Biomolecules

The One Thing

The biological function of a biomolecule depends on its chemical structure, including its bonds, three-dimensional shape and sequence. Carbohydrates, proteins, enzymes, vitamins and nucleic acids therefore perform distinct roles in energy provision, structure, catalysis, regulation and heredity.

Reactions, Processes and Experiments

Key Terms

• Biomolecules: Naturally occurring organic molecules in living systems, including carbohydrates, proteins, lipids, nucleic acids and vitamins.
• Carbohydrates: Polyhydroxy aldehydes or ketones, or compounds that yield them on hydrolysis; commonly represented approximately as Cx(H2O)y.
• Monosaccharide: The simplest carbohydrate that cannot be hydrolysed into a smaller carbohydrate, such as glucose, fructose and ribose.
• Oligosaccharide: A carbohydrate that produces two to ten monosaccharide units on hydrolysis; disaccharides include sucrose, maltose and lactose.
• Polysaccharide: A high-molecular-mass carbohydrate made of many monosaccharide units, such as starch, cellulose and glycogen.
• Reducing sugar: A sugar with a free aldehyde group or a free anomeric carbon capable of reducing Fehling's or Tollens' reagent; glucose, fructose, maltose and lactose are examples.
• Glycosidic linkage: The oxygen bridge formed when two monosaccharides join, usually by loss of water.
• Starch: A plant storage polysaccharide consisting mainly of amylose with alpha-1,4 linkages and amylopectin with alpha-1,4 and alpha-1,6 linkages.
• Cellulose: A structural polysaccharide of plants made of beta-D-glucose units joined by beta-1,4 glycosidic linkages.
• Glycogen: A highly branched storage polysaccharide found mainly in animals and fungi.
• Proteins: Nitrogen-containing polymers of alpha-amino acids joined by peptide bonds.
• Amino acid: An organic compound containing both an amino group and a carboxyl group; alpha-amino acids have both groups attached to the same carbon atom.
• Zwitterion: A dipolar form of an amino acid containing both a positively charged ammonium group and a negatively charged carboxylate group.
• Peptide bond: The -CO-NH- linkage formed by condensation between the carboxyl group of one amino acid and the amino group of another.
• Primary structure: The specific sequence of amino acids in a polypeptide chain.
• Secondary structure: Local folding of a polypeptide into structures such as alpha-helix and beta-pleated sheet, stabilised mainly by hydrogen bonds.
• Tertiary structure: The overall three-dimensional folding of a single polypeptide chain, maintained by hydrogen bonds, ionic interactions, disulfide bonds and other forces.
• Quaternary structure: The arrangement of two or more polypeptide chains in a functional protein.
• Denaturation: Loss of a protein's secondary, tertiary or quaternary structure due to heat, pH change or chemicals, generally without breaking peptide bonds.
• Enzyme: A biological catalyst, usually a globular protein, that increases reaction rate by lowering activation energy without being consumed.
• Active site: The specific region of an enzyme where the substrate binds and the reaction occurs.
• Lock-and-key model: A model stating that the substrate fits the enzyme active site with a complementary shape.
• Induced-fit model: A model in which substrate binding causes a slight change in the enzyme's active site, improving catalytic interaction.
• Cofactor: A non-protein component required for the activity of some enzymes; it may be an inorganic ion or an organic coenzyme.
• Vitamins: Organic compounds needed in very small quantities for normal growth and metabolism, generally not synthesised sufficiently by the body.
• Fat-soluble vitamins: Vitamins A, D, E and K, which can be stored in body tissues and may accumulate to toxic levels.
• Water-soluble vitamins: Vitamins of the B group and vitamin C, which are generally not stored extensively and require regular dietary intake.
• Nucleic acids: Polymers of nucleotides that function in the storage, transmission and expression of genetic information.
• Nucleotide: A unit of nucleic acid consisting of a nitrogenous base, a pentose sugar and one or more phosphate groups.
• Nucleoside: A nitrogenous base linked to a pentose sugar, without a phosphate group.
• DNA: Deoxyribonucleic acid, generally a double-stranded molecule that stores hereditary information.
• RNA: Ribonucleic acid, usually single-stranded, involved in protein synthesis and other cellular functions.
• Complementary base pairing: Specific hydrogen bonding in DNA in which adenine pairs with thymine and guanine pairs with cytosine; in RNA, uracil replaces thymine.

