Biomolecules Question Answers: NCERT Class 11 Biology

Macromolecules are large complex molecules present in colloidal state in intercellular fluid. They are formed by the condensation of low molecular weight micromolecules and hence are polymeric in nature.

Polysaccharides, proteins, and nucleic acids are common examples of macromolecules.

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

Structure of alanine

Gums are hetero-polysaccharides. They are made from two or more different types of derived monosaccharides. On the other hand, fevicol is polyvinyl alcohol (PVA) glue. It is not a polysaccharide.

(a) Test for protein

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

(b) Test for fats and oils

Grease or solubility test

(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.

Approximately, 100 billion tonnes of cellulose are made per year by all the plants in the biosphere and it takes 17 full grown trees to make one ton of paper. Trees are also used to fulfill the other requirements of man such as for timber, food, medicines, etc. Hence, it is difficult to calculate the annual consumption of plant material by man.

Properties of enzymes

(1) Enzymes are complex macromolecules with high molecular weight.

(2) They catalyze biochemical reactions in a cell. They help in the breakdown of large molecules into smaller molecules or bring together two smaller molecules to form a larger molecule.

(3) Enzymes do not start a reaction. However, they help in accelerating it.

(4) Enzymes affect the rate of biochemical reaction and not the direction of the reaction.

(5) Most of the enzymes have high turnover number. Turnover number of an enzyme is the number of molecules of a substance that is acted upon by an enzyme per minute under saturated substrate concensation. High turnover number of enzymes increases the efficiency of reaction.

(6) Enzymes are specific in action.

(7) Enzymatic activity decreases with increase in temperature and all enzymes show maximum activity at an optimum kmp of 30-40 0C.

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

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

(a) Glycosidic bond is formed normally between C-1 and C-4, of adjacent monosaccharide units.

(b) Peptide bond is a covalent bond that is formed between two adjacent amino acids by condensation of NH2 gp of ane amino acid and cp oh gp of other amino acid and hence it can be depicted as

P

II - OH

C

(c) Phosphodiester bond is a strong covalent bond formed between phosphate and two adjacent sugar groups. Such bonds form the sugar phosphate backbone of nucleic acids.

The helical polypeptide chain undergoes coiling and folding to form a complex three-dimensional shape referred to as tertiary structure of proteins. These coiling and folding hide the non-polar amino acid chains in the interior of molecule and expose the polar side chains to the exterior. The tertiary structure is held together by the weak non-covalent interacting formed between various parts of the polypeptide chain.

 (a)

(b)

Yes, if we are given a method to know the sequence of proteins, we can use this information to determine purity of a protein. It is known that an accurate sequence of a certain amino acid 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. So by knowing sequence of a given protien, we can determine its structure and compare it with any of the known correct protein sequence. Any change in the sequence can be linked to the purity or homogeneity of a protein.

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

Proteins used as therapeutic agents are as follows:

1. Thrombin and fibrinogen – They help in blood clotting.

2. Antigen (antibody) – It helps in blood transfusion.

3. Insulin – It helps in maintaining blood glucose level in the body.

4. Renin – It helps in osmoregulation.

Proteins are also commonly used in the manufacture of cosmetics, toxins, and as biological buffers.

Triglyceride which is formed by esprification 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

These three fatty acids can be same or different.

Like lactobacillus. These bacteria secrete lactic acid which changes the Ph of milk and proteins are very sensitive to change in Ph. So this changed Ph to destruction of secondary and tertiary structures of protein (called denaturation). Which leads to precipitation of milk proteins and lead to curd formation.

Milk has many globular proteins. Milk is converted into curd or yoghurt by action of bacteria.

Ball and stick models are 3-D molecular models that can be used to describe the structure of biomolecules.

In ball and stick model, the atoms are represented as balls whereas the bonds that hold the atoms are represented by the sticks. Double and triple bonds are represented by springs that form curved connections between the balls. The size and colour of various atoms are different and are depicted by the relative size of the balls. It is the most fundamental and common model of representing biomolecular structures.

In the above ball and stick model of D-glucose, the oxygen atoms are represented by red balls, hydrogen atoms by blue balls, while carbon atoms are represented by grey balls.

The combination of two is known as enzyme- substrate complex (ES). The enzyme molecule operated by chemically uniting with the substrate molecules. Steps that are followed during the catalytic cycle of an enzyme action are:

1. The substrate binds to the active site of the enzyme which fits into the active site.
2. Because of binding substrate, the enzyme induced to alter its size and will fit more tightly.
3. A new complex enzyme is formed when the active site of the enzyme breaks the chemical bonds of the substrate.
4. The enzyme releases the product of the reaction and the free enzyme is ready to bind another molecule of the substrate and run through catalytic cycle once again.
5. When the enzyme- substrate complex is made, the product leaves the enzymes surface and is now free to receive a new set of starting molecules. Thus, the same enzyme is being used again and again.

Some reactions are as follows:

Enzymes are divided into six different classes according to the general type of chemical reactions they catalyses:

Oxidoreductase, transferase, hydrolase, lyases, isomerase and ligase or synthetase.

1. Transferase: enzymes that catalysis a transfer of group other than hydrogen between a pair of substrate S and S’. Example: 

2. Ligases: enzymes catalysing the linking together of 2 compounds or substrates by getting energy from hydrolysis of ATP. Example:

Nucleic acid exhibit a variety of secondary structures. The famous Watson - Crick Model says that DNA exists as a double helix. DNA is made up of two polypeptide chains arranged in double helix. The successive units are joined by phosphodiester bonds in each strand. The backbone is formed by the sugar phosphate sugar chain which is present on the outer side. Nitrogenous bases are present on the inner side and paired to each other by H- bonds. The two chains are spirally coiled around a common axis to form a regular, right handed double helix.

The helix is 20  wide, it’s one complete turn is 34  long and contains 10 base pairs. These two chains are complementary to each other. The two chains are antiparallel. One aligned in 5’ to 3’ direction and the other aligned in 3’ to 5’ direction. Adenine joined to Thymine by two hydrogen bonds and Cytosine on one chain is linked to Guanine by three hydrogen bonds. DNA is dextrorotatory.

Lipids are classified into three categories:

1. Simple lipids: The esters of fatty acids with various alcohols are called simple lipids. They are of two types: Neutral or True fats (these are the esters of fatty acids with glycerol) and Waxes (esters of fatty acids with alcohol other than glycerol).
2. Compound lipids: Compound or conjugated lipids are the fatty acids and alcohol but contains other substances like additional group. Example: Phospholipids, glycolipids, lipoproteins and cutin and suberin.
3. Derived lipids: When lipids consist of hydrocarbon ring and long chain of hydrocarbons, they are called derivatives or derived lipids. Example: Steroids, terpenes and prostaglandins. 

- COOH, - NH₂, and – H are the common substituents.

• Cotton fibre: Cellulose
• Exoskeleton of cockroach: Chitin
• Liver: Glycogen
• Peeled potato: Starch