The shape of the lesson
By the end of the lesson, learners can
- Sources, sinks and the phloem: Extended.
- One organ, two roles: Extended.
- Translocation: sucrose and amino acids, in phloem, from sources to sinks.
- Source: releases sucrose or amino acids (e.g. a photosynthesising leaf).
- Sink: uses or stores them (growing tips, roots, fruits, seeds, storage organs).
- Same organ, two roles: a tuber is a sink in summer and a source in spring.
One picture every idea returns to
Extended. Translocation is the movement of sucrose and amino acids in the phloem from sources to sinks. A source is a part of the plant that releases sucrose or amino acids, such as a photosynthesising leaf. A sink is a part that uses or stores them, such as a growing tip, a root, a developing fruit or a storage organ. Because sources and sinks can be anywhere, phloem carries its contents both up and down the plant, and an organ such as a tuber can be a sink at one time and a source at another.
Forty-five minutes, phase by phase
| Time | Phase | What happens in the room | Grouping |
|---|---|---|---|
| 0 to 5 min | Hook: predict | Open with the question in the title, From source to sink.. The sugar a leaf makes is no use to a root tip or a ripening fruit unless it gets there. Put the figure on the board: it is the one picture every idea in the lesson returns to. | Whole class, then pairs |
| 5 to 17 min | Teach and check: Sources, sinks and the phloem | Extended. Project tab 7 of the lab (Translocation from sources to sinks) and let a student drive it while the class predicts. | Whole class, then pairs |
| 17 to 30 min | Teach and check: One organ, two roles | Extended. | Corners |
| 30 to 36 min | Check | The four Stage 1 questions from the student lesson, answered on mini whiteboards or by show of fingers. Re-teach the one most get wrong. | Individual, then whole class |
| 36 to 43 min | Plenary | The exam-style questions from Stage 2, one at a time: thirty seconds alone, then a random call. The feedback lines in the key give the examiner's wording. | Individual, random call |
| 43 to 45 min | Exit and homework | Set the worksheet below (6 questions, 20 marks). Learners self-assess against the objectives before they leave. | Individual |
Protect the parts that carry the learning
The sequence carries about fifty minutes of material, so the lab is driven by one student while the class predicts rather than explored freely. Times are a guide.
Protected: the check at 30 minutes. It is the only individual read on whether the key idea landed, and it is not cut.
Cut first if the build overruns: the last check in One organ, two roles, never the check or the plenary.
In a 60 minute block: run worksheet questions 1 to 3 in class as a Rally Coach set and model both the hardest worked example and its check live.
Corners
Put one answer or viewpoint in each corner of the room. Students move to the corner they agree with, justify the choice to a partner there, then hear one voice from each corner before the reveal.
The full structure, with its variations and the other twelve, is on the cooperative-learning page.
What to head off, and how
| Trap learners fall into | Teaching move that pre-empts it |
|---|---|
| Translocation happens in the phloem and carries sucrose and amino acids. | Do not say glucose is translocated (it is converted to sucrose first), and do not confuse it with transpiration, which is water loss through the xylem and leaves. |
| A storage organ is not "always a sink" and a leaf is not "always a source". | Exam questions often describe a season or a growth stage on purpose: decide whether the organ is releasing or receiving sucrose at that time. |
Translocation: worksheet
6 questions, 20 marks. Written in the style of the papers; no past paper question is reproduced.
Extended. Define translocation and state the meaning of source and sink.
Extended. Classify each of the following as a source or a sink in a bean plant in midsummer, and give a reason for each: (a) a fully grown leaf in sunlight, (b) a root tip, (c) a developing seed pod, (d) a young leaf at the top of the shoot.
Extended. Explain why a potato tuber can act as a sink at one time of year and as a source at another.
Extended. Radioactive carbon dioxide was supplied to one mature leaf of a plant for one hour. The table shows the radioactivity found in other parts of the plant after 6 hours.
| Part | Radioactivity / arbitrary units |
|---|---|
| shoot tip | 340 |
| developing fruit | 520 |
| roots | 290 |
| older leaf on the opposite side | 10 |
Use the data to explain what the results show about translocation.
Extended. Compare translocation with the movement of water in a plant. Give four differences.
Extended. Aphids push their mouthparts into the stems of plants and feed on the liquid in the phloem. Suggest why heavy aphid infestation reduces the size of fruits.
Answer key and marking notes
- Translocation is the movement of sucrose and amino acids in the phloem from sources to sinks.
- A source is a part of the plant that releases sucrose or amino acids.
- A sink is a part of the plant that uses or stores sucrose or amino acids.
- (a) Source: it photosynthesises and releases sucrose into the phloem.
- (b) Sink: it uses sucrose for respiration and growth.
- (c) Sink: it uses sucrose and amino acids to grow and stores them in the seeds.
- (d) Sink: it is still growing and cannot yet make enough sucrose for itself.
- In summer the leaves photosynthesise and sucrose is translocated in the phloem to the tuber.
- The tuber converts the sucrose to starch and stores it, so it is a sink.
- In spring the new shoots have few or no leaves, so they cannot make enough sucrose.
- The tuber converts its starch back to sucrose and releases it into the phloem for the growing shoots, so it is now a source.
- The labelled carbon was made into sucrose in the leaf (the source) by photosynthesis and translocated in the phloem.
- It reached parts both above (shoot tip, fruit) and below (roots) the leaf, so phloem carries sucrose up and down.
