MCQs

The iodine test requires a helix of sufficient length to stabilize the polyiodide chain. Glycogen's highly branched structure means its α-1,4 helical segments are very short. With iodine, it yields a reddish-brown color (not blue-black), which can be easily confused with a negative test if not careful.

Each monosaccharide has several polar -OH groups that can participate in hydrogen bonding with water molecules. This strong interaction (adhesion) overcomes the sugar-sugar interactions and allows the sugars to dissolve readily in water.

The D/L designation is fixed by the configuration of the highest-numbered chiral center (C-5 in hexoses). The α and β anomers are defined specifically by the orientation of the hydroxyl group on the newly formed chiral center, the anomeric carbon (C-1 in aldoses), relative to the ring.

Amylopectin is a homopolymer of D-glucose. Complete hydrolysis with strong acid will break all the α-1,4 and α-1,6 glycosidic bonds, yielding only D-glucose monomers as the final product.

Resistant starch is physically inaccessible or structurally resistant to pancreatic amylases. It passes to the colon, where it acts similarly to soluble fiber, being fermented by gut microbiota, producing beneficial short-chain fatty acids.

Agar melts at ~85°C and solidifies at ~32-40°C. Once gelled, it remains solid at typical incubation temperatures (e.g., 37°C) and is resistant to degradation by most microorganisms, making it an ideal, inert solidifying agent for culture media.

Both have the same molecular formula (C₆H₁₂O₆), but different functional groups—glucose is an aldehyde (aldose) and fructose is a ketone (ketose). This difference in the connectivity of atoms makes them structural (constitutional) isomers.

Branching enzyme is a transglycosylase. It cuts a short α-1,4-linked chain of 6-7 glucose units and transfers it to the 6-OH position of a glucose residue in the same or a nearby chain. This creates the α-1,6 branch points characteristic of amylopectin and glycogen.

Rumen microbes ferment cellulose and other carbohydrates to volatile fatty acids (VFAs) like acetate, propionate, and butyrate. These are absorbed through the rumen wall and serve as the primary energy source for the ruminant, not the glucose monomers of cellulose.

The Lobry de Bruyn–Alberta van Ekenstein transformation proceeds through the removal of a proton from C-2, forming an enediol (or enolate) intermediate. This intermediate can reprotonate to give either the original aldose (glucose), its C-2 epimer (mannose), or the ketose (fructose).

While providing no calories, the insoluble cellulose fibers absorb water, increasing fecal bulk. This bulk stimulates stretch receptors in the gut wall, promoting peristaltic contractions and helping prevent constipation and related disorders.

Glycogen phosphorylase cannot cleave near a branch point. The debranching enzyme has two activities: first, transferase activity moves a short α-1,4-linked chain to a nearby non-reducing end; second, α-1,6-glucosidase activity hydrolyzes the remaining α-1,6 bond, releasing a free glucose molecule.

In the full chemical name of a disaccharide, the glycosidic bond is specified, and the configuration at the anomeric carbon of the non-reducing sugar is named last. Here, "β-D-fructofuranoside" indicates that the fructose unit is in the β-configuration at its anomeric carbon (C-2).

Oxidation of the aldehyde group (C-1) of an aldose yields an aldonic acid (e.g., gluconic acid from glucose). Oxidation of the primary alcohol group (C-6) yields a uronic acid. Reduction yields a sugar alcohol (alditol).

Concentrated H₂SO₄ dehydrates pentoses to furfural and hexoses to hydroxymethylfurfural. These compounds react with α-naphthol (in Molisch's reagent) to form a purple/violet ring. This is a general test for all carbohydrates.

In plants, ADP-glucose is the activated form used by starch synthase. In animals, UDP-glucose is the glucosyl donor for glycogen synthesis. This is a fundamental biochemical distinction between the kingdoms.

Glycogen is primarily stored in the liver (for maintaining blood glucose levels) and skeletal muscles (as a local fuel reserve for contraction). The brain does not store significant glycogen and relies on blood glucose. Adipose tissue stores energy as triglycerides.

While capillary action in the lumen plays a minor role, the primary mechanism is the strong hydrogen bonding of water to the abundant -OH groups on the glucose units. This is especially effective in the less-ordered, amorphous regions of the cellulose microfibril where -OH groups are not already engaged in inter-chain H-bonds.

A reducing end of a polysaccharide is the terminal monosaccharide with a free anomeric carbon that can undergo ring-opening to expose a free aldehyde or ketone group. All other residues are locked in glycosidic bonds and are non-reducing.