9.10 Exercise Problems

Q1. What are macromolecules? Give examples.

Answer: Macromolecules are large complex molecules that occur in colloidal state in intercellular fluid. They are formed by the polymerization of low molecular weight macromolecules. Polysaccharides, proteins, and nucleic acids are common examples of macromolecules.

Q2. What is meant by the tertiary structure of proteins?

Answer: The helical polypeptide chain undergoes coiling and folding to form a complex three-dimensional shape referred to as the tertiary structure of proteins. These coils and folds are arranged to hide the non-polar amino acid chains and to expose the polar side chains. The tertiary structure is held together by the weak bonds formed between various parts of the polypeptide chain.

Q3. Find and write down structures of 10 interesting small molecular weight biomolecules. Find if there is any industry that manufactures the compounds by isolation. Find out who are the buyers.

Answer: 

(b)

Q4. Find out and make a list of proteins used as therapeutic agents. Find other applications of proteins (e.g., Cosmetics etc.)

Answer: Proteins used as therapeutic agents are as follows:

• Thrombin and fibrinogen – They help in blood clotting.
• Antigen (antibody) – It helps in blood transfusion.
• Insulin – It helps in maintaining blood glucose levels in the body.
• Renin – It helps in osmoregulation.
• Proteins are also commonly used in the manufacture of cosmetics, toxins, and as biological buffers.

Q5. Explain the composition of triglyceride.

Answer: Triglyceride is a glyceride, which is formed from a single molecule of glycerol, esterified with three fatty acids. It is mainly present in vegetable oils and animal fat.

The general chemical formula of triglyceride is

\(\text { R2COO-CH2CH(-OOCR1)CH2-OOCR3 }\)

Where \(\mathbf{R}_1, \mathbf{R}_2\), and \(\mathbf{R}_3\) are fatty acids. These three fatty acids can be same or different.

Q6. Can you attempt building models of biomolecules using commercially available atomic models (Ball and Stick models).

Answer: 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.

Q7. Draw the structure of the amino acid, alanine.

Answer: 

Q8. What are gums made of? Is Fevicol different?

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

Q9. Find out a qualitative test for proteins, fats and oils, amino acids and test any fruit juice, saliva, sweat and urine for them.

Answer: A qualitative test for proteins is the xanthoproteic test. When urine is tested for protein with the help of the xanthoproteic test, the presence of a yellow precipitate confirms the presence of protein in it.

The qualitative test for fats is emulsification test. In this test, the experimental material is treated with ethanol and then dissolved in water. Formation of emulsion confirms the presence of fats.

The qualitative test for oils is the paper test. The experimental material is put on a paper. If oil marks are left, the presence of oil is confirmed.

The qualitative test for amino acids is ninhydrin test.

Q10. 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!

Answer: 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.

Q11. Describe the important properties of enzymes.

Answer: Properties of enzymes

• Enzymes are complex macromolecules with high molecular weight.
• 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.
• Enzymes do not start a reaction. However, they help in accelerating it.
• Enzymes affect the rate of biochemical reaction and not the direction.
• 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. High turnover number of enzymes increases the efficiency of the reaction.
  (A) Enzymes are specific in action.
  (B) Enzymatic activity decreases with an increase in temperature.
  (C) They show maximum activity at an optimum pH of \(6-8\).
  (D) The velocity of the enzyme increases with an increase in substrate concentration and then, ultimately reaches maximum velocity.

Exemplar Section

VERY SHORT ANSWER TYPE QUESTIONS

Q1. Medicines are either man made (i.e., synthetic) or obtained from living organisms like plants, bacteria, animals etc. and hence the latter are called natural products. Sometimes natural products are chemically altered by man to reduce toxicity or side effects. Write against each of the following whether they were initially obtained as a natural product or as a synthetic chemical.
a. Penicillin ___________________________
b. Sulfonamide ___________________________
c. Vitamin C ___________________________
d. Growth Hormone ___________________________

Answer: a. Penicillin: Natural product
b. Sulfonamide: Synthetic chemical
c. Vitamin C: Natural product
d. Growth Hormone: Natural product

Q2. Select an appropriate chemical bond among ester bond, glycosidic bond, peptide bond and hydrogen bond and write against each of the following.
a. Polysaccharide ___________________________
b. Protein ___________________________
c. Fat ___________________________
d. Water ___________________________

Answer: a. Polysaccharide: Glycosidic bond
b. Protein: Peptide bond
c. Fat: Ester bond
d. Water: Hydrogen bond

Q3. Write the name of any one aminoacid, sugar, nucleotide and fatty acid.

