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Carbohydrate Digestion


We'll start with carbohydrates. We can approach them individually and make sure we understand those. And then when we put those two together we'll know everything we need to know. So salivary amylase. Salivary amylase starts in the mouth. So we kick carbohydrate digestion off in the mouth with salivary amylase.

And the whole entire point of carbohydrate digestion and absorption, and this an MCAT thing that has definitely been tested and will continue to be tested. We need to break down these complex carbohydrates we eat into monosaccharides. We could only absorb carbohydrate as monosaccharides. If we break them down only into disaccharides, we wont absorb them. And this is the case with people who are lactose intolerant.

Lactose, you can see down here, and we'll get to it, is a disaccharide. And people who are lactose intolerant don't have lactase which breaks lactose down into it's glucose and galactose constituents. And we'll just go ahead and throw that up here, and we'll get the rest later. The people who can't break the lactose, the disaccharide down into its two monosaccharide components, are not able to absorb it and they get gassy, and we'll talk about that.

So you need absolutely to know for the MCAT. Carbohydrates, you got to get them down into monosaccharides or you can't absorb them. Okay, so it starts in the mouth with salivary amylase. Again, all you need to know is it's just in the mouth, and it's breaking these complex carbohydrates down, gradually, slowly into smaller and smaller, that's until we eventually get them into monosaccharides.

Then we also have a pancreatic amylase. And again, if you remember the pancreas, I drew that bubble gummy looking thing that releases directly into the GI tract with the pancreatic amylase comes from that. And again, it breaks these things down smaller and smaller. By the time we get in to the small intestine, we are mostly into, thanks to the work of amylases, we're into disaccharides.

And the final thing that takes these disaccharides and turns them into monosaccharides, is these oligosaccharide hydrolases, better known as brush border enzymes. And these brush border enzymes, if this is our GI tract, I could put little projections here, and living in these projections are little brush border enzymes.

And these take all the disaccharides that amylases have broken us down into, and break them down further into monosaccharides, okay? Now the monosaccharides are glucose, galactose, and fructose. You need to know that for the MCAT. Commit that to memory, hopefully as we go through the disaccharides, you'll be able to retain it a bit more.

So, lactose is glucose and galactose. And the way to remember the disaccharides, they all have a glucose. So all you have to figure out is what the other thing is. So, lactose is easy. Glucose and galactose, galactose should make that easy to remember. Maltose, I also think is easy.

It's like chocolate malt, it's very sugary, so it's a glucose and another glucose. And then sucrose is just the last one then there, which is a fructose, the only monosaccharide that we haven't discussed. So you need to know that disaccharides, and above all you need to know we have to break things down into monosaccharides to absorb them.

Otherwise we have disaccharides going through the GI tract, and as I was mentioning with lactose, people who are lactose intolerant lack the enzyme lactase that breaks lactose down into its glucose and galactose components. And if you lack that now, instead of monosaccharides getting absorbed, you have disaccharides that travel through the rest of the colon. And in the colon you have bacteria that are gonna work on, now they have something that is not completely digested.

So they will work on this lactose and break it down, and in the process these bacteria release gases that then need to be expelled. And sure at some point in your life, you've met somebody, or if you yourself have lactose intolerant, have experience the gasiness. So, carbohydrates that is what you need to know for the MCAT.

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