A theoretical representation is developed for the relationship between photosynthesis and light in phytoplankton, based on a simple model of processes associated with electron flow through photosystem II. Photoinhibition is represented as an impediment to this electron flow. Examples are shown of fits to various typical light saturation curves for natural assemblages of marine phytoplankton. An important parameter is the size of the photosynthetic unit of PSII. This feature facilitates discussion of photoadaptation in terms of changes in the light saturation curves consequent on modifications in the number and/or size of the photosynthetic units.