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Summary

The question considered is whether pinocytosis is the principal means of transporting major nutrients across the plasmalemma.

It is argued that ultrastructural evidence previously advanced in favour of pinocytosis in plant cells can be interpreted in other ways and does not uniquely support the hypothesis of pinocytosis. Physiological evidence - particularly, selectivity in uptake, competition between substances, differential effects on influx of changes in internal states, and the associated electrical currents - is incompatible with the characteristics of pinocytosis in animal cells. The suggestion that pinocytosis in plants involves selective binding prior to uptake cannot be accepted because the required density of binding sites is at least three orders of magnitude greater than is possible.

On theoretical grounds it is shown that the water flow that would be associated with pinocytosis would generate high values of turgor which are not observed and probably could not be sustained. Further, it is calculated that during pinocytosis the probable energy to overcome turgor would be needed at a rate greater than the cell could supply. Thus it appears not only that pinocytosis does not occur, but also that it could not occur in plant cells.