NCERT Solutions for Class 11 Biology Chapter 9: Biomolecules

1. What are macromolecules? Give examples.

Ans. Macromolecules are large complex molecules that occur in colloidal state in intercellular fluid. They are formed by the polymerization of low molecular weight micromolecules. Examples of macromolecules are proteins, lipids, carbohydrate and nucleic acids.

2. Illustrate a glycosidic, peptide and a phosphodiester bond.

Ans. Glycosidic bond: A bond formed between the 1st carbon atom of one monosaccharide and 4th carbon atom of the neighbouring monosaccharide unit that results in the formation of polysaccharide is known as Glycosidic bond. A water molecule is released as a by-product.


Peptide bond: A covalent bond formed between the carboxyl group of one amino acid and the amino group of neighbouring amino acids is said to be a peptide bond. A water molecule is released as a by-product.


Phosphodiester bond: The phosphate-hydroxyl linkage between two nucleotides where the phosphate attached to the hydroxyl group at the 5' carbon of pentose sugar is in turn bonded to the hydroxyl group of the 3' carbon of the pentose sugar of the next nucleotide is known as a phosphodiester bond.


3. What is meant by tertiary structure of proteins?

Ans. The tertiary structure of a protein refers to the three-dimensional arrangement of its polypeptide chain in space. The tertiary structure of a protein is formed by coiling and folding of chains of proteins, which are held together by weak non-covalent interactions formed between various parts of the polypeptide chain.


4. Find and write down structures of 10 interesting small molecular weight biomolecules. Find if there is any industry which manufactures the compounds by isolation. Find out who are the buyers.
Ans. Structure of ten small molecular weight biomolecules are as follows:

Compound Manufacturer Buyer
Starch products Kosha Impex (P) Ltd. Research laboratories, educational institutes and food industry.
Liquid glucose Marudhar apparels Food industry, research laboratories.
Various enzymes such as amylase, protease, cellulase. Map (India) Ltd. Used in detergent, food industry, paper making industry.

5. Proteins have primary structure. If you are given a method to know which amino acid is at either of the two termini (ends) of a protein, can you connect this information to purity or homogeneity of a protein?

Ans. Yes, if we are given a method to know the sequence of proteins, we can use this information to determine the purity of a protein because, an accurate sequence of a certain amino acids is very important for the functioning of a protein. If there is any change in the sequence, it would alter its structure, thereby altering the function. Any change in the sequence can be linked to the purity or homogeneity of a protein.

For example, a single change at the 6th position in the sequence of P chain of haemoglobin can alter the normal haemoglobin structure to an abnormal structure that can cause sickle cell anaemia.

6. Find out and make a list of proteins used as therapeutic agents. Find other applications of proteins (e.g., Cosmetics etc.)

Ans. Proteins used as therapeutic agents are as follows:

  1. Thrombin and fibrinogen – Help in blood clotting.
  2. Antigen
    and antibody – Helps in blood transfusion.
  3. Insulin – Helps in maintaining blood glucose level in the body.
  4. Renin – Helps in osmoregulation.

Other applications of proteins include: they are commonly used in the manufacturing of cosmetics, toxins, and as biological buffers.

7. Explain the composition of triglyceride.

Ans. Triglyceride is formed by esterification of a single molecule of glycerol, with three molecules of fatty acids. It is mainly present in vegetable oils and animal fat.

Structure of Triglyceride:


8. Can you describe what happens when milk is converted into curd or yoghurt, from your understanding of proteins.

Ans. Milk contains a protein called casein. When we add a little amount of curd to the milk, the protein casein gets coagulated due to the action of lactic acid bacteria present in curd and as a result of this, milk is converted into curd. Coagulation disrupts the structure of the protein casein.

9. Can you attempt building models of biomolecules using commercially available atomic models (Ball and Stick models).

Ans. Yes, we can represent biomolecules by the ball and stick model. Because in biomolecules, the bonds which hold the atoms are represented by sticks, and the atoms are represented by balls.

Example: In the model of D-glucose, the oxygen atoms are represented by red balls, the hydrogen atoms by blue balls, while the carbon atoms are represented by grey balls.

10. Attempt titrating an amino acid against a weak base and discover the number of dissociating (ionizable) functional groups in the amino acid.

