Introduction to Biochemistry and Biological Elements
Biochemistry is the study of the chemical substances and processes in living organisms; about 96 % of the mass of the human body consists of just four elements (oxygen, carbon, hydrogen and nitrogen), which form the four major classes of biomolecules: carbohydrates, lipids, proteins and nucleic acids. In brief: a 70 kg person contains about 45 kg of oxygen, 12.6 kg of carbon, 7 kg of hydrogen and 2.1 kg of nitrogen, most of it in water (about 60 % of the body mass) and in organic molecules.
Living things are made of the same elements as the rest of the world, but arranged into a small set of complex molecules that carry out the functions of life. Understanding their composition and their chemistry is the foundation of medicine, nutrition, agriculture and biotechnology. This article lists the elements of life and their roles in a table, introduces the four classes of biomolecules, explains how large molecules are built from small ones and gives worked examples, with a short history of biochemistry.
Which elements are found in living things?
Of the 90 or so elements found in nature, about 25 are known to be essential for life. They are grouped as:
- Major elements (CHNOPS): carbon, hydrogen, oxygen, nitrogen, phosphorus and sulfur make up about 97 % of the mass of living matter.
- Macrominerals: calcium, potassium, sodium, chlorine and magnesium, needed in amounts of grams per day.
- Trace elements: iron, zinc, copper, manganese, iodine, selenium, cobalt, molybdenum and others, needed in milligrams or micrograms. They are often at the center of enzymes or of molecules such as hemoglobin and vitamin B12.
Elemental composition of the human body
| Element | Mass (%) | Main roles |
|---|---|---|
| Oxygen (O) | 65 | Water, all organic molecules, cellular respiration |
| Carbon (C) | 18 | Backbone of all organic molecules |
| Hydrogen (H) | 10 | Water, organic molecules, proton gradients |
| Nitrogen (N) | 3.0 | Proteins, nucleic acids |
| Calcium (Ca) | 1.5 | Bones and teeth, muscle contraction, signaling |
| Phosphorus (P) | 1.0 | Nucleic acids, ATP, phospholipids, bones |
| Potassium (K) | 0.35 | Nerve impulses, fluid balance |
| Sulfur (S) | 0.25 | Amino acids (cysteine, methionine), disulfide bonds |
| Sodium (Na) | 0.15 | Nerve impulses, fluid balance |
| Chlorine (Cl) | 0.15 | Fluid balance, stomach acid |
| Magnesium (Mg) | 0.05 | Enzyme cofactor, chlorophyll, ATP complexes |
The values are rounded and vary between people. Iron makes up only about 0.006 % (3–4 g in an adult), but is essential for oxygen transport.
Why is water so important?
Water makes up about 60 % of the body mass of an adult (the percentage is higher in infants and lower in people with more body fat). It is the solvent in which the reactions of life take place, it takes part in many of them (hydrolysis, condensation), it regulates the temperature because of its high heat capacity and heat of vaporization, and its polarity is the reason why lipid membranes form and proteins fold with their hydrophobic parts inside. Since water is so abundant, oxygen and hydrogen are the most abundant elements in the body by number of atoms: about 62 % of atoms are hydrogen, 24 % oxygen and 12 % carbon.
What are the four classes of biomolecules?
- Carbohydrates (sugars and starch): fuel and structural material.
- Lipids (fats, oils, phospholipids, steroids): energy storage and membranes.
- Proteins: enzymes, structure, transport, signaling and defense.
- Nucleic acids (DNA and RNA): storage and transfer of genetic information.
Three of the classes (carbohydrates, proteins and nucleic acids) are polymers built from small repeating units (monomers). Lipids are not true polymers, but are assembled from smaller building blocks, and held together in membranes by non-covalent forces. Large molecules are called macromolecules. Monomers are linked by condensation reactions, which remove a molecule of water, and are broken down by hydrolysis, which adds it.
