They are the simplest carbohydrates. Glucose, fructose and ribose are examples of monosaccharides.
Monosaccharides are carbohydrates that cannot be hydrolysed further to simpler polyhydroxy aldehyde or ketone units.
They contain a free aldehydic/ketonic group or can generate one in solution through the free anomeric carbon. All monosaccharides are reducing sugars; maltose and lactose are reducing disaccharides.
Reducing sugars are carbohydrates that reduce Fehling's solution and Tollens' reagent.
Starch is the main storage polysaccharide of plants. Cellulose forms the structural material of plant cell walls.
Carbohydrates store food as starch and provide structural support as cellulose.
Ribose, 2-deoxyribose, galactose and fructose are single sugar units. Maltose and lactose hydrolyse to two monosaccharide units, so they are disaccharides.
Monosaccharides: ribose, 2-deoxyribose, galactose and fructose. Disaccharides: maltose and lactose.
It is formed by loss of water between hydroxyl groups of monosaccharide units. Disaccharides and polysaccharides contain monosaccharides joined by glycosidic linkages.
A glycosidic linkage is an oxide linkage joining two monosaccharide units through an oxygen atom.
Both glycogen and starch are polymers of alpha-D-glucose. Starch is the storage carbohydrate of plants and contains amylose and amylopectin; glycogen is stored in animal tissues and has a more highly branched structure.
Glycogen is the storage polysaccharide of animals. It is more highly branched than starch.
Sucrose contains alpha-D-glucose and beta-D-fructose units. Lactose contains beta-D-galactose and beta-D-glucose units.
(i) Sucrose gives glucose and fructose. (ii) Lactose gives galactose and glucose.
In starch, glucose units are joined mainly by alpha glycosidic linkages. In cellulose, beta-D-glucose units are joined by beta-1,4 glycosidic linkages, giving straight chains suitable for structural support.
Starch is a polymer of alpha-D-glucose, while cellulose is a polymer of beta-D-glucose.
HI reduction shows that the six carbons of glucose are in a straight chain. Bromine water is a mild oxidising agent and oxidises the aldehyde group to –COOH. Nitric acid oxidises both terminal groups to form the dicarboxylic acid, saccharic acid.
(i) Prolonged heating with HI gives n-hexane. (ii) Bromine water oxidises D-glucose to gluconic acid. (iii) Nitric acid oxidises D-glucose to saccharic acid.
The open-chain aldehyde structure predicts normal aldehyde reactions, but glucose lacks a free –CHO group in its dominant cyclic hemiacetal forms. This explains its failure to give Schiff's test or NaHSO3 addition, the non-reaction of pentaacetate with hydroxylamine, and the existence of alpha and beta anomers.
D-glucose does not give Schiff's test, does not form a sodium hydrogensulphite addition product, glucose pentaacetate does not react with hydroxylamine, and glucose exists in alpha and beta crystalline forms.
The classification depends on dietary requirement. Essential amino acids are required from food, whereas non-essential amino acids are made in the body in sufficient amounts.
Essential amino acids cannot be synthesised by the body and must be supplied in diet; examples are valine and leucine. Non-essential amino acids can be synthesised by the body; examples are glycine and alanine.
A peptide bond forms by condensation between –COOH of one amino acid and –NH2 of another. The exact amino acid sequence defines primary structure. Heat or pH changes disturb hydrogen bonds and other interactions, causing denaturation.
(i) Peptide linkage is the –CO–NH– bond between amino acids. (ii) Primary structure is the sequence of amino acids in a polypeptide chain. (iii) Denaturation is loss of native secondary/tertiary structure and biological activity.
These arise from regular folding of the polypeptide backbone due to hydrogen bonding between peptide-bond –NH– and C=O groups.
The common secondary structures are alpha-helix and beta-pleated sheet.
The –NH group of each amino acid residue forms a hydrogen bond with the C=O group of an adjacent turn of the helix.
Hydrogen bonding stabilises the alpha-helix structure.
Fibrous proteins such as keratin and myosin have parallel chains held by hydrogen/disulphide bonds. Globular proteins such as insulin and albumins have coiled chains forming compact shapes.
Fibrous proteins have thread-like chains and are generally insoluble in water; globular proteins are folded into roughly spherical shapes and are usually water soluble.
In zwitterionic form, the molecule contains –NH3+ and –COO-. The carboxylate part can accept a proton in acidic medium and the ammonium part can lose a proton in basic medium, so amino acids react with both acids and bases.
Amino acids are amphoteric because they contain both acidic –COOH and basic –NH2 groups and exist as zwitterions.
Almost all enzymes are globular proteins. They are highly specific for their substrates and reduce the activation energy of biochemical reactions.
Enzymes are biological catalysts that speed up biochemical reactions under mild conditions.
Heat or pH changes disturb stabilising interactions such as hydrogen bonds. Globules unfold and helices uncoil, causing loss of biological activity, but peptide-bond sequence is not broken.
Denaturation destroys secondary and tertiary structures, while the primary structure remains intact.
Fat-soluble vitamins are A, D, E and K. Water-soluble vitamins are B group vitamins and vitamin C. Deficiency of vitamin K increases blood clotting time.
Vitamins are classified as fat soluble and water soluble. Vitamin K is responsible for blood coagulation.
Deficiency of vitamin A causes xerophthalmia and night blindness. Deficiency of vitamin C causes scurvy, including bleeding gums. These vitamins must therefore be supplied in diet from their common food sources.
Vitamin A is needed for healthy eyes and vision; vitamin C prevents scurvy and supports healthy gums/tissues. Vitamin A sources include fish liver oil, carrots, butter and milk; vitamin C sources include citrus fruits, amla and green leafy vegetables.
Nucleic acids are polymers of nucleotides containing a pentose sugar, phosphate and nitrogenous base. DNA stores genetic information and RNA molecules help express it during protein synthesis.
Nucleic acids are polynucleotides such as DNA and RNA. They transmit hereditary information and direct protein synthesis.
A base attached to the 1' position of a pentose sugar forms a nucleoside. When phosphoric acid is linked, usually at the 5' position of the sugar, the product is a nucleotide.
A nucleoside contains a base and a sugar; a nucleotide contains a base, a sugar and phosphate.
Hydrogen bonds form only between specific base pairs. Therefore, the sequence on one DNA strand determines the sequence on the other strand, even though the two sequences are not identical.
The strands are complementary because bases pair specifically: adenine pairs with thymine and cytosine pairs with guanine.
Both contain adenine, guanine and cytosine. DNA has thymine as the fourth base and deoxyribose sugar; RNA has uracil and ribose sugar. DNA is the chemical basis of heredity and carries coded information. mRNA, rRNA and tRNA help translate that information into proteins.
DNA contains 2-deoxyribose, thymine and usually a double helix; RNA contains ribose, uracil and is usually single stranded. DNA stores hereditary information, while RNA participates directly in protein synthesis.
These RNA molecules perform different roles in protein synthesis: mRNA carries the message, rRNA is part of ribosomes, and tRNA transfers amino acids during translation.
The three types are messenger RNA (mRNA), ribosomal RNA (rRNA) and transfer RNA (tRNA).