MCQs

The formation of a glycosidic bond between two monosaccharides is a dehydration synthesis (condensation). The hydroxyl group from one sugar's anomeric carbon combines with a hydrogen from a hydroxyl of another sugar, releasing one water molecule and forming a new covalent bond.

The free aldehyde or ketone group of a reducing sugar reduces the blue Cu²⁺ ions (cupric) in Benedict's reagent to Cu⁺ ions (cuprous). These precipitate out of the alkaline solution as an insoluble brick-red solid, copper(I) oxide (Cu₂O).

Cellulose is a linear, unbranched homopolymer of glucose. The β-1,4 linkage causes the chain to be straight. Adjacent chains align and form extensive inter-chain hydrogen bonds, creating rigid, high-tensile-strength microfibrils that provide structural integrity to the cell wall.

Glycogen is essentially the animal equivalent of amylopectin but is more extensively branched (branching every 8-12 residues). This extreme branching creates many non-reducing ends for glycogen phosphorylase to attack, enabling an extremely rapid release of glucose-1-phosphate to meet metabolic demands.

Amylose is a linear, helical polymer of glucose linked by α-1,4 bonds. Amylopectin is a highly branched polymer with an α-1,4 linked backbone and α-1,6 glycosidic bonds at branch points occurring approximately every 24-30 glucose units.

Human digestive enzymes (salivary and pancreatic amylase) can only hydrolyze the α-1,4 glycosidic bonds found in starch's amylose and amylopectin. Cellulose consists of glucose units linked by β-1,4 bonds, which require the enzyme cellulase, not produced in the human digestive tract.

By definition, oligosaccharides (oligo = few) are short polymers of 2 to about 10 monosaccharides. Common examples are disaccharides (sucrose, lactose, maltose). Polysaccharides contain hundreds or thousands of monosaccharide units.

Lactose is a reducing disaccharide. It is formed from β-D-galactose linked to the C4 of D-glucose via a β-1,4 glycosidic linkage. The glucose unit has a free anomeric carbon, giving lactose its reducing properties.

Sucrose consists of α-D-glucose and β-D-fructose linked via a glycosidic bond between their anomeric carbons (C1 of glucose and C2 of fructose). Since both anomeric carbons are involved, neither unit can open to expose a free carbonyl group, making it a non-reducing sugar.

Maltose is a reducing disaccharide formed from two D-glucose units linked by an α-1,4 glycosidic bond. The C1 of the first glucose (in α-configuration) is linked to the C4 of the second glucose. The second glucose retains a free anomeric carbon, making maltose a reducing sugar.

When a pure anomer (α or β) is dissolved in water, the specific rotation of the solution changes over time until a constant value is reached. This is mutarotation, resulting from the ring opening and reclosing, establishing an equilibrium mixture of α (36%), β (64%), and the open-chain form (<0.1%).

In the chair conformation, bulky substituents preferentially occupy equatorial positions (pointing out from the ring) rather than axial positions (perpendicular to the ring). β-D-glucose has all its -OH and -CH₂OH groups in equatorial positions, making it the most stable and abundant hexose.

Epimers are sugars that differ in configuration at only one chiral center. Glucose and galactose are identical in structure except for the orientation of the hydroxyl group on C-4, making them C-4 epimers. Glucose and mannose are C-2 epimers.

Enantiomers are a pair of molecules that are non-superimposable mirror images of each other. D-glucose and L-glucose are mirror images at all four chiral centers (C2, C3, C4, and C5), making them enantiomers. Diastereomers differ at one or more, but not all, chiral centers.

A reducing sugar has a free anomeric carbon whose carbonyl group can be oxidized, thereby reducing another agent like Cu²⁺ in Benedict's reagent. The free aldehyde or α-hydroxyketone group is essential for this property. Non-reducing sugars lack this free group.

The nucleophilic addition of an alcohol (hydroxyl group) to a carbonyl group forms a hemiacetal (from an aldehyde) or a hemiketal (from a ketone). This intramolecular reaction converts the linear monosaccharide into its cyclic form, creating a new chiral center (the anomeric carbon).

Monosaccharides are polyhydroxy carbonyl compounds. If the carbonyl group is at the end of the carbon chain (C1), it is an aldehyde and the sugar is an aldose (e.g., glucose). If the carbonyl group is on an internal carbon (C2), it is a ketone and the sugar is a ketose (e.g., fructose).

Antifreeze proteins adsorb to the surface of nascent ice nuclei. Their large, hydrophilic structure sterically hinders and geometrically mismatches the approach and orderly crystallization of further water molecules, effectively stopping ice crystal growth.

Heat energy disrupts the internal hydrogen bonds of the starch granule. Water molecules then penetrate and form new H-bonds with the exposed -OH groups of the starch polymers (imbibition). This swelling, called gelatinization, eventually ruptures the granules, thickening the mixture.