Everything alive is made of molecules. Your muscles contract because of proteins. Your cells store energy in carbohydrates and fats. Your genetic information is written in nucleic acids. Every signal your body sends, every structure it builds, every reaction it runs depends on the chemistry of a small set of molecular families.

This chapter introduces the four major families of biological molecules — carbohydrates, lipids, proteins, and nucleic acids — together with the one substance that makes all of them functional: water. These are sometimes called the molecules of life or biomolecules because they are found in every living system, from bacteria to blue whales.

Understanding biomolecules is not about memorizing lists of terms. It is about learning a fundamental biological principle: molecular structure determines biological function. The shape of a molecule, the way its atoms are arranged, the chemical groups it carries — all of these determine what that molecule can do inside a living cell. Change the structure, and you change the function. This principle will appear again and again throughout your study of biology.

Living things are made primarily of four elements: carbon (C), hydrogen (H), oxygen (O), and nitrogen (N). Smaller but essential amounts of phosphorus (P), sulfur (S), and other elements also appear in living systems. These elements are not rare — they are among the most common atoms on Earth. What makes living matter distinctive is not the elements themselves but the way they are assembled.

The molecules that make up life fall into four major categories. All four groups are often called the macromolecules of life because they tend to be large molecules built by linking smaller units together. The smaller units are called monomers. The large molecules built from monomers are called polymers.

Before examining any of the four macromolecule families, it is important to understand water. Water is not one of the four macromolecule groups, but it is the substance that makes all of them functional. Every biological process on Earth takes place in or near water. Cells are mostly water by mass. Reactions in cells happen in water. Nutrients are carried in water. Wastes are removed in water.

A water molecule consists of one oxygen atom bonded to two hydrogen atoms (H₂O). Oxygen attracts electrons more strongly than hydrogen does — the electrons in the O–H bonds are pulled toward the oxygen side. This creates a slight negative charge (δ−) at the oxygen end and a slight positive charge (δ+) at the hydrogen ends. A molecule with this kind of charge separation is called a polar molecule.

Because water is polar, the slightly positive hydrogen of one molecule is attracted to the slightly negative oxygen of a neighboring molecule. This attraction is called a hydrogen bond. Any individual hydrogen bond is weak, but water molecules form enormous numbers of them simultaneously. Collectively, hydrogen bonds give water a set of properties that make life possible.

Carbohydrates are molecules made of carbon, hydrogen, and oxygen, typically in roughly a 1:2:1 ratio (CₙH₂ₙOₙ). Sugars are the simplest carbohydrates. Carbohydrates serve two primary roles in living systems: energy (simple sugars are a cell's most readily available fuel) and structure (some carbohydrates form the rigid frameworks that support cells, tissues, and organisms).

The monomer unit of carbohydrates is the monosaccharide. Monosaccharides are simple sugars that cannot be broken down into smaller sugar units. The most important monosaccharide in biology is glucose (C₆H₁₂O₆) — the primary fuel molecule for cellular respiration. Glucose, fructose, and galactose all share the same molecular formula (C₆H₁₂O₆) but have different arrangements of atoms, which means cells treat them differently.

When two monosaccharides are joined together, the product is a disaccharide. The bond between them is called a glycosidic bond, and a water molecule is released (a condensation reaction). Disaccharides can be broken back into monosaccharides by adding water — a hydrolysis reaction.

Polysaccharides are polymers built from many monosaccharide monomers. The function of a polysaccharide depends on the specific way its monomers are connected — not merely what the monomers are.

Lipids are defined not by a common structure but by a common property: they do not dissolve in water. Lipids are hydrophobic — nonpolar molecules that are repelled by water. Unlike carbohydrates, proteins, and nucleic acids, lipids are not polymers built from repeating monomer units. They are a collection of related hydrophobic molecules that biology uses for a variety of purposes.

A triglyceride consists of one glycerol molecule linked to three fatty acid chains. Each fatty acid is a long hydrocarbon chain with an acid group at one end. The long hydrocarbon tails are strongly nonpolar, which is why triglycerides do not mix with water. Triglycerides store more energy per gram than carbohydrates, making them excellent long-term energy reserves.

Phospholipids are structurally similar to triglycerides, but one fatty acid chain is replaced by a phosphate-containing group. This creates a molecule with two distinct regions: a hydrophilic head (the glycerol + phosphate group) and a hydrophobic tail (the two fatty acid chains). A molecule with both water-loving and water-fearing regions is called amphipathic.

When phospholipids are placed in water, they spontaneously organize into a phospholipid bilayer: the hydrophobic tails face inward (away from water) and the hydrophilic heads face outward (toward the surrounding water). This bilayer is the structural basis of every cell membrane on Earth — and it forms spontaneously, driven by the physical properties of the molecule itself, without any additional instructions.

Steroids have a distinctive structure of four fused carbon rings. All steroids share this framework, but different functional groups give different steroids different roles. Cholesterol is found in animal cell membranes where it affects fluidity and stability; it also serves as the starting material for steroid hormones such as testosterone, estrogen, and cortisol. Because steroids are hydrophobic, they can pass directly through cell membranes and enter cells to influence gene activity.

Proteins are the most functionally diverse of the four macromolecule families. They serve as structural components, catalysts, transporters, regulators, receptors, immune defenders, motors, and signals. Nearly every biological process depends on proteins.

