Q3 LONG ANSWER 10M ★ High-yield

Define and classify carbohydrates with suitable examples. Add a note on the derived carbohydrates.

Biochemistry · 1st Professional — Pre-Clinical

Definition

Carbohydrates = polyhydroxy aldehydes or ketones, or compounds that yield these on hydrolysis.

General formula (CH2O)n for most, though deoxyribose and the amino sugars do not fit it
They are the chief source of dietary energy, supplying 4 kcal/g and 55–70% of calories in the Indian diet

Classification

Class	Definition	Examples
Monosaccharides	Cannot be hydrolysed to simpler sugars	Glucose, fructose, galactose, ribose
Disaccharides	Yield 2 monosaccharides	Maltose, lactose, sucrose
Oligosaccharides	Yield 3–10	Raffinose, stachyose
Polysaccharides	Yield more than 10	Starch, glycogen, cellulose, glycosaminoglycans

Monosaccharides

By number of carbons

Type	Carbons	Examples	Significance
Triose	3	Glyceraldehyde, dihydroxyacetone	Glycolytic intermediates
Tetrose	4	Erythrose	HMP shunt
Pentose	5	Ribose, deoxyribose, xylulose, arabinose	Nucleic acids, ATP, NAD, FAD
Hexose	6	Glucose, fructose, galactose, mannose	The nutritionally important sugars
Heptose	7	Sedoheptulose	HMP shunt

By functional group

Aldoses — carry an aldehyde group at C-1: glucose, galactose, ribose
Ketoses — carry a ketone group at C-2: fructose, ribulose, sedoheptulose

Disaccharides

Sugar	Components	Linkage	Reducing?	Source
Maltose	Glucose + glucose	α-1,4	Yes	Starch digestion; malt
Lactose	Galactose + glucose	β-1,4	Yes	Milk — the only sugar of animal origin
Sucrose	Glucose + fructose	α-1,2 β	NO	Cane sugar, beet
Isomaltose	Glucose + glucose	α-1,6	Yes	From amylopectin branch points
Trehalose	Glucose + glucose	α-1,1	NO	Fungi, insect haemolymph

CLINICAL PEARL

Sucrose is non-reducing because both anomeric carbons are involved in the linkage — C-1 of glucose to C-2 of fructose. There is no free aldehyde or ketone group left. This is why a patient with sucrose in the urine gives a negative Benedict test.

Polysaccharides

A. Homopolysaccharides — one kind of monosaccharide

Polysaccharide	Unit	Linkages	Significance
Starch	Glucose	Amylose — α-1,4 linear (15–20%); amylopectin — α-1,4 with α-1,6 branches	Chief dietary carbohydrate of plants
Glycogen	Glucose	α-1,4 with α-1,6 every 8–12 residues	Storage form in animals; more branched than starch
Cellulose	Glucose	β-1,4	Plant cell wall; indigestible by man — dietary fibre
Inulin	Fructose	β-2,1	Used to measure GFR — freely filtered, neither secreted nor reabsorbed
Dextrin	Glucose	α-1,4	Partial hydrolysis product of starch
Dextran	Glucose	α-1,6	Plasma volume expander
Chitin	N-acetylglucosamine	β-1,4	Exoskeleton of insects and crustacea

B. Heteropolysaccharides — more than one kind of unit

Glycosaminoglycans — hyaluronic acid, chondroitin sulphate, keratan sulphate, dermatan sulphate, heparan sulphate, heparin
Each is a repeating amino sugar + uronic acid disaccharide, usually sulphated
Agar and gum acacia are plant heteropolysaccharides

Derived Carbohydrates

Type	Example	Significance
Sugar acids	Gluconic acid, glucuronic acid, ascorbic acid	Glucuronic acid conjugates bilirubin and drugs
Sugar alcohols (polyols)	Sorbitol, mannitol, dulcitol, inositol	Diabetic cataract and neuropathy; mannitol as an osmotic diuretic
Amino sugars	Glucosamine, galactosamine, N-acetylneuraminic acid (sialic acid)	Glycosaminoglycans and glycoproteins
Deoxy sugars	Deoxyribose	DNA

Applied Aspects

Lactose intolerance — lactase deficiency, very common in Indian adults; the undigested sugar causes osmotic diarrhoea and flatulence
Cellulose cannot be digested because man has no β-glucosidase, yet it is nutritionally important as dietary fibre
Inulin clearance is the gold standard for GFR, though creatinine clearance is used in practice
Sialic acid on the RBC surface gives the negative charge that keeps cells apart; its loss with ageing marks the cell for splenic removal
Sugar alcohols are used as sweeteners but cause osmotic diarrhoea in quantity, and sorbitol must be avoided in hereditary fructose intolerance
Dextran is used as a plasma volume expander, and is also the polysaccharide of dental plaque formed by oral streptococci from sucrose

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