Question 1
Paper 4 short-answer style1 markName the plant tissue in which translocation takes place. (1 mark)
Model answer
The phloem.
Why this scores
One mark for 'phloem'.
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Detailed notes on Transport in Plants for Cambridge IGCSE Biology, covering key concepts, explanations, examples, and exam-focused revision points.
Phloem moves sugars and amino acids around the plant from where they're made or stored to where they're needed.
Mapped to the Cambridge IGCSE 0610 syllabus (2026-2028).
Phloem moves sucrose and amino acids around the plant.
Translocation is the movement of:
through the phloem, from sources to sinks.
Source: a tissue or organ that adds sugars to the phloem. Examples:
Sink: a tissue or organ that takes sugars OUT of the phloem to use or store. Examples:
Why sucrose, not glucose?
Worked qualitative. Why does an apple get sweeter as it ripens?
Cambridge tip. Memorise the keyword phrase: "movement of sucrose and amino acids in the phloem from sources to sinks". Cambridge marks this almost word-for-word.
The same organ can be a source one season and a sink another.
Sources and sinks aren't fixed — they SHIFT depending on the time of year and the plant's stage of life.
Same organ, different roles:
| Organ | Spring | Summer |
|---|---|---|
| Potato tuber | SOURCE (starch breaks down → fuels new shoots) | SINK (filling with starch from leaves) |
| Tree trunk | SOURCE (stored sugars feeding new buds) | SINK (storing carbohydrate as bark and wood develops) |
| Underground bulb | SOURCE (feeding new shoots above ground) | SINK (refilling) |
Direction of phloem flow:
This bidirectional flow is one of the key differences between phloem (variable direction) and xylem (water always UP).
Worked qualitative. A daffodil bulb in March vs September.
Cambridge tip. Cambridge often gives a scenario (a season + an organ) and asks you to identify "source or sink?". Remember: any organ that is RELEASING sugars is a source; any organ that is RECEIVING sugars is a sink.
Active loading at source + passive flow + unloading at sink. Energy required.
Step 1 — loading. At the SOURCE, COMPANION CELLS pump sucrose INTO the phloem sieve tube using ACTIVE TRANSPORT (energy needed).
Step 2 — water follows. With sucrose in the sieve tube, the water potential there drops. Water from neighbouring xylem moves IN by osmosis → high pressure builds in the sieve tube at the source.
Step 3 — flow. High pressure at source + low pressure at sink (where sucrose is being removed) → sap flows from source to sink along the pressure gradient.
Step 4 — unloading. At the SINK, sucrose is removed (used or stored). Water leaves the sieve tube → low pressure here.
Why energy needed.
Worked qualitative. Why are companion cells right next to sieve tubes?
Cambridge tip. At IGCSE level the focus is on source-to-sink and the role of phloem; full pressure-flow detail is for A-level. But knowing that ENERGY (active transport) is involved earns marks.
Verbatim phrases and definitions Cambridge mark schemes credit.
Translocation appears Paper 4 (5-7 marks). Common formats: define + identify source/sink in a scenario; compare to xylem flow. Examiner reports flag students saying glucose is the transport sugar, that flow is always down, or that translocation happens in xylem.
Sources: Cambridge IGCSE Biology 0610 syllabus 2026-2028 (8.4); 0610/42 Oct/Nov 2024 — Q6 (translocation); 0610 Examiner Reports 2022-2024. Last reviewed 2026-05-07.
Step-by-step solutions to past-paper-style questions on translocation, written exactly the way a tutor would explain them at the board.
Question
Define translocation and name the tissue in which it occurs.
Step-by-step solution
Step 1
Translocation is the movement of sucrose and amino acids in the PHLOEM.
Step 2
It moves these substances from sources (where they are made or released) to sinks (where they are used or stored).
Answer
Translocation is the movement of sucrose and amino acids in the phloem from sources to sinks.
Question
For each plant part, state whether it is a source or a sink: (a) a green leaf in summer; (b) a growing fruit; (c) a growing root tip; (d) a storage organ that is releasing its stored food.
