The two best-known photophysiological processes are photosynthesis and vision. In photosynthesis light energy is absorbed and transformed into ‘life energy’, in vision the information content of light is utilized by organsims. In photosynthesis chlorophyll and other pigments involved in light-gathering are not chemically changed (except that some of the chlorophyll or phaeophytin molecules are oxidized and immediately reduced again). In human vision the rhodopsin and other visual pigments undergo a reaction cycle induced by photon absorption. The pigment is brought back to its original state only after a long chain of chemical reactions.

One of the important problems of biophysics is to understand in detail the mechanisms by which animals produce acoustic signals, and the ways in which other animals, of the same or different species, detect and process these signals, through the masking effects of ambient noise, to extract their information content. At a primitive level this information relates simply to the presence of the other animal and perhaps to its proximity and direction. At the next level we find some sort of coding in the acoustic signal which identifies the species and perhaps the sex, and finally there is the whole range of information content and emotional overtone contained in a complex sound like human speech or music.

It has virtually become a truism that osmosis, and the inseparable array of related processes comprising ultrafiltration, diffusion, anomalous osmosis, etc., has found its final and lasting exposition to which anything can only be added by way of a few finishing touches. This view, as I shall attempt to show, is quite wrong, if for no other reason than that several of the different approaches to the problem are not complementary. We want two things: to gain an insight into the mechanisms operative in porous systems, and to derive equations which predict the flow rates from a minimal set of properties – if we can decide what these are.