All proteins are polymers built from monomers called amino acids. There are 20 different amino acids used by living organisms. Each has the same basic structure — a central carbon bonded to an amino group (–NH₂), a carboxyl group (–COOH), a hydrogen atom, and a distinctive side chain called an R group. The R group is what makes each amino acid different. Some R groups are hydrophobic; others are hydrophilic. Some carry charges. Some contain sulfur. These differences ultimately determine the shape and function of every protein.

Amino acids are joined by peptide bonds. A peptide bond forms when the carboxyl group of one amino acid reacts with the amino group of the next, releasing a water molecule (condensation). A chain of amino acids linked by peptide bonds is called a polypeptide. Most proteins consist of one or more polypeptide chains that fold into a specific three-dimensional shape. The sequence of amino acids — the primary structure — is determined by genetic information and is the starting point from which all protein properties emerge.

When a protein is exposed to extreme heat, strong acids or bases, or certain chemicals, it can lose its three-dimensional shape — a process called denaturation. A denatured protein cannot perform its normal function. Cooking an egg denatures its proteins; the white shifts from clear and fluid to white and solid. Very high fevers can denature proteins in the body, which is one reason they are medically dangerous. In many cases, if conditions return to normal quickly enough, some proteins can refold and regain function, but severe denaturation is often irreversible.

Nucleic acids are the molecules of heredity and information. They store genetic instructions, transmit those instructions from parents to offspring, and carry information within cells to direct protein synthesis. The two types are DNA (deoxyribonucleic acid) and RNA (ribonucleic acid).

Nucleic acids are polymers built from monomers called nucleotides. Each nucleotide consists of three components: a five-carbon sugar (deoxyribose in DNA; ribose in RNA), a phosphate group (negatively charged; links nucleotides in a chain), and a nitrogenous base (the information-carrying part).

DNA consists of two strands of nucleotides wound around each other in a double helix. The backbone of each strand alternates phosphate groups and deoxyribose sugars. The nitrogenous bases project inward, where they pair with bases on the opposite strand according to strict rules — complementary base pairing: A pairs with T, and C pairs with G. The two strands are held together by hydrogen bonds between the paired bases.

The sequence of bases along a DNA strand encodes genetic information, much as the sequence of letters on a page encodes language. Different sequences of bases specify different instructions, which ultimately direct the assembly of different proteins. This sequence is what is passed from parents to offspring.

RNA is similar to DNA but differs in three important ways: it is single-stranded; it contains ribose sugar instead of deoxyribose; and it uses uracil instead of thymine. RNA serves as an intermediary between the genetic instructions in DNA and the proteins built by ribosomes.

Not all nucleotides carry genetic information. Adenosine triphosphate (ATP) is a nucleotide that serves as the primary energy currency of cells. ATP consists of adenine, ribose, and three phosphate groups. When the bond between the second and third phosphate groups is broken — releasing one phosphate group and producing ADP (adenosine diphosphate) — energy is released and made available for cellular work. You will study ATP in depth in the chapters on photosynthesis and cellular respiration.

Three of the four macromolecule families — carbohydrates, proteins, and nucleic acids — share a common organizational pattern: large molecules are built by joining many smaller subunits. The small subunits are monomers; the large assembled molecules are polymers. (Lipids are hydrophobic biological molecules but do not fit the strict monomer-polymer pattern.)

Polymers are assembled by condensation reactions (also called dehydration synthesis). In a condensation reaction, two monomers are joined and a water molecule is released. This type of reaction requires an input of energy. Every peptide bond in a protein and every glycosidic bond in a polysaccharide was formed this way.

Polymers are broken apart by hydrolysis. In hydrolysis, a water molecule is added across the bond between monomers, breaking the bond and releasing the individual monomers. Digestion is largely a process of hydrolysis — your digestive system breaks the polymers in food into monomers small enough to absorb into the bloodstream.

The theme running through this entire chapter is one of the most important principles in all of biology: structure determines function. The properties and abilities of any biological molecule depend entirely on its structure — on the specific arrangement of its atoms, the types of chemical groups it carries, and the shape it adopts in three dimensions.

When you eat, you consume a mixture of carbohydrates, lipids, and proteins. Your digestive system breaks macromolecules into monomers through hydrolysis. Those monomers are then absorbed and either reassembled into new polymers your body needs or broken down further to release energy. Nutritional labels reflect macromolecule categories: carbohydrates and proteins provide about 4 kilocalories per gram; fats provide about 9 kilocalories per gram, making them the most energy-dense macromolecule. Dietary fiber — mostly cellulose and other plant polysaccharides that human enzymes cannot break — provides little energy but plays important roles in digestive health.

Insulin is a protein hormone produced today through genetic engineering: the human insulin gene is inserted into bacteria, which produce human insulin in large quantities. Before the 1980s, insulin for people with diabetes was extracted from animal pancreases.

Antibiotics often work by targeting specific biomolecules in bacteria. Penicillin interferes with enzymes that bacteria use to build cell walls. Because human cells lack those enzymes, penicillin harms bacteria without harming human cells — a direct application of structural differences between organisms.

Cholesterol is an essential lipid that every cell membrane needs, but excess cholesterol in the blood can contribute to plaque formation in arteries. Understanding cholesterol as a hydrophobic lipid helps explain why it can accumulate in blood vessel walls rather than remaining dissolved in blood plasma.

Enzymes are used industrially in enormous quantities: enzymes that break down lactose are added to dairy products; proteases are used in meat tenderizers and laundry detergents; amylases are used in baking and brewing. Carbohydrates from plant material (cellulose and starch) are the starting point for biofuels and biodegradable plastics. Understanding the chemistry of plant biomolecules is increasingly important as society looks for alternatives to petroleum-based products.