A mouse rod, held at body temperature , shows a single-photon current step of lasting…

PHYSICS · Multi-part · Fermi University Physics · Optics & Photonics · Biophysics & Medical Physics · Bioelectricity, the senses and the physics of the body

Forces, Machines and Signals, Ch Vision and photoreception: counting photons

Problem

A mouse rod, held at body temperature

310\,\mathrm{K}

, shows a single-photon current step of

3\,\mathrm{pA}

lasting

300\,\mathrm{ms}

. ((a)) Assumptions: treat the absorbed photon as having wavelength

500\,\mathrm{nm}

(near the peak rhodopsin absorbs), and treat the current step as a constant

3\,\mathrm{pA}

for the full

300\,\mathrm{ms}

. Estimate the photon's energy in units of

k_BT

310\,\mathrm{K}

. ((b)) Estimate the number of elementary charges kept out of the cell by this one absorbed photon. ((c)) Compare this charge count with the toad rod estimate of about

6\times10^6

charges per photon quoted for a

1\,\mathrm{pA}

1\,\mathrm{s}

response, and say briefly what accounts for any difference.

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