Phototransduction uses a signal transduction cascade similar to mechanisms stimu

Question

Phototransduction uses a signal transduction cascade similar to mechanisms stimulated by hormones and neurotransmitters. However, the visual sensory apparatus is stimulated by photons, and converts light energy into a nerve impulse. Determine whether each of the the following events increases phototransduction or does not increase phototransduction (either by not initiating a signal or by terminating the signal). The abbreviation cGMP is used for guanosine 3',5'-cyclic monophosphate (cyclic GMP), and PDE is used for phosphodiesterase.

Explanation / Answer

Phototransduction increases

11-cis-retinal converted to all-trans-retinal

transducing dissociates into T-GTP and T

T subunit encounters PDE

PED converts cGMP to biologically inactive 5’-GMP

[cGMP] decreases

cGMP-gated ion channels close

High [Ca2+] inhibits guanylyl cyclase

Phototransduction does not increase

T subunit hydrolyzes GTP

PDE bound to inhibitory subunit

Guanylyl cyclase convert GTP to cGMP

[Ca2+] declines

GDP bound to transducin

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