Definitions [5]
Define carbohydrates.
Carbohydrates are optically active polyhydroxy aldehydes or polyhydroxy ketones or compounds that can be hydrolysed to polyhydroxy aldehydes or polyhydroxy ketones.
Define α-amino acids.
α-Amino acids are carboxylic acids having an amino (–NH2) group bonded to the α-carbon, that is, the carbon next to the carboxyl (–COOH) group.
Define peptide bond.
The bond that connects α-amino acids to each other is called a peptide bond.
Define the term Protein.
Chemically proteins are polyamides which are high molecular weight polymers of the monomer units, i.e., α-amino acids. OR It can also be defined as proteins are the biopolymers of a large number of α-amino acids and they are naturally occurring polymeric nitrogenous organic compounds containing 16% nitrogen and peptide linkages (-CO-NH-)
Define enzymes.
Enzymes are biological catalysts that speed up chemical reactions in living cells without being consumed in the process.
Key Points
- Carbohydrates are organic biomolecules made of C, H and O, usually fitting the general formula Cx(H₂O)y and existing as aldoses or ketoses.
- They are classified into monosaccharides, disaccharides and polysaccharides; monosaccharides cannot be hydrolysed further, disaccharides are formed by two monosaccharides via glycosidic bonds, and polysaccharides are long polymers.
- Some sugars like digitoxose (C₆H₁₂O₄) and rhamnose (C₆H₁₂O₅) do not obey the typical Cx(H₂O)y formula.
- All monosaccharides are reducing sugars because they possess a free aldehyde or ketone group.
- Cellulose is a linear polymer of β‑D‑glucose, unlike starch and glycogen which are polymers of α‑glucose and show branching.
- Biologically, carbohydrates supply energy for metabolism; glucose is the main substrate for ATP synthesis and lactose provides energy to infants.
- Polysaccharides such as starch and glycogen act as storage products and also contribute to structural components of cell membranes and cell walls.
- Lipids are esters of fatty acids with a hydrogen-to-oxygen ratio greater than 2:1.
- They are classified into simple lipids (fats and waxes), compound lipids (phospholipids, glycolipids, lipoproteins) and sterols (derived lipids).
- Simple lipids are esters of fatty acids with various alcohols, while compound lipids typically contain 1 glycerol, 2 fatty acids and either 1 phosphate group (phospholipid) or 1 simple sugar (glycolipid).
- Glycolipids, also called cerebrosides, are abundant in the myelin sheath of nerve cells.
- In plants, sterols occur as phytosterols; the yam plant (Dioscorea) yields the sterol diosgenin, used to manufacture birth‑control pills.
- Proteins are polypeptides made of long chains of amino acids linked by peptide bonds.
- In a polypeptide, the first amino acid is the N‑terminal amino acid and the last is the C‑terminal amino acid.
- Protein structure is described at four levels: primary, secondary, tertiary and quaternary.
- Primary structure is the linear sequence of amino acids.
- Secondary structure involves local folding or coiling (α‑helix, β‑pleated sheet) stabilised by hydrogen bonds.
- Tertiary structure is the three‑dimensional folding of a polypeptide due to side‑chain interactions.
- Quaternary structure is the association of two or more polypeptide chains into a functional protein (e.g., haemoglobin).
- Enzymes are proteinaceous substances that alter (usually increase) the rate of chemical reactions without being used up.
- By composition they are of two types: simple enzymes made only of protein, and conjugated enzymes made of an apoenzyme plus a non‑protein cofactor or prosthetic group.
- They possess a specific three‑dimensional conformation with an active site, show high substrate specificity, and usually have names ending in “‑ase”.
- Enzymes catalyse reversible biochemical reactions, remain unchanged at the end, and are highly sensitive to changes in temperature and pH.
- In the catalytic cycle, the enzyme (E) binds substrate (S) at its active site to form an enzyme–substrate (ES) complex, converts it into an enzyme–product (EP) complex, then releases product (P) so the free enzyme can be reused.