Step-by-step solution
Step 1
(a) A green leaf in summer is a source — it is photosynthesising and making sugar.
Step 2
(b) A growing fruit is a sink — it receives sugars to grow and store.
Step 3
(c) A growing root tip is a sink — it needs sugars for respiration and new tissue.
Step 4
(d) A storage organ releasing its food is a source — it exports the sugars to other parts.
Answer
(a) Source; (b) sink; (c) sink; (d) source.
Examiner tip
A source releases sugars into the phloem; a sink takes them out. The same organ can be either, depending on the season.
Question
Explain why translocation can occur in different directions at different times of the year, unlike the flow of water in the xylem.
Step-by-step solution
Step 1
The xylem carries water in ONE direction only — upwards — driven by the transpiration pull.
Step 2
Phloem carries sugars to wherever there is a sink, which can be above or below the source.
Step 3
In summer, the leaves are the source and sugars flow DOWN to the roots, fruits and storage organs (sinks).
Step 4
In spring, storage organs become the source and sugars flow UP to the new growing shoots (sinks). So the direction of flow follows the source-to-sink need.
Answer
Phloem flow follows source-to-sink, and sources and sinks change with the season, so the flow can be up or down. Xylem flow is always up.
Question
Explain how a potato tuber can act as a sink at one time of year and a source at another.
Step-by-step solution
Step 1
In summer, the leaves photosynthesise and make sugars, which are translocated to the tuber.
Step 2
The tuber stores these sugars as starch, so it is acting as a sink (taking sugars out of the phloem).
Step 3
In spring, the stored starch is broken down to sugars and exported to the new shoots.
Step 4
Now the tuber is releasing sugars into the phloem, so it is acting as a source.
Answer
When the tuber stores sugars (summer) it is a sink; when it releases stored sugars to feed new growth (spring) it is a source. The same organ switches role with the season.
Question
Translocation needs energy, and phloem is made of living cells. Explain the link.
Step-by-step solution
Step 1
At a source, sucrose has to be loaded into the sieve tubes, often against a concentration gradient.
Step 2
Moving a substance against a gradient requires energy from respiration (active transport).
Step 3
The companion cells next to each sieve tube have many mitochondria and provide this energy, because the sieve tubes themselves have little cytoplasm.
Step 4
Because energy and living processes are needed, phloem must be made of living cells — unlike the dead xylem, which carries water passively.
Answer
Loading sucrose into the phloem needs energy from respiration (active transport), supplied by the living companion cells. That is why phloem is made of living cells, while the xylem can be dead.
Examiner tip
The contrast with xylem is the point: translocation is an active, energy-using process, so the tissue must be alive.
Question
A gardener notices that when many leaves are removed from a tomato plant, the fruits grow more slowly. Explain this using the idea of translocation.
Step-by-step solution
Step 1
The leaves are the source — they photosynthesise to make the sugars.
Step 2
The growing fruits are sinks — they receive sugars by translocation in the phloem to grow and store.
Step 3
Removing leaves reduces the amount of photosynthesis, so fewer sugars are made and available to translocate.
Step 4
With less sugar reaching the fruits, they have less material for growth, so they grow more slowly.
Answer
Leaves are the source of sugars; fruits are sinks. Removing leaves reduces photosynthesis, so fewer sugars are translocated to the fruits, and they grow more slowly.
Examiner tip
Link the observation to source (leaves), sink (fruits) and the reduced supply of translocated sugar.
High-scoring sample answers for translocation on the Cambridge IGCSE 0610 paper, with examiner-style notes mapping each response to the mark scheme and assessment objectives.
Name the plant tissue in which translocation takes place. (1 mark)
Model answer
The phloem.
Why this scores
One mark for 'phloem'.
Define the terms source and sink in relation to translocation. (2 marks)
Model answer
A source is a part of the plant that releases sucrose and amino acids into the phloem (where they are made or stored, such as a photosynthesising leaf). A sink is a part of the plant that takes sucrose and amino acids out of the phloem to use or store them (such as a growing root or fruit).
