Biological macromolecules are large, organic molecules constructed from smaller repeating units. These molecules are fundamental to life, comprising the majority of a cell’s dry mass and performing a vast array of tasks necessary for an organism’s survival, growth, and reproduction. All living things build and use these large compounds, which are sorted into four main classes.
The Building Blocks of Life
Most biological macromolecules are polymers, which are long chains assembled from repeating building blocks called monomers. This concept is similar to a train, where each car is a monomer, and the entire linked train is the polymer. This structure allows for immense diversity from a limited set of monomers; for instance, just 20 common monomers are used to build the vast array of proteins.
The assembly of polymers from monomers occurs through dehydration synthesis. In this reaction, two monomers join when one loses a hydrogen atom (H) and the other loses a hydroxyl group (OH). These combine to release a molecule of water, and a strong covalent bond forms between the monomers, linking them together.
Conversely, when organisms need to break down macromolecules, they use a reaction called hydrolysis. Hydrolysis is the reverse of dehydration synthesis; a water molecule is inserted across the bond linking two monomers, causing it to break. One monomer gains a hydrogen atom while the other gains a hydroxyl group, splitting the polymer into its smaller subunits.
Carbohydrates
Carbohydrates are composed of carbon, hydrogen, and oxygen, typically in a 1:2:1 ratio. Their primary purpose is to serve as a ready source of energy for cellular activities. The fundamental building blocks of carbohydrates are simple sugars called monosaccharides, with glucose being a primary example.
These macromolecules are categorized into three main subtypes: monosaccharides, disaccharides, and polysaccharides. Monosaccharides like glucose and fructose are single sugar units. Disaccharides, such as sucrose and lactose, consist of two sugar units linked together. Polysaccharides are formed when many monosaccharides are joined, serving functions related to energy storage and structural support.
In plants, excess glucose is stored as starch. Animals store excess glucose in the form of glycogen, which is found in the liver and muscles. Beyond energy, some polysaccharides have structural roles. Cellulose is a major component of plant cell walls, providing rigidity, while chitin forms the hard exoskeleton of insects and other arthropods.
Lipids
Lipids are a diverse group of molecules grouped together because of their hydrophobic, or “water-fearing,” nature. Unlike the other macromolecules, lipids are not true polymers. Instead, they are defined by their insolubility in water, a property that stems from their structure, which consists mainly of hydrocarbon chains. Their building blocks are often fatty acids and a glycerol molecule.
This group includes fats and oils, which are a form of long-term energy storage. A fat molecule, or triglyceride, is composed of a glycerol backbone attached to three fatty acid chains. Another type, phospholipids, is a main component of all cell membranes. Phospholipids are structurally similar to fats but have a phosphate group and only two fatty acid tails, giving them a hydrophilic (water-loving) head and hydrophobic tails.
This quality allows phospholipids to form the bilayer structure of a cell membrane, creating a barrier between the cell’s internal and external environments. Steroids are another class of lipids, characterized by a structure of four fused hydrocarbon rings. Many steroids, such as cholesterol and hormones like testosterone, act as signaling molecules that regulate physiological processes.
Proteins
Proteins are the most functionally diverse of the biological macromolecules, acting as the “workhorses” of the cell. They are polymers constructed from monomers called amino acids. The specific sequence of the 20 common amino acids, known as the primary structure, is determined by an organism’s genetic code and dictates the protein’s final shape and function.
The initial chain of amino acids folds into localized shapes, such as the alpha-helix or beta-pleated sheet, forming the protein’s secondary structure. The overall three-dimensional shape that a single polypeptide chain assumes is its tertiary structure, determined by interactions between the amino acid side chains. Some proteins consist of multiple polypeptide chains, and their arrangement constitutes the quaternary structure.
The functions of proteins are extensive.
- Enzymes catalyze biochemical reactions, such as those in digestion and metabolism.
- Structural proteins like keratin in hair and collagen in connective tissues provide support.
- Transport proteins, like hemoglobin, carry substances such as oxygen through the bloodstream.
- Other proteins are involved in movement, immune defense, and cell communication.
Nucleic Acids
Nucleic acids are macromolecules designed to store, transmit, and express hereditary information. The monomers that make up nucleic acids are called nucleotides. Each nucleotide consists of three components: a five-carbon sugar, a phosphate group, and a nitrogen-containing base. The two main types are deoxyribonucleic acid (DNA) and ribonucleic acid (RNA).
DNA is the molecule that carries the genetic blueprint for an organism. It exists as a double helix, with two polynucleotide chains connected by hydrogen bonds between their nitrogenous bases. The sequence of these bases—adenine (A), guanine (G), cytosine (C), and thymine (T)—encodes the genetic instructions. In eukaryotic cells, DNA is housed primarily within the nucleus.
RNA is typically a single-stranded molecule with a direct role in protein synthesis. Its primary function is to act as a messenger, carrying genetic instructions from the DNA to the cell’s protein-building machinery, the ribosomes. There are several types of RNA, including messenger RNA (mRNA), transfer RNA (tRNA), and ribosomal RNA (rRNA), each playing a part in translating the genetic code. In RNA, the base uracil (U) is used in place of thymine.