Monosaccharides: Study with Video Lessons, Practice Problems & Examples

Monosaccharides, the simplest sugars, can be classified as aldoses or ketoses based on their carbonyl groups. Aldoses contain an aldehyde group (–CHO), while ketoses have a ketone group (–C=O). Additionally, monosaccharides are categorized by the number of carbon atoms: trioses (3), tetroses (4), pentoses (5), hexoses (6), and heptoses (7). When numbering carbon atoms in linear monosaccharides, the carbonyl carbon must receive the lowest possible number, ensuring clarity in molecular structure and function.

Monosaccharides

In this video, we're going to begin talking in more detail about monosaccharides. Recall from our previous lesson videos, we already mentioned that monosaccharides are individual carbohydrate monomers, and that makes them the simplest form of sugar. Now, sugars have names that generally end with the suffix -ose. So, really, anything that ends with -ose, the likelihood is that it's going to be a sugar. For example, glucose ends with the suffix -ose, and so glucose is a sugar. Now, monosaccharides can actually be grouped in several different ways. Moving forward through our lesson, we're going to talk about all of the different ways that monosaccharides can be grouped. I'll see you guys in our next lesson video when we talk about how monosaccharides can be grouped either as aldoses or ketoses. I'll see you guys there.

Monosaccharides

So monosaccharides can be classified into 2 groups based on the nature of their carbonyl group, and those two groups are either an aldose or a ketose. Now notice that both aldose and ketose end with the suffix -ose, so we know that these are sugars. And, again, these two categories of sugars are going to be classifying the monosaccharides based on their carbonyl groups. And so an aldose is going to be a sugar whose carbonyl group is an aldehyde. And recall that aldehydes can be abbreviated with CHO. And then, of course, a ketose is going to be a sugar whose carbonyl group is a ketone.

And so if we take a look down below at our image over here on the left-hand side, we're showing you the difference between an aldehyde and a ketone group. And so recall that aldehydes have the H here in them, and so that reminds us that they're going to be carbonyl groups that are attached to a hydrogen. And so they're going to be found at the end of a molecule. And then, of course, ketones don't have that H in them, and so they're going to be attached to another R group. So they're going to be found in the middle of a molecule.

And so, if we take a look at our images over here of our monosaccharides, we can see this one over here on the left is Glyceraldehyde and notice that even in its name it tells us that it has an aldehyde group. And so, what we can say is that glyceraldehyde is an aldose. And then if we take a look at this sugar over here, notice it is Dihydroxyacetone and this one has its carbonyl group right in the middle. So it has 2 R groups going either way. And so that makes this sugar right here a ketose. And so we can classify all of our monosaccharides either as aldoses or as ketoses based on again the nature of the carbonyl group. So that concludes this introduction to aldoses and ketoses and we'll be able to get practice as we move along through our course.

Monosaccharides

Alright. So now that we already know from our last lesson video that monosaccharides can be grouped based on the nature of their carbonyl groups, either as aldoses or ketoses. In this video, we're going to talk about how monosaccharides can also be grouped based on the number of their carbon atoms. And so, monosaccharides that have 3, 4, 5, and 6 carbons are respectively called trioses, tetroses, pentoses, and hexoses. And so these prefixes here are revealing the number of carbon atoms that these monosaccharides have. Now, these highlighted prefixes can also be preceded by the prefixes "aldo" or "keto" to respectively indicate an aldose or a ketose. And so if we take a look down below at our image, we can classify the following monosaccharides based on both the number of the carbon atoms that they have and based on the nature of their carbonyl groups. And so if we look at this first sugar over here, we can see that it has a total of 1, 2, 3 carbon atoms and this is going to make it a triose, "tri" meaning 3. This sugar right here has 4 carbons. So, this is going to be a tetrose. This one here has 5. So, it's going to be a pentose. This one has 6. It's going to be a hexose and this one over here has 7 carbon atoms making this a heptose. And so really these are the prefixes that I would suggest committing to memory to indicate the number of carbon atoms that the monosaccharides have. Now, as we mentioned up above, we can also attach the prefixes "aldo" to indicate that all of these sugars that are bracketed here are going to be aldoses since they have the CH₂O aldehyde group. And then of course, this sugar over here is going to be a ketose because notice that it's going to have, a carbonyl group that is not at the end. Instead, it's going to be in the middle here and it has 2 R groups branching off. And so, technically, we can call all of these sugars aldotrioses, aldotetroses, aldopentoses, or aldohexoses. And then, of course, this sugar over here is going to be a ketoheptose. And so, now that we can understand how to group monosaccharides based on both the number of the carbon atoms and the number of their carbonyl groups, we can get some practice applying these concepts in our next few videos. So, I'll see you guys there.

