MCQs

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.

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.

Sugars are polar and partition between an organic mobile phase and a water stationary phase held by the paper. Slight differences in structure (e.g., number of -OH groups) cause them to partition differently and thus migrate at different rates, allowing for separation and identification.

The enzyme glucose (xylose) isomerase catalyzes the reversible isomerization of glucose to the sweeter fructose. This process is used to convert a portion of the glucose from corn starch into fructose, creating high-fructose corn syrup (HFCS).

Pancreatic α-amylase, like salivary amylase, is an endoglycosidase that hydrolyzes internal α-1,4 bonds. It cannot cleave α-1,6 bonds at branch points. Therefore, the products are the disaccharide maltose, the trisaccharide maltotriose, and oligosaccharides containing branch points called α-limit dextrins.

Chitin is the primary structural polysaccharide in the cell walls of fungi, the exoskeletons of arthropods, and the beaks of cephalopods. It provides rigidity and strength, analogous to the role of cellulose in plants.

The D/L nomenclature is a convention based on glyceraldehyde. A sugar is D if the -OH group on the chiral carbon farthest from the carbonyl group (the highest numbered chiral center) is drawn on the right side in a standard Fischer projection.

The vitrification (glass formation) hypothesis suggests that trehalose forms a stable glassy matrix. Its many -OH groups replace the hydrogen bonds normally provided by water to the polar head groups of membrane phospholipids and to the protein surface, maintaining their native conformation during dry periods.