Monosaccharides and Carbohydrates - Biochemistry

Sucrose is a disaccharide that is made up of a glucose and a fructose molecule, bound by a glycosidic linkage. A disaccahridase, called sucrase, breaks down sucrose molecules into their component monosaccharides (glucose and fructose), which can then by absorbed by the enterocytes in the small intestine.

Fructose is a monosaccharide that can be directly absorbed by enterocytes. Starch is a complex carbohydrate (polysaccharide) with many glucose molecules attached via glycosidic bonds.

Just like starch, glycogen and cellulose are complex carbohydrates that contain glucose molecules joined via glycosidic bonds. Both carbohydrates contain (1,4) glycosidic bonds. This means that the bond occurs between the 1-carbon of one glucose and the 4-carbon of the other glucose. Glycogen contains -(1,4) glycosidic bonds whereas cellulose contains -(1,4) bonds.

In addition, glycogen molecules contain -(1,6 ) glycosidic bonds. These bonds are typically found in branch points along the main chain of glucose molecules. Cellulose molecules do not have branching and, therefore, do not have -(1,6) glycosidic bonds.

As mentioned, glycogen contains bonds. Humans have enzymes capable of digesting complex carbohydrates with linkages; therefore, only glycogen and starch (which also has linkages) can be digested by humans. Amylin and amylopectin are forms of starch, not cellulose.

The question states that the molecule has the same structure as glucose, except at one stereogenic center. Recall that epimers are structural isomers that differ at one stereogenic center; therefore, glucose and the compound are epimers. The epimers of glucose include galactose and mannose. All three molecules have a molecular formula of .

Recall that aldoses are carbohydrates that have an aldehyde group at one of the carbons. Glucose and all of its epimers have an aldehyde group at the first carbon; therefore, this compound is an aldose.

Fructose is a ketose that has a ketone group in the second carbon. Recall that aldehydes and ketones have carbonyl groups; therefore, they are not considered stereogenic centers (to be a stereogenic center you need to have four distinct functional groups on a carbon). Since fructose differs from glucose at a non-stereogenic center, this molecule cannot be fructose. Note that fructose also has a different molecular formula ().

Only 1% of all sugars that have five or more carbons are found in an open chain, thus any sugar that has five or more carbons will be most likely found in its cyclic form. Of the four choices a triose is the only one that has less that five carbons (it has three) the others have 5 (pentose), 6 (hexose), and 7 (heptose).

Given no other information about how many carbons are in the chain, any monosaccharide with a carbonyl group on a carbon between two others and not at the terminal carbon of the chain is called a ketose. Aldoses are when the carbonyl group is at the end of the chain. Ribose and aldohexose are incorrect because 1) ribose is too specific, we do not know anything about the structure and ribose indicates a specific monosaccharide, and 2) aldohexose has an aldehyde group, not a ketone group.

A furanose is defined as a cyclical sugar structure with a five-membered ring. By contrast, a pyranose is a cyclical sugar structure with a six-membered ring.

Three common simple sugars are: glucose, fructose, and galactose. Combining these simple sugars leads to the formation of more complex sugar molecules. Glucose and fructose make sucrose. Glucose and galactose make lactose. Two glucose molecules make maltose.

A pyranose is a carbohydrate that includes a ring. It is not an open-chain carbohydrate. Additionally, this term is reserved for six-membered, not five-membered carbohydrate ring structures. A sugar which contains a five-membered ring as part of its cyclic structure is called a furanose.

As carbohydrates are broken down, they are oxidized to carbon dioxide . This process, cellular respiration, begins during glycolysis. Reduction is the opposite of oxidation, and would indicate a loss of oxygen. Elimination refers to the removal of a functional group, which does not occur during this process. In substitution, one functional group is replaced with another, which also does not accurately describe this process. Finally, proteolysis refers to the breakdown of proteins, not carbohydrates.

Most mammals are unable to digest beta glycosidic bonds, such as the bonds in cellobiose and lactose. Maltose and sucrose both are connected via alpha glycosidic bonds, and can be digested by mammals.

Note: Most mammals are unable to digest lactose after infancy. It is only through a genetic mutation that many humans now are able to ingest lactose well into adulthood.

This is a fact question. A lactone is a cyclic ester, which is a ring of two or more carbon atoms and one oxygen atom with a ketone at one of the carbons adjacent to the other oxygen. Lactones are usually formed via a reaction of a carboxylic acid with a hydroxyl group or halogen atom present in the same molecule.

Maltose is a disaccharide formed from two glucose molecules. Mammals use the enzyme maltase to digest maltose. Lactose is a disaccharide formed from one glucose and one galactose molecule. Mammals use the enzyme lactase to digest lactose. Sucrose is a disaccharide formed from one glucose and one fructose molecule. Mammals use the enzyme sucrase to digest sucrose. Mammals cannot digest cellulose because they do not have the necessary enzymes to do so in their digestive tracts.

Glycogen is a multi-branched glucose polysaccharide that functions in energy storage in animals. Its chemical formula is . In contrast, starch is a glucose polymer which functions in energy storage in plants. Glycogen is also not an artificial sweetener (synthetic sugar substitute). As stated, it is the main storage form of glucose in animals.

Beta-linked polysaccharides are tougher to break down than alpha-linked polysaccharides. This feature makes beta-linked polysaccharides a key component in the cell walls of many bacteria such as peptidoglycan. For even more added structure, beta-linked polysaccharides can also pack into tight crystalline conformations (i.e., cellulose in celery stalks). Thus, all of the answer choices are correct.

Reduction of carbonyl groups in aldoses and ketoses gives sugar alcohols.

The sugar we eat, or table sugar, is known as sucrose. It is a disaccharide made of one molecule each of glucose and fructose.

Fructose is "fruit sugar." It is a monosaccharide found in honey, fruits, and flowers. Glucose is "blood sugar." It is a monosaccharide that is (like the name suggests) found in our blood. Maltose is "malt sugar." It is a disaccharide formed from two molecules of glucose. It is found in seeds such as barley and is also produced when glucose is caramelized.

"Oligo-" is a prefix which means "few." "Poly-" is a prefix which means "many." Just as the names suggest, combining a few monosaccharides leads to the formation of _oligo_saccharides. In the same way, combining many monosaccharides leads to the formation of _poly_saccharides.

Lactase deficiency is common in humans, particularly those of European descent. Without this enzyme, products containing the sugar lactose cannot be broken down. When lactose cannot be digested, it serves as an osmotic agent in the intestines resulting in abdominal distention and subsequent diarrhea.

Glycogen is an example of a polysaccharide. It is composed of > 10 individual units of a base sugar. On the other hand, disaccharides are composed of only two base sugars. Lactose, sucrose, and maltose are important disaccharides.