IGCSE Biology / Transport in plants
Moving sugars from source to sink revision notes
Edexcel International GCSE Biology (4BI1), chapter 11: Transport in plants, specification point 2.53. Free lesson.
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Translocation is the movement of sucrose and amino acids through the phloem from a source to a sink. Its direction can change because an organ’s role as a source or sink depends on the season and what the plant needs.
What translocation does
Translocation moves sugars from where they are made or released to where they are needed or stored. The sugar is transported as sucrose, not glucose. Sucrose is less reactive and affects the cell’s water balance less, so it travels better. Translocation happens in the phloem. A solution of sucrose in the phloem is called phloem sap.
How sucrose moves from source to sink
A source is a place where sucrose is made or released. A sink is a place where sucrose is used or stored. Sucrose is loaded into phloem at the source and unloaded at the sink, so it moves from source to sink. Loading into the phloem uses active transport, which requires energy from companion cells. Sucrose travels through the sieve tubes and across the sieve plates. At the sink, sucrose may be used in respiration or converted to starch for storage.
Why the direction can change
An organ’s role depends on what it is doing at that time of year. In summer, leaves make sucrose by photosynthesis and potato tubers store surplus sugar. In early spring, the tuber releases stored starch as sucrose to support new shoot growth. The movement in the stem can therefore reverse. In summer, sucrose moves downwards from leaves to potato tubers. In early spring, sucrose moves upwards from tubers to shoots, before leaves open. A growing root tip is a sink because it needs sugars and does not make them. Unlike xylem, phloem can transport sucrose in either direction, depending on where…
How phloem differs from xylem
Keep the transport tissues and the form of sugar separate. Xylem carries water and mineral ions upwards only. Sugars are translocated in phloem as sucrose, in whichever direction the sink lies. Xylem is dead and hollow. Phloem loading uses active transport, so the cells involved must be alive.