This chapter covers the four types of macromolecules in biology: carbohydrates, lipids, proteins and nucleic acids. Each contain carbon and hydrogen, so they are organic compounds.
Synthesis of Molecules
Atoms join to build molecules, and monomers join to form polymers. Monomer is a general term for any molecule joining to form a larger molecule. The figure below shows one monomer joined to another by dehydration synthesis. This a chemical reaction. In this reaction, a molecule of water is one of the products shown in red. Because water is removed during the reaction, it is a dehydration synthesis reaction. The opposite is also true, as shown in the second figure. Polymers break down into monomers in a process called hydrolysis. Water is a reactant in hydrolysis reactions.
In the dehydration synthesis reaction depicted above, two molecules of glucose are linked together to form the disaccharide maltose. In the process, a water molecule is formed.
In the hydrolysis reaction shown here, the disaccharide maltose is broken down to form two glucose monomers with the addition of a water molecule. Note that this reaction is the reverse of the synthesis reaction shown above it.
Carbohydrates
Carbohydrates are an essential part of our diet. Monosaccharides join to form disaccharides and polysaccharides. Carbohydrates consist of carbon, hydrogen, and oxygen in a 1:2:1 ratio. Glucose is the most common carbohydrate C6H12O6 – it clearly demonstrates the ratio. The figure below shows sucrose (a disaccharide) formed by a monomer of glucose and a monomer of fructose joined by dehydration synthesis. Large polysaccharides, like starch and cellulose, may have 100’s of monosaccharides in their structure. Cellulose is a vital plant structure and makes up plant cells walls. Humans do not have an enzyme to digest cellulose, so it is considered dietary fiber. Starch and other polysaccharides store energy. These can be utilized by our cells in cellular respiration reactions that are enzyme driven.
Sucrose is formed when a monomer of glucose and a monomer of fructose are joined in a dehydration reaction to form a glycosidic bond. In the process, a water molecule is lost.
Lipids
Lipids are hydrophobic, so they do not dissolve in water. The phospholipid forms cell membranes. One gram of fat stores more than twice the energy of a gram of carbohydrate. As a result, fatty foods are high in calories. A calorie is the amount of heat required to raise 1g of water by 1 degree Celsius, it is a measure of energy stored in food. Lipids mainly consist of long hydrocarbon chains. Lipids consist of carbon, hydrogen, and oxygen but not in the 1:2:1 carbohydrate ratio. Lipids are categorized as fats, oils, or waxes.
If there are only single bonds between neighboring carbons in the hydrocarbon chain, a fatty acid is said to be saturated. These are solid at room temperature and typically come from animals. When the hydrocarbon chain has a double bond, the fatty acid is said to be unsaturated, as it now has fewer hydrogens. These are liquid at room temperature, and typically come from plants or fish.
A phospholipid molecule has a phosphate head and two fatty acid tails. The phosphate head is hydrophilic while the hydrocarbon tails are hydrophobic. Phospholipids align tail to tail to form cell membranes.
A phospholipid is a molecule with two fatty acids and a modified phosphate group attached to a glycerol backbone. The phosphate may be modified by the addition of charged or polar chemical groups.
Proteins
Proteins are one of the most abundant organic molecules in living systems and have the most diverse range of functions of all macromolecules. Proteins may be structural, regulatory, contractile, or protective. Others serve in transport, storage, or function in membranes. There are even classes of protein toxins and thousands of enzymes. Every cell in a living system may contain thousands of proteins, each with a unique function. Their structures, like their functions, vary greatly. They are all polymers composed of amino acids, arranged in a linear sequence.
Enzymes speed up reactions. Enzymes can be used over and over again. Each enzyme is shaped specifically to fit perfectly with another molecule and will only speed up reactions with that molecule.
Amino acids are the monomer of proteins. There are 20 types of amino acids, and their order in a protein determines the protein’s shape and its function. All of the 20 amino acids have one part in common and an R group that is not the same. R group chemical structure is what distinguishes one amino acid from another.
The bonds holding one amino acid to another are called peptide bonds. Amino acids join together to form polypeptides through dehydration synthesis. The figure below shows a polypeptide processed and folded into the exact, 3D shape required to function. Structure determines function. As a result, changing a protein’s structure can limit its function. Denatured proteins are unfolded and cannot function.
The primary protein structure is the unique sequence of amino acids, forming α-helix and β-pleated sheet secondary structures. The overall three-dimensional structure is the tertiary structure. When two or more polypeptides combine to form the complete protein structure, the configuration is known as the quaternary structure of a protein.
The four levels of protein structure can be observed in these illustrations. (credit: modification of work by National Human Genome Research Institute)
Nucleic Acids
Nucleic acids are DNA (deoxyribonucleic acid) and RNA (ribonucleic acid). DNA is common to all life forms, even those without a nucleus. RNA has several sub-types that help deliver instructions for making proteins, deliver amino acids for making proteins, and activate ribosomes. DNA stores all of the information necessary to build proteins, including cell parts. RNA moves copies of those instructions out of the nucleus and functions to help construct proteins at the ribosome.
The monomer for nucleic acids is the nucleotide, containing a phosphate, a sugar (ribose in RNA, deoxyribose in DNA), and a nitrogenous base. The nucleotides join in a single strand for RNA and in a double helix for DNA.
There are two categories of bases in nucleic acids: the purines (adenine, guanine) and pyrimidines (thymine, cytosine, and uracil). In DNA, A binds to T and G binds to C. In RNA, there is no thymine. RNA contains uracil (U), which binds to adenine (A). You can explore more DNA topics here.
Three components comprise a nucleotide: a nitrogenous base, a pentose sugar, and one or more phosphate groups.
Carbohydrates, lipids, proteins, and nucleic acids are the molecules of life.
Monomers join by dehydration synthesis to form polymers, which can be broken down by hydrolysis.
Carbohydrates provide fuel to cells, lipids form membranes (compartments), proteins all sorts of cellular work, and nucleic acids hold the instructions for building proteins.
