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  • Hi. It's Mr. Andersen and in this podcast I'm going to talk about carbohydrates.

  • When I say the word carbohydrates you might think about the starch that's found in this

  • bread or maybe in this pasta. As a biology teacher I immediately think of sugar because

  • that's going to be the building block upon which most carbohydrates are made. But you

  • should also know that aside from providing energy for us they also provide structure.

  • And so cellulose that's found in plants is going to be a polysaccharide or the chiton

  • that's found in the exoskeleton of an insect or the building block of fungi is going to

  • be a carbohydrate. And so they give us energy but they also give us structure. Sugars, in

  • science we call saccharides. And the reason I wrote carbohydrates inside these hexagons

  • is that's how sugars essentially are put together. And so if we have just one of these sugar

  • molecules, we call that a monosaccharide. So an example could be glucose. If we have

  • two of them together, we call that a disaccharide And example could be table sugar or sucrose.

  • It's actually one glucose and one fructose molecule. If we have about three to ten sugar

  • molecules we call that an oligosaccharide. And then if we have a whole bunch of sugar

  • molecules attached together we call that a polysaccharide. And so glycogen would be an

  • example of that. And so basically the empirical formula of all carbohydrates is going to be

  • the same. In other words we have a ratio of 1 to 2 to 1 in the amounts of carbon, hydrogen

  • and oxygen. So we have twice as much hydrogen as we do carbon and oxygen. Can you see why

  • they're called carbohydrates? We've got a carbon out here, and then we have water so

  • it's a carbohydrate, a good way to remember that. If we look at a simple monosaccharide

  • it's going to have 6 carbon 12 hydrogen and 6 oxygen. And so the simplest, the sugar upon

  • which life is built is called glucose. And so glucose has 6 carbons. We could look at

  • them. There's one here, here, here, here, here and here. So they're going to be at the

  • junction points on this ring. In an aqueous solution, or in water they're going to form

  • these rings. But you can also see that there's going to be a lot of oxygen. So we have all

  • of these hydroxyl groups around the outside and that makes sugars readily dissolvable

  • in water. And so glucose is used in cellular respiration, it's produced by plants in photosynthesis

  • so they can use it in respiration. So it's the building block. A lot of the different

  • sugars I'm going to show you in this podcast are built on glucose. But there are other

  • ones. We've got fructose. Fructose is going to be a five, you can see it's a five sided

  • sugar. It's found, it's going to be a little sweeter than glucose, and it's going to be

  • found like in fruit or high fructose corn syrup. And then we have galactose. And galactose

  • is going to be a little less sweet than glucose. But these are the basic three monosaccharides.

  • What's cool about these, they all can readily be moved into our blood supply. And so these

  • are flowing through your blood right now, these little monosaccharides. If you ate pasta

  • for example we first have to break that down into it's monosaccharides before we can move

  • it through the blood and the into the cells in our body. So what are disaccharides then?

  • Disaccharides are going to be two sugar molecules attached together. And so the table sugar

  • that's found in these sugar cubes is going to be sucrose. And so it is a glucose molecule

  • attached to a fructose. And so when that goes into my body, I have an enzyme called sucrase

  • that has to break that down into it's monosaccharides before I can actually use it. Or here's another

  • one. The milk sugar, so lactose, is going to be a glucose and a galactose chemically

  • bonded together. And so if you want to break down lactose you have to have an enzyme called

  • lactase. Now if you're lactose intolerant what does that mean? You just lack the enzyme

  • to break lactose down into it's two monosaccharides. And so you're going to feel a little irritation

  • in your gut and that's because we can't break it down. Now that seems to be, there's some

  • really cool studies you could read on lactose tolerance or intolerance and it's been naturally

  • selected. In other words if your ancestors had domesticated cattle it made sense for

  • them to drink milk later on in their life. But most people just drank milk when they

  • were young and so they quit producing that lactase enzyme. Okay. Let's go to oligosaccharides.

