What is translocation in the context of plant biology?

Translocation in plant biology refers to the movement of soluble organic substances, primarily sugars like sucrose, from the leaves to other parts of the plant. This process occurs in the phloem tissue and is essential for distributing the products of photosynthesis to areas of growth, storage, and respiration. Understanding translocation is crucial for Cambridge IGCSE Biology students studying the transport systems in plants.

How does translocation differ from transpiration in plants?

Translocation and transpiration are both processes involved in the transport system of plants, but they serve different functions. Translocation is the movement of nutrients, particularly sugars, through the phloem from sources (like leaves) to sinks (such as roots and fruits). In contrast, transpiration is the process of water movement through the plant and its evaporation from aerial parts, primarily through the stomata in the leaves. Transpiration occurs in the xylem and is driven by water potential gradients, while translocation is an active process requiring energy.

What role does the phloem play in translocation?

The phloem is a type of vascular tissue in plants that plays a crucial role in translocation. It consists of sieve tubes and companion cells that facilitate the transport of organic nutrients, especially sucrose, from the leaves to other parts of the plant. The phloem's structure allows for the efficient movement of these substances, which is vital for plant growth and development. For Cambridge IGCSE Biology students, understanding the function of phloem is key to grasping how plants distribute nutrients.

What are the sources and sinks in the process of translocation?

In the process of translocation, 'sources' refer to the parts of the plant that produce or release sugars, such as the leaves where photosynthesis occurs. 'Sinks' are the parts of the plant that require or store these sugars, such as roots, fruits, and seeds. The movement of sugars from sources to sinks is essential for plant growth, storage, and energy supply. Cambridge IGCSE Biology students should understand the dynamic nature of sources and sinks, as they can change depending on the plant's developmental stage and environmental conditions.

Why is translocation considered an active process in plants?

Translocation is considered an active process because it requires energy in the form of ATP to transport sugars against concentration gradients. This energy is used to load sugars into the phloem at the source and unload them at the sink. The active transport of sugars into the phloem creates a high osmotic pressure that drives the flow of sap through the phloem. Understanding this active transport mechanism is important for Cambridge IGCSE Biology students studying plant physiology and the transport of nutrients.

Why is "Saying GLUCOSE is the transport sugar." a common mistake in Translocation?

Why it happens: Photosynthesis makes glucose first. How to avoid it: Glucose is converted to SUCROSE for transport. Sucrose is more stable (less reactive in transit). Glucose is regenerated at the sink.

Why is "Saying phloem flow is always DOWNWARDS." a common mistake in Translocation?

Why it happens: In summer, sugars go from leaves down to roots/fruits. How to avoid it: Flow follows source-to-sink. In spring, sugars from storage organs flow UP to feed new growth.

Why is "Saying translocation happens in xylem." a common mistake in Translocation?

Why it happens: Both are transport tissues. How to avoid it: Translocation is PHLOEM-specific. Xylem carries water + minerals up only — that's the transpiration stream, not translocation.

Transport in PlantsCambridge IGCSE Biology 0610 Extended

Translocation

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Translocation — Cambridge IGCSE 0610 Biology Extended (2026)

Phloem moves sugars and amino acids around the plant from where they're made or stored to where they're needed.

What you’ll learn

Mapped to the Cambridge IGCSE 0610 syllabus (2026-2028).

8.4 — Define translocation.
8.4 — Identify sources and sinks in a plant.
8.4 — Explain how source/sink relationships change throughout the year.

What is translocation?

Phloem moves sucrose and amino acids around the plant.

Translocation is the movement of:

Sucrose (the main transport sugar)
Amino acids (raw materials for protein synthesis)

through the phloem, from sources to sinks.

Source: a tissue or organ that adds sugars to the phloem. Examples:

A photosynthesising leaf (making sugar from CO₂).
A potato tuber in SPRING (its starch is being broken down to glucose, then sucrose, for export).
A storage organ being mobilised.

Sink: a tissue or organ that takes sugars OUT of the phloem to use or store. Examples:

Growing tips (buds, root tips).
Developing fruits (filling with sugar).
Storage organs being filled (potato in summer, beetroot maturing).

Why sucrose, not glucose?

Glucose is reactive — gets used up by respiration as it travels.
Sucrose is more stable — survives the journey unreacted.
At the sink, sucrose is split back into glucose + fructose for use.

Phloem carries sucrose from a source to any sink — flow direction is not fixed.

Worked qualitative. Why does an apple get sweeter as it ripens?

The apple is a SINK during ripening.
Sugars travel from leaves → apple via phloem.
Inside the apple, sucrose accumulates → sweeter taste.

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.

Sucrose + amino acids in phloem.
Source → sink.
Sources: leaves, mobilised storage.
Sinks: growing tissues, filling storage.

How sources and sinks change

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:

Spring: sugars flow UP from underground stores to feeding shoots.
Summer: sugars flow DOWN from leaves to roots, fruits, storage organs.
Late summer: sugars flow into seeds and fruits.

This bidirectional flow is one of the key differences between phloem (variable direction) and xylem (water always UP).

The same potato tuber is a source in spring and a sink in summer — flow direction reverses.

Worked qualitative. A daffodil bulb in March vs September.

March: bulb is releasing stored sugars → SOURCE → flow upward, fueling green shoot growth.
September: bulb is filling with sugars produced by summer leaves → SINK → flow downward into the bulb.

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.

