IGCSE · Practice questions · Translocation

Wherever the sink is.

Six original Cambridge-style questions on translocation in the phloem, identifying sources and sinks, the seasonal switch of storage organs and comparing translocation with transpiration.

Original questions All questions on this page are original work, written in the Cambridge IGCSE style. They are not from past papers. They test the same concepts and skills the syllabus rewards.

Keep these straight

Release or receive?

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.

Recall

[3 marks]

Extended. Define translocation and state the meaning of source and sink.

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. ✓

Analysis

[4 marks]

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.

(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. ✓

Explain

[4 marks]

Extended. Explain why a potato tuber can act as a sink at one time of year and as a source at another.

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. ✓

Data

[3 marks]

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.

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. ✓

Analysis

[4 marks]

Extended. Compare translocation with the movement of water in a plant. Give four differences.

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. ✓

Explain

[2 marks]

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.

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. ✓

Mark this once you have attempted all six and checked your working. It records a Practiced badge on the topic and adds a one-time bonus. Revealing the solutions alone does not count.