The symplast of a plant is the region enclosed by the cell membranes, within which water and solutes can diffuse freely. By contrast the apoplast is any fluid-filled space within the cell wall and extracellular space.[1] Neighbouring cells are interconnected by microscopic channels known as plasmodesmata that traverse the cell walls. These channels, allow the flow of small molecules such as sugars, amino acids, and ions between cells (from the inner part of one cell to the inner part of the next cell). Larger molecules, including transcription factors and plant viruses, can also be transported through with the help of actin structures. The symplast allows direct cytoplasm-to-cytoplasm flow of water and other nutrients along concentration gradients. In particular, symplastic flow is used in the root systems to bring in nutrients from soil.[clarification needed] Nutrient solutes move in this way through three skin layers of the roots: from cells of the epidermis, the outermost layer, through the cortex into the endodermis.
Once solutes in the soil water reach the endodermal cells through apoplastic flow, they are forced into the symplastic pathway due to the presence of the Casparian strip. Once the solutes are passively filtered[clarification needed], they eventually reach the pericycle, where they can be moved into the xylem for long-distance transport.