Disaccharides - Video Tutorials & Practice Problems
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Disaccharides
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In this video, we're going to introduce some specific disaccharides. And so recall that disaccharides are defined as sugars that consist of exactly 2 monosaccharides means 2 and of course, saccharides means sugars. And so there are many, many, many disaccharides that exist in nature, so you're definitely not expected to memorize all of the disaccharides. But the most abundant disaccharides are the following 4 that we have listed which are maltose, cellobiose, lactose, and sucrose. And so notice down below in our image here, we're just giving you information about these 4 disaccharides. And of course, the disaccharide consists of a sugar 1 plus a sugar 2, a glycosidic linkage that is linking those 2 sugars. And so in this table, we have sugar 1 here in the first row, sugar 2 here in the second, and the glycosidic linkage here in the third. And then we'll put the name of the disaccharide here in this column, give you a little bit of information about the disaccharide and then let you know if the disaccharide is digestible or not with these check boxes. And then also notice that the table that you see up above here is color coordinated to the structures of the disaccharides that you see down below. So the yellow corresponds with the yellow, green corresponds with the green, the white corresponds with the white, and of course the blue here corresponds with the blue. So let's go on and get started here with our first row, which is actually going to be the disaccharide maltose. And so notice that maltose is a disaccharide that, And of course, the And, of course, the glycosidic linkage that's used to link together these 2 d glucose residues, as identified here in this image, is an alpha 14glycosidic linkage. Now as we'll see moving forward in our course, maltose is a disaccharide that we're going to find can be found in the polysaccharide starch. And of course, starches are found in many plant based foods including potatoes and so of course, maltose and starches are going to be digestible. So we can go ahead and give this box a green check mark since maltose is digestible. Now the next row that we have here is the disaccharide Cellobios. And so what you'll notice about Cellobios is that it also consists of the same exact sugar residues as maltose. It consists of 2 d glucose residues. And so if you take a look down below at our image of Cellobios, again, notice that it consists of 2 d glucose residues just like maltose. And so the real difference between maltose and cellobiose is actually the glycosidic linkage. And so instead of having an Alpha 1 4 glycosidic linkage, Cellobios is actually going to have a Beta 14glycosidic Now, Cellobios, as you can see from its name, is actually gonna be a disaccharide that's found in the polysaccharide cellulose. So you can see it's got these same prefixes here. And cellulose again is a polysaccharide that we'll talk more about later in our course, but important to start associating cellobiose and cellulose. Now interestingly enough, even though maltose and cellobiose only differ literally in the configuration of the anomeric carbon in the glycosidic linkage. Maltose is digestible, but cellobiose is actually not digestible by most mammals. And so, of course, what this means is that the configuration of the glycosidic bond can actually make a big, big difference because it can determine whether or not the disaccharide will be digestible or not. And so, that is the main takeaway here with this digestibility that configurations actually matter. Now moving on to our 3rd row here, what we have is the disaccharide lactose. And so notice that lactose will actually swap out the D glucose residue with another residue, D galactose. And so if you take a look down below at the lactose structure, notice that it swaps its first residue here, with a galactose sugar. However, the second sugar is still a D glucose sugar just like all of the other ones that we've covered so far. And also notice that the glycosidic linkage that's used to link together the 2 sugars in lactose is a Beta 14glycosidic linkage. Just like Cello Bios had a Beta 14glycosidic linkage. And so what's interesting to point out here is that, the Beta 14glycosidic linkages can be shown in different ways. So notice that Cellobios has a beta 14glycosidic linkage and it seems to be having 1 sugar above, the other sugar. Whereas lactose has a Beta 14glycosidic linkage as well, but the 2 sugars seem to be perfectly aligned. And that's because of the curvature here that you see with the, beta 14glycosidic linkage. So, it's possible to show Beta 14glycosidic linkages with this curvature or you could just show it, straight up and down like this. So, you will see it both ways in your textbooks and both ways, in your practice problems. Now one thing to note about, lactose here is that lactose is a disaccharide that's commonly found in milk. And so, of course, we know that for most mammals, milk is definitely going to be digestible. So we can go ahead and give it a green check mark. Now, of course, there are some people that are lactose intolerant and so they can't really digest lactose or milk very well but that's a special case that we'll talk about later in our course. For the most part, lactose and milk are digestible by most mammals. So now, moving on to our 4th and final disaccharide, we have the disaccharide, sucrose. Now notice that sucrose keeps the D glucose residue for its first sugar, But instead of swapping out the first sugar again, it's gonna swap out the second sugar. And so it uses a d fructose sugar as its second sugar. So if you take a look down below at sucrose's structure over here, notice the first residue is still D glucose just like the one here and the ones over here. But notice that it's actually swapping out the second sugar with a defructose molecule. Now, the glycosidic linkage and sucrose structure is actually the most complicated because it includes 2 anomeric carbons in its glycosidic linkage. And so the first anomeric carbon has an alpha one glycosidic linkage. But the second anomeric carbon, notice is, it's going down on the same side of its highest numbered carbon. So it's gonna have a beta 14glycosidic. Oh, I'm sorry. It's gonna be a beta 12 glycosidic linkage because of the second carbon here. And so, really what you'll see is that sucrose is a disaccharide that's found in a lot of processed foods. So you'll find it in lots of your cereals and milkshakes and things like that. And so, of course, that means that sucrose is going to be digestible by most mammals. And so this is quite a lot of information to remember about these 4 disaccharides, but if you look for very specific patterns, then there's not that much that, you really need to memorize. And so notice that most of the sugars that we see on this list are actually d glucose residues. And so the only ones that aren't d glucose residues are the d galactose and lactose and the d fructose and sucrose. And then also notice that for the glycosidic linkages, that all of them are 14 glycosidic linkages except for sucrose is a 12 glycosidic linkage. And then, of course, you'll notice that, maltose is alpha, whereas cellobiose and lactose are beta, and then sucrose is a combination of both, alpha and beta. And then the only sugar, the only disaccharide that is not digestible on this list is of course, Cellobios. And so just spending a little bit of time looking at this table and trying to commit these structures, and components of the structures to memory, you guys will be able to get this down in no time. And so moving forward, we'll be able to get some practice applying these concepts that we've learned. So I'll see you guys in our next video.
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Problem
Problem
Which of the following contains galactose as one of the sugar subunits?
A
Glucose.
B
Ribose.
C
Maltose.
D
Lactose.
E
Cellobiose.
3
Problem
Problem
Which disaccharide forms a 1,1-glycosidic linkage?
A
Lactose.
B
Trehalose.
C
Maltose.
D
Sucrose.
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Problem
Problem
What is the identity of the disaccharide below?
A
Cellulose.
B
Lactose.
C
Chitin.
D
Maltose.
E
Sucrose.
F
Cellobiose.
G
Glycogen.
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Problem
Problem
The structure of a disaccharide is shown below. Which statement applies?
A
Both sugar rings A and B are in equilibrium with their linear chain forms.
B
Only sugar ring A is in equilibrium with its linear chain form.
C
Only sugar ring B is in equilibrium with its linear chain form.
D
Neither sugar ring is in equilibrium with their linear chain form.
E
None of the above statements are correct.
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Problem
Problem
Name each monosaccharide unit & the glycosidic linkage in the following disaccharide (gentiobiulose).
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7
Problem
Problem
Given the following information, draw a Haworth projection for the disaccharide gentibiose:
1. Gentibiose is a dimer of glucopyranoses.
2. The glycosidic linkage is β(1 → 6).
3. The anomeric carbon not involved in the glycosidic linkage is in the α configuration.