- Most went to the developing fruit, the largest sink; the older mature leaf received almost none because it is a source itself, not a sink.
- Translocation happens in phloem; water moves in xylem.
- Translocation carries sucrose and amino acids; xylem carries water and mineral ions.
- Translocation can go up or down (source to sink); water in the xylem moves only upwards, from roots to leaves.
- Translocation takes place in living cells (sieve tubes); xylem vessels are dead, empty tubes. Also accept: water movement is driven by transpiration pull.
- Aphids remove sucrose and amino acids from the phloem before they reach the sinks.
- Less sucrose and fewer amino acids reach the developing fruits, so they have less for growth and storage and stay small.
8 multiple-choice questions on this lesson
From the topic question bank. Assign them in the app for a marked score in your gradebook, or run the topic as a class game.
Extended: which two substances are moved in translocation?
- ASucrose and amino acids
- BWater and mineral ions
- CGlucose and oxygen
- DStarch and proteins
Extended: which part of a plant is acting as a source?
- AA mature leaf photosynthesising in sunlight
- BA root tip that is growing quickly
- CA seed developing inside a pod
- DA potato tuber filling with starch in summer
Extended: a sink is a part of a plant that:
- Auses or stores sucrose or amino acids
- Breleases sucrose or amino acids
- Cabsorbs water and mineral ions from soil
- Dloses water vapour through stomata
Extended: early in spring, before its leaves have opened, sucrose moves upwards in the phloem of a young tree. Where is the sucrose most likely coming from?
- AStarch stored in the roots and stem over winter
- BPhotosynthesis in the new leaves
- CSucrose absorbed from the soil by root hairs
- DWater taken up from the soil by osmosis
Extended: in which direction does sucrose move in the phloem of a stem?
- AUp or down, from wherever the sources are to wherever the sinks are
- BOnly upwards, from the roots to the leaves
- COnly downwards, from the leaves to the roots
- DOnly sideways, from the xylem into the phloem
Extended: a young leaf at the tip of a shoot is a sink, but the same leaf is a source a month later. Why?
- AOnce fully grown it photosynthesises enough to export sucrose
- BOnce fully grown it stops respiring and stores sucrose
- COnce fully grown its xylem starts carrying sucrose
- DOnce fully grown it absorbs sucrose from the air
Extended: a bean seed germinates in the dark. Its stored food is in two large seed leaves (cotyledons). Which row correctly identifies a source and a sink during the first week?
- Asource: the cotyledons; sink: the growing root and shoot
- Bsource: the growing shoot; sink: the cotyledons
- Csource: the growing root; sink: the growing shoot
- Dsource: the soil water; sink: the cotyledons
Extended: a ring of bark (containing the phloem) is removed from the trunk of an apple tree, between the leaves and the roots. A few weeks later, the fruit above the ring is larger than usual. Which explanation is best?
- ASucrose cannot pass the ring to the roots, so more reaches the fruit above it
- BWater cannot pass the ring, so the fruit fills with stored water
- CThe roots now send extra sucrose up to the fruit
- DThe leaves stop photosynthesising, so the fruit makes its own sucrose
Quiz key
Translocation moves sucrose and amino acids in the phloem. Water and mineral ions travel in the xylem. Glucose is converted to sucrose before it is transported, and oxygen leaves by diffusion. Starch and proteins are large, insoluble storage molecules that are not transported.
A photosynthesising mature leaf makes more sucrose than it uses and releases it into the phloem, so it is a source. A growing root tip uses sucrose, a developing seed stores it, and a tuber filling with starch in summer stores it, so all three are sinks.
A sink uses (for respiration and growth) or stores sucrose or amino acids. Releasing them is the definition of a source. Absorbing water and ions describes root hair cells, and losing water vapour describes transpiration.
Before the leaves open, the stored starch in the roots and stem is converted back to sucrose and transported up to the growing buds, so the storage tissue is the source. Leaves that have not opened cannot photosynthesise much. Root hairs absorb water and ions, not sucrose. Water is not a source of sucrose.
Phloem carries sucrose from sources to sinks, which may be above or below the source, so movement can be up or down. Only upwards describes water in the xylem. Only downwards ignores sinks such as fruits and shoot tips above the leaves. Sucrose does not come from the xylem.
A young growing leaf cannot make enough sucrose for itself, so it imports it (sink). When mature, it makes more sucrose by photosynthesis than it uses and exports the surplus (source). Living cells never stop respiring, xylem does not carry sucrose, and sucrose is not absorbed from the air.
The cotyledons break down their stored food and release sucrose and amino acids, so they are the source; the growing root and shoot use them for respiration and growth, so they are sinks. The shoot cannot photosynthesise in the dark, so it is not a source, and the cotyledons are emptying, not filling. The root uses sucrose rather than releasing it. Soil water supplies water, not sucrose.
With the phloem cut, sucrose from the leaves cannot travel down to the roots, so more of it goes to the sinks above the ring, including the fruit. The xylem is intact, so water still flows. The roots cannot send sucrose up past the ring, and they are now short of sucrose themselves. The leaves keep photosynthesising, and fruits are sinks, not sources.
What to project
The student lesson embeds these tabs. Open them full screen on the projector; every slider has a drag handle on the stage.
Slides for this lesson
A projectable deck for every pack is on the way. Until then, present the student lesson full screen: its sections, figure and lab tabs follow the same order as the plan above.