Translocation

Translocation is the movement of sucrose and amino acids (in phloem) from the leaves (sources) to regions of respiration, growth and storage (sinks).
- Unlike transpiration which is a passive process driven by the evaporation of water, translocation is an active process. It requires metabolic energy from the plant’s living phloem cells to move dissolved nutrientts (sap) throughout the organism.
- Transport in translocation is bidirectional, where it can move up or down the plant depending on where the materials are needed.

Sources and sinks

The parts of a plant that release sucrose or amino acids into the phloem.
Examples include:
- fully grown, photosynthesizing green leaves (producing sucrose)
- germinating seeds or storage organs (like tubers) when they break down stored food reserves.

The parts of a plant that use or store sucrose or amino acids.
Examples include:
- for growth/usage, growing shoot and root tips, developing buds, flowers and expanding leaves (requires materials to build new cells + respire)
- for storage, developing fruits, seeds, or specialized underground storage organs (e.g. potato tubers, carrots, flower bulbs)

The leaves are the SOURCE. The leaves are fully developed and photosynthesizing rapidly. They produce far more glucose (converted to sucrose) than they need.
The roots/tubers are the SINK. The excess sucrose is sent downward to the roots or underground storage organs (e.g. potato tuber) where it is converted into insoluble starch and stored for the winter.

The roots/tubers become the SOURCE. The plant has lost its leaves over winter and cannot photosynthesize. To grow new leaves, it must break down the insoluble starch stored in its roots/tubers back into soluble sucrose. The roots release this sucrose into the phloem.
The new buds/leaves become the SINK. The newly forming buds and young leaves absorb this sucrose from the phloem, using it for cellular respiration and rapid growth until they are mature enough to photosynthesize on their own.