What are the similarities and differences between the phototropic response of a plant shoot and the response of the pupil of a human eyes in response to light?
The similarity is that plant shoots and eye pupils respond to the sun light. But the function, mechanism and regulation of this response are different. Plant shoots positively response to the light. In the conditions of high intensity of light plant leaves turn their surface perpendicular to the sun light while flowers open. This photo response is promoted by the changes in the cell permeability to water: this is the main mechanism of all plant movements. Of course the main function of the phototropism is to increase the amount of the sunlight incoming to the plant: the increase of light positively regulates and activates photosynthesis. Pupil of eye negatively respond to the light: with the increase of sunlight intensity pupil diameter is decreased. The mechanism of this regulation is the pupillary light reflex that includes neuron activity. Great amounts of light are harmful for retina. That is way the main purpose of this reflect is the decreasing of the amounts of light reaching the retina.