Molecules of Life

The One Thing

The structure of a biomolecule—including its atoms, functional groups, bonds and three-dimensional shape—determines its biological function. Monomers can join by condensation reactions to form polymers, while polymers can be broken down by hydrolysis.

Reactions, Processes and Experiments

Key Terms

• Biomolecules: Organic and inorganic molecules present in living organisms that perform structural, metabolic, genetic or regulatory functions.
• Macromolecules: Very large biological molecules, usually polymers, formed by the joining of smaller subunits.
• Monomer: A small molecular unit that can combine with similar units to form a polymer.
• Polymer: A large molecule made of repeating monomer units linked by covalent bonds.
• Carbohydrates: Sugars and their polymers, mainly composed of carbon, hydrogen and oxygen; they provide energy and structural support.
• Monosaccharide: The simplest carbohydrate unit, such as glucose, fructose or ribose.
• Disaccharide: A carbohydrate formed by joining two monosaccharides through a glycosidic bond, usually with the loss of water.
• Polysaccharide: A long chain of monosaccharides, such as starch, glycogen, cellulose or chitin.
• Proteins: Nitrogen-containing polymers of amino acids that perform structural, enzymatic, transport, defensive and regulatory functions.
• Amino acid: The basic unit of a protein, containing an amino group, a carboxyl group, a hydrogen atom and a variable R group attached to a central carbon.
• Peptide bond: The covalent bond formed between the carboxyl group of one amino acid and the amino group of another, releasing water.
• Protein structure: The organisation of a protein at primary, secondary, tertiary and, in some cases, quaternary levels.
• Primary structure: The specific sequence of amino acids in a polypeptide chain.
• Secondary structure: Local folding of a polypeptide into alpha helices or beta sheets, mainly stabilised by hydrogen bonds.
• Tertiary structure: The complete three-dimensional folding of a single polypeptide chain.
• Quaternary structure: The arrangement of two or more polypeptide chains in a functional protein.
• Denaturation: The loss of a protein’s natural shape and activity due to heat, extreme pH, chemicals or other conditions, generally without breaking its peptide bonds.
• Lipids: Water-insoluble or poorly soluble organic molecules including fats, oils, phospholipids, waxes and steroids.
• Triglyceride: A lipid formed from one glycerol molecule and three fatty acid molecules joined by ester bonds.
• Phospholipid: A lipid containing glycerol, fatty acids and a phosphate group; it forms the basic bilayer of cell membranes.
• Saturated fatty acid: A fatty acid containing only single carbon-carbon bonds.
• Unsaturated fatty acid: A fatty acid containing one or more carbon-carbon double bonds.
• Nucleic acids: Polymers that store, transmit and help express genetic information.
• Nucleotide: The basic unit of a nucleic acid, made of a nitrogenous base, a five-carbon sugar and one or more phosphate groups.
• DNA: Deoxyribonucleic acid, usually a double-stranded molecule that stores hereditary information.
• RNA: Ribonucleic acid, usually single-stranded and involved in gene expression, protein synthesis and regulation.
• Hydrogen bond: A weak attraction important for water properties, nucleic acid base pairing and protein folding.
• Condensation reaction: A reaction in which monomers join to form a larger molecule with the removal of a water molecule.
• Hydrolysis: A reaction in which water is used to break a covalent bond in a polymer or other molecule.
• Functional group: A specific group of atoms, such as hydroxyl, amino, carboxyl or phosphate, that gives a molecule characteristic chemical properties.

Easily Confused

• Monomer and polymer: A monomer is a small molecular unit; a polymer is a large molecule made from repeating monomer units.
• Condensation and hydrolysis: Condensation joins molecules by removing water; hydrolysis breaks bonds by using water.
• Monosaccharide, disaccharide and polysaccharide: A monosaccharide is one sugar unit, a disaccharide contains two, and a polysaccharide contains a long chain of sugar units.
• Starch and cellulose: Both are made of glucose units, but different glycosidic linkages produce different structures and functions.
• Saturated and unsaturated fatty acids: Saturated fatty acids contain only single carbon-carbon bonds; unsaturated fatty acids contain one or more carbon-carbon double bonds.
• DNA and RNA: DNA usually has two strands, contains deoxyribose and uses thymine; RNA is usually single-stranded, contains ribose and uses uracil.
• Peptide and phosphodiester bonds: Peptide bonds join amino acids in proteins; 3'-5' phosphodiester bonds join nucleotides in nucleic acids.
• Hydrogen bonds and covalent bonds: Hydrogen bonds are weak attractions important in folding and base pairing; covalent bonds link atoms or monomers directly.
• Denaturation and hydrolysis: Denaturation generally changes protein shape without breaking peptide bonds; hydrolysis breaks covalent bonds using water.
• Organic and inorganic substances: Organic biomolecules generally contain carbon and hydrogen and may also contain oxygen, nitrogen, phosphorus or sulphur; important inorganic substances include water, mineral ions, gases and salts.

What Gets Asked

• Identify the major biological elements: Questions may require the abbreviation CHNOPS for carbon, hydrogen, oxygen, nitrogen, phosphorus and sulphur. Marks are lost by omitting sulphur or confusing elements with biomolecule classes.
• Explain why carbon is important: Carbon forms four covalent bonds and can make chains, branches and rings. Answers that state only that carbon is “present in living organisms” omit the structural reason.
• Classify carbohydrate examples: Questions may distinguish glucose, fructose and ribose as monosaccharides; sucrose, lactose and maltose as disaccharides; and starch, glycogen, cellulose and chitin as polysaccharides.
• Compare starch and cellulose: Both contain glucose units, but different glycosidic linkages produce different structures and functions. Treating them as interchangeable loses the structure–function distinction.
• Describe protein formation and structure: Questions may require the general amino acid structure H2N-CHR-COOH, peptide-bond formation between -COOH and -NH2, and the four levels of protein structure. A common error is to omit water released during peptide-bond formation.
• Distinguish lipid types and functions: Questions may test triglyceride formation, phospholipid amphipathic structure, and saturated versus unsaturated fatty acids. Marks are lost by assigning membrane formation to triglycerides or reversing hydrophilic heads and hydrophobic tails.
• Compare DNA and RNA: Answers must distinguish deoxyribose from ribose, thymine from uracil, and usually double-stranded DNA from usually single-stranded RNA.
• Explain biological structure–function relationships: Examples include enzymes lowering activation energy, phospholipid bilayers forming membranes, DNA base pairing and protein folding. Answers that describe structure without linking it to function are incomplete.