BRAINCAKEPhloem Unloading
Phloem unloading is the removal of sucrose and other assimilates from the sieve tubes at a sink, such as a growing root tip, a developing fruit or a storage organ.
1. Sucrose leaves the sieve tube elements and moves into the surrounding sink cells. In many sinks this happens by diffusion through plasmodesmata, down a concentration gradient. In some sinks, such as developing seeds, sucrose is moved out by active transport.
2. In the sink cells, sucrose is used or converted into other substances. The enzyme invertase hydrolyses sucrose into glucose and fructose, which are used in respiration or for growth. In storage organs such as potato tubers, sugars are converted into starch.
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Key terms in this lesson
- plasmodesmata
- Fine channels through plant cell walls that connect the cytoplasm of neighbouring cells, allowing transport and communication between them.
- hydrostatic pressure
- The pressure a fluid exerts on the walls of its container; in a capillary it pushes fluid out through the gaps between the endothelial cells.
More in Transport in Plants
- Factors Affecting Transpiration
- Root Pressure
- Translocation in the Phloem
- Phloem Loading
- The Symplast Pathway
- The Apoplast Pathway
- Mass Flow Hypothesis
- Pressure Flow Hypothesis
All 21 lessons in Transport in Plants · All Edexcel AS-level Biology topics