Why this scores
One mark each: source = where sugars enter the phloem / are made; sink = where they are removed / used or stored.
Explain what is meant by translocation. (3 marks)
Model answer
Translocation is the movement of sucrose and amino acids in the phloem, from sources (parts of the plant where these substances are made or stored and released) to sinks (parts where they are used or stored).
Why this scores
Three marks: sucrose and amino acids; in the phloem; from source to sink.
Explain how a storage organ, such as a potato tuber, can act as a sink at one time of year and a source at another. (4 marks)
Model answer
In the growing season (summer), the leaves photosynthesise and make sugars, which are translocated to the tuber and stored as starch; here the tuber is taking sugars out of the phloem, so it acts as a sink. Later, in spring, the stored starch is broken down into sugars and released into the phloem to be carried to the new growing shoots; now the tuber is supplying sugars, so it acts as a source.
Why this scores
Four marks: sink when it stores sugars from the leaves; stores as starch; source when it releases stored sugars; to feed new growth.
Explain why translocation in the phloem can occur in both directions, whereas transport in the xylem is in one direction only. (5 marks)
Model answer
Transport in the xylem is driven by transpiration pull, which always draws water upwards from the roots to the leaves, so it is one-way. Translocation in the phloem, however, moves sugars and amino acids from wherever there is a source to wherever there is a sink, and sinks can be above or below the source. For example, in summer the leaves are the source and sugars move down to the roots and fruits, while in spring the storage organs are the source and sugars move up to the new shoots. Because the positions of sources and sinks change, and because phloem is made of living cells that can load and unload sugars at different points, the flow can occur in either direction.
Why this scores
Five marks: xylem one-way driven by transpiration; phloem moves source to sink; sinks can be above or below; seasonal examples (up in spring, down in summer); phloem living so can load/unload either way.
Describe the movement of sugars in a plant during the growing season, using the terms source, sink and translocation. (6 marks)
Model answer
During the growing season, the leaves photosynthesise and make glucose, which is converted to sucrose for transport. The leaves are therefore acting as a source, releasing the sucrose (and amino acids) into the phloem. The process of moving these substances through the phloem is called translocation. The sucrose is carried to the various sinks of the plant — such as the growing root and shoot tips, where it is used in respiration and to make new tissue, and the fruits and storage organs, where it is stored (for example as starch in a tuber). Because the flow always runs from source to sink, and the leaves are the main source while many parts are sinks, sugars are distributed throughout the plant during the growing season to support growth and storage.
Why this scores
Up to 6 marks: leaves photosynthesise/make sugar; leaves are the source; sucrose is the transport form; translocation in the phloem; sinks named (growing tips, fruits, storage organs); use of sugar (respiration, growth, storage).
Definitions to memorise and the exact keywords mark schemes credit for translocation answers — sharpened from recent examiner reports for the 2026 0610 sitting.
Movement of sucrose and amino acids in the phloem from sources (where they are made or stored) to sinks (where they are used or stored).
A part of the plant that releases sucrose and amino acids into the phloem — e.g. photosynthesising leaves, or a storage organ being broken down.
A part of the plant that uses or stores sucrose and amino acids from the phloem — e.g. growing buds, fruits, root tips, storage organs being filled.
The sugar in which carbohydrate is transported in the phloem. Stable and unreactive, so suited to transport.
The traps other students keep falling into on translocation questions — taken from recent Cambridge IGCSE 0610 examiner reports and mark schemes — and how to avoid them.
Why it happens
Photosynthesis makes glucose first.
How to avoid it
Glucose is converted to SUCROSE for transport. Translocation moves sucrose (and amino acids).
Why it happens
In summer, sugars often go from leaves down to roots and fruits.
How to avoid it
Flow follows source-to-sink. In spring, sugars from storage organs flow UP to new growth.
Why it happens
Both are transport tissues.
How to avoid it
Translocation is PHLOEM-specific. The xylem carries water and minerals upwards (the transpiration stream), not sugars.
The things students keep getting wrong in this sub-topic, answered.