How does the light set off the electrical charge?
In the absence of light, a rod cell maintains a electrically neutral state by allowing sodium ions to flow freely in and out of the cell. (An ion is an atom or group of atoms that carries an electric charge.) It does this by means of two proteins embedded in the cell membrane. One protein, called an ion channel, acts like a gate, regulating the inflow of sodium ions. Another protein acts as a pump, pushing the sodium ions back out of the cell. The ion channel opens and closes in response to another biomolecule, called cGMP. For convenience, we’ll call it the opener. When the opener attaches to the ion channel, the channel opens up and allows positively charged sodium ions to flow into the cell. When the opener falls off, the channel shuts and the flow of ions stops. Under normal circumstances, there is a high concentration of opener molecules in the cell, and they are continually attaching to the channel and then falling off. As a result, the channel is continually opening and closing.
