What Are Macromolecules?

What are macromolecules? Here is an overview of the structure and function four main classes of macromolecules: carbohydrates, lipids, proteins, and nucleic acids.

There are four classes of macromolecules that constitute all living matter: carbohydrates, lipids, proteins, and nucleic acids. While they have different structures and functions, they are all composed of long complex chains of molecules (polymers) made up of simpler, smaller subunits (monomers). They are joined together in a process known as dehydration synthesis, in which a covalent bond is formed between two monomers by releasing a water molecule.


Carbohydrates are made up of sugars and their polymers. Simple sugars (monosaccharides) are hydrocarbon chains of varying length that possess a hydroxyl (OH) group on each carbon. The most common monosaccharide is glucose, a valuable sugar for all living things. Monosaccharides bond together, via glycosidic linkage, to form polysaccharides, the polymers of carbohydrates.

Carbohydrates are essential for both energy storage and structure. Starch is the chief energy source for plants and glycogen is the main energy source for animals. Cellulose in plants and chitin in invertebrate animals help to provide structure and support.


Lipids fall into three main categories: fats, steroids, and phospholipids. A fat is composed of a glycerol molecule (a short hydrocarbon) bonded to three fatty acids through dehydration synthesis. These fatty acids are long, nonpolar, hydrocarbon chains, responsible for fats' insolubility in water. If there are no double covalent bonds between the carbons of the fatty acids, the fat is said to be saturated, because the maximum number of hydrogens are bonded to the carbon skeleton. IF the fatty acid has any double bonds, it is said to be unsaturated. Saturated fats are solids at room temperature and are found mostly in animals, while unsaturated fats are liquids (oils) at room temperature and occur chiefly in plants. Phospholipids are composed of a glycerol molecule bonded to two fatty acids and a phosphate group. This phosphate group is polar although the rest of the molecule is hydrophobic. Steroids are made of four interconnecting carbon rings and cholesterol is the most common steroid.

Lipids are excellent sources of energy, insulation, and, in the case of the dual nature of the phospholipid, they are crucial elements of membranes.


Proteins have many levels of structure. Their primary level of structure is the sequence of amino acids linked together in a peptide chain. There are only 20 amino acids, each with a hydrogen, an amino group (NH2 -), a carboxyl group (COO -), and an R group. This R group is known as a side chain and is composed of varying molecules. It is the only distinction between the 20 amino acids. The secondary level of structure in proteins is the bending of this peptide chain into either an alpha helix (coil) or a beta sheet (plaited sheet) as a result of hydrogen bonding. The tertiary structure is based on the folding of the secondary structure caused by interactions between amino acid side chains. These include ionic and covalent bonds, disulphide bonds, and hydrophobic interactions. A protein's quaternary structure is based on the interaction between many peptide chains.

Proteins have many vital functions, including: structure, support, movement, energy transfer, and defense.


Nucleic acids include DNA (deoxyribonucleic acid) and RNA (ribonucleic acid). Each is composed of a nitrogenous base, a fvie-carbon sugar (a pentose), and a phosphate group. There are five nitrogenous bases: adnine, guanine, thymine, uracil, and cytosine. It is important to remember that cytosine will only bond with guanine and that adnine will only bond with thymine. Also, uracil is only found in RNA and thymine is only found in DNA. The pentose in RNA is ribose and deoxyribose in DNA. A phosphate group is linked to the sugar via a phosphodiester bond and the three nucleotides have become a nucleic acid.

DNA holds the generic information necessary for protein synthesis and RNA carries this information to the actual site of protein production.

Together, these macromolecules are responsible for all of life's many processes.

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