Translocation (Cambridge (CIE) IGCSE Biology): Revision Note

Translocation: Extended

• The main substances transported in the phloem are sugars (mainly sucrose, made from the glucose produced in photosynthesis) and amino acids (made using glucose and nitrates absorbed from the soil)

• These are transported around the plant in the phloem tubes which are made of living cells (as opposed to xylem vessels which are made of dead cells (opens in a new tab))

• The cells are joined end to end and contain holes in the end cell walls (called sieve plates) which allow easy flow of substances from one cell to the next

• The transport of sucrose and amino acids in the phloem, from regions of production to regions of storage or use, is called translocation

• The direction of transport in the phloem depends on the stage of development of the plant or the time of year; however dissolved substances are always transported from a source (where they are made or released) to a sink (where they're stored or used)

  • During winter, when many plants have no leaves, the phloem tubes may transport dissolved sucrose from the storage organs to other parts of the plant so that respiration can continue, and amino acids for the maintenance and repair of cells

  • During a growth period (e.g. during the spring), the storage organs (roots) would be the source and the many growing areas of the plant would be the sinks

  • After the plant has grown (usually during the summer), the leaves are photosynthesising and producing large quantities of sugars; so they become the source and the roots become the sinks, where the sucrose is converted back into glucose and stored as starch (opens in a new tab) until it is needed again

Comparison between xylem and phloem tissue table