The noble gases are left out of the trends in atomic radii because there is great debate over the experimental values of their atomic radii.

The SI units for measuring atomic radii are the nanometer (nm) and the picometer (pm). 1 nm = 1 X 109 m; 1 pm = 1 X 10-12 m.

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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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Depolarized state of a resting rod cell is due to the presence of a large number of open nonselective ion channels that admit both Na+ & Ca²+ into the cell

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Nonselective cation channels open in response to binding of the second messenger cGMP (cGMP-gated channels)

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3 or 4 cGMP molecules must bind to each channel in order to open it; because of this cooperative allosteric interaction, channel opening is very sensitive to small changes in cGMP levels.