Mini-review. On the possible importance of an intracellular circulation.

Since ultrastructural, biophysical and other studies continue to demonstrate that the internum of the cell is highly structured, the question raised and discussed in this review is whether an intracellular circulatory system is essential for the maintenance of active metabolism. Although cytoplasmic streaming is evident in large animal and plant cells, it is argued that it probably occurs in all cells irrespective of size, and is of particular importance in bringing together interacting molecules fast enough for metabolic processes to occur which would otherwise be far too slow if diffusion were the only form of motion. A common intracellular system would suffice for most metabolic processes and would also help to dissipate waste products. Interruption of this internal circulation would result in the inhibition of metabolic functioning, including for example protein turnover, for which evidence is presented to substantiate this hypothesis.