Carbohydrates serve as the primary energy source for humans, and are found almost everywhere in life. They are made up of carbon atoms (carbo- ), along with water (hydrates), so they are usually of the form Cn (H2O)n, like glucose (C6H12O6).
Simple sugars, or monosaccharides, (mono = single, sacchar = sugar), include glucose and fructose. The molecular formulae for glucose and fructose are the same, but they are arranged differently. They are examples of isomers, or molecules that have the same molecular formula but have different structures. In this case, the small difference between the structure of glucose and fructose makes fructose taste much sweeter.
A disaccharide, or double sugar, is constructed from two monosaccharides through dehydration synthesis. That's when two monosaccharides combine to create a disaccharide, while also releasing water. For example, two glucoses combine to create a maltose, while releasing H2O. In the same way, water can be added to maltose to yield two glucoses, called a hydrolysis reaction. Also, an important thing to note is that the bond between a carbohydrate and something else, possibly another carbohydrate is called a glycosidic bond.
Another common disaccharide is lactose, made from glucose and galactose, both monosaccharides. It's very common in milk, and people can't drink milk because they are lactose intolerant. What this means is that their bodies don't produce enough lactase, a molecule produced in the small intestine that breaks down lactose into glucose and galactose through a hydrolysis reaction.
The most common disaccharide is sucrose, or table sugar. This consists of a glucose linked to a fructose. This is the main carbohydrate in plant sap, and it's extracted from sugarcane or sugar beets.
Another type of a carbohydrate is a polysaccharide, or a complex carbohydrate. A polysaccharide is a chain of monosaccharides. One example is starch, found in roots. They consist of glucose monomers strung together. Plant cells store starch, where it's available to break down as needed to provide energy for building other molecules. Humans and other animals use plant starch as food by hydrolyzing the bonds between monomers within their digestive systems.
Animals store excess sugar in the form of a polysaccharide called glycogen. Glycogen is similar in structure to starch, but it's more extensively branched. Most of it is stored in our liver and muscles. This is why people "carbo load", when athletes eat a lot of starchy foods the night before an athletic event.
The most abundant organic compound on Earth is called cellulose. It forms cable=like fibrils in the tough walls that enclose plant cells and is a major part of wood. Its glucose monomers are linked differently than starch, so people can't digest them. They pass unchanged through our digestive tract, and they're known as dietary fibers.
Simple sugars and double sugars (like table sugar) dissolve easily in water, forming sugary solutions. In contrast, cellulose and starch are so large that they don't dissolve in water. In spite of this, almost all carbohydrates are hydrophilic, meaning they adhere water to their surface.
Types of Glucose
This goes against the organization of this post, but this was important to put in. There are two types of glucose, alpha-glucose and beta-glucose.
It is important to remember that if they hydrogen is on top, it's alpha-glucose. If the hydroxyl group is on top, it's beta-glucose. In addition, it's important to note the small numbers near the vertices of the diagram. Those are the numbers of the six carbons. So the "1" means the circled group has a C, OH, and H. The numbering goes clockwise, and the last vertex only has an oxygen.