Biochemistry : Identifying Disaccharides

Study concepts, example questions & explanations for Biochemistry

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Example Questions

Example Question #1 : Identifying Disaccharides

Which of the following is not a disaccharide?

Possible Answers:

Cellobiose

Maltose

All of these are disaccharides

Lactose

Sucrose

Correct answer:

All of these are disaccharides

Explanation:

Review the specific types of monosaccharides and glycosidic bonds that are involved in these disaccharides. Remember that disaccharides are formed upon dehydration synthesis of two monosaccharides.

Maltose is composed of two glucose monomers with an 1-4 linkage.

Cellobiose is composed of two glucose monomers with a 1-4 linkage.

Sucrose is composed of one glucose monomer and one fructose monomer with an 1-2 linkage.

Lactose is composed of one galactose monomer and one glucose monomer with a 1-4 linkage.

Example Question #2 : Identifying Disaccharides

Maltose is a disaccharide composed of which two monosaccharides?

Possible Answers:

Fructose + fructose

Glucose + fructose

Glucose + galactose

Galactose + galactose

Glucose + glucose

Correct answer:

Glucose + glucose

Explanation:

Maltose, which is made up of two glucose monomers joined by an alpha-1,4 linkage, is the correct answer. On the other hand, lactose is made up of one glucose monomer and one galactose monomer, and sucrose is made up of one glucose monomer and one fructose monomer. 

Example Question #2 : Identifying Disaccharides

Which of the following substances are found in fructose?

Possible Answers:

Glucose

None of these are found in fructose

Lactose

Galactose

Correct answer:

None of these are found in fructose

Explanation:

Fructose is a monosaccharide; therefore, it is only made up of one type of carbohydrate. Disaccharides are made up of two types of monosaccharides. For example, lactose is made up of galactose and glucose whereas sucrose is made up of glucose and fructose. Polysaccharides are made up of multiple units of monosaccaharides.

Example Question #3 : Identifying Disaccharides

Lactose intolerance is a very common disorder, which involves a decrease in the function of the enzyme lactase. Which of the following should be avoided in a patient with lactose intolerance?

Possible Answers:

Galactose

Glucose

Both should be avoided

Neither needs to be avoided

Correct answer:

Neither needs to be avoided

Explanation:

Lactose is a disaccharide that needs to be broken down to its monosaccharide components in the gut (so that it can be absorbed). Lactose is made up of galactose and glucose monosaccharide units. An enzyme called lactase, found on the intestinal walls, is used to break down lactose to galactose and glucose. A person with lactose intolerance lacks lactase and, therefore, cannot break down lactose to its components. This leads to malabsorption of lactose. However, there is no problem with galactose and glucose absorption. This means that a patient with lactose intolerance can still digest galactose and glucose if given separately.

Example Question #4 : Identifying Disaccharides

Hypoglycemia is a disorder that signifies low levels of glucose in the blood. In some cases of hypoglycemia, glucose levels can be brought back to normal via consumption of sugars. What sugar(s) can be administered to a patient with hypoglycemia?

I. Lactose

II. Galactose

III. Sucrose

Possible Answers:

II only

I and II

I only

I and III

Correct answer:

I and III

Explanation:

As the question states, hypoglycemia is characterized by low levels of glucose in the blood. To bring the glucose levels back to normal, one can ingest foods that contain high amounts of glucose. To solve this question we need to decide which of the stated options can be broken down to glucose. Lactose is a disaccharide that is made up of glucose and galactose. An enzyme, called lactase, breaks down lactose to its components, which can then be absorbed by the intestinal cells; therefore, ingesting glucose will increase glucose levels in the blood. Similarly, sucrose is a disaccharide made up of fructose and glucose; therefore, ingesting sucrose will also increase glucose levels in the blood.

Galactose is a monosaccharide and, therefore, cannot be broken down to any further. It is absorbed as galactose by the intestinal cells. This will not increase the concentration of glucose in the blood.

Example Question #6 : Identifying Disaccharides

Which reducing sugar(s) is/are formed from two glucose molecules connected by a 1 to 4 linkage?

Possible Answers:

More than one of these

Cellobiose

Sucrose

Lactose

Maltose

Correct answer:

More than one of these

Explanation:

Maltose, made from the breakdown of starch, contains a 1 to 4 alpha linkage, and cellobiose, made from the breakdown of cellulose, contains a 1 to 4 beta linkage. Both are disaccharides with different shapes and different properties.

Example Question #7 : Identifying Disaccharides

Which of the following sugars is composed of glucose and galactose?

Possible Answers:

Fructose

Lactose

Xylose

Maltose

Sucrose

Correct answer:

Lactose

Explanation:

To answer this question, we'll need to have some background information on each of the sugars shown in the answer choices.

A glycosidic bond between glucose and galactose will produce the disaccharide lactose, which is thus the correct answer.

Sucrose is a disaccharide composed of glucose and fructose. Maltose is also a disaccharide composed of two glucose molecules. Moreover, fructose and xylose are monosaccharides.

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