Biomolecules

The One Thing

Biomolecular structure determines biological function: the specific bonds, shapes and sequences of carbohydrates, proteins, vitamins and nucleic acids enable their biological roles. Condensation forms key biomolecular linkages, whereas hydrolysis breaks them using water.

Reactions, Processes and Experiments

Key Terms

• Biomolecules: Complex organic substances found in living organisms, including carbohydrates, proteins, lipids, nucleic acids and vitamins.
• Carbohydrates: Polyhydroxy aldehydes or ketones, or compounds that produce them on hydrolysis; they commonly follow the approximate formula Cx(H2O)y.
• Monosaccharides: Simple sugars that cannot be hydrolysed into smaller carbohydrates, such as glucose, fructose and ribose.
• Oligosaccharides: Carbohydrates that yield two to ten monosaccharide units on hydrolysis; disaccharides such as sucrose, maltose and lactose are common examples.
• Polysaccharides: High-molecular-mass carbohydrates made of many monosaccharide units, such as starch, cellulose and glycogen.
• Aldose: A monosaccharide containing an aldehyde group, such as glucose.
• Ketose: A monosaccharide containing a ketone group, such as fructose.
• Reducing sugar: A sugar with a free aldehyde group or a free anomeric carbon that can reduce Fehling's or Tollens' reagent; glucose, fructose, lactose and maltose are reducing sugars.
• Non-reducing sugar: A sugar whose anomeric carbons are involved in the glycosidic bond and therefore cannot reduce Fehling's or Tollens' reagent; sucrose is a common example.
• Glycosidic linkage: The oxygen bridge formed when two monosaccharides join by loss of water, such as the linkage connecting glucose and fructose in sucrose.
• Starch: A plant storage polysaccharide made mainly of amylose and amylopectin, both composed of glucose units.
• Glycogen: A highly branched storage polysaccharide of animals, often called animal starch.
• Cellulose: A structural polysaccharide of plants made of beta-D-glucose units joined by beta(1→4) linkages.
• Amino acid: An organic compound containing amino and carboxyl groups; alpha-amino acids have both groups attached to the same carbon atom.
• Zwitterion: A dipolar form of an amino acid containing a positively charged ammonium group and a negatively charged carboxylate group, written as +NH3-CHR-COO−.
• Peptide bond: The -CO-NH- linkage formed when the carboxyl group of one amino acid reacts with the amino group of another, releasing water.
• Protein: A natural polymer composed of amino acid residues joined by peptide bonds.
• Primary structure: The specific sequence of amino acids in a polypeptide chain.
• Secondary structure: Local folding of a polypeptide into alpha-helix or beta-pleated sheet structures, mainly stabilised by hydrogen bonds.
• Tertiary structure: The overall three-dimensional shape of a single polypeptide chain, stabilised by hydrogen bonds, ionic interactions, disulfide bonds and other forces.
• Quaternary structure: The arrangement of two or more polypeptide chains in a functional protein, as in haemoglobin.
• Denaturation: Loss of a protein's secondary, tertiary or quaternary structure due to heat, pH change or chemicals, usually without breaking peptide bonds.
• Enzyme: A biological catalyst, generally a protein, that increases the rate of a biochemical reaction without being consumed.
• Vitamins: Organic compounds required in very small amounts for normal growth and metabolism, usually obtained through the diet.
• Fat-soluble vitamins: Vitamins A, D, E and K, which are stored in body tissues and may accumulate to harmful levels.
• Water-soluble vitamins: Vitamins of the B group and vitamin C, which are generally not stored extensively and need regular dietary supply.
• Nucleoside: A nitrogenous base linked to a pentose sugar without a phosphate group.
• Nucleotide: A nucleoside containing one or more phosphate groups; it is the basic unit of nucleic acids.
• DNA: Deoxyribonucleic acid, usually a double-stranded polymer of deoxyribonucleotides that stores hereditary information.
• RNA: Ribonucleic acid, usually a single-stranded polymer of ribonucleotides involved in protein synthesis and gene regulation.
• Complementary base pairing: Specific hydrogen bonding between bases: adenine pairs with thymine in DNA or uracil in RNA, while guanine pairs with cytosine.
• Central dogma: The general flow of genetic information from DNA to RNA to protein.

Easily Confused

• Glucose and fructose: Both have the formula C6H12O6, but glucose is an aldohexose whereas fructose is a ketohexose.
• Ribose and 2-deoxyribose: Ribose occurs in RNA, whereas 2-deoxyribose occurs in DNA.
• Reducing and non-reducing sugars: Glucose, fructose, lactose and maltose are reducing sugars; sucrose is non-reducing because both anomeric carbons are involved in its glycosidic linkage.
• Maltose and lactose: Maltose consists of two glucose units; lactose consists of galactose and glucose.
• Amylopectin and glycogen: Both are branched glucose polymers, but glycogen is more highly branched and is stored mainly in animal liver and muscle.
• Starch and cellulose: Starch is a plant storage polysaccharide containing amylose and amylopectin; cellulose is a structural polysaccharide made of beta-D-glucose units.
• Peptide bond and phosphodiester linkage: Peptide bonds join amino acids in proteins; phosphodiester linkages join nucleotides in nucleic acids.
• Denaturation and hydrolysis: Denaturation disrupts protein folding without usually breaking peptide bonds; hydrolysis breaks bonds using water.
• Nucleoside and nucleotide: A nucleoside contains a base and pentose sugar; a nucleotide also contains one or more phosphate groups.
• DNA and RNA: DNA contains deoxyribose and thymine and is usually double-stranded; RNA contains ribose and uracil and is usually single-stranded.
• Fat-soluble and water-soluble vitamins: Vitamins A, D, E and K are stored in body tissues, whereas B-group vitamins and vitamin C are generally not stored extensively.

What Gets Asked

• Classifying carbohydrates: Questions may require glucose, fructose, ribose, sucrose, maltose, lactose, starch, glycogen or cellulose to be identified as monosaccharides, oligosaccharides or polysaccharides. Marks are lost by confusing maltose with lactose or by treating sucrose as a reducing sugar.
• Comparing carbohydrate structures: Questions may ask for the distinction between amylose, amylopectin, glycogen and cellulose. The relevant slips are omitting alpha(1→6) branches in amylopectin, failing to note that glycogen is more highly branched, or using alpha(1→4) rather than beta(1→4) for cellulose.
• Describing amino-acid and protein formation: Questions may require the general alpha-amino-acid structure, peptide-bond formation or the relationship between n amino acids, n−1 peptide bonds and n−1 water molecules. Marks are lost by giving the wrong bond or omitting the condensation of water.
• Explaining protein structure and denaturation: Questions may distinguish primary, secondary, tertiary and quaternary structure or ask about protein denaturation, including coagulation of egg white on heating. The key slip is stating that denaturation normally breaks peptide bonds.
• Matching vitamins with functions and deficiency diseases: Questions may link vitamin A with vision and night blindness, vitamin D with calcium absorption, bone health and rickets, vitamin B1 with beriberi, vitamin B12 with pernicious anaemia, and vitamin C with scurvy. Confusing fat-soluble vitamins with water-soluble vitamins also costs marks.
• Comparing DNA and RNA: Questions may test sugars, bases, strand structure, complementary pairing, phosphodiester linkages and the central dogma. The specific errors are substituting thymine for uracil in RNA, omitting antiparallel DNA strands, or reversing the information flow DNA → RNA → protein.