Translocation — answer key

1. TQ1 · Practice

Model answer. Translocation is the movement of sucrose and amino acids in the phloem, from sources, which release them, to sinks, which use or store them. Water moving up the xylem is part of the pathway of water, and water vapour leaving through the stomata is transpiration.

A full answer includes

• Translocation: 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 in the xylem.” — It is the movement of sucrose and amino acids, in the phloem.
• “Translocation moves glucose.” — Glucose is changed into sucrose before it is carried.

2. TQ2 · Practice

Options as numbered on the pupil sheet: 1 — source; 2 — sink.

Model answer. In summer the full-grown leaves in the light are the source: they make more sugar than they use and release sucrose into the phloem. The growing shoot tip, the flowers and the roots use sucrose and amino acids, and the new tubers store sucrose as starch: they are all sinks.

A full answer includes

• Source: a full-grown leaf in the light.
• Sinks: the growing shoot tip, the flowers, the roots, the new tubers.

Also accept

• leaf: source; shoot tip, flowers, roots, new tubers: sinks

Watch for

• “The roots are a source, because they take things in.” — Roots take in water and mineral ions, but for sucrose they are a sink.
• “The tubers are a source, because potatoes are food.” — In summer the tubers are being filled: they store sucrose as starch, so they are sinks.

3. TQ3 · Practice

Model answer. A source is a part of a plant that releases sucrose or amino acids. A sink is a part of a plant that uses sucrose or amino acids, for example to grow, or stores them, for example as starch in a tuber.

A full answer includes

• g1: releases.
• g2: uses.
• g3: stores.

Also accept

• releases; uses; stores

Watch for

• “A source is a part that makes sucrose by photosynthesis.” — A source releases sucrose or amino acids into the phloem. A potato tuber in spring releases sucrose without photosynthesising.
• “A sink only stores.” — A sink uses or stores them.

4. TQ4 · Practice

Model answer. Roots grow underground in the dark and cannot photosynthesise, so they cannot make sucrose. They need it for respiration, to release energy, and to grow new cells. Some roots also store it as starch. So sucrose and amino acids are carried to them in the phloem: they are sinks.

A full answer includes

• Roots cannot photosynthesise (no light underground, no chloroplasts), so they cannot make sucrose.
• They use sucrose for respiration and growth (and some store it as starch).

Also accept

• roots cannot photosynthesise; they use sucrose for respiration and growth

Watch for

• “Roots are a source because they take in water.” — Water and mineral ions go up in the xylem; for sucrose the roots are a sink.
• “Roots make their own sucrose from the soil.” — Sucrose is made from the sugar of photosynthesis, in the leaves.

5. TQ5 · Practice

Model answer. (a) Up the stem, to the shoot tip. (b) Down the stem, to the roots. (c) The phloem carries sucrose and amino acids from a source to wherever the sinks are, which may be above it or below it. Water always enters at the roots and is lost from the leaves, so in the xylem it moves only up.

A full answer includes

• (a) Up.
• (b) Down.
• (c) The phloem carries sucrose from a source to wherever the sinks are, above or below it; water enters at the roots and is lost from the leaves, so in the xylem it moves only up.

Also accept

• (a) up; (b) down; (c) source to sink, up or down; water only up from roots to leaves

Watch for

• “Phloem carries things only down.” — It carries sucrose up to the shoot tip and the flowers too.
• “Translocation always goes up, like water.” — It goes from source to sink, up or down.

6. TQ6 · Practice

Model answer. In spring the tuber sprouts. Its new shoots have no green leaves yet and cannot photosynthesise. The tuber breaks down its stored starch into sucrose and releases it into the phloem, which carries it to the growing shoots. A part that releases sucrose is a source.

A full answer includes

• In spring it sprouts; its new shoots cannot yet photosynthesise, so the tuber breaks its starch down to sucrose and releases it into the phloem for the shoots: it is a source.

Also accept

• it breaks down its starch to sucrose and releases it to the new shoots

Watch for

• “The tuber photosynthesises in the soil.” — There is no light underground; the tuber uses its store of starch.
• “A tuber is always a sink, because it is a store.” — It is a sink while it stores starch in summer, and a source when it releases sucrose in spring.

7. TQ7 · Practice

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

A full answer includes

• (a) A young leaf cannot yet make enough sugar for its own growth, so it uses sucrose and amino acids brought in the phloem.
• (b) A full-grown leaf makes more sugar by photosynthesis than it uses, and releases sucrose into the phloem.

Also accept

• (a) cannot make enough for its growth, takes sucrose in; (b) makes more than it uses, releases sucrose

Watch for

• “All leaves are sources.” — A young, growing leaf is a sink until it makes more sugar than it uses.
• “A young leaf is a sink because it has no phloem.” — It has phloem; sucrose reaches it through the phloem because it cannot yet make enough for its own growth.

8. TQ8 · Practice

Options as numbered on the pupil sheet: 1 — source; 2 — sink.

Model answer. C, the full-grown leaf, is the source: it makes sugar by photosynthesis and releases sucrose into the phloem. A, the growing shoot tip, B, the flowers, D, the roots, and E, the new tuber, are sinks: they use or store sucrose and amino acids.

