Practice Questions

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.

In alkaline conditions, monosaccharides undergo keto-enol tautomerism (Lobry de Bruyn–Alberta van Ekenstein transformation). For example, glucose can form an enediol intermediate that can then convert to either glucose, fructose, or mannose. This results in the epimerization of glucose to mannose at C-2.