Q.1. What are carbohydrates?
Ans. Carbohydrates are organic substances containing C, H and O. Hydrogen atoms are present usually in the ratio of 2:1 as it occurs in a water molecule.
Example: glucose, fructose, lactose, starch etc. There may be exception to the above, e.g. C2H4O2 is acetic acid and not a carbohydrate though H and O are in the ratio of 2:1.
Q.2. Define carbohydrates in chemical term.
Ans. Carbohydrates are defined chemically as aldehyde or ketone derivatives of the higher polyhydric alcohols or compounds which yield these derivatives on hydrolysis.
Q.3. How will you classify carbohydrates?
Ans. Carbohydrates are classified into four major groups:
- Monosaccharide (“simple sugars”): They cannot be hydrolyzed into simpler forms.
- Disaccharides: They yield two molecules of same or different monosaccharide units on hydrolysis.
- Oligosaccharides: They yield three to six molecules of monosaccharides on hydrolysis.
- Polysaccharides (glycans): They yield more than 6 molecules of monosaccharides on hydrolysis.
Q.4. How are monosaccharides further classified?
Ans. Monosaccharides are further classified into two groups depending on:
- The number of carbon atoms they possess, e.g. trioses,tetroses, pentoses, hexoses, etc.
- Whether aldehyde (-CHO) or ketone (-CO) group is present, e.g. aldoses, ketoses.
Q.5. Give an example of an aldohexose and a ketohexose which is of biological importance.
Ans. Example of an aldohexose and a ketohexose :
- Aldhexose: D-glucose
- Ketohexose: D-fructose
Q.6. How will you classify polysaccharides?
Ans. Polysaccharides are classified into two main groups:
- Homopolydisaccharides (hemoglycans): Polymer of same monosaccharide units, e.g. starch, glycogen, insulin, dextrins, cellulose, etc.
- Heteropolysaccharides (heteroglycans): Polymer of different monosaccharide units or their derivatives, e.g. mucopolysaccharides (MPS).
Q.7. What is an asymmetric carbon?
Ans. A carbon atom to which four different atoms or groups of atoms are attached is said to be an asymmetric carbon.
Q.8. What are the effects of presence of asymmetric carbon in a compound?
Ans. The presence of asymmetric carbon atoms in a compound produces the following effects:
- Gives rise to the formation of stereoisomers of that compound.
- Also confers optical activity to the compound.
Q.9. What are stereoisomers?
Ans. The compounds which are identical in composition and differs only in spatial configuration are called stereoisomers. Two such stereoisomers of glucose, D-glucose and L-glucose are mirror-images of each other.
Q.10. What are optical isomers?
Ans. When a beam of plane polarized light is passed through sugar solution exhibiting optical activity, it will be rotated to the right or left according to the type of the compound. Such compounds are called “optical isomers” (or “enantiomorphs”).
Q.11. Explain ‘D’ and ‘L’ forms of sugar and d(+) and l(–) sugars.
Ans.
- D and L (capital letters) are used to represent configuration of the sugar molecules.
The orientation of H and OH groups around the carbon atom just adjacent to the terminal primary alcohol group -CH2OH, i.e. C atom 5 in glucose determines the series.
When the -OH group is on the right it belongs to D-Series and -OH
Group is on the left it is a member of L-series.
- d(+) and l(–) denote the nature of optical activity.
When the plane polarized light is rotated to the right, the compound is called “dextrorotatory” or d(+) and when rotated to left, the compound is called “laevorotatory” or l(–).
Q.12. What is Vant Hoff’s rule of n?
Ans. According to Vant Hoff’s rule of ‘n’; 2n gives the possible stereoisomers of that compound; where n represents thenumber of asymmetric carbon atoms in a compound.
Q.13. How many stereoisomers are formed by glucose?
Ans. Glucose has 4 asymmetric carbon atoms, hence as per Vant Hoff’s rule of ‘n’; glucose will have 2n = 16 stereoisomers; of which 8 belongs to D-series and 8 belongs to L-series.
Q.14. What are α and β forms of glucose?
Ans. When aldehyde group of C-1, on cyclization, condenses with alcoholic-OH group on C-5 on the same molecule, two different cyclic forms are produced. When -OH group on C-1 of cyclic form extends to right, it is called α-D-glucose and when -OH group extends to left, it is called β-D-glucose.
Q.15. What is an anomer and what is an anomeric carbon?
Ans.
- After cyclization, the two cyclic compounds α and β have different optical rotation but they are not “mirror-image” of each other. Such compounds are called “anomers”.
- Carbon 1 after cyclization becomes asymmetric as it has new 4 different groups attached to it and is called “anomeric carbon”.