Color Sensitive Chemicals in the Cones

Color is the result of a chain of complex interactions. One of the key interactions is between wavelengths of light and the cone cells in the human eye. A little background on the interaction between light and matter and a little about signalling mechanisms in neurons will help in understanding the mechanism of light absorption --- and why humans see in three different colors.

Light and Matter

• Different wavelengths of light carry different amounts of energy. Longer wavelengths mean less energy; shorter wavelengths mean more. When a photon --- the smallest possible amount of light --- travels through a material, either one of three things happens: it transmits through, it reflects off, or it is absorbed. The fate of the photon depends upon the characteristics of the material through which it travels. If the atom or molecule has two different energy states that are separated by the amount of energy carried in the photon, its probability of being absorbed is higher.

Visible Light

• Atoms are surrounded by electrons. Electrons can be in different energy states around the atom, although they typically like to be in their lowest energy configuration. It turns out that the energies of visible light are right in the range of the energy difference of electrons around atoms. So when visible light travels through a material, if the color of the light matches exactly the energy needed to move an electron from one configuration to another, then the light gets absorbed and the electron ends up in its new configuration.

Cell Signalling

• Neurons transmit signals by opening and closing channels in their membranes.

  Neurons transmit signals by transmitting chemicals through channels in their cell membrane. As specific molecules change their configuration, they modify the distribution of their atoms and trigger other molecules to respond. In what's called an "enzymatic cascade," an initially small signal ends up getting amplified through the cell, and triggers the opening of the channels in the cell membrane. The open channels let other signalling molecules out, and the message is sent. In the cone cells in the eye, the message is "I saw some light."

Retinal and Cone Cells

• There are three different types of cone cells in the normal human eye (people with fewer types suffer color deficiency and people with four types are called tetrachromic). Each type of cone cell absorbs a slightly different color of light: blue, green, and yellow-green. As discussed in Sections 1 and 2, light is best absorbed when it matches perfectly to an energy difference in the electrons in an atom. In cones, that molecule is called retinal. Retinal absorbs visible light, moving an electron from one energy to another. The change in energy of that one electron changes the shape of the retinal. That's the heart of all human vision --- and just about all vision on Earth: an absorbed photon changes the shape of retinal.

Retinal in the Eye

• The eye focusses light onto the retina, where photons interact with molecules of retinal.

  Retinal in its normal configuration is locked into a different molecule at the surface of a cone cell. Opsin is a larger molecule that holds on to a retinal molecule. When the retinal changes shape, it bounces out of the opsin. The opsin then changes its configuration, and starts a signalling cascade. One photon results in a signal of about 100,000 molecules further down the chain. The opsin is eventually returned to shape by another molecule within the cell. Meanwhile, the distorted retinal moves out of the retina, gets recycled back to its other shape, and eventually is free to bind to another opsin.

Different Colors

• We see different colors because different cones create just slightly different environments for the retinal.

  If the light is all being absorbed by the same molecule --- retinal --- then how do photons of different energy get absorbed? The answer is that the three different cone cells in the eye produce just ever-so-slightly different versions of the opsin molecule. Those opsin molecules, which are tightly holding the retinal, modify the electron energies possible in the retinal. Modified energy means a different photon is absorbed, and that's why humans have three different color sensors in the eye.