The elements involved in life processes can, and do, form millions of different compounds. Thankfully, these millions of compounds fall into four major groups: carbohydrates, proteins, lipids, and nucleic acids. Though all of these groups are organized around carbon, each group has its own special structure and function.
Carbohydrates
Carbohydrates are compounds that have carbon, hydrogen, and oxygen atoms in a ratio of about 1:2:1. If you’re stuck on an SAT II Biology question about whether a compound is a carbohydrate, just count up the atoms and see if they fit this ratio. Carbohydrates are often sugars, which provide energy for cellular processes.
Like all of the biologically important classes of compounds, carbohydrates can be monomers, dimers, or polymers. The names of most carbohydrates end in “-ose”: glucose, fructose, sucrose, and maltose are some common examples.
Monosaccharides
Carbohydrate monomers are known as monosaccharides. This group includes glucose, C6H12O6, which is a key substance in biochemistry. Sugars that an animal eats are converted into glucose, which is then converted into energy to fuel the animal’s activities by respiration (see Cell Processes).
Glucose has a cousin called fructose with the same chemical formula. But these two compounds have different structures:
Glucose and fructose differ in one important way: glucose has a double-bonded oxygen on the top carbon, while fructose has its double-bonded carbon on the second carbon. This difference is most apparent when the two monosaccharides are in their ring forms. Glucose generally forms a hexagonal ring (six sided), while fructose forms a pentagonal ring (five sided). Whereas fructose is the sugar most often found in fruits, glucose is most often used as the major source of energy for cellular activities.
Disaccharides
Disaccharides are carbohydrate dimers. These dimers are formed from two monomers by dehydration synthesis. Any two monosaccharides can form a disaccharide. For example, maltose is formed by the dehydration synthesis of two glucose molecules. Sucrose, common table sugar, comes from the linkage of one molecule of glucose and one of fructose.
Polysaccharides
Polysaccharides can consist of as few as three and as many as several thousand monosaccharides. Depending on their structure and the monosaccharides they contain, polysaccharides can function as a means of storing excess energy or provide structural support.
When cells ingest more carbohydrates than they need for fuel, they link the sugars together to form polysaccharides. The structure of these polysaccharides is different in plants and animals: in plants, polysaccharides take the form of starch, whereas in animals, they are linked in a structure called glycogen.
Polysaccharides can also have structural roles in plants and animals. Cellulose, which forms the cell walls of plant cells, is a structural polysaccharide. In animals, the polysaccharide chitin forms the hard outer armor of insects, crabs, spiders, and other arthropods. Many fungi also use chitin as a structural carbohydrate.
