MCQs

Trioses are 3-carbon monosaccharides. The simplest aldose (aldehyde-containing) triose is glyceraldehyde. The simplest ketose (ketone-containing) triose is dihydroxyacetone. Both are key intermediates in glycolysis and photosynthesis.

Sucrose is dextrorotatory (+66.5°). Upon hydrolysis, the resulting fructose is strongly levorotatory (-92°), which outweighs the dextrorotation of glucose (+52.7°). The net optical rotation of the mixture (invert sugar) is negative (-19.8°), thus the rotation is inverted.

Starch digestion hydrolyzes starch to glucose, which is then phosphorylated to glucose-6-phosphate and isomerized to glucose-1-phosphate for entry into glycolysis. Cellulose is synthesized in plants from the activated monomer UDP-glucose, which is derived from glucose-1-phosphate.

Glycogen's extreme branching creates a compact, highly soluble granule with thousands of terminal non-reducing ends. Enzymes like glycogen phosphorylase and debranching enzyme can work simultaneously at multiple ends, releasing glucose-1-phosphate far faster than from the less branched amylopectin of starch.

Amylose forms a left-handed helix. Iodine (as I₃⁻ or I₅⁻ ions) fits into the central hydrophobic channel of this helix. The resulting charge-transfer complex absorbs light strongly, giving a characteristic deep blue-black color.

Chitin is a linear homopolymer of N-acetyl-D-glucosamine, which is glucose with an N-acetylamino group at the C-2 position. It is linked by β-1,4 glycosidic bonds, analogous to the structure of cellulose.

This change in optical rotation is due to mutarotation. α-D-glucose (+112°) undergoes ring opening and reclosing to form an equilibrium mixture of β-D-glucose (+18.7°) and α-D-glucose, with the overall specific rotation settling at +52.7°.

A six-membered ring (pyranose) forms when the carbonyl carbon (C1 in aldoses) reacts with the -OH group on C5. A five-membered ring (furanose) would involve a reaction with the -OH on C4. Glucose predominantly forms a pyranose ring.

In lactose intolerance, lactase (β-galactosidase) is deficient. Undigested lactose passes to the colon, where gut microbiota ferment it. This produces gases like H₂, CO₂, and methane (causing bloating) and short-chain fatty acids, which draw water into the bowel (osmotic diarrhea).

The D/L classification is based on the configuration of the chiral carbon farthest from the most oxidized end (the carbonyl group). If the -OH on this carbon is on the right in a Fischer projection, it is D; if on the left, it is L. For glucose, this is C-5.

Cellulose is a homopolymer of β-D-glucose and constitutes the majority of plant biomass. Its annual production is estimated to be over 10¹¹ tons, making it the most abundant organic compound on Earth. Starch and glycogen are storage, not structural, polymers.

Sucrose is a disaccharide of glucose and fructose. Hydrolysis of the glycosidic bond (e.g., by sucrase/invertase) yields one molecule of D-glucose and one molecule of D-fructose. This 1:1 mixture is known as invert sugar.

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.