
• g3: inside (inner side, towards the middle).

Also accept

• ring; outside; inside

Watch for

• “In each bundle the xylem is towards the outside.” — In a stem bundle the phloem is on the outer side and the xylem on the inner side.
• “The bundles are in the middle of the stem.” — In a root the xylem is in the middle; in a stem the bundles form a ring near the outside.

4. RR4 · Return

Model answer. A xylem vessel has thick walls strengthened with lignin, which keep the tube open and help to support the plant. It has no cell contents, so nothing inside blocks the water. It forms from cells joined end to end with no cross walls, so it is one long, continuous tube.

A full answer includes

• g1: lignin.
• g2: cell contents (cytoplasm and nucleus).
• g3: cross walls.

Also accept

• lignin; cell contents; cross walls

Watch for

• “A xylem vessel is full of living cytoplasm.” — A finished vessel has no cell contents: it is an empty tube.
• “Its walls are thin, so that water can soak through them.” — The walls are thick and strengthened with lignin, so the tube stays open and does not collapse.

5. HR1 · Return

Model answer. (a) Root hair cells take in water and mineral ions from the soil. (b) The many root hairs give a large surface area, which increases the uptake of water and mineral ions.

A full answer includes

• (a) Taking in water; taking in mineral ions.
• (b) They give a large surface area, which increases the uptake of water and mineral ions.

Also accept

• (a) water, mineral ions; (b) a large surface area

Watch for

• “Root hair cells carry water up to the leaves.” — They take water in; the xylem carries it up.
• “Root hairs make the root stronger.” — Their job is uptake, through a large surface area.

6. HR2 · Return

Model answer. Root hair cells, root cortex cells, xylem, mesophyll cells.

A full answer includes

• g1: root cortex cells.
• g2: xylem.

Also accept

• root cortex cells; xylem

Watch for

• “Root hair cells, then xylem, then root cortex cells.” — The cortex is between the root hairs and the xylem in the middle.
• “The second gap is phloem.” — Water travels in the xylem.

7. HR3 · Return

Model answer. In a root the xylem is in the middle, with the phloem between its arms. In a leaf vein the xylem is towards the upper surface and the phloem towards the lower surface. In a stem the bundles form a ring near the outside, with the xylem on the inner side of each bundle and the phloem on the outer side.

A full answer includes

• Correct: in a root the xylem is in the middle; in a root the phloem is between the arms of the xylem; in a leaf vein the xylem is towards the upper surface.
• Wrong: in a stem the xylem is on the outer side of each bundle; in a leaf vein the phloem is towards the upper surface; in a root the bundles form a ring near the outside.

Also accept

• root: xylem in the middle, phloem between the arms; leaf: xylem towards the upper surface

Watch for

• “In a stem, the xylem is on the outer side of each bundle.” — The phloem is on the outer side; the xylem on the inner side.
• “A root has a ring of bundles, like a stem.” — A root has its xylem in the middle.

8. TR1 · Return

Model answer. Translocation is the movement of sucrose and amino acids in the phloem, from sources to sinks.

A full answer includes

• The movement of sucrose and amino acids in the phloem, from sources to sinks.

Also accept

• sucrose and amino acids, in the phloem, from sources to sinks

Watch for

• “Translocation is the movement of water through a plant.” — Water moves in the xylem; translocation is sucrose and amino acids in the phloem.
• “Translocation moves food only downwards.” — From sources to sinks, up or down.

9. TR2 · Return

Model answer. A growing root tip, a ripening fruit, a developing seed and an opening flower bud all use or store sucrose and amino acids: they are sinks. A full-grown leaf in sunlight and a potato tuber sprouting in spring release sucrose: they are sources.

A full answer includes

• Sinks: a growing root tip, a ripening fruit, a developing seed, an opening flower bud.
• Not sinks (sources): a full-grown leaf in sunlight, a potato tuber sprouting in spring.

Also accept

• root tip, ripening fruit, developing seed, flower bud

Watch for

• “A sprouting potato tuber is a sink, because it is a store.” — When it sprouts it releases sucrose from its store: a source.
• “A full-grown leaf is a sink, because it uses sucrose in respiration.” — It uses some, but releases much more: it is a source.

10. TR3 · Return

Model answer. In summer the tuber stores sucrose, brought from the leaves in the phloem, as starch: it is a sink. In spring, when it sprouts, it breaks down its starch to sucrose and releases it into the phloem for the new shoots, which cannot yet photosynthesise: it is a source.

A full answer includes

• Summer: it stores sucrose (as starch): a sink.
• Spring: it breaks down its starch to sucrose and releases it to the new shoots: a source.

Also accept

• summer: stores, a sink; spring: releases sucrose, a source

Watch for

• “A tuber is always a store, so it is always a sink.” — In spring it releases its store: then it is a source.
• “In spring the tuber makes new sugar by photosynthesis.” — It is underground; it breaks down its starch.