Biomolecules compared
| Class | Building block | Linkage | Main elements | Examples |
|---|---|---|---|---|
| Carbohydrates | Monosaccharides | Glycosidic bond | C, H, O | Glucose, starch, cellulose |
| Lipids | Fatty acids, glycerol | Ester bond | C, H, O (P, N in phospholipids) | Fats, phospholipids, cholesterol |
| Proteins | Amino acids | Peptide bond | C, H, O, N, S | Enzymes, collagen, hemoglobin |
| Nucleic acids | Nucleotides | Phosphodiester bond | C, H, O, N, P | DNA, RNA |
Why is carbon the basis of life?
Carbon can form four strong covalent bonds, link into long chains and rings, and bond with H, O, N, S and P to give an enormous variety of molecules with different shapes and functional groups. The bonds of carbon are strong enough to be stable in water, but weak enough to be broken by enzymes at body temperature. Silicon, its neighbor in the periodic table, forms much weaker chains, and its oxide, SiO2, is a solid, while CO2 is a gas that can be exchanged by organisms. The functional groups (–OH, –COOH, –NH2, –SH, –PO42−) give biomolecules their chemical properties, and determine how they interact with water and each other.
Worked examples
Example 1: elements in a 70 kg person
Using the percentages: oxygen 0.65 × 70 = 45.5 kg; carbon 0.18 × 70 = 12.6 kg; hydrogen 0.10 × 70 = 7.0 kg; nitrogen 0.030 × 70 = 2.1 kg; calcium 0.015 × 70 = 1.05 kg; phosphorus 0.010 × 70 = 0.70 kg. The 12.6 kg of carbon is 12,600 / 12.011 = 1050 mol, or 6.3 × 1026 atoms. The hydrogen amounts to 7000 / 1.008 = 6940 mol, or 4.2 × 1027 atoms.
Example 2: water in the body
An adult of 70 kg with 60 % water contains 42 kg, or 42,000 g / 18.02 g/mol = 2330 mol of water. A glass of water of 250 mL (250 g) has a total of 13.9 mol, equal to 0.6 % of the total.
Example 3: identifying the biomolecule class
- A compound has the formula C6H12O6 (C : H : O = 1 : 2 : 1) and many –OH groups: a carbohydrate (a monosaccharide such as glucose).
- A long molecule with repeating –CO–NH– links and N and S: a protein.
- A molecule with a long hydrocarbon chain and an ester link to glycerol: a lipid.
- A polymer with sugar–phosphate backbone and N-containing bases: a nucleic acid.
Example 4: formation of a polymer
When 100 monomers are joined by condensation, 99 water molecules are released. For glucose units forming starch, the repeat unit has the formula C6H10O5 (162.14 g/mol), the formula of glucose (180.16 g/mol) minus H2O (18.02 g/mol).
How did biochemistry develop?
Friedrich Wöhler’s synthesis of urea from inorganic starting materials in 1828 weakened the belief in a “vital force” that was thought to be needed to make the compounds of life. In 1897 Eduard Buchner showed that a cell-free extract of yeast can ferment sugar, which demonstrated that the chemistry of life can take place outside living cells, and he received the Nobel Prize in Chemistry in 1907. The word “biochemistry” was popularized by Carl Neuberg in 1903. Throughout the 20th century, the structures of proteins (the first, insulin, by Frederick Sanger in 1955) and of DNA (Watson and Crick, 1953) were found, and the metabolic pathways were mapped out, leading to molecular biology and genomics.
Which mistakes are common?
- Thinking that living things contain special elements: they use the same elements as non-living matter.
- Confusing the percentage by mass and by atoms: hydrogen is the most abundant element by atoms, oxygen by mass.
- Calling lipids polymers: they are not made of repeating monomers.
- Forgetting the role of water in polymer formation and breakdown: condensation releases it, hydrolysis uses it.
- Ignoring trace elements: they are needed in small amounts but are essential.