What is the apoplast?
Apoplast is the space outside the plasma membrane (in plant cells) within which materials can diffuse freely.
Structurally, the apoplast is formed by the continuum of cell walls of adjacent cells as well as the extracellular spaces, forming a tissue level compartment comparable to the symplast. This route facilitates the transfer of water and solutes across a tissue or organ.
The apoplast is important for all the plant's interaction with its environment. The main carbon source needs to be soluble in the apoplast before it diffuses through the plasma membrane. In the roots, ions diffuse into the apoplast of the epidermis before diffusing into the symplast. It is also the site for cell to cell communication.