Measurements of Photoreceptor Membrane Properties Allow the Calculation of Metabolic Cost

<div><p>(A) The membrane potential (mean ± standard error of the mean) of R1–6 photoreceptors in the dark and at different effective photon rates, measured in four species S. carnaria (blue), C. vicina (red), D. virilis (green), and D. melanogaster (black).</p> <p>(B) The corresponding resistances (mean ± standard error of the mean) of R1–6 photoreceptor in the dark and at different effective photon rates.</p> <p>(C) The electrical model circuit of the photoreceptors. The model calculates from the measurements of membrane potential and resistance the rate at which the Na⁺/K⁺ pump, P, hydrolyses ATP molecules: <i>g</i>_L = light-gated conductance; <i>E</i>_L = reversal potential for light-gated current; <i>i</i>_L = light-gated current; <i>g</i>_K = potassium conductance; <i>E</i>_K = potassium reversal potential; <i>i</i>_K = potassium current.</p> <p>(D) The rate of hydrolysis of ATP molecules calculated at each effective photon rate for R1–6 photoreceptor of the four species (mean).</p>