A Matter of Taste (beware of educational post!)

Today's AP Bio class was so incredibly fascinating and appropriate for this blog that I knew I had to do a post on it.
Our current unit is on macromolecules (i.e. proteins, carbohydrates, fats, etc.). Ms. Housinger, our teacher, said, "I have an activity today, but not everybody has to do it, since it involves eating chemicals... but don't worry, don't be like 'oh no, it's bugs' brains!' because all of these are perfectly safe and don't cause allergies. Then again, don't take a big spoonful, because it could taste really gross."
My ears perked up. Tasting? In class? Sure! Not everybody raised their hands, but almost the whole class wanted to participate. We got to taste the contents of four paper cups, labeled A, B, C, and D. We knew they were all various types of carbohydrates, but what I thought was funny was how people used what they were positive each cup contained to base their descriptions on. For example, everyone was convinced B was some kind of artificial sweetener, just because they knew it was sweet but also knew it wasn't table sugar, so they said, "hmm... it tastes kind of... artificial. Not like table sugar... but with a weird undertone." Yeah, way to be obvious.
Anyways, here were the various gathered comments on the samples:
A-- Tasteless, powdery (tastes like cardboard or flour), bitter aftertaste (actually, I was the only one who said it tasted bitter. Apparently about only one person per class thinks it's bitter, and we'll get to wh that is later)
B-- Slightly sweet, off putting twinge, not like table sugar (the crystals were much finer)
C-- Good, sweeter than B (there were also people who said it was less sweet)
D-- Some said overy sweet, some said "fabulous", like honey, almost like vanilla (this one had large crystals, like rock candy).
It would have been better if it had been a blind taste test, but our guesses weren't entirely right.
A turned out to be starch, not flour, since flour is a mixture and not a compound. B was glucose, which we're not used to tasting on its own, so we had trouble recognizing the flavor. C, as we all guessed, was sucrose, and D was fructose, so it was good that someone thought it tasted like honey.
BUT WHY DO THINGS TASTE THE WAY THEY DO? That's what this class was about, and for me it was fascinating and almost enlightening.
It's a lie when people say different parts of your tongue taste different flavors (i.e. bitter, sour, sweet, etc.). Here's how it works: the cells on your tongue have protein receptors on the surface. To taste something specific, a protein with the correct shape has to fit into the receptor. For example, only glucose can fit into a certain tastebud... but the interesting part is is that the taste is all in your head-- a nerve connected from your tongue will send a message to your brain, who determines that the taste is "sweet." Starch didn't taste good because the molecule is too long-- as a polysaccharide, it doesn't fit into the receptors.
Of course, there is also something called a structural analog, which is a completely different protein molecule that happens to have hydrogen bonds in the same place, so it will fit into the "wrong receptor"-- this is how cocaine and other drugs trigger messages to your brain.
BUT WHY DON'T THINGS THAT ARE GOOD FOR US NECESSARILY TASTE GOOD?
This is kind of a complicated answer, but it has to do with evolution. Let's use peppers as an example. Mammals think that they're spicy, due to a protein called capsacin that irritates the nerves. However, the ancestors of the pepper plant were berries intended to attract birds to that they could eat the berry and poop out the seeds in order for more berry plants to grow. It tastes spicy to us because we weren't originally intended to eat it-- while birds swallow the seeds whole, our teeth with crush them and destroy them.
And why do most people enjoy sweet things but not necessarily vegetables? Our ancestors, primates, received bigger calorie bonuses from fruit. Because fruit was rarer, they needed a reason to want to find the fruit, so therefore it tasted good. They were surrounded by leaves, so when there was no fruit, they had no choice but to eat the leaves, so there was no strong selective pressure to have some delicious taste for vegetation.
When people say something is an "acquired tasted," you can continue to eat something until your taste receptors recognize the taste. Likewise, you can also eat so much of something that your receptors become a bit "immune" to the taste and you can no longer detect the flavor of the food as much. If you don't have it for a while and try it again, you may find that you no longer like it.
When you try something with artificial banana flavor, that flavor is a combination of molecules that activate the subsets for what your brain recognizes as "banana." However, there is no way that it can activate all the subsets, so while it reminds you of banana, you still know that it isn't really.
More science stuff later, unless you're too bored!