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Georg Nagel studies membrane proteins that respond to light. His work on channelrhodopsin-2 helped establish the molecular basis of optogenetics, in which introduced light-sensitive proteins allow light to control cellular electrical activity.

In 2003, Nagel and colleagues expressed channelrhodopsin-2 from the microalga Chlamydomonas reinhardtii in Xenopus laevis oocytes and mammalian cells. Electrical recordings showed a rapidly opening ion channel permeable to cations. Experiments on isolated membrane patches supported direct gating by light, without requiring a freely diffusible intracellular intermediary. During sustained illumination the initial electric current declined to a lower maintained level.^[ref]

This gives a useful distinction between an ion channel and a molecular pump: an open channelrhodopsin-2 permits ions to move down their electrochemical gradient. The resulting change in membrane potential depends on the cell and its ionic conditions; a flash of light does not imply an identical electrical response in every cell.^[ref]