  • Oligosaccharides are going to be like three to ten different sugar molecules. They're

  • important in biology in one pretty important part and that is in the production of these

  • which are called glycoproteins. So we're in a cell membrane and these which are going

  • to be glycolipids, if you look at the glyco part or the sugar part, that's going to be

  • a few sugar molecules attached together and these are really important, for example, attaching

  • to the extra cellular matrix. They're important in identifying what type of a cell it is.

  • Here's an interesting note I learned on wikipedia, it you're to eat carrots, carrots are a wonderful

  • source of oligosaccharides, however, you can't get the sugar molecules out of it until you've

  • cooked the carrots for about an hour to release those oligosaccharides. But again, if you're

  • not getting them in your diet, we can synthesize those inside the cell. Now let's look at this

  • number right here as we go from oligosaccharides to polysaccharides. And look how much that

  • jumped. And so when we're talking about starch for example, what is starch? Starch is going

  • to be hundreds of these glucose molecules attached over and over and over again. And

  • so the starch that's found in a potato or if we dry it out it's going to look like this,

  • is going to be hundreds of sugar molecules attached over and over and over again. Now

  • why are plants doing this? Why are they making these large molecules? They're storing energy

  • in the starch molecule so they can use it by chopping it down into individual monosaccharides.

  • Now can we do that? You bet. We've got glycogen. So glycogen is essentially a macro macro molecule.

  • And so it's going to have thousands of glucose molecules attracted together or chemically

  • bonded together. You can see how monstrous this looks with all these individual glucose

  • molecules. And we're going to store that in the liver. And so if you are carbo loading

  • what are you really doing? You're eating a bunch of starch. You're breaking those down

  • into monosaccharides and then you're reattaching those again and you're storing them in our

  • liver as glycogen. And so we can get to those stores eventually when we need it. We can

  • chop up those monosaccharides and we can use them in the cell. But we also get structure

  • remember. And so cellulose that makes up that structure in a lot of plants, you can see

  • here, it's going to be a bunch of sugar molecules attached over and over again. But we're going

  • to have these hydrogen bonds that cross bond between the different polysaccharides and

  • makes them incredibly durable. If you were to eat wood, don't, but you don't have the

  • enzymes to break it done inside your gut. And so it's going to go in as wood and it's

  • going to come out as wood. And so if we want to break down cellulose we have to get help.

  • And we have to get microscopic help. And so like a cow for example is going to have a

  • bunch of bacteria and other microscopic life that lives in their gut that can break down

  • that cellulose and so they can eventually get to sugars. But it's not that easy. So

  • how do we do all this building and how do we do all this breaking. Well there's basically

  • two processes. Since it's a polymer we can use hydrolysis. Hydrolysis is simply breaking

  • the sugars and so right here we have a glucose, or excuse me, a lactose molecule. You can

  • see it's a disaccharide. And so what we can do is we can add a water and when we add a

  • water we can break this bond right here and we can make two monosaccharides. And so hydrolysis's

  • simply breaking them apart. Enzymes help on this as well. And then a dehydration reaction

  • is when we're going to have two monosaccharides and we go in the other direction. So when

  • we're actually making lactose we're taking two monosaccharides, we're losing a water

  • and then we're making that covalent bond between them. And so again we can build, we can make

  • them smaller and then we can eventually break them down in respiration. Now if we were to

  • look at sugars, are they a good thing? Well evolutionarily they're very important. Why

  • do we love sugar so much? It's because sugar is usually an indicator of fruit and fruit

  • is going to have a lot of other vitamins in it that we need. And so humans are essentially

  • programmed to love sugar. Sadly what we've done is we started to put sugar in everything.

  • And so this, I didn't even notice that they made this, this is a double big gulp. So if

  • you had this much soda that's made of high fructose corn syrup. So basically we're enzymatically

  • breaking down corn to make this fructose, this really sweet sugar, and it's killing

  • us. We're seeing an increase in heart disease, an increase in diabetes as a result of that.

  • And so a little bit of sugar is good. We need it for energy obviously, but too much is probably

  • bad. And I hope that was helpful.

Hi. It's Mr. Andersen and in this podcast I'm going to talk about carbohydrates.

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B1 中級

碳水化合物 (Carbohydrates)

  • 49 13
    Cheng-Hong Liu 發佈於 2021 年 01 月 14 日
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