Biomolecules

The One Thing

Biomolecules are carbon-based compounds whose chemical structures and three-dimensional shapes determine their biological functions. Carbohydrates, proteins, lipids and nucleic acids support energy provision, structure, information storage and regulation, while enzymes catalyse biochemical reactions without being consumed.

Reactions, Processes and Experiments

Key Terms

• Biomolecules: Organic compounds produced by living organisms, including carbohydrates, proteins, lipids and nucleic acids.
• Monomer: A small molecular unit that can join with similar units to form a larger molecule or polymer.
• Polymer: A large molecule made by linking many repeating monomer units.
• Carbohydrates: Polyhydroxy aldehydes or ketones, or compounds that produce them on hydrolysis; they commonly provide energy and structural support.
• Monosaccharide: The simplest carbohydrate that cannot be hydrolysed into a smaller carbohydrate, such as glucose, fructose and ribose.
• Disaccharide: A carbohydrate formed when two monosaccharides join through a glycosidic bond, such as sucrose, maltose and lactose.
• Polysaccharide: A carbohydrate made of many monosaccharide units, such as starch, glycogen, cellulose and chitin.
• Glycosidic bond: The covalent bond joining monosaccharide units, usually formed by removal of water.
• Reducing sugar: A sugar with a free aldehyde or ketone group capable of reducing certain chemical reagents; glucose, fructose, lactose and maltose are examples.
• Proteins: Nitrogen-containing polymers of amino acids that carry out structural, catalytic, transport and regulatory functions.
• Amino acid: An organic compound containing an amino group, a carboxyl group, a hydrogen atom and a variable R group attached to the same carbon atom.
• Peptide bond: The covalent bond formed between the carboxyl group of one amino acid and the amino group of another, with removal of water.
• Primary structure: The specific linear sequence of amino acids in a polypeptide chain.
• Secondary structure: Local folding of a polypeptide into structures such as an alpha helix or beta-pleated sheet, mainly due to hydrogen bonding.
• Tertiary structure: The overall three-dimensional folding of a single polypeptide chain.
• Quaternary structure: The arrangement of two or more polypeptide chains in a functional protein.
• Denaturation: Loss of a protein’s natural three-dimensional structure and activity due to factors such as high temperature, extreme pH or chemicals.
• Lipids: Water-insoluble organic compounds soluble in non-polar solvents; they include fats, oils, waxes, phospholipids and steroids.
• Triglyceride: A lipid formed from one glycerol molecule and three fatty acid molecules.
• Ester bond: The bond formed between the hydroxyl group of glycerol and the carboxyl group of a fatty acid.
• Saturated fatty acid: A fatty acid containing no carbon-carbon double bonds; its chains are generally straighter and often form solid fats.
• Unsaturated fatty acid: A fatty acid containing one or more carbon-carbon double bonds; it is commonly found in liquid oils.
• Phospholipid: A lipid containing glycerol, fatty acids, phosphate and usually another polar group; it forms the basic structure of cell membranes.
• Nucleic acids: Polymers of nucleotides that store, transmit and help express genetic information.
• Nucleotide: The basic unit of a nucleic acid, consisting of a nitrogenous base, a pentose sugar and a phosphate group.
• Nucleoside: A nitrogenous base joined to a pentose sugar, without a phosphate group.
• DNA: Deoxyribonucleic acid, usually a double-stranded molecule that stores hereditary information.
• RNA: Ribonucleic acid, usually single-stranded, involved in gene expression and protein synthesis.
• Phosphodiester bond: The bond joining nucleotides through the phosphate group in a nucleic acid strand.
• Complementary base pairing: Specific pairing of bases through hydrogen bonds: adenine pairs with thymine in DNA or uracil in RNA, while guanine pairs with cytosine.
• Enzyme: A biological catalyst that increases the rate of a biochemical reaction without being permanently changed.
• Substrate: The reactant on which an enzyme acts.
• Active site: The specific region of an enzyme where the substrate binds and the reaction occurs.
• Activation energy: The minimum energy required for a chemical reaction to begin.
• Cofactor: A non-protein component required for the activity of some enzymes; it may be an inorganic ion or an organic molecule.
• Apoenzyme: The inactive protein part of an enzyme without its required cofactor.
• Holoenzyme: The complete, active enzyme consisting of an apoenzyme and its cofactor.
• Specificity: The ability of an enzyme to act on a particular substrate or catalyse a particular reaction.
• ATP: A nucleotide derivative that acts as an important immediate energy carrier in cells.
• Ribozyme: An RNA molecule that acts as a catalyst.

