Gluconeogenesis occurs in all animals, plants, fungi, and microorganisms. The reactions are essentially the same in all tissues and all species.

The important precursors of glucose in animals are three-carbon compounds such as lactate, pyruvate, and glycerol, as well as certain amino acids

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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.

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In resting rod cells high cGMP concentration generated from GTP catalyzed by guanylyl cyclase.

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Light absorption by rhodopsin→activation of a cGMP PDE → hydrolyzes cGMP to 5′-GMP→ cGMP levels  drop →channel close →membrane hyperpolarization→reduction in NT release→transmitted to brain

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