Now we've moved onto carbohydrates! (Actually, we covered nucleic acids after proteins, but there was more new material in the carbohydrates lectures so I'm going to talk about them first.)
Learn the terminology associated with carbohydrates.
Be able to classify the different
types of carbohydrates.
Wow, this dot point is broad. A carbohydrate is basically a sugar or a chain of sugar molecules, but the formal definition is something like "a polyhydroxyaldehyde or polyhydroxyketone, or a substance that gives these compounds on hydrolysis." Basically what this means is that a carbohydrate is either an aldehyde or a ketone with plenty of hydroxyl groups, or it's made up of several compounds like these.
A single polyhydroxyaldehyde or polyhydroxyketone is known more simply as a monosaccharide. Two monosaccharides can join together to make a disaccharide. Short chains of monosaccharides (generally around 2-10 monosaccharides) are known as oligosaccharides, whereas longer chains are known as polysaccharides.
Learn the general formula.
Simple. The general formula of a monosaccharide is CnH2nOn, where n is between 3 and 8.
Be able to give the number of
carbons for the category name.
Carbohydrates can also be categorised according to the number of carbons that they have. The number of carbons is given by the prefix tri-, tetra- etc., which is then attached to the suffix -ose to give triose (3-carbon sugar), tetrose (4-carbon sugar), pentose (5-carbon sugar) and so on.
Be able to identify common functional groups associated with carbohydrates.
I think this dot point here is referring to talking about aldehyde and ketone groups in sugars. When sugars are in their linear form, they are either aldehydes, with a C=O bond at the end of the molecule, or ketones, with C=O bonds somewhere in the middle of the molecule. Sugars that are aldehydes are known as aldoses, whereas sugars that are ketones are known as ketoses. You can combine the aldo- and keto- prefixes with the numbering prefixes to give ketohexose, aldetriose etc. However, normally the keto- or aldo- parts are omitted.
There are several other functional groups that may be attached to carbohydrates. One of these is the acetyl group, which is a C double bonded to an O and single bonded to a methyl group. There are also amino (-NH3+/-NH2) groups and amido groups (like acetyl groups but with an amine group instead of a methyl group).
Be able to describe chirality.
The definition of a chiral molecule is a molecule that cannot be superimposed on its mirror image. For example, your right hand is roughly a mirror image of your left hand, but you can't rotate it so that it looks the same as your left hand. Chirality generally occurs when a carbon atom is bonded to four different groups.
Be able explain the difference between D & L carbohydrates.
Know that the D form is the biological form.
D and L carbohydrates are named according to the orientation of the asymmetric carbon furthest from the carbonyl carbon in a Fischer projection (i.e. when the molecule is drawn in its linear form). In a D sugar, the hydroxyl group is on the right, whereas in an L sugar, the hydroxyl group is on the left. Most if not all sugars exist in the D form.
Understand the terminology associated with chirality.
Be able to distinguish between stereoisomers when presented with a figure.
A stereogenic centre is the centre around which chirality can take place. As I mentioned earlier, chirality generally occurs when a carbon atom is bonded to four different groups. In this case, the carbon atom is a stereogenic centre.
Enantiomers are pairs of stereoisomers that are mirror images of each other. Diastereomers, on the other hand, differ between one or more chiral centres in their molecules, but they are not complete mirror images of each other.
A meso compound is a molecule that has one or more chiral centres, but the molecule as a whole is symmetrical and therefore achiral.
Be able to number the carbons in a carbohydrate.
The carbons in a carbohydrate are simply numbered 1, 2, 3 etc., starting from the carbonyl carbon of an aldose, or starting from the end carbon nearest to the carbonyl carbon of a ketose.
Be able to describe an anomer.
Anomers are isomers that differ only in their orientation around the carbonyl carbon (a.k.a. the anomeric carbon) when the carbohydrate is in its ring form (I think).
Recognise the difference between the α and β anomers.
When the carbohydrate is in its ring form, there will usually be a terminal -CH2OH group. If the -OH on the carbonyl carbon is facing the side of the ring opposite to the -CH2OH group, it's in the alpha form, whereas if the -OH group is on the same side of the ring, it's in the beta form.
Know the difference between a Fischer and Haworth projection.
I've already alluded to this before, but a Fischer projection is basically a way of drawing the carbohydrate when it's in its linear form, whereas the Haworth projection is a way of drawing the carbohydrate when it's in its ring form.
Be able to recognise the difference between a pyranose and a furanose.
Simply put, a pyranose is a 6-membered ring whereas a furanose is a 5-membered ring. Generally one "member" of these rings is an O atom while all the other members are C atoms. Hexoses tend to be pyranoses and pentoses tend to be furanoses.
Be able to identify the different structural conformations of a pyranose sugar.
Pyranoses are not completely flat. If one carbon is above the plane of the ring, and another carbon on the opposite side is below the plane of the ring, the sugar is said to be in the "chair" conformation (if you search up some diagrams you'll see why). If one carbon is above the plane of the ring, and another carbon on the opposite side is also above the plane of the ring, the sugar is said to be in the "boat" conformation.
Be familiar with the structure of glucose, an important monosaccharide.
Glucose is a hexose- that is, it is a sugar with six carbons. It forms pyranose rings. Generally, in a Haworth projection, the hydroxyl groups on the 2nd, 3rd and 4th carbons will be in a "down up down" configuration, respectively.
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