Easily Confused

• Monomer and polymer: A monomer is a small molecular unit; a polymer consists of many linked monomer units.
• Monosaccharide and disaccharide: A monosaccharide cannot be hydrolysed into a smaller carbohydrate, whereas a disaccharide contains two joined monosaccharides.
• Glycosidic bond and peptide bond: A glycosidic bond joins monosaccharides; a peptide bond joins amino acids.
• Condensation and hydrolysis: Condensation removes water to build larger molecules; hydrolysis adds water to break them down.
• Reducing and non-reducing sugars: Reducing sugars have a free aldehyde or ketone group capable of reducing certain chemical reagents; glucose, fructose, lactose and maltose are reducing sugars.
• Starch and glycogen: Starch is the main storage polysaccharide in plants; glycogen is the storage polysaccharide in animals and fungi.
• Cellulose and chitin: Cellulose provides strength in plant cell walls; chitin occurs in fungal cell walls and arthropod exoskeletons.
• Saturated and unsaturated fatty acids: Saturated fatty acids have no carbon-carbon double bonds and often form solid fats; unsaturated fatty acids contain one or more double bonds and are commonly found in liquid oils.
• Phospholipid and triglyceride: A phospholipid contains phosphate and forms membrane bilayers; a triglyceride contains glycerol and three fatty acids and stores energy.
• Nucleotide and nucleoside: A nucleotide contains a nitrogenous base, pentose sugar and phosphate; a nucleoside lacks the phosphate group.
• DNA and RNA: DNA contains deoxyribose and thymine and is usually double-stranded; RNA contains ribose and uracil and is usually single-stranded.
• Apoenzyme and holoenzyme: An apoenzyme is the inactive protein component without its cofactor; a holoenzyme is the complete active enzyme with its cofactor.
• Competitive and non-competitive inhibition: Competitive inhibitors bind at the active site; non-competitive inhibitors bind elsewhere and alter enzyme shape.
• Denaturation and catalysis: Denaturation changes or destroys protein structure and activity; catalysis accelerates a reaction without permanently changing the enzyme.
• Activation energy and overall reaction energy: Enzymes lower activation energy but do not change the overall energy released or absorbed by a reaction.

What Gets Asked

• Classifying carbohydrates and identifying examples: Questions may distinguish monosaccharides, disaccharides and polysaccharides, including glucose, fructose, ribose, sucrose, maltose, lactose, starch, glycogen, cellulose and chitin. Marks are lost by confusing storage polysaccharides with structural polysaccharides.
• Writing or describing biomolecule formation and breakdown: Questions may require condensation and hydrolysis, including glycosidic, peptide and ester bond formation. Marks are lost by reversing condensation and hydrolysis or omitting the removal or addition of water.
• Describing protein structure and denaturation: Questions may ask for primary, secondary, tertiary and quaternary structure or the effects of high temperature, extreme pH and chemicals. Marks are lost by treating denaturation as the breakdown of the amino acid sequence rather than loss of three-dimensional structure and activity.
• Comparing lipid types: Questions may involve triglycerides, saturated and unsaturated fatty acids, phospholipids and steroids. Marks are lost by describing lipids as true polymers or confusing phospholipid membrane structure with triglyceride energy storage.
• Comparing DNA and RNA: Questions may test sugar type, bases, strand structure, complementary base pairing and biological role. Marks are lost by using thymine in RNA, uracil in DNA, or failing to state that adenine-thymine pairing has two hydrogen bonds while guanine-cytosine pairing has three.
• Explaining enzyme action and inhibition: Questions may involve active sites, enzyme-substrate complexes, activation energy, optimum temperature and pH, cofactors, competitive inhibition and non-competitive inhibition. Marks are lost by stating that enzymes are consumed, change the overall reaction energy, or that all inhibitors bind to the active site.