Ch 14Biomolecules — Class 12 Chemistry, concept-first.

Living organisms are built from a small number of large, repeating classes of molecules, and this chapter studies three of the most important: carbohydrates, proteins and nucleic acids.

Carbohydrates are defined chemically, not just by taste: they are polyhydroxy aldehydes or ketones, or compounds that yield such polyhydroxy carbonyl units on hydrolysis.

Carbohydrates fall into three broad groups according to how they behave on hydrolysis. Monosaccharides do not hydrolyse into any smaller carbohydrate units at all -- they ARE the smallest unit (exampl…

The IUPAC system gives every monosaccharide a systematic name built from two pieces of information: whether its single carbonyl group is an aldehyde or a ketone, and how many carbon atoms the molecule…

Glucose is one of the most abundant and important monosaccharides, occurring in nature both in its free form (as in fruit and honey) and combined into larger carbohydrates such as sucrose, starch and…

Sucrose is hydrolysed by warming it with dilute hydrochloric acid or dilute sulfuric acid for about two hours; this splits its single glycosidic linkage and converts the sucrose completely into an equ…

On a commercial scale, glucose is manufactured by hydrolysing starch rather than sucrose, because starch (from grains such as maize) is a cheaper and more abundant raw material.

Glucose's open-chain structure -- an aldohexose, CHO-(CHOH)4-CH2OH, with one aldehydic carbonyl at C-1 and a hydroxyl group on each of the other five carbons -- was worked out entirely from chemical e…

Glucose's structural formula, CHO-(CHOH)4-CH2OH, contains four chiral (stereogenic) carbons -- C-2, C-3, C-4 and C-5, each bonded to four different groups (C-1 and C-6 are not chiral).

The open-chain Fischer structure of glucose, though it correctly explains many of glucose's properties, cannot account for every chemical property glucose is observed to show -- most notably, glucose…

The cyclic hemiacetal group present in glucopyranose is not a permanently 'closed' structure -- it sits in a fast equilibrium with a small, ever-present amount of the open-chain form carrying a genuin…

Fructose (C6H12O6) is the second monosaccharide studied in detail in this chapter -- a laevorotatory ketohexose, with specific rotation [alpha]D20 = -92.4 degrees; this strong laevorotation is why fru…

Disaccharides are the most familiar class of oligosaccharide: on hydrolysis with either dilute acid or a specific enzyme, each disaccharide molecule splits into exactly two monosaccharide units, which…

Sucrose, C12H22O11, is the most common disaccharide of everyday life (ordinary table sugar) and is strongly dextrorotatory, with specific rotation +66.5 degrees.

Maltose, C12H22O11, is a disaccharide built from two units of D-glucose, joined by a glycosidic linkage that runs from C-1 of one glucose ring to C-4 of the other.

Lactose, C12H22O11, is the disaccharide naturally present in milk (both human and animal), and is built from one unit of D-galactose and one unit of D-glucose -- two DIFFERENT monosaccharides, unlike…

Polysaccharides are the third and largest class of carbohydrate defined in section 14.2.1: they form when a very large number of monosaccharide units -- hundreds to many thousands -- are linked togeth…

Starch, the plant storage polysaccharide, is itself a polymer built entirely of alpha-D-glucose units, but it is not a single uniform substance -- it is a mixture of two distinct components with diffe…

Cellulose, the principal structural polysaccharide of plants, is -- like amylose -- an unbranched, straight-chain polysaccharide, but built from beta-glucose units (rather than starch's alpha-glucose…

Glycogen is the carbohydrate reserve that animals store, functionally the direct animal-kingdom counterpart of plant starch.

Proteins are the fundamental structural materials of animal bodies -- they build muscle, skin, hair, nails and connective tissue -- and, in the specialised form of enzymes, they also drive essentially…

Complete hydrolysis of any protein breaks every peptide linkage in it and yields a mixture of free alpha-amino acids -- the monomer units from which every protein is built.

The chemical bond that links one alpha-amino acid to the next, building up the polymer chain of a protein, is called the peptide bond.

Beyond their chemical composition (the sequence of amino-acid residues, discussed from section 14.3.4a onward), proteins can also be classified more simply by the overall molecular SHAPE their polypep…

Globular proteins are proteins whose individual polypeptide-chain molecules adopt an overall compact, roughly SPHERICAL shape.

Fibrous proteins are proteins whose individual polypeptide-chain molecules adopt an overall elongated, ROD-like shape, rather than the compact sphere of a globular protein.

Beyond simple shape classification (globular vs fibrous, section 14.3.3), the true diversity of protein shapes and functions -- from the load-bearing fibres of hair and muscle, to the catalytic pocket…

The primary structure of a protein is, most simply, the exact SEQUENCE in which its constituent alpha-amino-acid residues are strung together, one after another, joined by peptide bonds -- it is the m…

The secondary structure of a protein describes the regular, LOCAL three-dimensional shape that a stretch of the polypeptide backbone folds into, distinct from (and nested within) the larger overall te…

The tertiary structure of a protein is the overall, non-repeating three-dimensional shape adopted by one ENTIRE polypeptide chain -- the result of the whole chain folding up in a specific way that bot…

The quaternary structure of a protein is the highest of the four structural levels, and applies only to proteins built from MORE than one separately-folded polypeptide chain: when two or more chains,…

Denaturation is the process by which a protein's carefully folded, specific three-dimensional shape is disrupted and destroyed by an external physical or chemical stress -- commonly high temperature,…

Enzymes are the biological catalysts responsible for the enormous number of chemical reactions that must occur inside a living body, all proceeding efficiently under the comparatively mild, constant c…

Nucleic acids are the third and final class of biomolecule studied in this chapter, and are distinguished from carbohydrates and proteins by their unique biological role: they are the molecules respon…

Nucleic acids -- both DNA and RNA -- are unbranched polymers built from repeating monomer units called nucleotides; because of this, nucleic acids are said to have a polynucleotide structure.

Both DNA and RNA are true polymers of their nucleotide monomer units, built by repeatedly joining the free 3'-OH group on one nucleotide's sugar to the free 5'-phosphate group of the NEXT nucleotide i…

In 1953, the scientists James Watson and Francis Crick proposed a now-famous double-helix model for the overall three-dimensional structure of DNA, a model later confirmed directly by electron-microsc…

The end-of-chapter questions of this group, exactly as the book prints them.

The end-of-chapter questions of this group, exactly as the book prints them.

The end-of-chapter questions of this group, exactly as the book prints them.

The end-of-chapter questions of this group, exactly as the book prints them.

The end-of-chapter questions of this group, exactly as the book prints them.

Sample papers and previous-year board questions for this subject.