Case study (4 Marks)

Question 14 Marks

Write detailed note on: Starch

Answer

$\rightarrow$ Starch is the main storage polysaccharide of plants.
$\rightarrow$ It is the most important dietary source for human beings.
$\rightarrow$ High content of starch is found in cereals, roots, tubers and some vegetables.
$\rightarrow$ It is a polymer of $a-$glucose and consists of two components amylose and amylopectin.
$\rightarrow$ Amylose is water soluble component which constitutes about $15-20\%$ of starch.
$\rightarrow$ Chemically amylose is along unbranched chain with $200-1000 \alpha-D-(+)-$glucose units held together by $C_1-C_4$ glycosidic linkage.

$\rightarrow$ Amylopectin is insoluble in water and constituents about $80-85\%$ of starch.
$\rightarrow$ It is a branched chain polymer of $\alpha-D-$ glucose units in which chain is formed by $C_1-C_4$ glycosidic linkage where as branching occurs by $C_1-C_6$ glycosidic linkage.

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Question 24 Marks

What is fructose? Explain the structure of fructose.

Answer

→Fructose is an important ketohexose. It is obtained along with glucose by the hydrolysis of disaccharide sucrose.
→It is natural monosaccharide found in fruits, honey and vegetables. In it's pure form it is used as a sweetner.
Structure of fructose:
→Fructose has the molecular formula $C _6 H _{12} O _6$.
→On the basis of it's reactions it was to contain a ketonic functional group at carbon number 2 and six carbons in straight chain as in the case of glucose.
It belongs to D-series and is a laevorotatory compound. It is appropriately written as D(-) fructose.
→Its open chain structure is as shown.

→It also exists in two cyclic forms which are obtained by the addition ofOH at $C_5$ to the

group.
→The ring, thus formed is a five membered ring and is named as furanose with analogy to the compound furan.
→Furan is a five membered cyclic compound with one oxygen and four carbon atoms.

→The cyclic structures of two anomers of fructose are represented by Haworth structures as given.

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Question 34 Marks

Describe detailed information on classification of carbohydrates.

Answer

→Carbohydrates are classified on the basis of their behaviour on hydrolysis. They have been broadly divided into following 3 groups
(i) Monosaccharide compounds :
→"A carbohydrate that can not be hydrolysed further to give simpler unit of polyhydroxy aldehyde or ketone is called a monosaccharide."
→About 20 monosaccharides are known to occur in nature. Some common examples are glucose, fructose, ribose, etc.
(ii) Oligosaccharide compounds :
→"Carbohydrates that yield two to ten monosaccharide units, on hydrolysis are called oligosaccharides."
→They are further classified as disaccharides, trisaccharides, tetrasaccharides, etc., depending upon the number of monosaccharides they provide on hydrolysis.
→Amongst these the most common are disaccharides. The two monosaccharide units obtained on hydrolysis of a disaccharide may be same or different.
→For example, one molecule of sucrose on hydrolysis gives one molecule of glucose and one molecule of fructose where as maltose gives two molecules of only glucose.
(iii) Polysaccharide compounds:
→"Carbohydrate which yield a large number of monosaccharide units on hydrolysis are called polysaccharides."
→Some common examples are starch, cellulose, glycogen, gums etc. Polysaccharides are not sweet in taste. Hence, they are also called non- sugars.

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Question 44 Marks

Explain the D and L notation method of spatial arrangement with respect to glucose.

Answer

→The letters 'D' or 'L' before the name of any compound indicate the relative configuration of a particular stereoisomer of a compound with respect to configuration of some other compound, configuration of which is known.
→In the case of carbohydrates, this refers to their relation with a particular isomer of glyceraldehyde.
→Glyceraldehyde contains one asymmetric carbon atom and exists in two enantiomeric forms as shown below

→(+) Isomer of glyceraldehyde has 'D' configuration it means that the -OH group lies on right hand side in the structure.
→All those compounds which can be chemically correlated to D(+) isomer of glyceraldehyde are said to have D-configuration.
→Where as those which can be correlated to L (-) isomer of glyceraldehyde are said to have L-configuration. In 'L(-)' isomer -OH group is on left hand side.
→For assigning the configuration of monosaccharides it is the lowest asymmetric carbon atom (as shown below) which is compared. As in (+) glucose -OH on the lowest assymmetric carbon is on the right side which is compared to (+) glyceraldehyde. So, (+) glucose is assigned D-configuration.

→Other asymmetric carbon atoms of glucose are not considered for this comparison. Also, the structure of glucose and glyceraldehyde is written in a way that most oxidised carbon (in this case -CHO) is at the top.

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Question 54 Marks

Explain the structure of nucleic acid compounds.

Answer

→"A unit formed by the attachment of a base to 1' position of sugar is known as nucleoside."
→In nucleosides the sugar carbons are numberd as 1', 2', 3' etc. In order to distinguish these from the bases - when nucleoside is linked to phosphoric acid at 5'-position of sugar moiety, we got a nucleotide.

→Nucleotides are joined together by phosphodiester linkage between 5' and 3' carbon atoms of the pentose sugar. The formation of a typical dinucleotide is shown in figure.

A simplified version of nucleic acid chain is as shown below.

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Chemistry Case study (4 Marks)

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