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Chapter: Biochemistry : Introduction to Carbohydrates

Monosaccharides containing an aldehyde group are called aldoses, and those with a keto group are called ketoses.


Monosaccharides (Figure 7.12 ) containing an aldehyde group are called aldoses, and those with a keto group are called ketoses. Disaccharides, oligosaccharides, and polysaccharides consist of monosaccharides linked by glycosidic bonds. Compounds with the same chemical formula but different structures are called isomers. If two monosaccharide isomers differ in configuration around one specific carbon atom (with the exception of the carbonyl carbon), they are defined as epimers of each other. If a pair of sugars are mirror images (enantiomers), the two members of the pair are designated as D- and L-sugars. If the aldehyde group on an acyclic sugar gets oxidized as a chromogenic agent gets reduced, that sugar is a reducing sugar. When a sugar cyclizes, an anomeric carbon is created from the aldehyde group of an aldose or keto group of a ketose. The sugar can have two configurations, α or β. A sugar with its anomeric carbon linked to another structure forms a glycoside. Sugars can be attached either to an – NH2 or an –OH group, producing N- and O-glycosides. Salivary α-amylase acts o n dietary polysaccharides (starch, glycogen), producing oligosaccharides. Pancreatic α-amylase continues the process of carbohydrate digestion. The final digestive processes occur at the mucosal lining of the small intestine. Several disaccharidases (for example, lactase [β-galactosidase] , sucrase, isomaltase, and maltase) produce monosaccharides (glucose, galactose, and fructose). These enzymes are transmembrane proteins of the luminal brush border of intestinal mucosal cells. Absorption of the monosaccharides requires specific transporters. If carbohydrate degradation is deficient (as a result of heredity, disease, or drugs that injure the intestinal mucosa), undigested carbohydrate will pass into the large intestine, where it can cause osmotic diarrhea. Bacterial fermentation of the material produces large volumes of CO2 and H2, causing abdominal cramps, diarrhea, and flatulence. Lactose intolerance, dependent loss of lactase (adult hypolactasia), these deficiencies.

Figure 7.12 Key concept map for the classification and structure of monosaccharides and the digestion of dietary carbohydrates.

Study Question

Choose the ONE best answer.


7.1 Which of the following statements best describes glucose?

A. It is a C-4 epimer of galactose.

B. It is a ketose and usually exists as a furanose ring in solution.

C. It is produced from dietary starch by the action of α-amylase.

D. It is utilized in biological systems only in the L-isomeric form.

Correct answer = A. Glucose and galactose differ only in configuration around carbon 4 and so are C-4 epimers that are interconvertible by the action of an epimerase. Glucose is an aldose sugar that typically exists as a pyranose ring in solution. Fructose, however, is a ketose with a furanose ring. α-Amylase does not produce monosaccharides. The D-isomeric form of carbohydrates is most typically the form found in biologic systems, in contrast to amino acids.


7.2 A young man entered his physician’s office complaining of bloating and diarrhea. His eyes were sunken, and the physician noted additional signs of dehydration. The patient’s temperature was normal. He explained that the episode had occurred following a birthday party at which he had participated in an ice cream–eating contest. The patient reported prior episodes of a similar nature following ingestion of a significant amount of dairy products. This clinical picture is most probably due to a deficiency in the activity of:

A. isomaltase.

B. lactase.

C. pancreatic α-amylase.

D. salivary α-amylase.

E. sucrase.

Correct answer = B. The physical symptoms suggest a deficiency in an enzyme responsible for carbohydrate degradation. The symptoms observed following the ingestion of dairy products suggest that the patient is deficient in lactase.


7.3 Routine examination of the urine of an asymptomatic pediatric patient showed a positive reaction with Clinitest (a copper reduction method of detecting reducing sugars) but a negative reaction with the glucose oxidase test for detecting glucose.

Using these data, show on the chart below which of the sugars could (YES) or could not (NO) be present in the urine of this individual.

Each of the listed sugars, except for sucrose and glucose, could be present in the urine of this individual. Clinitest is a nonspecific test that produces a change in color if urine is positive for reducing substances such as reducing sugars (fructose, galactose, glucose, lactose, xylulose). Because sucrose is not a reducing sugar, it is not detected by Clinitest. The glucose oxidase test will detect only glucose, and it cannot detect other sugars. The negative glucose oxidase test in the face of a positive reducing sugar test means that glucose cannot be the reducing sugar in the patient’s urine.


7.4 Why are α-glucosidase inhibitors that are taken with meals, such as acarbose and miglitol, used in the treatment of diabetes? What effect should these drugs have on the digestion of lactose?

α-Glucosidase inhibitors slow the production of glucose from dietary carbohydrates, thereby reducing the postprandial rise in blood glucose and facilitating better blood glucose control in diabetics. These drugs have no effect on lactose digestion because the disaccharide lactose contains a β-glycosidic bond, not an α-glycosidic bond.

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