Biomolecules

14.18

Open structure of D-glucose could not explain the following reactions:

(i) Despite having the aldehyde group, glucose does not give 2,4 DNP test and Schiff's

(ii) Glucose does not react with sodium hydrogen sulphite to form addition

(iii) The absence of free –CHO group is shown when the penta-acetate of glucose does not react with hydroxyl

(iv) When glucose is heated with methanol in the presence of dry HCl gas, it forms two isomeric monomethyl derivatives known as? -D-glucoside and? -D-glucoside. Since only one molecule of methanol is used for the formation of methyl glucosided, these must be hemiacetals.

14.17

When D-glucose is heated and treated with HI for a long period of time, then n-hexane is formed, which shows that all the six-carbon atoms are linked in a straight

2. When D-glucose is treated with Br₂ water i.e., bromine water which is a mild oxidising agent, then we get D-gluconic acid as one of the product. This reaction assures the presence of carbonyl group which is available as an aldehydic group.

3. On being treated with HNO₃, D-glucose get oxidised which gives saccharic acid as final Saccharic acid is a di-carboxylic acid. This reaction confirms the presence of alcoholic group (- OH) in the glucose.

14.16

Starch consists of two components – amylase and amylopectin. Amylose is a long linear chain of α–D-(+)-glucose units joined by C₁-C₄glycosidic linkage (α -link).

Amylopectin is a branched-chain polymer of –D-glucose units, in which the chain is formed by C1-C4 glycosidic linkage and the branching occurs by C₁-C₆ glycosidic linkage.

On the other hand, cellulose is the main structural material of tree and other plants. Wood is 50% cellulose, while cotton wool is almost pure cellulose. It is linear chain natural polymers of β-D-glucose units joined by 1, 4-glycosidic linkage (natural linear polymers).

14.15

Hydrolysis is the process of using water to break down a molecule into two parts. It is usually a type of decomposition reaction where one reactant is water, where water is used to break chemical bonds in the other reactant. It can be considered as reverse of a condensation reaction.

The general formula of a hydrolysis reaction is:

XY + H₂O → XH + YOH

(i) On hydrolysis with dilute acids, sucrose yields an equimolecular mixture of α –D glucose and β–D- fructose.

(ii) The hydrolysis of lactose gives β–D-galactose and β–D-glucose as final products.

14.14

Glycogen is a polysaccharide-type of carbohydrate. In animals, carbohydrates are stored as glycogen.

But starch is a carbohydrate which consists of two components –amylase (15 -20 %) and amylopectin (80 – 85%). However, glycogen is also like amylopectin but branching will take place after every 5 to 6 glucose unit. Also, glycogen is highly branched.

14.13 

The condensation of the hydroxyl group of two monosaccharides to form a link between them is called glycosidic linkage. In other words, it refers to linkage developed between two different monosaccharide units through an oxygen atom by the loss of a water molecule. For example, in a sucrose molecule, two monosaccharide units, α-glucose and β–fructose, are joined together by a glycosidic linkage.

14.12 

i. Ribose, 2-deoxyribose, galactose, and fructose are

Monosaccharides are the simplest units of carbohydrates which cannot be hydrolyzed into simpler compounds.

ii. Maltose and lactose are

A disaccharide is a carbohydrate that is formed when two monosaccharides are joined together and a molecule of water is removed from the structure. Lactose is a disaccharide formed from the combination of galactose and glucose.

14.10 

All those carbohydrates which reduce Fehling's solution to red precipitate of Cu₂O or Tollen's reagent to metallic Ag is called reducing sugars. All monosaccharides (both aldoses and ketoses) and disaccharides except Sucrose are reducing sugars.