Within a plant, the apoplast is the free diffusional space outside the plasma membrane. It is interrupted by the casparian strip in roots, air spaces between plant cells and the cuticula of the plant. 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. The apoplastic route facilitates the transport of water and solutes across a tissue or organ. This process is known as apoplastic transport.
The symplast of a plant is the inner side of the plasma membrane in which water can freely diffuse. The plasmodesmata allow the direct flow of small molecules such as sugar, amino acids and ions between cells. Larger molecules, including transcription factors and plant viruses ! can also be transported through with the help of actin structures.
