Glossary: Chemical Composition in a Cell

This chapter's terms in context

Chapter 4 groups five terms belonging to the biochemistry of the cell: carbohydrate, protein, lipid, amino acid and deoxyribonucleic acid (DNA). A carbohydrate is an organic compound built from carbon, hydrogen and oxygen, usually in the ratio of two hydrogen atoms to one oxygen atom, and functions mainly as an energy source and, in the form of cellulose, as a structural material in plant cell walls. Monosaccharides such as glucose are the building blocks that join by condensation reactions into disaccharides such as sucrose and polysaccharides such as starch, glycogen and cellulose, and a very common exam error is describing starch and cellulose as chemically identical because both are polymers of glucose, when in fact their different glucose linkages give them very different properties, starch is a storage molecule that can be digested by amylase, while cellulose is a structural molecule humans cannot digest.

Protein is built from amino acids joined by peptide bonds into long chains that fold into a specific three-dimensional shape, and that shape determines the protein's function completely, as a structural material such as keratin, as an enzyme that catalyses reactions, as a transport molecule such as haemoglobin, or as a hormone such as insulin. An amino acid itself is the basic monomer unit of a protein, and every amino acid shares the same basic structure, an amino group, a carboxyl group and a variable side chain attached to a central carbon atom, with the side chain being the only part that differs between the twenty naturally occurring amino acids and gives each protein its distinctive shape and function.

Lipids, unlike carbohydrates and proteins, are not built from a single repeating monomer type; a typical lipid such as a fat or oil is formed from one glycerol molecule joined to three fatty acid molecules by condensation reactions, releasing three water molecules in the process. Lipids store roughly twice as much energy per gram as carbohydrates, and also form the core structural material of every plasma membrane in the phospholipid bilayer, a role no carbohydrate or protein alone can perform.

DNA is the nucleic acid that carries the genetic information of an organism, built from a double helix of two nucleotide strands, where each nucleotide contains a sugar, a phosphate group and one of four nitrogenous bases, adenine, thymine, cytosine and guanine, that pair specifically, adenine with thymine and cytosine with guanine. This complementary base pairing is exactly why the sequence of one DNA strand allows the other strand's sequence to be predicted, and it is the property that makes both DNA replication and genetic inheritance in later chapters possible. Confusing DNA with a protein, or describing a gene as a type of protein rather than a segment of DNA that codes for one, is a recurring error worth eliminating early.

Together, these five terms describe the chemical hierarchy tested repeatedly at SPM level: a molecule's structure (its monomers and bonds) determines its properties, and its properties determine its biological role, whether that role is providing energy, building structure, catalysing reactions or storing genetic information.

Paper 2 questions on this chapter frequently supply a food test result, for example, a blue-black colour with iodine solution, or a lilac colour with Biuret solution, and ask a candidate to name the food group present and explain the chemical basis of the test rather than simply naming the colour change. A common source of lost marks is stating the test result correctly but failing to link it back to the underlying molecule being tested for, such as recognising that a positive Biuret test detects the presence of peptide bonds specifically, not protein in some vague general sense. Linking each of the five chemical terms in this chapter to the specific food test that identifies it, iodine for starch, Benedict's solution for reducing sugar, Biuret for protein, and the emulsion test for lipid, turns abstract biochemistry into a set of concrete, testable observations.