Select the appropriate label for each of the three following monosaccharides:

Alright. So this practice problem wants us to select the appropriate label for each of the 3 following monosaccharides: arabinose, threose, and erythralose. And so we're going to start with arabinose over here on the far left. First, we're going to classify arabinose based on the nature of its carbonyl group, which recall a carbonyl group is just a carbon double bonded to an oxygen. However, when I take a look at this arabinose molecule here, I don't see any carbon atoms that are double bonded to anything. And so that's when we need to realize that this CH2O group that we have here is a condensed form of an aldehyde group, a condensed form of a carbonyl group. And so this is an aldehyde group because it's at the very end of the molecule, which means that we can eliminate any answer options that suggest this might be a keto group. So that means that we can eliminate option B and we can eliminate option D. Next, we need to count up the number of carbon atoms that there are. So we have 1, 2, 3, 4, 5 carbon atoms, which suggest that this is going to be a pentose. And so, we can eliminate option C here, which suggests that it is a tetrose. It would be a tetrose if it had 4 carbon atoms. And so the correct answer here is actually going to be option A. This is an aldopentose. An aldo because it has an aldehyde group and a pentose because it has 5 carbons. So we can indicate that A is the correct answer for part A. So now moving on to part B, threose, what we can see is that there is another aldehyde group here at the end, and so it's going to be an aldose, not a ketose. So we can eliminate any answer options that suggest it's a ketose. This time, it's going to be a tetrose because it has 4 carbon atoms. We can eliminate the pentose option since that would be if it had 5 carbon atoms. So, we can indicate that the correct answer for threose is that it is an Aldotetrose. So then moving on to our final option here, erythralose, what we can see is that its carbonyl group is right here in the middle. And this is a ketone group because it has an R group, 2 R groups branching off. And so because this is a ketone, it's going to be a ketose and we can go ahead and eliminate any answer option that suggests that it is an aldose such as option A and option C. And then counting the number of carbons, we can see there are 1, 2, 3, 4 carbon atoms making this a tetrose. And so, we can go ahead and indicate that option D, ketotetrose, is correct. Ketopentose would be again if it had 5 carbon atoms, but that's not the case here. So then we can say that erythralose is indeed a keto tetra. And so that concludes this practice problem and I'll see you guys in our next video.

Monosaccharides

Alright. So now that we've already learned how to group monosaccharides based on the nature of their carbonyl groups, and based on the number of carbon atoms that they have, in this video, we're going to move on to how to assign numbers to all of those carbon atoms of linear monosaccharides. And so later in our course, we'll focus on cyclic monosaccharides. But in this video, we're focused on linear monosaccharides. And so, the carbon atoms in linear monosaccharides are numbered based on the positioning of the carbonyl group. Or in other words, they're numbered based on the positioning of the aldehyde or ketone group. And so the key is that the carbonyl carbon atom, that's part of either the aldehyde or ketone should always be assigned the lowest possible number. And that way, we can designate the numbered carbon atoms as _C1 for the first carbon, _C2 for the second one, _C3 for the third carbon, and so on. And so in our example, we're going to appropriately number the carbon atoms in each of the following 3 linear monosaccharides, starting with glucose over here on the far left. And again, the key to assigning numbers to the carbon atoms of linear monosaccharides is right here in this second line. And that is that we want the aldehyde or ketone to be assigned the lowest possible number. And so, when we take a look at glucose's structure, we can see that it has an aldehyde group. And that is the CH₂O and it's up at the top. So, we want this carbon atom right here to be assigned the lowest possible number when we're trying to assign numbers to all of the carbon atoms. And so, of course, that means that this carbon is gonna be number 1 and all of the other carbon atoms that go down are sequentially going to be carbon ₂, carbon ₃, carbon ₄, carbon ₅, and carbon ₆. And so this is the appropriate numbering of the carbon atoms in the glucose molecule. So let's move on to mannose over here, and again, we can see that it has an aldehyde group here at the top just like glucose. And so we want the carbon atom of this aldehyde group to be the lowest possible number. So we're gonna give it a one again and then again, we're going to assign each of the atoms going down sequentially as carbons ₂, ₃, ₄, carbon ₅, and carbon ₆. So the numbering is very similar to the carbon numbering of glucose. So then, of course, we can move on to our last but not least monosaccharide over here, and we're just gonna number these carbon atoms from bottom to top just like we did for the previous 2. Right? No. Not right. So remember that the key to assigning numbers to the carbon atoms of linear monosaccharides is that we have to identify the aldehyde or ketone and then assign the lowest possible number. And so we can see that the fructose molecule on this page has its ketone group towards the bottom of the Fischer projection. And so if we were to number the carbon atoms from top to bottom, like what we did with both mannose and with glucose, then notice that the ketone carbon atom is assigned carbon number ₅. However, if we number the carbon atoms from the bottom towards the top, then notice that the ketone carbon atom would be assigned carbon number ₂. And of course, ₂ is lower than ₅ and ₂ would be the lowest possible number. And so that means that we're not going to be numbering the carbon atoms from top to bottom on this fructose molecule. Instead, we're gonna be numbering from the bottom to the top. And so the carbon numbering that you see here is the correct carbon numbering for this fructose molecule on this page. And so, again, the main takeaway here is that when we are assigning numbers to the carbon atoms of linear monosaccharides, we want to look for the aldehyde or ketone and make sure that the carbon atom is assigned the lowest possible number. And so we'll be able to get some practice applying the concepts that we've learned in this video in our next practice video. So I'll see you guys there.

