Final answer:
A modified mechanoreceptor in each cell is responsible for photoreception in the retina and fluid movement detection in the kidney tubules. Photoreceptors like rods contain rhodopsin for light detection, while osmoreceptors in the kidney's macula densa monitor sodium ion concentrations.
Step-by-step explanation:
Photoreception by the retina of the eyes and detection of fluid movement in kidney tubules is performed by a modified mechanoreceptor on each cell.
In the retina, these mechanoreceptors are specialized as photoreceptors known as rods and cones. Rods are particularly sensitive to dim light and are instrumental in peripheral and nighttime vision. They contain the photopigment rhodopsin, which undergoes a shape change when light is detected, initiating a signal cascade that ultimately results in visual perception.
Meanwhile, in the kidney tubules, osmoreceptors – a type of mechanoreceptor – detect fluid movement based on osmotic pressure differences. The macula densa, a group of cells in the kidney, contains osmoreceptors that sense changes in the concentration of sodium ions, crucial for the regulation of blood pressure and volume.