In this blog post, we will explore the principles of how cone cells in the human eye recognize and distinguish various colors.
You may have heard the story that dogs and cats are color blind and only humans can distinguish colors. In fact, pets see the world differently from us. They may not be able to detect certain colors or see only within a limited range of colors. Nevertheless, animals perceive the world through senses other than color, and they have better hearing and smell than humans. Humans, on the other hand, have an excellent ability to distinguish colors. While this story is not 100% true, it is true that humans are more sensitive to color than other animals. This is thanks to the two types of photoreceptors we have: cone cells. The cone cells in the retina of the human eye respond to red, green, and blue light, depending on the type, and transmit the information to the brain. It is widely known that cone cells play a role in distinguishing colors. However, most people do not think deeply about how this process takes place. So, how do cone cells respond to light? And how are cone cells divided into three types of cells: red, green, and blue (RGB)? These questions may seem difficult to answer at first glance, but they can actually be solved with knowledge from higher education. Let’s now 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 cells to understand how cone cells distinguish colors. Let’s just look at the light-absorbing part at the very end of the cone cells. Vision begins with opsin, a protein, and retinol, which binds to it. Retinol is a type of vitamin A. Vision begins when retinol receives light energy and changes into an isomer. An isomer is a molecule that has different physical and chemical properties despite having the same molecular formula and structure. For a molecule to become an isomer, it needs the energy to change its structure, which in the case of retinol is light energy. At this point, it needs the right amount of energy to become an isomer and it will only absorb specific wavelengths. In this way, cone cells can distinguish colors by absorbing specific wavelengths of red, blue, and green.
What is interesting here is that the process of detecting color is not just a visual phenomenon, but also occurs through a complex interaction between the nerves and the brain. When light reaches the cone cells, this information is immediately transmitted to the brain and converted into the color we perceive. At this time, the brain creates the various colors we see by comparing the intensity and wavelength of the light absorbed by each cone cell. In other words, it is thanks to the brain’s excellent computational ability that it can create an almost infinite number of color combinations using only three colors: red, green, and blue. This process is done quickly and accurately, allowing us to recognize colors instantly in our daily lives.
Now, let’s find out how cone cells are divided into red, blue, and green. Since all the retinal used in cone cells is the same, RGB cannot be distinguished by retinal alone. At this point, opsin, which is combined with retinal, creates the difference between the types of cone cells. Opsin is a type of protein, and the basic unit of protein is amino acid. All amino acids have a central carbon that shares a covalent bond with an amino group, a carboxyl group, and a hydrogen. The nature of the amino acid is determined by what the remaining single covalent bond, called the R group, is bonded to. The molecules that bind to the R group of opsin differ for each type of cone cell. These molecules determine the strength of the interaction with the retinal molecule. Let’s go back to the process of retinal turning into an isomer. When retinal turns into an isomer, the energy required is affected by the attraction of surrounding molecules. This is because the structure must be changed by overcoming the attraction. Therefore, the required energy varies depending on how much the interaction with the R group of opsin is affected, and it absorbs light of different wavelengths.
So far, we have talked about how cone cells absorb light of a specific wavelength, focusing on their chemical structure. When the retina of a cone cell receives light, it changes into an isomer, which detects light, and the color of the light is distinguished by the R group of opsin. As mentioned earlier, this could be explained as part of the chemistry course I learned in high school. In fact, most of the phenomena that occur around us can be explained at the high school science level. I hope that readers of this article will feel more familiar with science that is applied to real life.
