Animals and plants respond to changes in their environment. The mammalian eye contains photoreceptor cells which respond to light. The photographs show a grey-bellied night monkey and a squirrel monkey. (Source: © rod williams/Alamy Stock Photo) (Source: © Jesse Kraft/Alamy Stock Photo) The grey-bellied night monkey is active at night and the squirrel monkey is active during the day. Rod cells are photoreceptors located in the retina. Rod cells detect low light intensities. Cone cells are also photoreceptors located in the retina. They are responsible for colour vision and require higher light intensities. The numbers of different photoreceptors in the retina of one eye of each of these two species of monkey were compared. The results are shown in the table. A section of squirrel monkey retinal tissue contained a total of 5000 photoreceptors. Calculate how many of these photoreceptors would be rod cells. Assume an even distribution of rod cells and cone cells in the retina. (2)

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