Carbohydrate Chemistry l Biochemistry MCQs

carbohydrate chemistry l Biochemistry MCQs (multiple choice question) for dental students

Carbohydrates:

•  Are the most abundant organic molecules in nature.
•  They are primarily composed of the elements carbon, hydrogen and oxygen.
•  the name of carbohydrates literally means hydrates of carbon.   

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•           Carbo(C)……Hydrate  (hydrates of carbon)
•           Most have general formula CnH2nOn

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 Carbohydrate Definition

• Polyhydroxyaldehyde or polyhydroxyketone, or a substance that can be hydrolyzed to form these compounds.
• are carbon compounds that contain large quantities of hydroxyl groups and also contain either aldehyde or ketone group.
• the term "sugar“ is applied to carbohydrates soluble in water and sweet to taste.

Functions of Carbohydrates

1.          They are most abundant dietary source of energy for all organisms.
2.          Are precursors for many organic compounds (fats, amino acids).
3.          Participate in the structure of cell membrane (as glycoproteins and glycolipids).
4.          Structural component of many important molecules such as nucleic acids.
5.          They are structural components of many organisms. These include the fiber (Cellulose) of plants, exoskeleton of some insects (Chitin).
6.          Serve as storage form of energy (glycogen) to meet the immediate energy demands of the body.

 

Classification of Carbohydrates

1. ·         Monosaccharides - simple sugars
2. ·         Disaccharides - 2 monosaccharides covalently linked.
3. ·         Oligosaccharides – 3 to 10 monosaccharides covalently linked.
4. ·         Polysaccharides - more than 10 monosaccharide units covalently linked

Monosaccharides

A monosaccharide is a carbohydrate that cannot be hydrolyzed to a simpler carbohydrate (simple sugar) biologically important ones are:

•          Glucose: most common sugar,  also blood sugar .
•          Fructose : fruit sugar          
•          Galactose: sugar found in milk        

Glucose is the most important monosaccharide:

•          Most dietary carbohydrate is absorbed as glucose.
•          Is the blood sugar.
•          Other sugars can be converted to glucose in the liver.
•          Is the precursor for synthesis of other carbohydrates in the body such as:

1-       glycogen for storage.

2-       ribose and deoxyribose in nucleic acids.

3-       galactose in lactose of milk.

4-       Is the major metabolic fuel of mammals.

5-     A constituent in glycolipids and glycoproteins .

Properties of Monosaccharides

•          Simplest of carbohydrates
•          Sweet-tasting
•          Dissolve in water
•          Straight chain or ring structure
•          All in body are of the D type.

 Some Important Reaction of Monosaccharide:

 Reduction to form alcohols:

• When treated with reducing agents, the aldehyde or keto group of monosaccharide is reduced to corresponding alcohol

1.  glucose →  sorbitol
2. ribose  →  ribitol

b) Oxidation to form acid:

• depending on the oxidized agent used, the aldehyde (or keto) or the terminal alcohol or both groups may be oxidized. For instance, glucose:
• Oxidation of aldehyde group (CHO→ COOH) results in the formation of gluconic acid.
• Oxidation of terminal alcohol group (CH2OH→COOH) leads to the production of glucuronic acid.
• Oxidation of both groups (aldehyde and alcohol) glucosaccharic acid (glucaric acid).