The Four Major Biomolecules

Strip away the water from a living cell and almost everything left belongs to one of four families of molecules: carbohydrates, lipids, proteins and nucleic acids. They’re sometimes called the macromolecules of life because many of them are very large. Each family has its own building blocks, bonds and jobs, but they share some common chemistry. Once you see the pattern, the whole of introductory biochemistry becomes much easier to organise.

The common thread: monomers, polymers and water

Three of the four families are polymers: long molecules built by linking many small units, called monomers.

• Monomers join by a condensation reaction: an –OH from one unit and an –H from the other are removed as a molecule of water, and a new covalent bond forms between the units.
• Polymers are broken down by hydrolysis, the reverse reaction: water is added across the bond, splitting it.

Family	Monomer	Polymer	Bond formed
Carbohydrates	Monosaccharides (e.g. glucose)	Polysaccharides (starch, glycogen, cellulose)	Glycosidic bond
Proteins	Amino acids	Polypeptides / proteins	Peptide bond
Nucleic acids	Nucleotides	DNA, RNA	Phosphodiester bond

Lipids are the exception. They aren’t true polymers: triglycerides are built from glycerol and three fatty acids joined by ester bonds, also by condensation, but they don’t form long repeating chains.

Digestion is essentially hydrolysis on a massive scale. Enzymes in your gut break polymers in food down into monomers small enough to absorb, and your cells then use condensation to build their own polymers.

1. Carbohydrates

Elements: carbon, hydrogen and oxygen, typically in the ratio C:H:O = 1:2:1. Glucose, for example, is C₆H₁₂O₆. (The name means “hydrated carbon”, from the formula Cₙ(H₂O)ₙ.)

Building blocks: monosaccharides (simple sugars) such as glucose, fructose and galactose. Two joined together make a disaccharide (sucrose, lactose, maltose); many make a polysaccharide.

Main functions:

• Quick energy. Glucose is the main fuel for respiration.
• Energy storage. Starch in plants, glycogen in animals.
• Structure. Cellulose in plant cell walls; chitin in insect exoskeletons.
• Recognition. Short sugar chains on cell surfaces act as identity tags, for example in the ABO blood groups.

Food sources: bread, rice, pasta, potatoes, fruit, sugar, vegetables.

Chemical tests: Benedict’s test for reducing sugars; the iodine test for starch.

2. Lipids

Elements: carbon, hydrogen and oxygen, but with far less oxygen than carbohydrates. That makes them richer in energy per gram.

Main types:

• Triglycerides (fats and oils): one glycerol plus three fatty acids.
• Phospholipids: like triglycerides but with one fatty acid replaced by a phosphate-containing group. They form cell membranes.
• Steroids: four fused carbon rings, including cholesterol and hormones such as testosterone and oestrogen.
• Waxes: long-chain esters that waterproof leaves, feathers and skin.

Main functions:

• Long-term energy storage. Fats release about 37 kJ per gram when oxidised, more than twice the roughly 17 kJ per gram of carbohydrates or proteins.
• Insulation and protection of organs.
• Cell membranes (phospholipids and cholesterol).
• Signalling (steroid hormones).

Key property: lipids are hydrophobic (or partly so) and don’t dissolve in water, because they’re mostly non-polar hydrocarbon chains (see polar vs non-polar molecules).

Food sources: butter, oils, nuts, seeds, oily fish, meat, avocados.

Chemical test: the emulsion test (shaking with ethanol, then adding water, gives a cloudy white emulsion).

3. Proteins

Elements: carbon, hydrogen, oxygen and nitrogen, and usually sulfur.

Building blocks: 20 standard amino acids, each with an amino group, a carboxylic acid group and a distinctive side chain. Amino acids are joined by peptide bonds into polypeptide chains, which fold into precise three-dimensional shapes. Amino acids themselves have interesting acid–base chemistry, covered in amino acids as acids and bases.

Main functions: proteins are the most versatile biomolecules.

