MCQs

The notation specifies the configuration (α) of the anomeric carbon (C-1) of the first sugar, and the carbon (C-4) of the second sugar to which it is linked. This precise nomenclature is essential for describing the specific, biologically active structure of an oligo- or polysaccharide.

Cellobiose is the repeating disaccharide unit of cellulose and is formed by partial hydrolysis. It consists of two β-D-glucose molecules linked by a β-1,4 glycosidic bond. It is an isomer of maltose, which has an α-1,4 linkage.

The cyclic form, a hemiacetal, is much lower in energy than the free aldehyde. The equilibrium thus lies heavily on the side of the cyclic forms. The open form is a high-energy, transient intermediate that exists only briefly to allow anomeric interconversion.

The size of the ring is determined by which hydroxyl attacks the carbonyl. Reaction with the C4-OH forms a five-membered ring (furanose). Reaction with the C5-OH forms a six-membered ring (pyranose). The pyranose form is favored for most aldohexoses due to lower steric strain.

A homopolysaccharide is composed of a single type of monosaccharide monomer (e.g., starch, cellulose, glycogen are all made of glucose). A heteropolysaccharide contains two or more different types of monosaccharide units (e.g., peptidoglycan, agar, hyaluronic acid).

This is a classic example of a complex carbohydrate playing a specific biological regulatory role. The unique sequence in heparan sulfate binds to antithrombin III, inducing a conformational change that dramatically increases its affinity for thrombin and Factor Xa, preventing blood clotting.

In cellulose, every glucose residue is flipped 180° relative to the next to accommodate the β-1,4 linkage. This creates a straight, ribbon-like structure with cellobiose as the repeating unit. In amylose, the α-1,4 linkage does not require this flip, causing the chain to adopt a helical twist.

In β-D-glucose, the anomeric -OH on C1 is equatorial. In the chair form, bulky substituents in equatorial positions have more space and experience less steric strain (1,3-diaxial interactions), making this conformation thermodynamically more stable than the α-anomer (axial -OH).

Saccharification is the step where the polysaccharide starch is chemically or enzymatically hydrolyzed into simple sugars (glucose and maltose). These sugars then serve as substrates for the subsequent fermentation by microorganisms to produce ethanol.

O-linked glycosylation involves the formation of a glycosidic bond between the anomeric carbon of a sugar (often N-acetylgalactosamine) and the hydroxyl group of a serine or threonine residue in the protein. N-linked glycosylation links to the amide nitrogen of asparagine.

The anomeric carbon in a glycoside is part of an acetal (or ketal) functional group, which is stable and cannot open to the free carbonyl form in a neutral/basic aqueous solution. Since no free carbonyl can form, the sugar cannot act as a reducing agent.

S. mutans produces glucosyltransferases that specifically use sucrose to synthesize sticky, water-insoluble glucan polymers (dental plaque). The sucrose is then fermented to lactic acid within this plaque, causing localized demineralization of tooth enamel.

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.