Describe how sucrose is transported around the plant by translocation

The idea

Translocation is the movement of dissolved sugars, especially sucrose, through the phloem from a source to a sink. A source is where sucrose is made or released, such as a photosynthesising leaf; a sink is where it is used or stored, such as a growing root, shoot tip, developing fruit or storage organ. Sucrose enters phloem sieve tubes and is carried to sinks, where it is unloaded and used for growth, respiration or storage. Phloem tissue is living and its cells are joined end to end.

What earns the marks

• Translocation is the transport of dissolved sugars, mainly sucrose, through the phloem.
• Sucrose moves from a source, where it is made or released, to a sink, where it is used or stored.
• Examples of sources include photosynthesising leaves; examples of sinks include growing roots, shoot tips, fruits and storage organs.
• Phloem is living tissue with cells joined end to end, allowing sucrose solution to flow between them.
• Unlike the upward xylem flow of water, translocation can carry sucrose to sinks in different directions.

Common mistakes and how to put them right

• Naming xylem as the tissue for sucrose.

  Put it right: Sucrose travels in phloem, while xylem carries water and mineral ions.

• Reversing source and sink.

  Put it right: Sucrose leaves the source and arrives at the sink, not the other way round.

• Describing translocation as movement of water vapour.

  Put it right: That is transpiration; translocation concerns dissolved sugars.

• Assuming translocation only moves upward.

  Put it right: Sucrose can be carried to sinks above or below the source.

Exam tips

• Check which tissue each option names before deciding; phloem is the key word for sucrose transport.
• Trace the direction in each option from source to sink and reject any that reverse it.
• Use a familiar example, such as a leaf supplying a growing root, to test whether an option makes sense.