质外体和同质体

Plants take up water and dissolved minerals from the soil and transport these resources to distant tissues. The resources enter and move in two distinct compartments: the apoplast and the symplast. 

The apoplast is a continuous system of extracellular spaces, such as cell walls and air spaces between the cells. Water, gases, and minerals move freely and passively through the apoplast without crossing plasma membranes.

Plant cells may also excrete excess protons into the apoplast due to changes in pH. 

Additionally, chemicals that enter the apoplast in the root may trigger the secretion of molecules that bind and excrete toxins.  

The cytoplasm of plant cells forms another continuous system, called the symplast. Neighboring plant cells are connected by channels allowing water and small molecules to move from one cell to the next. 

The continuum of interconnected plant cells facilitates the transport of signal molecules, such as hormones. For example, the hormone cytokinin moves freely through the symplast to regulate cell proliferation.

The symplast is also a conduit for small RNAs, which activate defense responses against plant pathogens.

Resources usually travel on both the apopla