Chemical composition of the cell. Proteins. Nucleic acids

Cells are made of C, N, O and H, and 99 per cent of their molecules are water molecules. Those four elements can form almost all known organic biomolecules: nucleic acids, proteins and lipids.

Proteins are large molecules built from about 20 different amino acids. They are classified structurally into simple and compound proteins, and functionally into structural, transport, enzyme, storage and defensive proteins. Their structure is described at four levels: primary, secondary, tertiary and quaternary.

Nucleic acids are macromolecules composed of nucleotides, each nucleotide being a pentose sugar, a phosphate group and a nitrogenous base. DNA carries A, G, C and T; RNA carries A, G, C and U.

Cells are made of C, N, O and H, and 99 per cent of their molecules are water molecules.

These elements can form almost all known organic biomolecules:

• nucleic acids;
• proteins;
• lipids.

Proteins are large molecules made of about 20 different amino acids, which are the protein building blocks.

Structurally:

• Simple proteins, for example collagen and histones.
• Compound proteins, for example glycoproteins and lipoproteins.

By functional class:

• structural proteins;
• transport proteins;
• enzymes;
• storage proteins;
• defensive proteins.

Proteins perform nearly every task of cellular life. In particular they:

• determine the shape and structure of cells and organelles;
• catalyse biological reactions, as enzymes.

Named examples by location:

Primary structure is the amino acid sequence.

Secondary structure is the local structure stabilised by hydrogen bonds, giving the α-helices and β-sheets.

Tertiary structure is the formation of protein subunits by folding.

Quaternary structure is a protein consisting of more than one amino acid chain, that is more than one polypeptide chain.

The progression is worth reading as a sequence of scales: a sequence, then a local shape, then a whole folded subunit, then several subunits together.

Nucleic acids are macromolecules composed of nucleotides.

There are two: RNA and DNA. RNA occurs as three types:

• rRNA;
• tRNA;
• mRNA.

Each nucleotide is composed of:

• a pentose sugar;
• a phosphate group;
• a nitrogenous base.

The pentose sugar is what distinguishes the two nucleic acids: deoxyribose in DNA, ribose in RNA.

The bases fall into two families:

• Purines: adenine and guanine.
• Pyrimidines: cytosine and thymine.

Which four appear depends on the nucleic acid:

• DNA: A, G, C, T.
• RNA: A, G, C, U.

Three bases are shared. The one that differs is thymine in DNA against uracil in RNA, and it is the pair most often confused under examination conditions.