An Immense World: How Animal Senses Reveal the Hidden Realms Around Us

by Ed Yong

At the start of this chapter, we saw that dogs and other animals detect smells using proteins called odorant receptors. These are part of a much larger group of proteins called G-protein-coupled receptors, or GPCRs. Ignore the convoluted name; it doesn’t matter. What matters is that they are chemical sensors. They sit on the surface of cells, grabbing specific molecules that float past. Through their actions, cells can detect and react to the substances around them. This process is temporary: After the GPCRs are done, they either release or destroy the molecules that they’ve grabbed. But one group of them bucks this trend: opsins. They are special because they keep hold of their target molecules, and because those molecules absorb light. This is the entire basis of vision. This is how all animals see—using light-sensitive proteins that are actually modified chemical sensors. In a way, we see by smelling light.