It has long been recognized that measurements of the permeability characteristics of membranes (and of epithelia) may be strongly influenced by the presence of unstirred, or diffusion, layers adjacent to the membranes: see Dainty (1963) for the first systematic discussion of the subject in a biological context and Barry & Diamond (1983) for an important recent review. Consider a homogeneous, plane, uncharged membrane separating two solutions of the same solute at different bulk concentrations Cb+ and Cb- (Fig. 1), but at the same hydrostatic pressure and electrostatic potential (note that in this paper subscripts + and − refer to the two sides of the membrane, not to cation and anion parameters; electroneutrality is assumed throughout).

In a living cell membrane-bound compartments are continuously either separated or united through fusion reactions, and literally thousands of such reactions take place every minute. The formation of membrane vesicles from pre-existing membranes, and their fusion with specific acceptor membranes, constitute a prerequisite for the transport of most impermeant molecules and macromolecules into the cell by endocytosis, out of the cell by exocytosis, and between the cellular organelles (Palade, 1975; Silverstein, 1978; Evered & Collins, 1982). Less frequent, but equally crucial, are fusion events in fertilization, cell division, polykaryon formation, enucleation, etc. (for reviews see Poste & Nicholson, 1978). Although a great deal is known about the properties and consequences of individual forms of membrane fusion in cellular systems, and about fusion in artificial lipid membranes, the molecular basis for the reactions remain largely unclear.

Photosynthesis in green plants or algae may be represented by an overall equation:

The energy necessary to promote this overall reaction is derived from light through absorption by pigment molecules — chiefly the chlorophylls.

Photosynthesis occurs in chloroplasts - subcellular organelles in which all the chlorophyll pigments are located. The chloroplasts comprise membranous, structures, and can be classified into two types. To the first type belong chloroplasts with appressed stacks of lamellar membranes, termed grana. These chloroplasts occur in mesophyll cells (C3 plants). The second type of chloroplasts are those with lamellar membranes that do not form the grana structures; they occur in bundle sheath cells of maize and other monocotyledons (C4plants, Hatch & Slack, 1970). In algae a greater diversity of structure occurs (Kirk & Tilney-Basset, 1978). Fluorescence microscopy indicates that chlorophyll molecules are localized mainly in the grana membrane regions of mesophyll-type chloroplasts and uniformly throughout the bundle sheath cells (Spencer & Wildman, 1962). Mesophyll chloroplasts are flattened saucer-shaped organelles (20 or more in each cell) of between 5000 and 10000 nm in diameter, and of thickness 1000–2000 nm, whilst the individual grana are each of the order of 300–500 nm in diameter. The available evidence suggests that individual lamellar membranes are arranged to form vesicles, or sacks where the internal space is completely delimited from the external space. These individual closed membrane structures were termed thylakoids (Menke, 1962).