How do cones allow humans to distinguish an infinite number of colors?

This article explains that we owe our ability to distinguish colors to cones and discusses how cones perceive colors. In particular, it details the chemical structure of cone cells, how they absorb wavelengths of light, and how red, green, and blue cells distinguish colors through the role of opsin proteins.

 

We’ve all heard that dogs and cats are color blind and that only humans can distinguish colors. In reality, pets see the world in a different way than we do: they can’t detect certain colors, or they can only see within a limited range of colors. Nevertheless, animals perceive the world through senses other than color, and they often have better hearing and smell than humans. Humans, on the other hand, are very good at distinguishing colors. While this story isn’t 100% true, it’s true that humans are more sensitive to color than other animals. This is thanks to the cones, which are one of the two types of visual cells we have. Cone cells in the retina of the human eye respond to red, green, and blue light, depending on the type, and relay the information to the brain. It’s a well-known fact that cones are responsible for color discrimination, but most people don’t give much thought to how this process works. So, how do cones respond to light, and how do they divide into the three RGB (red, green, and blue) cell types? These questions may seem like they have difficult answers at first glance, but they can actually be answered with higher education knowledge. Let’s take a look at the answers to these questions, focusing on the chemical structure of cone cells.
We don’t need to dig into every part of the cell to understand how cone cells distinguish colors. Let’s just look at the very end of the cone, the part that absorbs light. It’s a protein called opsin, and the retinal that binds to it. Retinal is a type of vitamin A. Vision begins when retinal is energized by light and turns into isomers. Isomers are molecules with the same molecular formula but different structures that have different physical and chemical properties. In order for a molecule to become a different isomer, it needs energy to change its structure, which in the case of retinal is light energy. In this case, it requires the right energy to become an isomer, and it specializes in absorbing certain wavelengths. In this way, cone cells absorb specific wavelengths of red, blue, and green and are able to discriminate colors.
What’s interesting here is that the process of detecting color is not just a visual phenomenon, but a complex interaction between the nerves and the brain. When light reaches the cones, this information is immediately transmitted to the brain, where it is converted into the colors we perceive. The brain then compares the intensity and wavelength of light absorbed by each cone to create the different colors we see. In other words, the brain’s ability to create an almost infinite number of color combinations with just three colors-red, green, and blue-is due to its incredible computational power. This process is so fast and accurate that we can recognize colors instantly in our daily lives.
Now, let’s take a look at how cone cells are divided into red, blue, and green colors. Since all cone cells have the same retinal, the retinal alone cannot distinguish between RGB colors. The opsin that binds to the retinal is what makes the difference between the different types of cone cells. Opsin is a type of protein, and the basic unit of protein is an amino acid. All amino acids have a central carbon covalently bonded to an amino group, a carboxy group, and hydrogen, and their properties are determined by what the remaining covalent bond, called the R group, is. Different types of cone cells have different molecules that bind to the R group of opsin. This determines the size of the interaction with the retinal molecule. Let’s go back to the process of retinal isomerization. When retinal isomerizes, the energy required is affected by the attraction of neighboring molecules. Therefore, the energy required depends on how strong the interaction with the R group of opsin is, and the different wavelengths of light are absorbed.
So far, we’ve talked about how cone cells absorb certain wavelengths of light, focusing on their chemical structure. The idea is that when the retinal of the cone cell receives light, it turns into an isomer that detects the light, and the color of the light is identified by the R group of the opsin. As I mentioned earlier, this could be explained as part of my high school chemistry course. In fact, most of the phenomena around us can be explained at a high school science level. I hope this article has made you feel more comfortable with this kind of real-world science.