Carbohydrate, a Primer

Some people are surprised to learn that the pasta and cookies they eat are carbohydrate, and basically sugar. Others talk about slow and fast sugars. Here slow sugars are the stuff like bread and waffles, and fast sugars are found in things like soda and glucose tablets. It’s especially interesting when talking to someone about diabetics and their diet. Diabetics are allowed, by their doctor, to eat pasta or fries with the associated insulin shot. Diabetics have a problem with carbohydrate metabolism. They can’t metabolize sugar properly because they have a problem with insulin production. It’s strange to me that one may eat something that causes such problems. But that is something to go into deeper when we talk about carbohydrate metabolism. Here’s a short overview of what carbohydrate is.

A carbohydrate is made up of carbon (hence, carbo-), hydrogen and oxygen (hence, -hydrate). The simplest ones are called monosaccharides, also known as a simple sugar. The word saccharide comes from the Greek meaning “sugar”. They have the general formula (CH2O)n where n is 3 or more. They consist of a carbon chain and a number of hydroxyl (OH) groups and, either an aldehyde group or a ketone group. The sugar with an aldehyde group is called an aldose. The sugar with a ketone group is called a ketose. For example: glyceraldehyde is an aldose with the formula C3H6O3, or (CH2O)3.

The aldehyde and the ketone group can react with a hydroxyl group to form a bond. This allows these simple sugars to become complex sugars. If two monosaccharides combine they form a disaccharide, also known as a double sugar. The names of monosaccharides and disaccharides end very often in the suffix -ose. For example: the monosaccharides glucose and fructose, and the disaccharides sucrose and lactose. Note here that sucrose is a bond between glucose and fructose. This will be come back when we talk about carbohydrate metabolism.

Polysaccharides are chains of many sugars joined together. These chains can be linear or branched. Glycogen is a large, chained polysaccharide that is found in animals as a way to store excess glucose. The numerous branches allow for greater accessibility when the molecule needs to be broken down. This allows for rapid use in time of need. Note these long chains and their breakdown for when we talk about carbohydrate metabolism. Most plants store excess glucose in the polysaccharide called starch. In plants it exists as insoluble starch granules in chloroplasts. These granules contain a mix of two polysaccharides, amylose and amylopectin. Polysaccharides can also be used for structure, as is the case for cellulose, which makes up the cell walls of plants. Cellulose is an unbranched polysaccharide of glucose units. Long chains are formed that form fibrils. Mammals, including humans, lack enzymes to digest cellulose linkages and cannot digest plant cell walls. However, some bacteria can.

The last group is oligosaccharides. These are short chains of monosaccharides. They can be linked to proteins (glycoproteins) or lipids (glycolipids).

Class

Component

Found in

Monosaccharides

Glucose

Many fruits

Fructose

Disaccharides

Sucrose

Table sugar

Lactose

Milk

Polysaccharides

Glycogen

In humans it is stored primarily in the cells of the liver and skeletal muscle

Starch (amylose, amylopectin)

Cereals, potatoes, foods based on cereals (bread, pasta)

Cellulose

Plant cell walls, main part of insoluble dietary fiber

Further Reading

Carbohydrate. Encyclopedia Britannica. https://www.britannica.com/science/carbohydrate