Translocation, sources and sinks

Transport in plants · Translocation · note 1 of 1

Spec 8.4.1, 8.4.2, 8.4.3

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Translocation, sources and sinksSpec 8.4.1, 8.4.2, 8.4.3

In short

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 mature green leaf. A sink uses or stores them, such as a growing root tip or developing seed. A potato tuber can be both at different times.

Translocation is the movement of sucrose and amino acids in the phloem from sources to sinks.

Source: a part of the plant that releases sucrose or amino acids
Sink: a part of the plant that uses or stores sucrose or amino acids

A plant with arrows showing sucrose moving in the phloem from the leaves, where it is made, up to the growing shoot tip, along to a fruit and down to the root. — Sucrose moves in the phloem from the source (the leaves) to the sinks, up and down the plant.

Examples of sources and sinks
Part of the plant	Source or sink	Why
Mature green leaf (in the light)	source	releases sucrose made in photosynthesis
Growing shoot tip or root tip	sink	uses sucrose for growth
Developing fruit or seed	sink	stores or uses sucrose
Storage organ such as a potato tuber, while the plant is photosynthesising	sink	stores the sucrose it receives (as starch)

Some parts are both

A part of a plant can be a source at one time and a sink at another. A potato tuber receives sucrose while the plant is growing and photosynthesising, and stores it as starch, so it is a sink. When the tuber starts to grow new shoots, the starch is broken down and sucrose is released and moved to the growing shoots, so the same organ becomes a source.

A young leaf is a sink because it uses sucrose to grow. Once it is fully grown, it makes more sucrose than it uses, so it becomes a source.

Exam tip:

Use 'releases' for a source and 'uses or stores' for a sink. Always say which direction sucrose moves: from source to sink.

Quick check

Supplement Define translocation.
Show answer
The movement of sucrose and amino acids in the phloem from sources to sinks.
Supplement What is a sink?
Show answer
A part of the plant that uses or stores sucrose or amino acids.
Supplement Give an example of a source.
Show answer
A fully grown green leaf.
Supplement Why can a tuber be both?
Show answer
It stores sucrose (as starch) as a sink, then releases sucrose as a source when new shoots grow.

Written and checked against the Cambridge IGCSE Biology (0610) specification · Updated October 2026

Frequently asked questions

What is translocation in plants?

Translocation is the movement of sucrose and amino acids in the phloem from sources to sinks. A source, such as a mature green leaf in the light, releases sucrose made in photosynthesis. A sink, such as a growing shoot tip or a developing fruit, uses or stores the sucrose it receives.

What is the difference between a source and a sink in a plant?

A source is a part of the plant that releases sucrose or amino acids, such as a mature green leaf. A sink is a part of the plant that uses or stores sucrose or amino acids, such as a root tip, a developing seed or a potato tuber. Sucrose always moves from source to sink.

How can a potato tuber be both a source and a sink?

A potato tuber is a sink while the plant is growing and photosynthesising, because it receives sucrose and stores it as starch. When the tuber starts to grow new shoots, the starch is broken down and sucrose is released and moved to the shoots, so the same organ becomes a source.

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