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Organic Chemistry - Carbohydrates

Carbohydrates are composed only of carbon, hydrogen, and oxygen. And, when we talk about carbohydrates, these three elements are always present in this ratio, Cn,H2n,On. So, we can always tell when we are dealing with carbohydrate just by noting that the molecule is made only of carbon, hydrogen and oxygen. And, the number of carbons is equal to the number of oxygens. And, the number of hydrogens is twice the number of carbons. 

The simplest carbohydrates in the world are monosaccharides. Some monosaccharides contain three carbons. Some contain five carbons and some contain six carbons. If a monosaccharide contains three carbons, it is called a triose. If it contains five carbons it is called a pentose. If it contains six carbons it’s called a hexose. 

Glucose and fructose are two hexoses. Glucose has a C double bond O group on its terminal carbon which makes it an aldehyde. We call glucose an aldose or an aldohexose. Fructose has a C double bond O group on the carbon that is next to the terminal carbon which makes it a ketone. We call fructose a ketose or a ketohexose. Every hexose is either an aldohexose or a ketohexose. In fact, every monosaccharide is either a ketose or an aldose; whether it is a triose, a pentose or a hexose. 

Hexoses assume the conformation of both a straight chain molecule and a ring. So, a glucose can turn itself from a straight chain structure to a ring structure. The ring structure of glucose is called glucopyranose. The glucopyranose ring contains five carbons and one oxygen. The sixth carbon is bonded to the ring. Hexoses are chiral molecules because four of their carbons are asymmetric. Every hexose can exist as a D, dextro- or L, levo- isomer. Here is what that means. When you’re looking at a Fischer Projection of a hexose, look at carbon number five and see whether the OH is on the right or the left. If the OH is on the right then you’re looking at the D isomer. If the OH is on the left then you’re looking at the L isomer. 

Fructose can also form a ring structure and it is called fructofuranose. Since fructose is ketohexose, its ring conformation will contain four carbons and one oxygen. Two carbon atoms will be bonded to the ring. When two monosaccharides get together they form a disaccharide. And, when it comes to the hexoses you should know that a molecule of D-glucose and the molecule of D-fructose get together to form a disaccharide called sucrose. You should also know the following combinations of D-glucose. If two D-glucose molecules get together, they will make a molecule of maltose. Lots of D-glucose molecules linked together can make either cellulose or glycogen. The difference between cellulose and glycogen has to do with the way the glycosidic bond is formed between the glucose molecules. If D-glucose molecules are linked up by something called an alpha 1, 4-glycosidic bond then the result is glycogen. When we say alpha 1, 4-glycosidic bond, we only mean that the glycosidic bond links carbon number one of one glucose molecule to carbon number four of the next. The alpha means that the glycosidic bond arises below the carbon number one. If D-glucose molecules are linked together by a beta 1, 4-glycosidic bond, then the result is cellulose. The body stores its glucose in the form of glycogen which is made of many glucose molecules linked together by alpha 1, 4-glycosidic bonds. If these glycosidic linkages are broken, glucose is generated. There are two steps in breaking these linkages. 

Step One: 
Orthophosphate comes and splits the linkage between one glucose molecule and the next leaving glucose, one phosphate and the rest of the glycogen molecule. 

Step Two: 
The phosphate is separated from the glucose by hydrolysis to yield a glucose molecule and a molecule of phosphate. 

When these two steps are complete, we are left with a molecule of glucose that has been split off from a long molecule of glycogen. The rest of the glycogen molecule keeps undergoing the same two steps over and over again. And, glucose molecules are removed one at a time by the breakage of glycosidic bonds.