The movement of food molecules through phloem tissue is called…

The idea

Translocation is the transport of dissolved food molecules, mainly sucrose and amino acids, through phloem tissue from sources to sinks. Sources are regions that produce or release sugars, such as mature leaves carrying out photosynthesis. Sinks are regions that use or store them, such as growing tips, roots, fruits and storage organs. The food molecules are dissolved in cell sap and travel through living phloem cells, so translocation requires energy from respiration. This distinguishes it from transpiration, which is the loss of water vapour and the movement of water and mineral ions through xylem. A useful method is to trace a sugar made in a leaf to a growing bud or root, naming the source, the phloem pathway and the sink.

What earns the marks

• Translocation is defined as the movement of dissolved food molecules, such as sucrose and amino acids, through phloem tissue.
• The food molecules are carried in cell sap rather than as solid particles or as water alone.
• Movement is from a source, for example a mature leaf, to a sink, for example a growing tip, root or storage organ.
• Phloem tissue is living, so translocation depends on energy released by respiration.
• Translocation is distinct from transpiration, which involves water and mineral ions moving through xylem.
• Naming a source and a sink in a described example shows understanding of direction of movement.

Common mistakes and how to put them right

• Confusing translocation with transpiration.

  Put it right: Translocation moves food molecules through phloem, whereas transpiration is water loss and xylem transport.

• Stating that food molecules move through xylem.

  Put it right: Xylem carries water and mineral ions, while phloem carries dissolved food.

• Describing movement only from roots to leaves.

  Put it right: Translocation moves from any source to any sink, which may be downward, upward or sideways.

Exam tips

• Define translocation in one precise sentence, then add a named source and sink to show understanding.
• Use the terms phloem, cell sap, source and sink accurately rather than writing 'food tubes' or 'plant veins'.
• If asked to compare transport tissues, set out phloem and xylem in a table or paired sentences to keep the differences clear.