Number the carbon atoms in the following linear form of fructose below:

Alright. So this practice problem wants us to number the carbon atoms in the following linear form of fructose below. And so what we need to recall from our previous lesson videos is that when it comes to assigning numbers to the carbon atoms of linear monosaccharides, it's all about finding that aldehyde or ketone group and giving it the lowest possible number. And so notice that fructose here has a carbonyl group that is right here. And so we have two options. We can either start numbering the carbon atoms from left to right, or we could number the carbon atoms starting on the right and number them from right to left. If we start from left to right, what we'll see is that this carbon ends up getting carbon number 5. However, if we number from right to left, this carbon atom gets carbon number 2. And so this carbon number 2 is the lowest possible number. And so the appropriate numbering here is going to be numbering from right to left. So, this would be carbon 1, this is carbon 2,

Here’s what students ask on this topic:

What are monosaccharides and how are they classified?

Monosaccharides are the simplest form of carbohydrates and are often referred to as simple sugars. They are the building blocks of more complex carbohydrates. Monosaccharides can be classified based on the nature of their carbonyl group into aldoses and ketoses. Aldoses contain an aldehyde group (–CHO), while ketoses have a ketone group (–C=O). Additionally, monosaccharides are categorized by the number of carbon atoms they contain: trioses (3 carbons), tetroses (4 carbons), pentoses (5 carbons), hexoses (6 carbons), and heptoses (7 carbons).

How do you number the carbon atoms in linear monosaccharides?

When numbering the carbon atoms in linear monosaccharides, the carbonyl carbon (either part of an aldehyde or ketone group) must be assigned the lowest possible number. This ensures clarity in the molecular structure. For example, in glucose, which has an aldehyde group at the top, the carbonyl carbon is assigned as C1, and the numbering continues sequentially down the chain. In fructose, which has a ketone group, the numbering starts from the end closest to the ketone group to give it the lowest possible number, typically C2.

What is the difference between aldoses and ketoses?

Aldoses and ketoses are two types of monosaccharides classified based on their carbonyl group. Aldoses contain an aldehyde group (–CHO) at the end of the carbon chain, making them terminal carbonyl compounds. Ketoses, on the other hand, have a ketone group (–C=O) located within the carbon chain, making them internal carbonyl compounds. This structural difference affects their chemical properties and reactivity.

How are monosaccharides named based on the number of carbon atoms?

Monosaccharides are named based on the number of carbon atoms they contain using specific prefixes: trioses (3 carbons), tetroses (4 carbons), pentoses (5 carbons), hexoses (6 carbons), and heptoses (7 carbons). These prefixes can be combined with 'aldo-' or 'keto-' to indicate the presence of an aldehyde or ketone group, respectively. For example, a monosaccharide with 5 carbons and an aldehyde group is called an aldopentose, while one with 6 carbons and a ketone group is called a ketohexose.

What are some examples of aldoses and ketoses?

Examples of aldoses include glucose and glyceraldehyde. Glucose is a hexose with an aldehyde group, making it an aldohexose. Glyceraldehyde is a triose with an aldehyde group, making it an aldotriose. Examples of ketoses include fructose and dihydroxyacetone. Fructose is a hexose with a ketone group, making it a ketohexose. Dihydroxyacetone is a triose with a ketone group, making it a ketotriose.