In the physical world, large structures are built from simple, repeating units, like a brick wall made of individual bricks. Nature uses a similar strategy at the molecular level to construct the intricate molecules for life from smaller building blocks. These large molecules are known as macromolecules. When they consist of repeating subunits, they are also called polymers, and the individual units are monomers. This principle of joining monomers to create polymers underlies the structure of living organisms and many man–made materials.
The Assembly Process of Macromolecules
The construction of macromolecules from monomers is called polymerization. This assembly uses a chemical reaction known as dehydration synthesis. In this reaction, a hydrogen atom from one monomer joins with a hydroxyl group from another. This bonding releases a water molecule and forms a strong covalent bond between the two monomers.
This process repeats, adding more monomers to the growing chain to create long and complex polymers. The energy required to form these bonds is supplied by the cell’s metabolic processes. The reverse process, breaking down polymers back into their monomer units, is called hydrolysis.
Hydrolysis, meaning “to split with water,” involves a water molecule being inserted across the covalent bond that links two monomers. This action breaks the bond and separates the units. One monomer gains a hydrogen atom while the other gains a hydroxyl group, reversing the dehydration synthesis reaction. This breakdown process also releases energy.
The Four Major Classes of Biological Macromolecules
Life is built upon four main types of biological macromolecules: carbohydrates, lipids, proteins, and nucleic acids. Together, these molecules constitute the majority of a cell’s dry mass and perform most of its functions.
Carbohydrates are a primary source of energy and structural support for cells. Their monomers are simple sugars called monosaccharides, like glucose. When two link together, they form a disaccharide, while long chains create polysaccharides. Examples include starch for energy storage in plants, cellulose for plant cell walls, and glycogen for energy storage in animals.
Proteins are highly versatile macromolecules whose monomers are amino acids. They act as enzymes, provide structural support like collagen, enable movement, and transport substances. The sequence of amino acids determines a protein’s unique three-dimensional structure, which in turn dictates its specific function.
Nucleic acids carry a cell’s genetic blueprint. The two main types are deoxyribonucleic acid (DNA) and ribonucleic acid (RNA), and their monomers are called nucleotides. DNA contains the hereditary information for an organism’s development, survival, and reproduction. RNA is involved in synthesizing proteins based on the instructions encoded in DNA.
Lipids are a diverse group of hydrophobic molecules, meaning they do not mix with water. Unlike the other classes, lipids are not true polymers as they are not formed from repeating monomer chains. They are classified as macromolecules due to their large size and their functions. These functions include forming cell membranes, storing energy long-term, and acting as signaling molecules.
Man-Made Macromolecules
The principles of polymerization extend beyond biology to synthetic materials. Scientists create man-made macromolecules, or synthetic polymers, by linking monomers. These materials have properties tailored for specific applications based on their molecular structure.
A common example is plastic, like polyethylene, used in packaging. It is formed by linking many ethylene monomers into long, flexible chains. This structure gives the material its durability and pliability. The repeating polymer chain allows it to be molded into various shapes, making it versatile.
Other synthetic polymers are designed for specific properties. Nylon, for instance, is known for its strength and is used in textiles for clothing and ropes. Its resilience comes from strong bonds holding its polymer chains together. Polytetrafluoroethylene (Teflon) is non-reactive with low friction, making it ideal for non-stick coatings on cookware.
Big Molecules and Human Nutrition
The macromolecules for life are the same ones we consume in our diets. Foods like meats, breads, and vegetables are rich in proteins, carbohydrates, and lipids. Digestion is the process of breaking down these large dietary polymers into their monomer units. This breakdown is achieved through the chemical reaction of hydrolysis.
During digestion, specific enzymes target different macromolecules. For instance, amylase breaks down starches into simple sugars, while proteases break down proteins into amino acids. These smaller monomers are then absorbed from the intestine into the bloodstream and transported to cells throughout the body.
Inside our cells, these monomers become raw materials for building new macromolecules. The amino acids from consumed protein can be reassembled to create human proteins for muscle repair or to form enzymes. Simple sugars can be used immediately for energy or linked together to form glycogen for storage. This cycle of breaking down and rebuilding macromolecules is fundamental to sustaining life.