How the Eye Detects Light
In order for light to be perceived, it is necessary to convert the energy of an arriving photon into an electrical signal. The photoreceptor pigments which enable this are found in rods and cones in the retina. They are a combination of a short chromophore molecule called retinal, embedded in a membrane protein called an opsin.
The absorption of the energy of a photon by an electron in retinal causes the retinal to change shape, from bent to straight. This in turn causes it to change the shape of the opsin protein, being ejected from the opsin pocket in the process. The new shape of the opsin protein exposes a previously hidden reactive portion, which triggers a cascade of chemical reactions, opening and closing ion channels in the photoreceptor membrane. This results in changes of the electrical gradient across the membrane, signaling the detection of the photon.
More detail
The ability of retinal to change its shape when it absorbs a photon is due to the property of carbon-to-carbon double bonds. Retinal has a backbone of 9 carbon atoms that are connected by double bonds. Carbon atoms have 4 binding electrons that are trying to pair up with electrons from other atoms. When forming a double bond, 3 of the electron orbitals are in a plane, so that there is a 120 degree angle between them (think of the orbitals at numbers 4 – 8 – 12 on a clock face, with the carbon nucleus in the center where the hands are attached). Therefore, connected carbon atoms in a chain cannot link in a straight line – the chain must bend up or down by 60 degrees. If consecutive bonds bend in opposite directions (trans conformation) then the bend will cancel out and that segment of the molecule will be straight. However if 2 consecutive bends occur in the same direction (cis conformation), the molecule will have a 120 degree bend.
In the double bond, the 4th electron orbital is perpendicular to the plane of the other 3 electron orbitals. It is shaped like an hourglass, extending above and below the clock face plane. The bond with this orbital locks the molecule into the trans or cis conformation, since it prevents the chain from rotating around the linear bond axis. It is the bent cis retinal conformation that is able to detect a photon of light.
This bent conformation of retinal is less stable, since the perpendicular electron orbital is asymmetrical. This puts a torsional tension on cis retinal, trying to twist it to the more stable trans form. If a photon arrives with just the exact right amount of energy (quantum) to push one of the perpendicular electrons to a higher energy orbital, this momentarily breaks the double bond and allows the retinal backbone to rotate from the bent (cis) to the straight (trans) shape. This happens very rapidly, measured in femtoseconds.
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