In this blog post, we’ll explore the principles behind how the cone cells in the human eye perceive and distinguish various colors.
You’ve probably heard at least once that dogs and cats are colorblind, and that only humans can distinguish colors. In fact, pets see the world differently than we do. They either cannot detect certain colors or can only see within a limited color range. Nevertheless, animals perceive the world through senses other than color and often possess superior hearing and sense of smell compared to humans. On the other hand, humans are exceptionally skilled at distinguishing colors. While this isn’t 100% accurate, it is true that humans are more sensitive to color than other animals. This is thanks to the two types of photoreceptor cells we possess: the cone cells. The cone cells in the human retina respond to red, green, and blue light, depending on their type, and transmit this information to the brain. The fact that cone cells play this role in distinguishing colors is widely known to the public. However, most people do not think deeply about how this process actually works. So, how exactly do cone cells respond to light? And how are cone cells divided into the three types—RGB (red, green, and blue)? These questions may seem to have complicated answers at first glance, but they can actually be answered using knowledge from high school-level science. Let’s explore the answers to these questions by focusing on the chemical structure of cone cells.
To understand how cone cells distinguish colors, we don’t need to dissect every part of the cell. Let’s focus only on the light-absorbing portion at the very tip of the cone cell. Vision begins with a protein called opsin and retinal, which binds to it. Retinal is a form of vitamin A. Vision begins when retinal absorbs light energy and transforms into an isomer. An isomer refers to molecules that have the same molecular formula but different structures, resulting in distinct physical and chemical properties. For a molecule to transform into a different isomer, it requires energy to alter its structure; in the case of retinal, this energy comes from light. At this point, the specific energy required for isomerization is needed, causing the molecule to absorb only certain wavelengths of light. In this way, cone cells absorb specific wavelengths of red, blue, and green light, enabling them to distinguish colors.
What is interesting here is that the process of color perception is not merely a visual phenomenon but is achieved through complex interactions between the nervous system and the brain. When light reaches the cone cells, this information is immediately transmitted to the brain and converted into the colors we perceive. At this point, the brain compares the intensity and wavelength of the light absorbed by each cone cell to produce the various colors we see. In other words, it is thanks to the brain’s exceptional computational ability that we can create an almost infinite number of color combinations using just three colors: red, green, and blue. Because this process occurs quickly and accurately, we can instantly recognize colors in our daily lives.
Now, let’s examine how cone cells are divided into red, blue, and green types. Since the retinal used in all cone cells is the same, retinal alone cannot distinguish between RGB. It is the opsin bound to the retinal that creates the difference between the types of cone cells. Opsin is a type of protein, and the basic unit of a protein is the amino acid. In every amino acid, the central carbon atom is covalently bonded to an amino group, a carboxyl group, and a hydrogen atom. The nature of the amino acid is determined by what the remaining covalent bond—known as the R-group—is bonded to. In this case, the molecule bound to the R-group of the opsin differs for each type of cone cell. This molecule determines the strength of the interaction with the retinal molecule.
Let’s return to the process by which retinal transforms into an isomer. When retinal transforms into an isomer, the energy required is influenced by the attractive forces of surrounding molecules. This is because it must overcome these forces to change its structure. Therefore, the energy required varies depending on the strength of the interaction with the R-group of opsin, leading to the absorption of light at different wavelengths.
So far, we have discussed how cone cells absorb light of specific wavelengths, focusing on their chemical structure. The key points were that when retinal in cone cells absorbs light, it converts into an isomer to detect light, and the color of that light is distinguished by the R-group of opsin. As mentioned earlier, this can be explained using the chemistry curriculum taught in high school. In fact, most phenomena occurring around us can be explained at the high school science level. I hope that readers of this article have become a little more familiar with science as it applies to everyday life.
