Question

Which of these sugars is laevorotatory?
a. Fructose
b. Glucose
c. Maltose
d. Sucrose

Answer 1

Hint: Carbohydrates are mainly compounds of carbon, hydrogen, and oxygen. They are of three types- monosaccharides, disaccharides and polysaccharides. Monosaccharides are the simplest carbohydrates.

Complete answer:
- Monosaccharides are the simplest type of carbohydrates formed of single aldehyde or ketone derivatives of polyhydroxy alcohol. These are also called sugars because of their sweet taste. Fructose and glucose are monosaccharides with six carbon atoms i.e they are hexoses.

- Maltose – It is the simplest of all disaccharides which is made up of two units of glucose molecules interlinked by alpha (1, 4) glycosidic bond. Its sweetening power is 32. It is a major product in enzymatic hydrolysis of starch. It is formed in germinating starchy seeds and malt. It is also called malt sugar.

- Sucrose is a disaccharide composed of alpha-D-glucose and beta-D-fructose attached together by the aldehyde and ketone carbon by alpha (1, 2) glycosidic bond. Sucrose is a non-reducing sugar and its sweetening power is 100. It is also called common cane or table sugar.

- Monosaccharides show the phenomenon of rotation of light, called mutarotation. The monosaccharides that rotate the plane polarized light towards anti clock direction are called levorotatory while the monosaccharides that rotate it towards the clockwise direction are called dextrorotatory.

- Fructose which is mainly found in fruits, and nuts as the monosaccharide unit of sucrose, is a levorotatory sugar as it rotates the plane polarized light to the left. Glucose is a dextrorotatory sugar.

Hence, the correct answer is option (A).

Note: Carbohydrates constitute about 80% of dry weight of plants. Glucose or grape sugar and fructose or sweetest sugar are reserved foods of grapes and other fruits. All monosaccharides and disaccharides are reducing in nature i.e. they can carry out reduction reactions. Benedict’s test and Fehling’s test are the two reducing tests for sugars.