TranslocationCambridge IGCSE Biology: Revision notes

Section 1

Phloem's Role in Transport

Phloem is the plant tissue responsible for transporting dissolved food substances around the plant.

Phloem transports sucrose and amino acids
Unlike xylem, which only moves substances upwards, phloem can transport substances in either direction, depending on where they are needed

Key termsphloem

Section 2

What Is Translocation?

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

It is the process by which plants distribute the products of photosynthesis and protein building to wherever they are needed
Translocation happens throughout the plant, not just in one direction, because sources and sinks can be located anywhere in the plant

Key termstranslocation

Common mistake

Do not confuse translocation (movement of sucrose/amino acids in phloem) with transpiration (loss of water vapour from leaves) - they are different processes in different tissues.

Section 3

Sources and Sinks

Translocation is described in terms of sources and sinks:

A source is a part of the plant that releases sucrose or amino acids into the phloem (e.g. a photosynthesising leaf, releasing sucrose)
A sink is a part of the plant that uses or stores sucrose or amino acids (e.g. a growing root, a developing fruit, or a storage organ)
Substances always move from a source to a sink in the phloem

Key termssourcesink

Example

A mature leaf making sugar by photosynthesis is a source; a growing bud using that sugar for growth is a sink.

Section 4

Can a Plant Part Be Both a Source and a Sink?

Some parts of a plant can act as a source at one time and a sink at another, depending on the plant's needs.

A storage organ (such as a root tuber) may act as a sink while it is storing sucrose during the growing season
The same storage organ may later act as a source, releasing stored sucrose to support new growth (for example, in spring)

Example

A potato tuber stores sucrose as a sink during the growing season, then acts as a source, supplying sugar to a new shoot the following spring.

Must Know

Phloem transports sucrose and amino acids, in either direction
Translocation is the movement of sucrose and amino acids from sources to sinks
A source releases sucrose/amino acids (e.g. a photosynthesising leaf)
A sink uses or stores sucrose/amino acids (e.g. a growing root or fruit)
Some plant parts can act as a source at one time and a sink at another, depending on the plant's needs

That's the notes covered.

Carry on to the next subtopic.