The photosynthesis-irradiance curve (or PI-curve, for short) in phytoplankton is highly plastic, in response to changes in the environment. The response to changes in the long term average irradiance is usually designated by the term 'photoacclimation'. On a much shorter time scale, high irradiances can lead to diminished rates of photosynthesis, a phenomenon referred to as 'photoinhibition'. Photoacclimation and photoinhibition both affect the PI-curve, but existing models for the PI-curve do not account for both phenomena at once. In this paper I propose a model that aims to remedy this situation. The model is couched in differential equations that describe changes in the photosynthetic machinery relevant at the short time scale characteristic of photoinhibition. Photoacclimation is accounted for by variations in parameter values in response to changes in the chlorophyll content of the cells, the hallmark of photoacclimation. The rate of photosynthesis depends on the photon absorption rate rather than on irradiance. The photon absorption rate is the product of the irradiance and the cellular absorption cross section. Photoacclimation affects the cellular absorption cross section through its effects on the chlorophyll content of the cells. As a result, photoacclimation affects the photon absorption rate, and so indirectly the sensitivity to photoinhibition at a particular irradiance.