Phototransduction (3 marks) The conversion of physical energy to electrical activity in the nervous system is the essential first stage in sensory function. In the visual system, the initial conversion of light energy to changes in membrane potential occurs in photoreceptors of the retina. The figure above illustrates the steps in this process. Light is absorbed by a photopigment, which undergoes an isomerisation, which in turn causes a conformational change in the opsin (or rhodopsin) protein in which the photopigment resides. Activation of the photopigment and associated protein activates a G-Protein - as indicated by the arrow numbered “1” in the figure: this step involves an exchange of the bound GTP molecule with the cytoplasmic GDP molecule. The activated subunit of the G-protein complex (transducin) translocates in the membrane (indicated by the arrow labeled “2” in the figure) and activates an enzyme, this enzyme (indicated by the number “3” in the figure) is [ Select ]   phosphodiesterase  protein kinase  GTPase  GMPase  . The process indicated by the number “4” in the figure shows ionic conductance through an ion channel. This conductance occurs in the dark. The ion channel is closed when the concentration of [ Select ]   ATP  GTP  cGMP  cAMP  is reduced in response to stimulation by light The process indicated by the number “5” in the figure shows the inactivation of an ion channel. When this occurs in photoreceptors as a result of light stimulation, the membrane becomes [ Select ]   depolarised  stabilised  destabilised  hyperpolarised  and this causes a decrease in neurotransmitter release.  Multiple dropdown selections