Test: Carbohydrate Structure and Function - MCAT MCQ

Glucose is an aldohexose, meaning that it has one aldehyde group and six carbons. Given this information, choices (A) and (B) can be eliminated. In aldose sugars, each nonterminal carbon is chiral. Therefore, glucose has four chiral centers, not five, as mentioned in choice (C). The number of stereoisomers possible for a chiral molecule is 2ⁿ, where n is the number of chiral carbons. Because glucose has four chiral centers, there are 2⁴ = 16 possible stereoisomers.

Aldonic acids form after the aldehyde group on a reducing sugar reduces another compound, becoming oxidized in the process.

Ketose sugars undergo tautomerization, a rearrangement of bonds, to undergo keto-enol shifts. This forms an aldose, which then allows them to act as reducing sugars. A ketone group alone cannot be oxidized. Anomerization, mentioned in choice (C), refers to ring closure of a monosaccharide, creating an anomeric carbon.

β-Amylase cleaves amylose at the nonreducing end of the polymer to yield maltose exclusively, while α-amylase, choice (A), cleaves amylose anywhere along the chain to yield short polysaccharides, maltose, and glucose. Debranching enzyme, choice (C), removes oligosaccharides from a branch in glycogen or starches, while glycogen phosphorylase, choice (D), yields glucose 1-phosphate.

The hydroxyl group on the anomeric carbon of the β-anomer is equatorial, thereby creating less steric hindrance than the α-anomer, which has the hydroxyl group of the anomeric carbon in axial position.

While maltose and cellobiose both have the same glucose subunits, only maltose is digestible by humans because β-glycosidic linkages cannot be cleaved in the human body.

Monosaccharides can exist as hemiacetals or hemiketals, depending on whether they are aldoses or ketoses. When a monosaccharide is in its cyclic form, the anomeric carbon is attached to the oxygen in the ring and a hydroxyl group. Hence, it is only a hemiacetal or hemiketal because an acetal or ketal would require the –OH group to be converted to another –OR group.

Carbohydrates are organic compounds composed of carbon, hydrogen, and oxygen atoms. They serve as a major source of energy in living organisms.

The simplest form of carbohydrates consists of aldehyde and ketone groups. These functional groups are present in monosaccharides, the basic building blocks of carbohydrates.

Monosaccharides are the simplest form of carbohydrates and cannot be further hydrolyzed into smaller units. Glucose is a common monosaccharide and an important energy source in biological systems.