Ans. Titrating a neutral or basic amino acid against a weak base will dissociate only one functional group (carboxylic acid group), whereas titration between acidic amino acid and a weak base will dissociate two or more functional groups.

11. Draw the structure of the amino acid, alanine.

Ans. Structure of alanine:


12. What are gums made of? Is Fevicol different?

Ans. Gums are hetero-polysaccharides. They are made from two or more different types of derived monosaccharaides units which are linked together through Glycosidic bond. On the other hand, fevicol is polyvinyl alcohol (PVA) glue. Thus it’s a polymer not a polysaccharide.

13. Find out a qualitative test for proteins, fats and oils, amino acids and test any fruit juice, saliva, sweat and urine for them.

Ans. (a) Test for protein

Biuret’s test: If Biuret’s reagent is added to the protein, then the colour of the reagent changes from light blue to purple.

(b) Test for fats and oils

Emulsion test: Add ethanol to a very small amount of the test substance. Shake it in order to dissolve. Filter or dilute the test substance to obtain a fairly clear liquid. Take another tube containing water and pour the ethanolic solution into the top. A white (milk like) emulsion indicates the presence of fats or oils.

(c) Test for amino acid

Ninhydrin test: If Ninhydrin reagent is added to the solution, then the colourless solution changes to pink, blue, or purple, depending on the amino acid.

(d) Fruit juice

Biuret’s test: Fruit juice + Biuret’s reagent – Colour changes from light blue to purple. Protein is present.
Grease test: To a brown paper, add a few drops of fruit juice – No translucent spot. Fats and oils are absent or are in negligible amounts.
Ninhydrin test: Fruit juice + Ninhydrin reagent + boil for 5 minutes – Colourless solution changes to pink, blue, or purple colour. Amino acids are present.

(e) Saliva

Biuret’s test: Saliva + Biuret’s reagent –Colour changes from light blue to purple. Proteins are present.
Grease test: On a brown paper, add a drop of saliva – No translucent spot. Fats/oils are absent.

Ninhydrin test: Saliva + Ninhydrin reagent + boil for 5 minutes - Colourless solution changes to pink, blue, or purple colour. Amino acids are present.

(f) Sweat

Biuret’s test: Sweat + Biuret’s reagent – No colour change. Proteins are absent.
Solubility test: Sweat + Water – Oily appearance – Fats/oil may be present.

Ninhydrin test: Sweat + Ninhydrin reagent + boil for 5 minutes – No colour change, the solution remains colourless. Amino acids are absent.

(g) Urine

Biuret’s test: Few drops of urine + Biuret’s reagent – Colour changes from light blue to purple- Proteins are present.

Solubility test: Few drops of urine + Water – Little bit of oily appearance. Fats may or may not be present.

Ninhydrin test: Few drops of urine + Ninhydrin reagent + boil for 5 minutes.

Colourless solution changes to pink, blue, or purple colour depending on the type of amino acid – Amino acids are present.

14. Find out how much cellulose is made by all the plants in the biosphere and compare it with how much of paper is manufactured by man and hence what is the consumption of plant material by man annually. What a loss of vegetation!

Ans. Approximately, 100 billion tonnes of cellulose produced per year by all the plants present in the biosphere and approx. eighteen full grown trees are used to make one ton of paper. Trees are also used to fulfill other requirements of man such as they are used for timber, food, medicines, etc. Hence, it is difficult to calculate the annual consumption of plant material by human being. According to UN report of 2006, 312 million tons of carbon is stored as biomass in plants and about 10% of the total available cellulose is used in making paper. This is a small figure compared to the total mass but this is a huge loss of vegetation.

15. Describe the important properties of enzymes.

Ans. Important Properties of Enzymes:

(a) Enzymes are complex macromolecules with higher molecular weight.

(b) They are biocatalyst which catalyse biochemical reactions in a cell and help in the breakdown of large molecules into smaller molecules or bring together two smaller molecules to form a larger molecule.

(c) Enzymes do not start a reaction. However, they help in accelerating the reaction.

(d) Enzymes are specific in nature.

(e) Enzymatic activity decreases with increase in temperature and all enzymes show maximum activity at an optimum temperature range of 30°C-40°C.

(f) They show maximum activity at an optimum pH range of 6 – 8.

(g) The velocity of enzyme increases with increase in substrate concentration and then, ultimately reaches its maximum velocity.