There are two possible pathways for solute movement through plant tissues, the cell wall pathway and the cytoplasmic pathway. Movement may take place extracellularly through channels in the cell walls, thus bypassing the protoplasts and obviating the need to cross membrane barriers. Solutes in these cell-wall channels can be freely exchanged with an external solution and therefore this pathway is often referred to as the free space, the whole cell-wall continuity being termed the apoplast or apoplasm. In parallel with the free-space pathway is a route through the cytoplasm of the protoplasts, which are linked through the plasmodesmata to form a three-dimensional cytoplasmic continuity known as the symplast or symplasm. Solutes entering the symplast must first cross the plasma membrane, imparting a high degree of selectivity upon this pathway.
KeywordsTransfer Cell External Solution Sieve Element Wall Ingrowth Xylem Parenchyma
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