• Enzymes catalyse almost every reaction in the cell.
• Structural proteins such as collagen and keratin give strength to skin, tendons, hair and nails.
• Transport proteins such as haemoglobin carry oxygen.
• Hormones such as insulin carry signals.
• Antibodies defend against infection.
• Motor proteins such as actin and myosin make muscles contract.

Key idea: a protein’s function depends on its shape, and its shape depends on its amino acid sequence. Heat or extreme pH can disrupt the shape (denaturation) and destroy function.

Food sources: meat, fish, eggs, dairy, beans, lentils, tofu, nuts.

Chemical test: the biuret test (purple colour with proteins).

4. Nucleic acids

Elements: carbon, hydrogen, oxygen, nitrogen and phosphorus.

Building blocks: nucleotides, each made of three parts: a five-carbon sugar (deoxyribose in DNA, ribose in RNA), a phosphate group, and a nitrogen-containing base (A, T, G, C in DNA; A, U, G, C in RNA).

Structure: nucleotides link through phosphodiester bonds into a sugar–phosphate backbone with bases sticking out. DNA forms a double helix, with bases pairing between the two strands by hydrogen bonds (A with T, G with C).

Main functions:

• DNA stores genetic information.
• RNA carries and translates that information into proteins, and some RNAs act as catalysts.
• Related nucleotides have other jobs: ATP carries energy, and molecules such as NAD⁺ carry electrons in metabolism.

Summary table

	Carbohydrates	Lipids	Proteins	Nucleic acids
Elements	C, H, O	C, H, O (+P in phospholipids)	C, H, O, N (+S)	C, H, O, N, P
Monomer	Monosaccharide	(Glycerol + fatty acids)	Amino acid	Nucleotide
Key bond	Glycosidic	Ester	Peptide	Phosphodiester
Main role	Energy, structure	Energy store, membranes	Enzymes, structure, transport	Genetic information
Energy (kJ g⁻¹)	~17	~37	~17	Not used as fuel
School test	Benedict’s, iodine	Emulsion	Biuret	(DNA extraction)

How the four families work together

A single process can involve all four. Take muscle contraction: proteins (actin and myosin) do the pulling, powered by ATP, a nucleotide, made by oxidising glucose from glycogen or fatty acids from fat. The instructions for making the proteins came from DNA. Biochemistry is the study of how these molecules cooperate.

Why size matters: macromolecules and water

Most biomolecules are large, but they don’t all behave the same way in water, and that shapes where they’re found in a cell. Carbohydrates are covered in –OH groups, so small ones dissolve easily and even huge ones like starch interact with water. Proteins fold so that most of their water-loving side chains face outwards and their oily side chains hide inside, which lets many proteins dissolve in the watery cytoplasm while others sit in membranes. Nucleic acids carry a negative charge on every phosphate group, which makes them very water-soluble and explains why DNA binds to positively charged proteins in the nucleus. Lipids are the odd ones out: their long hydrocarbon chains avoid water, so they gather into droplets and membranes. Knowing how each family interacts with water predicts a surprising amount of cell structure.

Common misconceptions

• “Fats are bad and carbohydrates are good” (or vice versa). Both are essential; health depends on types and amounts.
• “Proteins are just for muscles.” Nearly every process in the body relies on proteins, especially enzymes.
• “DNA is a protein.” DNA is a nucleic acid; it carries the instructions for making proteins.
• “Lipids are polymers.” Triglycerides and steroids are not polymers, although they’re large molecules.

Key takeaways

• The four major biomolecules are carbohydrates, lipids, proteins and nucleic acids.
• Carbohydrates, proteins and nucleic acids are polymers made by condensation and broken down by hydrolysis.
• Carbohydrates provide quick energy and structure; lipids store energy and form membranes; proteins do most of the work; nucleic acids store and use genetic information.
• Differences in elements, bonds and polarity explain their different properties.
• Start with what is biochemistry? for the bigger picture.