Translocation: Extended (CIE IGCSE Biology: Co-ordinated Sciences (Double Award))

Extended Tier Only

• The soluble products of photosynthesis are sugars and amino acids
  • Glucose is converted into sucrose for transport
• These are transported around the plant in the phloem vesels
  • The cells of the phloem tubes are joined end-to-end and contain holes in their end cell walls 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
• Transport in the phloem occurs from a region of the plant described as the source to a region described as the sink
  • The source is any part of the plant that releases sucrose or amino acids
  • The sink is any part of the plant that uses or stores sucrose or amino acids
• Phloem transport can occur in different directions depending on the stage of development of the plant or the time of year

Translocation at different times of year

• During winter, when many plants have no leaves, the phloem tubes may transport dissolved sucrose and amino acids from the storage organs (e.g. roots) to other parts of the plant so that respiration can continue
• During a growth period (e.g. during the spring), the storage organs 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 – storing sucrose as starch until it is needed again

Translocation occurs in the phloem