 Hey everybody, Dr. Rowe here. We just finished talking about the monosaccharides, glucose, fructose, and galactose. Now we're going to look at how they join together and bond to form disaccharides. So dye means two, so disaccharides are going to be two monosaccharides put together. They're actually built by a dehydration synthesis reaction, and we'll look at the combinations in just a moment. So that's going to be how we build these. They're held together by what are called glycosidic bonds, glyco meaning sugar. They are 1, 4 alpha or beta glycosidic bonds. The most important bond of these three is going to be the one right there in the middle of lactose. It's actually a harder bond to get at, and it's harder to enzymatically break down. So between that and the fact that many humans don't have enough of the enzyme lactase, that's why you see many, many people are lactose intolerant. So we'll come back to that. If you're going to break them down, so you eat these things. You've all heard of sucrose and lactose for sure. You eat these things, but your body has to digest them and break them down before you can absorb them and use them. So it was dehydration synthesis reactions that built them. It is hydrolysis or decomposition reactions that break them down. The enzymes we would use would be sucrase to break down sucrose, lactase to break down lactose, and maltase to break down maltose. Let's go ahead and look at each one, though. So here we see sucrose, also known as sugar, table sugar, cane sugar, or beet sugar. So this is going to be something that we obviously talk a lot about. Now we're just talking about biochemistry here. I'm not going to get into the nutritional implications of a high sugar diet and those kind of things. So you'll see that sucrose is made of the monomers glucose plus fructose. We all love sugar and your brain loves it for sure. You may have heard of high fructose corn syrup. That's going to be similar to sucrose, but it's going to be 55% or more fructose. So you see that in a lot of your processed foods. So again, I'm not going to get into the health implications of sugar consumption right now, but that is sucrose or table sugar. Next we have lactose. So lactose is also known as milk sugar. It occurs only in mammary glands and mammary tissue. So as most carbohydrates come from plants, this one comes from mammary gland tissue. Lactose is not near as sweet as sucrose, about 1 sixth as sweet as sucrose. When you think about lactose, you probably think of dairy products, but you also probably think of infants. So about 40% of an infant's diet in its first year of life would be lactose if they're being breastfed for that whole time. Infants and children usually have a very strong version of the lactase enzyme. As we get older, we either get a weaker version of the enzyme lactase or we don't have much or don't have any of it at all, which is why up to 70% of the world's population is lactose intolerant, at least at some point in their life. What else? So what is lactose intolerance? If you can't break this down properly, then it passes into your large intestine. And that can draw some water with it, which is where the diarrhea comes from lactose intolerance, and then also the microbes living in your gut. They can start to break this down. They produce gas and organic acids, and that leads to the abdominal distention, bloating, you know, gas, pain, and diarrhea that comes from lactose intolerance. For most of human history, no one would have consumed lactose except for breastfed babies. But as far as your chances of being lactose intolerant, it really depends on where your genes are from. If your ancestors were herder gatherers and had been consuming dairy products for a long time, chances are you are not lactose intolerant. If your genes come from a part of the world where dairy products have not been a big part of the diet, you're much more likely to be lactose intolerant. But we'll cover all of that in the nutritional video series done in the future. So that is lactose, and I already mentioned earlier. The enzyme lactase is what breaks it down. Then you have maltose or malt sugar. You consume some of this. It's not near as sweet. It's about 30% as sweet as sucrose. You know, malted milk balls, malt liquor, malt powder. These kind of things will have some maltose, because maltose is liberated by malt, which is germinating barley on starch. So germinating barley does break starch down into some maltose. So there is a little bit of maltose in your diet. But the huge majority of maltose that you're going to have inside your body is going to come from consuming the polysaccharides like starch. So as your body takes this huge starch molecule, which can be hundreds or thousands of glucose units long and breaks them down into smaller, smaller chunks, at some point you'll get down to maltose, which is a glucose bonded to a second glucose. Then you'll use the enzyme maltase to break them apart into individual glucoses, and that's what will be absorbed and used in your body. All right. So quick reminder, we covered this on the monosaccharide video. Remember, so the three monosaccharides are glucose, fructose, and galactose, but glucose is the only one your body can use for fuel. So when you break maltose down, you now have two glucoses that can be used for fuel. When you break lactose down, you have a glucose and a galactose, and that galactose will have to be converted to glucose by the liver. Same thing with sucrose. When you break sucrose down, you have a glucose and a fructose. That fructose will have to be converted to glucose in the liver. All right, so those are your three disaccharides. I hope this helps. Have a wonderful day. Be blessed.