MCQs

Peptidoglycan (murein) is unique to bacteria. Its carbohydrate backbone is a linear chain of alternating N-acetylglucosamine (NAG) and N-acetylmuramic acid (NAM) linked by β-1,4 glycosidic bonds. Lysozyme hydrolyzes this specific bond.

Fructose is phosphorylated to fructose-1-phosphate, which is cleaved to glyceraldehyde and dihydroxyacetone phosphate, entering glycolysis downstream of PFK-1. This bypasses a critical regulatory checkpoint, allowing a rapid, unregulated influx of carbon that can overload the TCA cycle and lead to increased de novo lipogenesis.

A non-reducing sugar lacks a free carbonyl group, so it cannot participate in non-enzymatic glycation (Maillard reaction) with amino groups. This stability is crucial in the hemolymph, where high concentrations of trehalose could otherwise react with and damage circulating proteins.

Starch is synthesized in plastids. Transitory starch is synthesized in chloroplasts during photosynthesis and broken down at night. Storage starch is synthesized and stored in amyloplasts (non-pigmented plastids) in tissues like tubers, seeds, and roots.

Dextran is a branched bacterial polysaccharide of D-glucose, with a backbone of α-1,6 glycosidic linkages and α-1,3 branch points. Its high molecular weight and colloidal osmotic properties make it useful for drawing fluid into the circulatory system.

Cellulose's linear, unbranched chains allow them to lie parallel and form extremely stable, regular intermolecular hydrogen bonds. These strong lateral interactions create crystalline microfibrils that exclude water and are highly resistant to hydrolysis.

The osazone formation involves C-1 and C-2 of a reducing sugar. Glucose and fructose differ only in the configuration at C-1 and C-2 (glucose is an aldose, fructose is a ketose). The reaction eliminates these differences, forming an identical phenylosazone. The rest of the carbon skeleton is identical.

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.