Source/sink labels are dynamic.
Spring: stores release → flow UP.
Summer: leaves make → flow DOWN.
Same organ, different season = different role.

How does phloem move sap?

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.

Active loading at the source builds pressure that drives sap to the sink, where unloading lowers it.

Why energy needed.

Active loading at source uses ATP — that's why companion cells have so many MITOCHONDRIA.
Without energy, no flow.

Worked qualitative. Why are companion cells right next to sieve tubes?

They're like service vehicles — provide energy + control for the sieve tube (which itself is reduced, lacks a nucleus).
Without companion cells, sieve tubes can't load sugar against the gradient.

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.

Active loading at source.
Pressure gradient drives flow.
Unloading at sink.
Companion cells supply ATP.

How it’s examined

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.

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Step-by-step worked examples — Translocation

Step-by-step solutions to past-paper-style questions on translocation, written exactly the way a tutor would explain them at the board.

1Define translocation
Core• definition
Question
Define translocation and state which tissue carries it.
Step-by-step solution
1. Step 1
  Translocation = the movement of sucrose and amino acids in the PHLOEM from SOURCES to SINKS.
2. Step 2
  Source = where the substance is MADE or STORED (and being released). E.g. green leaves (making sugar) or a potato in spring (releasing stored starch as glucose).
3. Step 3
  Sink = where the substance is USED or STORED (filling up). E.g. growing buds, developing fruits, root tips, a potato in summer.
Answer
Translocation: movement of sucrose and amino acids in phloem from source to sink.
2Identify sources and sinks
Core• Adapted from 0610/42 Oct/Nov 2024 Q6• source and sink
Question
For each plant part, state whether it's a SOURCE or SINK and explain why.
Step-by-step solution
1. Step 1
  A green leaf in summer: SOURCE — photosynthesising → making sugar.
2. Step 2
  A growing fruit: SINK — receiving sugars to store/grow.
3. Step 3
  A potato tuber when sprouting (spring): SOURCE — stored starch breaks down to glucose, exported via phloem to new shoots.
4. Step 4
  A potato tuber being filled (summer): SINK — receiving sugars from leaves, storing as starch.
5. Step 5
  Root tip (growing): SINK — needs sugars for respiration + new tissue.
Answer
Source: anywhere making/releasing sugar (green leaf, sprouting tuber). Sink: anywhere using/storing (fruit, root tip, filling tuber). Same organ can be either, depending on season.
3Why is phloem flow BIDIRECTIONAL?
Extended• bidirectional flow
Question
Explain why translocation can go in DIFFERENT DIRECTIONS at different times of year — unlike xylem flow.
Step-by-step solution
1. Step 1
  Xylem only carries water UP — driven by transpiration pull, one-way.
2. Step 2
  Phloem carries SUGARS that are needed wherever there's a sink — anywhere in the plant.
3. Step 3
  In SPRING: sugars stored in roots/tubers/bulbs become the source. They flow UPWARD to feed new growth (sinks at the top).
4. Step 4
  In SUMMER: leaves are sources (photosynthesising). Sugars flow DOWN to roots, fruits, and storage organs (sinks).
5. Step 5
  Same phloem cells can carry sap UP or DOWN at different times — flow follows the source-to-sink need.
Answer
Phloem flow follows source-to-sink. Sources and sinks change with season → flow direction can reverse. Xylem is always up.

Key Definitions and Keywords — Translocation

Definitions to memorise and the exact keywords mark schemes credit for translocation answers — sharpened from recent examiner reports for the 2026 0610 sitting.

Translocation
Examiner keyword
Movement of sucrose and amino acids in the phloem from sources (where they're made or stored) to sinks (where they're used or stored).
Source
Examiner keyword
A part of the plant that releases sucrose and amino acids into the phloem. Examples: green leaves (photosynthesising), storage organs being broken down (e.g. potato in spring).
Sink
Examiner keyword
A part of the plant that USES or STORES sucrose and amino acids from the phloem. Examples: growing buds, fruits, root tips, storage organs being filled.
Sucrose (transport sugar)
Disaccharide of glucose + fructose. Stable and unreactive — the form in which sugar is transported in phloem.

Common Mistakes and Misconceptions — Translocation

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.

✕Saying GLUCOSE is the transport sugar.
Why it happens
Photosynthesis makes glucose first.
How to avoid it
Glucose is converted to SUCROSE for transport. Sucrose is more stable (less reactive in transit). Glucose is regenerated at the sink.
✕Saying phloem flow is always DOWNWARDS.
Why it happens
In summer, sugars go from leaves down to roots/fruits.
How to avoid it
Flow follows source-to-sink. In spring, sugars from storage organs flow UP to feed new growth.
✕Saying translocation happens in xylem.
Why it happens
Both are transport tissues.
How to avoid it
Translocation is PHLOEM-specific. Xylem carries water + minerals up only — that's the transpiration stream, not translocation.

Practice questions

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Translocation — frequently asked questions

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Translocation

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Short Study Notes — Translocation

Detailed Notes

What you’ll learn

How it’s examined

1Define translocation

2Identify sources and sinks

3Why is phloem flow BIDIRECTIONAL?

Translocation

Source

Sink

Sucrose (transport sugar)

✕Saying GLUCOSE is the transport sugar.

✕Saying phloem flow is always DOWNWARDS.

✕Saying translocation happens in xylem.

Practice questions

Past paper style quiz

Assess your understanding

Video lesson

Translocation — frequently asked questions