As promised, today we’ll be talking about the sense of taste. The actual, biological sense of taste, not the good taste you’re showing by reading Science Stories right now (although that one’s pretty good too!).
So! We have tongues, for the most part, and those tongues are a lot longer than you’d expect them to be, but that’s not the moral of this story. Your tongue is one of the strongest muscles in your body relative to its size, what with all the talking and eating and drinking and singing we do on a daily basis. I could write a whole story on why tongues are strange, but today we’ll focus on the most famous part of the tongue: the taste buds.
When I was little, I was taught that the tongue is divided into five sections, one for each of the five tastes we can sense: sweet, salty, bitter, sour, and umami. Unfortunately, this turns out to have been totally false. But that’s okay, because what taste buds are actually like is even cooler.
First of all, we have taste buds kind of everywhere on the tongue, but they’re most densely packed towards the tip of the tongue. There are three kinds of taste buds: fungiform, foliate, and circumvallate. The main difference between these is their shape and where they tend to be found on the tongue, but other than that they’re pretty similar. I’m going to use a fungiform taste bud as an example because the name means “mushroom-like” and this makes me happy.
Okay, so what is a taste bud? In short, they’re the little bumpy things on our tongues. Each little bump has pores in it, and each of these pores contains a number of cells.
In turn, each of these cells has a bunch of receptors in it. Now, these receptors can be sensitive to one of the following things — sweet, salty, bitter, sour, or umami compounds. As anyone who read my last story knows, any other flavor is actually a smell and not a taste.
In other words, within every taste bud, there are cells that can sense each of the five tastes we have. Which means that, assuming everything’s working properly, any given taste bud is capable of tasting any flavor you throw at it.
How does this work, though? Let’s say you’re a glucose molecule from a cookie. You’ve gotten chewed up and dissolved by saliva, and now you’re calmly floating towards a taste bud. As you approach the nearest pore, you notice a lovely little receptor that’s just your size and shape, waiting for you on the surface of one of the cells occupying the pore.
“Ah,” you think, “That must be the sweet cell, made just for me.” And you’d be right.
So you go up to the glucose receptor and get gently hugged by the proteins that make it up thanks to some complicated chemistry. The receptor has now changed shape slightly, because of this hug, and this causes another protein to leave its side and go elsewhere in the cell. This other protein, in the proud tradition of molecular biologists having terrible names for things, is called a G-protein, and so the receptor you’ve just hugged is called a G-protein-coupled receptor. Catchy.
So the G-protein goes off and lets a bunch of ions (electrically charged atoms) into the cell, which is the trigger for sending off an action potential that will travel through the cell’s long tail and straight to the brain. Interestingly, every taste has its own location in the brain; “sweet” signals end up somewhere different than “salty” or “bitter” signals.
The story is slightly different if you’re a salty or sour compound. In that case, there’s no G-protein involved. Instead, the receptor on your designated cell is an ion channel, which is a sort of gate that lets certain ions in. You bind to the channel, which makes it open up, more ions flow into the cell, the cell fires an action potential, and the rest is the same.
I’m not entirely sure what happens to you after the information gets sent to the brain — I imagine you’re absorbed by the body, and that’s how we get our nutrients. But at that point we’re no longer in the realm of taste, so I’ll stop there for now.
And there we go! Only one sense left, the lovely sense of touch. Until then, go and taste some things.
At the risk of repeating myself, thank you to Khan Academy for their lovely explanations of just about everything.