What Is Biochemistry?

Every second, your cells carry out millions of chemical reactions. They break down sugar for energy, copy DNA, build proteins, send signals and repair damage, all in water, at body temperature, and at a speed no chemical factory could match. Biochemistry is the study of those reactions and the molecules that take part in them. It sits exactly where chemistry meets biology, and it explains life in terms of atoms, bonds and energy.

A working definition

Biochemistry is the branch of science that studies the chemical substances and processes that occur in living organisms. Biochemists ask questions such as:

• What molecules are living things made of, and what are their structures?
• How do those molecules react, and what speeds the reactions up?
• How do cells capture, store and use energy?
• How is genetic information stored, copied and turned into working molecules?
• What goes wrong chemically in disease, and how can drugs put it right?

Biochemistry is sometimes called “the chemistry of life”, which is accurate, but it’s also a mindset: explaining biological observations, such as why muscles tire or why a fever can be dangerous, in terms of molecules.

Life is made of ordinary elements

Living things aren’t made of special matter. They’re built from the same elements as rocks and air, just arranged in very particular ways. About 96% of the mass of the human body comes from four elements:

Element	Approximate % of body mass	Main roles
Oxygen	65	Water, and part of nearly every biomolecule
Carbon	18	The backbone of all organic molecules
Hydrogen	10	Water and organic molecules
Nitrogen	3	Proteins and nucleic acids

Calcium, phosphorus, potassium, sulfur, sodium, chlorine and magnesium make up most of the rest, and trace elements such as iron, zinc, iodine and copper are essential in tiny amounts. Our article on elements in the human body goes through them in detail.

Carbon is special because each carbon atom forms four strong covalent bonds and can bond to other carbon atoms in long chains, branches and rings. That makes an almost unlimited variety of stable molecular shapes possible. Life on Earth is carbon-based for this reason. You can explore carbon’s properties on its element page.

Water: the stage for life’s chemistry

Around 60% of an adult human’s body mass is water. Almost every biochemical reaction happens in water, and water’s unusual properties come from its polarity and hydrogen bonding:

• It dissolves ions and polar molecules, so nutrients and waste can be carried around.
• It has a high heat capacity, which buffers organisms against temperature swings.
• It pushes non-polar molecules together (the hydrophobic effect), which drives the formation of cell membranes and the folding of proteins.
• It takes part directly in many reactions, such as the hydrolysis reactions that break down food.

The four major classes of biomolecules

Most of the dry mass of a cell comes from four families of large molecules. Each is covered in its own article in this library; here’s the overview (see also the four major biomolecules).

1. Carbohydrates: sugars and their polymers, such as starch and cellulose. They provide energy and structure.
2. Lipids: fats, oils, phospholipids and steroids. They store energy, form membranes and act as signals.
3. Proteins: chains of amino acids folded into precise shapes. They act as enzymes, transporters, structural fibres, hormones and antibodies.
4. Nucleic acids: DNA and RNA, which store and express genetic information.

Three of these (carbohydrates, proteins and nucleic acids) are polymers built from smaller units (monomers) joined by condensation reactions, which release water. They’re broken down again by hydrolysis, which adds water back. This single pair of reactions, repeated with different building blocks, explains a huge amount of biochemistry.

Key ideas that run through biochemistry

Structure determines function

A protein’s job depends on its three-dimensional shape; DNA’s double helix explains how it can be copied; the shape of a drug determines which receptor it fits. Biochemists spend a lot of effort working out molecular structures because function follows from them.

Enzymes make life fast enough

Most biochemical reactions would be far too slow without catalysts. Enzymes, usually proteins, can speed reactions up by factors of millions or more, and each one is highly specific for its reactions. Controlling which enzymes are active is how cells control their chemistry. See reaction rates and catalysts for the chemistry of catalysis in general.

Energy flows through coupled reactions

Cells obey the same laws of thermodynamics as any chemical system. They run energy-requiring reactions (like building proteins) by coupling them to energy-releasing ones, often using a carrier molecule called ATP (adenosine triphosphate) as a kind of energy currency. Respiration releases energy from food; photosynthesis captures energy from light. Both are long sequences of redox reactions, the same kind of chemistry described in oxidation and reduction.

pH and buffers matter

Enzymes and other proteins work only within narrow pH ranges. Blood is held at pH 7.35 to 7.45 by buffer systems, and small shifts can be life-threatening. The chemistry is explained in the blood buffer system and pH of the human body.

Information is chemical

Genetic information is stored in the sequence of four bases along DNA. It’s copied by pairing between bases held together by hydrogen bonds, and it’s translated into proteins by molecular machines in the cell. The flow of information from DNA to RNA to protein is sometimes called the central dogma of molecular biology.

Where biochemistry is used

• Medicine: understanding diseases such as diabetes or cancer at the molecular level; designing drugs that block particular enzymes; developing vaccines and diagnostic tests.
• Nutrition: working out how the body digests and uses carbohydrates, fats, proteins, vitamins and minerals.
• Agriculture: improving crops, understanding soil chemistry and plant nutrition, developing safer pest control.
• Food science: browning, fermentation, ripening and spoilage are all biochemical processes.
• Industrial biotechnology: enzymes in washing powders, microbes that make insulin, biofuels and biodegradable plastics.
• Forensics: DNA profiling and toxicology.

How biochemists study molecules

Biochemists use many of the tools of analytical chemistry: spectroscopy to follow reactions, chromatography and electrophoresis to separate mixtures of proteins and DNA, mass spectrometry to identify molecules, and X-ray crystallography and cryo-electron microscopy to determine structures. Simple school tests, such as Benedict’s test for reducing sugars or the biuret test for proteins, use the same principle: a chemical reaction that reveals a type of molecule.

Common misconceptions

• “Biochemistry is just biology.” It uses the full toolkit of chemistry (bonding, thermodynamics, kinetics, acid–base chemistry) to explain biological facts.
• “Natural chemicals and artificial chemicals are fundamentally different.” A molecule is the same whether it was made in a cell or a lab. Vitamin C from an orange and from a factory are identical.
• “Living things break the laws of chemistry.” They don’t. They use enzymes, energy coupling and compartments to control ordinary chemistry very precisely.

Key takeaways

• Biochemistry is the study of the molecules and chemical reactions of living things.
• Life is built mostly from carbon, hydrogen, oxygen and nitrogen, with water as the medium for its reactions.
• The four major classes of biomolecules are carbohydrates, lipids, proteins and nucleic acids.
• Recurring ideas include structure determines function, enzymes as catalysts, energy coupling through ATP, pH control and chemical information in DNA.
• Biochemistry underpins medicine, nutrition, agriculture, food science and biotechnology.