Population density is the number of individuals per unit area. D=N/S  

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There are about 90 million rod cells in the human retina, and each contains about 4x107 molecules of rhodopsin that are very densely packed in the membrane disks.

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A single activated rhodopsin protein activates 

~13 transducin molecules in the few seconds before the covalent bond between the opsin protein and all-trans-retinal is spontaneously cleaved

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RPE65 is the enzyme in the pigment epithelial cells of the retina that catalyzes the isomerization of the intermediate esterified all-trans retinol to 11-cis-retinol.

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Autosomal recessive mutations in the RPE65 gene are a frequent cause of blindness in children.

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In the dark, 

membrane potential of a rod cell is about −30 mV (Depolarized state)

Rod cells release neurotransmitters (glutamate) constantly

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