What is phloem loading and unloading?
The transfer of sugar from mesophyll cells (source) to sieve tube elements of phloem is called loading of phloem, and the transfer of sugar from sieve tube elements to roots or other storage cells (sink) is called unloading of phloem.
What is the phloem unloading?
Phloem unloading represents a series of cell-to-cell transport steps transferring phloem-mobile constituents from phloem to sink tissues/organs to fuel their development or resource storage. Our analysis focuses on unloading of two major phloem-mobile constituents, sugars and water.
What is phloem loading in plants?
Phloem loading is the process of loading carbon into the phloem for transport to different ‘sinks’ in a plant. Sinks include metabolism, growth, storage, and other processes or organs that need carbon solutes to persist.
Why phloem unloading is active?
Phloem unloading is typically symplastic in growing and respiring sinks such as meristems roots, and young leaves etc. in which sucrose can be rapidly metabolized. (Young leaves act as sink until their photosynthetic machinery is fully developed, at which point they become sources).
Why does phloem need active transport?
Active transport is used to load organic compounds into phloem sieve tubes at the source. High concentrations of solutes in the phloem at the source lead to water uptake by osmosis.
Why phloem unloading is active or passive?
(C) Phloem unloading is a passive transport mechanism from the sieve tubes to the cells at the root tip. It takes place passively down a concentration gradient of sucrose. The transfer cells are often present at unloading sites.
Does phloem loading require energy?
Phloem loading requires energization of the plasma membrane by a proton gradient that is generated through the activity of phloem-specific H+-ATPase to drive the proton-coupled sucrose uptake (Frommer and Sonnewald, 1995).