Biomolecules

Carbohydrates are optically active polyhydroxy aldehydes or ketones, or compounds that produce them on hydrolysis. They are the most abundant biomolecules, providing energy (glucose), storage (starch, glycogen) and structure (cellulose). They are classified by how many simple sugar units they give on hydrolysis: monosaccharides cannot be hydrolysed further (glucose, fructose, ribose), oligosaccharides give 2–10 units (the disaccharides sucrose, maltose and lactose), and polysaccharides give many units (starch, cellulose, glycogen).

The key monosaccharide is glucose, an aldohexose ($\text{C}_6\text{H}_{12}\text{O}_6$ with an aldehyde group). It exists mostly as a six-membered cyclic pyranose ring, which creates a new stereocentre at C-1 (the anomeric carbon) giving two anomers, $\alpha$ and $\beta$; their interconversion in solution is called mutarotation. Fructose is a ketohexose (same formula but a keto group). Sugar units join through a glycosidic linkage (an ether bond between the anomeric carbon of one unit and a hydroxyl of another) — this is how di- and polysaccharides are built.

A distinction NEET tests often is reducing versus non-reducing sugars. A reducing sugar has a free anomeric (aldehyde/keto) group and so reduces Tollens' and Fehling's reagents — glucose, fructose, maltose and lactose are reducing. Sucrose (table sugar, glucose + fructose) is non-reducing because the glycosidic bond ties up the anomeric carbons of both units, leaving no free group. Sucrose's hydrolysis to glucose + fructose is called inversion (the optical rotation changes sign), giving 'invert sugar'.

The polysaccharides are all polymers of glucose but differ crucially in their links. Starch (the storage carbohydrate of plants) is made of $\alpha$-glucose units and has two parts — linear amylose and branched amylopectin; its $\alpha$-glycosidic links are digestible by humans. Cellulose (the structural material of plant cell walls) is made of $\beta$-glucose units in long straight chains; humans lack the enzyme for $\beta$-links, so we cannot digest cellulose (it is dietary fibre). Glycogen is 'animal starch', a highly branched glucose store in liver and muscle. The starch-vs-cellulose ($\alpha$ vs $\beta$) contrast is a guaranteed NEET point.

Proteins are polymers of $\alpha$-amino acids — molecules that contain both an amino group ($-\text{NH}_2$) and a carboxyl group ($-\text{COOH}$) on the same ($\alpha$) carbon. About 20 standard amino acids build all proteins; those the body cannot make and must obtain from food are essential amino acids, the rest are non-essential. Because they carry both an acidic and a basic group, amino acids are amphoteric and usually exist as a zwitterion (an internal salt, $\text{H}_3\text{N}^+\text{–CHR–COO}^-$); the pH at which the zwitterion has no net charge is the isoelectric point.

Amino acids link through the peptide bond, an amide linkage ($-\text{CO}-\text{NH}-$) formed when the $-\text{COOH}$ of one acid condenses with the $-\text{NH}_2$ of the next, losing water. Two amino acids give a dipeptide, many give a polypeptide, and large polypeptides are proteins. The sequence of amino acids and the way the chain folds determine everything about a protein's function.

Protein structure is described at four levels. The primary structure is the exact sequence of amino acids. The secondary structure is local folding into an $\alpha$-helix or a $\beta$-pleated sheet, held together by hydrogen bonds. The tertiary structure is the overall three-dimensional shape of the whole chain (folded and held by various interactions). The quaternary structure is the assembly of two or more folded chains into one functional unit (haemoglobin, with four chains, is the classic example). NEET frequently asks which forces or features belong to which level.

When a protein is exposed to heat, strong acids or other stresses, it undergoes denaturation: its secondary and tertiary structures break down (the primary sequence remains), the molecule loses its specific shape, and it loses biological activity — boiling an egg or curdling milk are everyday examples. Finally, enzymes are specialised proteins that act as biological catalysts: they are highly specific, work under mild body conditions, and speed up reactions by lowering the activation energy (without being consumed). Vitamins and metal ions often help enzymes as coenzymes/cofactors — a link to the next module.

Nucleic acids are the information molecules of the cell: DNA (deoxyribonucleic acid) stores and transmits genetic information, and RNA (ribonucleic acid) carries out protein synthesis. Both are long polymers (polynucleotides) built from repeating units called nucleotides.

Each nucleotide has three parts: a nitrogenous base, a pentose sugar, and a phosphate group. A base joined to the sugar alone (no phosphate) is a nucleoside; adding phosphate makes it a nucleotide — a distinction NEET likes to test. The nucleotides are linked through phosphodiester bonds between the phosphate of one and the sugar of the next, forming the sugar–phosphate backbone.

The bases are of two kinds: purines (double-ring) — adenine (A) and guanine (G); and pyrimidines (single-ring) — cytosine (C), plus thymine (T) in DNA and uracil (U) in RNA. The sugar is deoxyribose in DNA and ribose in RNA. So the three differences between DNA and RNA are: sugar (deoxyribose vs ribose), one base (thymine vs uracil), and strandedness (DNA is usually double-stranded, RNA usually single-stranded).

DNA's famous double helix (Watson and Crick) consists of two strands wound around each other and held by hydrogen bonds between complementary base pairs: A pairs with T (two hydrogen bonds) and G pairs with C (three hydrogen bonds). This base-pairing rule means the two strands carry the same information in complementary form, which is the basis of accurate replication. RNA comes in three working types — messenger (mRNA), transfer (tRNA) and ribosomal (rRNA) — that together translate the DNA message into proteins. The A–T (2 bonds) and G–C (3 bonds) pairing and the DNA-vs-RNA differences are perennial NEET questions.

Vitamins are organic compounds that the body needs in small amounts for normal metabolism but mostly cannot synthesise, so they must come from the diet. They are grouped by solubility, which controls how they are stored and how often they are needed. Fat-soluble vitamins (A, D, E, K) dissolve in fats and are stored in the liver and fatty tissue, so they need not be eaten daily. Water-soluble vitamins (the B group and C) are not stored (excess is excreted in urine), so they must be supplied regularly.

Each vitamin has specific roles and a characteristic deficiency disease — a favourite NEET recall set. Vitamin A deficiency causes night blindness; vitamin B1 (thiamine) deficiency causes beriberi; vitamin C (ascorbic acid) deficiency causes scurvy (bleeding gums); vitamin D deficiency causes rickets in children and osteomalacia in adults; vitamin K deficiency impairs blood clotting; vitamin E is an antioxidant. Knowing the vitamin → disease pairs is essentially guaranteed marks.

Hormones are chemical messengers secreted directly into the blood by ductless (endocrine) glands; they travel to target organs and regulate processes such as growth, metabolism and reproduction. Chemically they are varied: some are steroids (the sex hormones testosterone and oestrogen, and cortisol), some are amino-acid derivatives (adrenaline, thyroxine), and some are proteins/peptides (insulin). Key examples are insulin (lowers blood glucose; its lack causes diabetes), adrenaline (the fight-or-flight hormone), and thyroxine (regulates metabolic rate; needs iodine, whose deficiency causes goitre).

It is important to distinguish the two. Vitamins are nutrients obtained from food and act mainly as coenzymes assisting enzymes, whereas hormones are made within the body by glands and act as regulatory signals. Both are needed in small quantities and both connect chemistry to physiology — the reason this applied topic appears in NEET. Together with carbohydrates, proteins and nucleic acids, vitamins and hormones complete the picture of the molecules that run living systems.