Molecular transport of substances in the presence or absence of membranes is an ubiquitous phenomenon. Research workers from various disciplines in the biological and phys ical sciences are actively pursuing problems of transport. One of the important questions arising in numerous biological transport situations concerns the differentiation between passive and active transport. The latter is a more complex phenomenon involving metabolic processes, and the economy of thinking requires that attempts to explain any transport pro cess from passive mechanisms should be carried as far as pos sible...