Easily Confused

• Monosaccharides and oligosaccharides: Monosaccharides cannot be hydrolysed into smaller carbohydrates, whereas oligosaccharides yield two to ten monosaccharide units.
• Starch and glycogen: Starch is mainly a plant storage polysaccharide and gives a blue colour with iodine; glycogen is mainly found in animals and fungi and gives a reddish-brown colour.
• Starch and cellulose: Starch contains mainly alpha linkages, whereas cellulose contains beta-1,4 glycosidic linkages and is not digested by humans because humans lack cellulase.
• Glucose and fructose: Both have formula C6H12O6, but glucose is an aldohexose and fructose is a ketohexose; they are structural isomers.
• Glucose and galactose: They are epimers differing at one chiral carbon, rather than structural isomers.
• Reducing and non-reducing sugars: Reducing sugars have a free aldehyde group or free anomeric carbon; sucrose is non-reducing because both anomeric carbons are involved in the glycosidic linkage.
• Maltose and lactose: Maltose consists of two glucose units and is reducing; lactose consists of galactose and glucose and is also reducing.
• Amino acid and zwitterion: An amino acid contains amino and carboxyl groups, whereas a zwitterion is its dipolar form with an ammonium group and a carboxylate group.
• Peptide bond and glycosidic linkage: Peptide bonds join amino acids in proteins; glycosidic linkages join monosaccharides in carbohydrates.
• Denaturation and peptide-bond cleavage: Denaturation disrupts secondary, tertiary or quaternary structure generally without breaking peptide bonds; peptide-bond cleavage changes the primary structure.
• Lock-and-key and induced-fit models: The lock-and-key model treats the active site as complementary in a fixed shape; the induced-fit model involves a slight change in the active site after substrate binding.
• Competitive inhibition and denaturation: Competitive inhibitors compete with substrates for the active site, whereas denaturation alters enzyme structure and may destroy catalytic function.
• Fat-soluble and water-soluble vitamins: Vitamins A, D, E and K may be stored and accumulate to toxic levels; B-group vitamins and vitamin C are generally not stored extensively.
• Nucleotide and nucleoside: A nucleotide contains a base, pentose sugar and one or more phosphate groups; a nucleoside contains only a base and sugar.
• DNA and RNA: DNA generally has two strands, contains deoxyribose and thymine, and stores hereditary information; RNA is usually single-stranded, contains ribose and uracil, and is involved in protein synthesis and other cellular functions.
• Purines and pyrimidines: Adenine and guanine are purines; cytosine, thymine and uracil are pyrimidines.
• mRNA, tRNA and rRNA: mRNA carries genetic information, tRNA brings amino acids, and rRNA forms an important part of ribosomes.

What Gets Asked

• Classification of carbohydrates: Questions may require classification as monosaccharides, oligosaccharides or polysaccharides, using examples such as glucose, fructose, ribose, sucrose, maltose, lactose, starch, cellulose and glycogen. Marks are lost by confusing the number of hydrolysis products or omitting the relevant example.
• Comparison of carbohydrate structures and properties: Questions may compare starch, cellulose and glycogen, including their linkages, biological roles, iodine colours and digestibility. Marks are lost by assigning starch’s blue iodine colour to glycogen or overlooking the absence of cellulase in humans.
• Reducing-sugar reasoning: Questions may ask why glucose, fructose, maltose and lactose are reducing sugars but sucrose is non-reducing. The key slip is failing to state that both anomeric carbons in sucrose are involved in the glycosidic linkage.
• Protein structure: Questions may require the representation of alpha-amino acids as H2N-CH(R)-COOH, identification of the peptide bond, or explanation of primary, secondary, tertiary and quaternary structure. Marks are lost by treating all amino acids as identical or failing to note that proline has a secondary amino group in a ring.
• Enzyme activity: Questions may examine the effects of temperature, pH, substrate concentration, enzyme concentration, inhibitors and cofactors. Marks are lost by stating only that temperature increases activity without identifying the optimum and the subsequent decrease caused by denaturation, or by confusing competitive inhibition with denaturation.
• Vitamins and nucleic acids: Questions may link particular vitamins to functions or deficiency diseases and may compare DNA and RNA, nucleotides and nucleosides, or complementary base pairing. Marks are lost by confusing vitamin functions and deficiency diseases, or by replacing thymine with uracil in DNA or failing to state that RNA contains uracil.