Unit 27: Biochemistry — Long Questions

Polysaccharides are large molecules made of a large number of monosaccharide units joined together by glycosidic bonds, with the general molecular formula (C₆H₁₀O₅)ₙ; they yield simple sugars (monosaccharides) on hydrolysis. Common examples are starch, glycogen, cellulose and pectin.

Starch the main energy-storage polysaccharide in plants, made of glucose units, with two components — amylose, which has long, unbranched chains with many glycosidic linkages, and amylopectin, a compact, highly branched structure of many glucose units. Amylopectin is water-soluble, while amylose is insoluble. Potato, sugar beet, rice and grains are rich in starch.

Cellulose the major structural material of plant cell walls, consisting of up to 2500 glucose units joined into long, unbranched chains linked by glycosidic bonds. These chains form strong hydrogen bonds with each other, making cellulose rigid and insoluble in water. It makes up most of plant leaves, branches and trunks, and in human food acts as dietary fibre that improves gut health.

Glycogen ('animal starch') a polymer of glucose stored in the liver and muscles of animals as an energy reserve. Its structure resembles amylopectin but is even more highly branched, distinguishing it from starch.

Proteins are organized into four structural levels, each contributing to how the final protein functions.

Primary structure the specific linear sequence of amino acids linked by peptide bonds in a polypeptide chain. This sequence determines how the protein folds into its three-dimensional shape, which in turn defines its biological function; even a single amino acid change can alter the structure and impair function (e.g. in sickle cell anaemia, abnormal haemoglobin deforms red blood cells into a sickle shape, disrupting blood circulation). Proteins such as albumin and insulin depend on their precise sequence to function properly.

Secondary structure arises when backbone atoms form hydrogen bonds within a polypeptide chain, producing regular patterns such as α-helices (hydrogen bonds along a single chain, giving a coiled spiral, e.g. keratin) and β-pleated sheets (hydrogen bonds between adjacent strands, giving a folded sheet, e.g. silk fibroin). These arrangements provide stability and flexibility that contribute to the overall 3-D shape.

Tertiary structure the overall three-dimensional shape formed when the polypeptide chain folds due to interactions between side chains (R-groups) — hydrogen bonds, ionic bonds, hydrophobic interactions and disulphide bridges. This folding, often into a globular shape, determines function directly; for example, in enzymes, the precise shape of the active site formed by tertiary folding allows specific binding to the substrate and enables catalysis. Any disturbance of this structure affects enzyme activity.

Quaternary structure present in some proteins, where two or more folded polypeptide chains (subunits) associate through hydrogen bonds, ionic (salt) bridges, hydrophobic interactions and sometimes disulphide bonds, giving the complete functional protein its final assembled form.

In each case, the level of structure builds on the one before it, so that the specific 3-D shape produced ultimately determines what the protein can do in the body.

Lipids are broadly classified into simple lipids, complex (compound) lipids, and derived (associated) lipids, based on their chemical composition.

Simple Lipids esters of fatty acids with different types of alcohols, sub-classified as fats, oils and waxes. Triglycerides (fats and oils) are esters of one glycerol molecule with three fatty acids; saturated fatty acids give solid fats, while unsaturated fatty acids give liquid oils. They act as efficient energy-storage molecules and provide insulation and protection of organs. Waxes are esters of long-chain fatty acids with long-chain alcohols, both nonpolar and hydrophobic, serving as protective, water-repelling coatings, e.g. beeswax.

Compound (Complex) Lipids esters of fatty acids with alcohols that carry an additional group such as phosphate, a nitrogenous base, carbohydrate, or protein. Phospholipids resemble fats but have only two fatty acids attached to glycerol, with the third hydroxyl linked to a phosphate group, making the molecule polar (hydrophilic head, hydrophobic tails); in water they form the bilayer structure of all cell membranes. Lipoproteins are complexes of lipids and proteins that transport hydrophobic lipids like cholesterol and triglycerides in the blood. Glycolipids are lipids with carbohydrate groups attached, mostly found in nervous tissue.

Derived (Associated) Lipids the hydrolytic products of simple and compound lipids, which still have lipid-like characteristics, e.g. fatty acids, glycerol, steroids, and lipid-soluble vitamins. Steroids (e.g. cholesterol, bile salts, vitamin D) consist of a steroid nucleus of three fused cyclohexane rings and one cyclopentane ring; many carry an –OH group, placing them among the sterols. Terpenes are plant hydrocarbons built from isoprene units, with fewer than 40 carbons (e.g. limonene); terpenes with 40 carbons are called carotenoids.

Nucleic acids are large biomolecules that store and transmit genetic information. They are polymers of nucleotides, each nucleotide made of a five-membered sugar (deoxyribose in DNA, ribose in RNA), a phosphate group, and a nitrogenous base. Nucleotides are joined by phosphodiester linkages between the 3' and 5' carbons of successive sugars, forming long polynucleotide chains with a sugar-phosphate backbone; these covalent linkages give the chain strength and stability.

DNA is present in the nucleus of every living cell and acts as the coded blueprint for making proteins; a specific protein is made by a specific gene, a section of DNA. DNA consists of two polynucleotide chains that coil around each other in a right-handed double helix, running in opposite (antiparallel) directions. The bases project inward and pair across the helix through hydrogen bonds — adenine with thymine (2 hydrogen bonds) and guanine with cytosine (3 hydrogen bonds). DNA undergoes self-replication, copying itself so that each new cell receives a complete set of genetic material, passing genetic information to daughter cells and the next generation.

RNA is usually single-stranded, containing ribose sugar and the bases A, U, G and C. There are three major types: messenger RNA (mRNA), which carries the genetic blueprint copied from DNA in the nucleus; ribosomal RNA (rRNA), which helps form the ribosome; and transfer RNA (tRNA), which translates the nucleotide sequence of mRNA into the corresponding sequence of amino acids.

Role in protein synthesis the process begins with DNA replication in the nucleus. The genetic instructions for a protein are copied from a DNA gene into mRNA — a process called transcription. The completed mRNA detaches from DNA, enters the cytoplasm, and attaches to ribosomes. There, the code carried by mRNA is read and converted into a specific sequence of amino acids — a process called translation. tRNA molecules deliver amino acids in the order specified by the mRNA sequence; these amino acids join together via peptide bonds, forming a growing polypeptide chain, which finally becomes the complete protein.