Four Categories of Biological Macromolecules

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A macromolecule is a large molecule made up of smaller units called monomers. These monomers can be combined in thousands of different ways to create a multitude of macromolecules. There are four basic types of macromolecules: proteins, nucleic acids, carbohydrates and lipids. The first three form polymers composed of monomers that are connected by covalent bonds. These bonds are formed through a dehydration reaction, the loss of a water molecule. Lipids are somewhat different. The feature that unites them is their hydrophobic tendencies. They are not water soluble. Lipids have a high hydrocarbon content and tend to form non-polar covalent bonds.

Lipids are composed of carbon, hydrogen and oxygen. Storage lipids include fats, oils and waxes. Chemically, they are monocarboxylic acids with long hydrocarbon side chains attached to them. The composition of the hydrocarbon chain and its length are important and differentiate one lipid from the next. If the hydrocarbon bonds are double bonds, then the lipid is an unsaturated fatty acid. Unsaturated fatty acids are oils and remain in a liquid state at room temperature. The kinks in the double bonds prevent the molecules from binding tightly enough to solidify. Saturated fatty acids do not have double bonds. When a product, such as peanut butter or margarine, has been hydrogenated, it means that hydrogen has been added to unsaturated fats, turning them into saturated fats. This prevents the oils in peanut butter and similar products from separating from the rest of the product.

The main function of lipids is energy storage.  They are much better at storing energy than carbohydrates. One gram of lipid contains twice as much energy as a gram of a starch. Phospholipids contain two fatty acids, a glycerol and a phosphate group. They are a major component of cell membranes. Lipids with a carbon skeleton composed of four fused rings are called steroids. Different functional groups attached to the rings create the diversity of steroids. Cholesterol is a steroid. It`s also an essential part of all animal cell membranes and a precursor to all the other steroids.

Carbohydrates are made up of carbon, hydrogen and oxygen in the basic formula of CnH2nOn. There are three basic types of carbohydrates. Monosaccharides are simple sugars such as glucose, galactose and fructose. Disaccharides are formed from two sugars and include lactose (a galactose and a glucose combined) and sucrose (a glucose combined with a fructose molecule). When there are more than two sugar molecules in the polymer, it is called a polysaccharide. Examples of polysaccharides include glycogen, starch, dextrin, cellulose and pectin. Animal glycogen is considered an important aspect of energy storage in the cell. Cellulose is involved in structural support in plant cells. In general, sugars provide a cellular energy source.

Proteins involve carbon, nitrogen, hydrogen and oxygen atoms. They are composed of amino acids. There are twenty different amino acids which are, in turn, made up  of of an amino group, a carboxyl group, a central carbon and hydrogen, and an R group. Variations in the R group are what differentiate one amino acid from another. More than 50 percent of the dry mass of a cell is proteins. They are involved in almost every aspect of the cell. Proteins are involved in the synthesis of enzymes and the transportation of molecules within the cell and through the cell membrane. They catalyze chemical reactions, defend the cell and maintain the cell structure.

Nucleic acids involve carbon, hydrogen, nitrogen and phosphates. They are a composite of nucleotides, each of which has a nitrogenous base, a phosphate group and a sugar. Nucleic acids form two very important products: deoxyribonucleic acid (DNA) and ribonucleic acid (RNA). There are five nucleotides: adenine, guanine, cytosine, thymine (used in DNA only) and uracil (used in RNA only). DNA is a long, linear, double-stranded molecule while RNA is a single-stranded, shorter molecule. The DNA contains the genetic coding for every aspect of the cell. It contains not only the instructions for its own replication but also the code to synthesize RNA. In turn, RNA contains the codes to produce proteins.

The four main types of macromolecules are nucleic acids, proteins, carbohydrates and lipids. They are complicated combinations of smaller molecules, and their importance to every aspect of cell function, and therefore every aspect of an organism, cannot be overestimated. All of them are absolutely necessary to a healthy, functioning body. Although you often think of proteins, carbohydrates, and lipids in terms of what you ingest and try to limit them in accordance with dietary fads, it is very important to realize that they all have a role to play in the body.

More about this author: Heather Brennan

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