A full answer includes

• C, the leaf: source.
• A (shoot tip), B (flowers), D (roots) and E (new tuber): sinks.

Also accept

• C source; A, B, D, E sinks

Watch for

• “E is a source, because a tuber is a food store.” — In summer it is being filled: it stores sucrose as starch, so it is a sink.
• “A is a source, because it is at the top.” — The shoot tip is growing; it uses sucrose, so it is a sink.

9. TQ9 · Practice

Model answer. The sugar is made by photosynthesis in the full-grown leaves, the source, and changed into sucrose. The phloem carries the sucrose from the leaves to the fruit: this is translocation. The growing fruit is a sink: it uses some of the sucrose to grow and respire, and stores the rest, which makes it sweet.

A full answer includes

• Sugar is made by photosynthesis in the full-grown leaves (the source) and carried as sucrose in the phloem (translocation) to the fruit (a sink), which uses and stores it.

Also accept

• made in the leaves, carried as sucrose in the phloem to the fruit, a sink

Watch for

• “The fruit makes all its own sugar.” — A green fruit makes only a little; most of its sugar comes from the leaves, in the phloem.
• “The sugar comes up from the soil in the xylem.” — Sugar is made in the leaves and carried in the phloem.

10. TQ10 · Practice

Model answer. In its first summer the leaves make sugar by photosynthesis, and the phloem carries sucrose down to the root, which stores it as starch. The root stores sucrose, so it is a sink. In the second spring the plant grows a flowering shoot. The root breaks down its store and releases sucrose into the phloem, which carries it up to the growing shoot: now the root is a source.

A full answer includes

• First summer: the root stores sucrose (as starch), brought in the phloem from the leaves: a sink.
• Second spring: the store is broken down and released as sucrose, carried to the growing flowering shoot: a source.

Also accept

• summer: stores sucrose, a sink; spring: releases sucrose to the new shoot, a source

Watch for

• “The carrot root is always a sink, because it is a root.” — It is a source when it releases sucrose from its store.
• “In spring the root photosynthesises.” — It is underground; it releases sucrose from its store.

11. TT1 · Apply

Model answer. (a) The phloem carries sucrose and amino acids down from the leaves. At the ring the phloem has been removed, so they cannot pass, and they collect just above the ring, which swells. (b) The roots are a sink: they need sucrose for respiration and growth. No sucrose reaches them now, so they slowly starve and die. The xylem is left, so at first water still goes up to the leaves.

A full answer includes

• (a) Sucrose and amino acids moving down in the phloem cannot pass the ring, so they collect above it.
• (b) The roots, a sink, no longer get sucrose from the leaves; without it they cannot respire and grow, so they die (the xylem still carries water up for a while).

Also accept

• (a) sucrose moving down in the phloem collects above the ring; (b) the roots get no sucrose

Watch for

• “The leaves die first, because water cannot go up.” — The xylem, deeper in, was not cut, so water still goes up.
• “The swelling is water collecting above the ring.” — Water goes up in the xylem, which was not cut. The swelling is from sucrose that cannot move down in the phloem.

12. TT2 · Apply

Model answer. (a) In early spring there are no full-grown leaves to photosynthesise. Starch stored in the roots and trunk over the winter is broken down to sucrose and carried in the phloem to the opening buds. The stores are sources; the buds are sinks. (b) By midsummer the full-grown leaves photosynthesise and are the sources. Sucrose is carried to the growing parts and the fruits, and down to the roots and trunk, which store it again as starch: now the stores are sinks.

A full answer includes

• (a) Starch stored over winter (in the roots and trunk) is broken down to sucrose and carried to the buds: the stores are sources, the buds are sinks.
• (b) By midsummer the full-grown leaves photosynthesise and are the sources; the roots and trunk store sucrose again as starch, so they are sinks.

Also accept

• (a) winter stores are sources, buds are sinks; (b) leaves become sources, stores become sinks

Watch for

• “The closed buds photosynthesise to feed themselves.” — Closed buds have no full-grown leaves; their sucrose comes from the stores.
• “The roots are always sinks.” — In spring the stores in the roots release sucrose: they are sources.

13. TT3 · Apply

Model answer. (a) The phloem carries sucrose and amino acids, so the aphids get both. (b) The aphids take sucrose and amino acids out of the phloem on their way from the leaves, the source, to the fruits, a sink. Less reaches the fruits, so they have less to use for growth and to store, and they grow more slowly.

A full answer includes

• (a) Sucrose and amino acids.
• (b) They take sucrose and amino acids on their way from the leaves (source) to the fruits (sink), so less reaches the fruits for growth and storage.

Also accept

• (a) sucrose and amino acids; (b) they take what was being carried from the leaves to the fruits

Watch for

• “Aphids drink water from the phloem.” — Water goes up in the xylem; the phloem carries sucrose and amino acids.
• “Aphids slow the fruits by blocking the xylem.” — They feed from the phloem, taking the sucrose meant for the fruits.