Answer: Glycine (amino acid), Ribose (sugar), Cytidylic acid (nucleotide) and Arachidonic acid (fatty acid).

Q4. Reaction given below is catalysed by oxidoreductase between two substrates A and \(A ^{\prime}\), complete the reaction.
A reduced \(+ A ^{\prime}\) oxidised \(\longrightarrow\)

Answer: \(\text { A reduced }+A^{\prime} \text { oxidised } \longrightarrow A \text { oxidised }+A^{\prime} \text { reduced }\)

Q5. How are prosthetic groups different from co-factors? 

Answer: Prosthetic groups are organic compounds and are distinguished from other cofactors in that they are tightly bound to the apoenzyme. For example, in peroxidase and catalase, which catalyze the breakdown of hydrogen peroxide to water and oxygen, haem is the prosthetic group and it is a part of the active site of the enzyme. Cofactor may be organic or inorganic (metal ions).

Q6. Starch, Cellulose, Glycogen, Chitin are polysaccharides found among the following. Choose the one appropriate and write against each.
Cotton fibre __________________________
Exoskeleton of cockroach __________________________
Liver __________________________
Peeled potato __________________________ 

Answer: (a) Cotton fibre – Cellulose (b) Exoskeleton of cockroach – Chitin
(c) Liver – Glycogen
(d) Peeled potato – Starch

Cellulose is a long chain of linked glucose molecules and is the main component of plant cell walls. Cotton is the purest natural form of cellulose.
The cellulose content of cotton fibre is \(90 \%\)
Chitin is a long chain polymer that forms the hard part of the outer integument or exoskeleton of crustaceans and insects such as cockroach. It is also the main component of the cell walls of fungi.
Glycogen is a multibranched polysaccharide of glucose acting as a form of stored energy in the liver of animals. It is also found in some stored fugni.
Starch is a carbohydrate consisting of along chain of glucose units joined by glycosidic bonds. This polysaccharide is produced mostly by green plants for energy storage, e.g., peeled potato.

SHORT ANSWER TYPE QUESTIONS

Q1. Enzymes are proteins. Proteins are long chains of aminoacids linked to each other by peptide bonds. Aminoacids have many functional groups in their structure. These functional groups are, many of them at least, ionisable. As they are weak acids and bases in chemical nature, this ionization is influenced by pH of the solution. For many enzymes, activity is influenced by surrounding pH. This is depicted in the curve below, explain briefly.

Answer: Given here is a plot between pH (in the x -axis) and the enzyme activity ( y axis). The graph shows the change in the activity of an enzyme with change in the value of pH .
A bell-shaped curve is observed between enzyme activity and pH .

Enzyme activity at optimal pH

A peak is represented by a pH value at which the enzyme activity is maximum (the optimal pH).
This is due to the retention of proper structural configuration and conformation of the enzyme which renders proper functionality to it.

Enzyme activity beyond optimal pH

When the pH rises more than the optimum pH or drops down the optimum pH , the enzymatic activity decreases as is evident from the decline in the reaction velocity. This corresponds to either side of the maxima.

This is because changes in pH results in the changes in the charge of many amino acids present in the protein (enzyme). This leads to the formation of new bonds and breakage of older bonds present in the enzymes.

This results in the changing of the shape/ conformation of the enzyme and hence, affects enzyme activity.

Q2. Is rubber a primary metabolite or a secondary metabolite? Write four sentences about rubber.

Answer: Those compounds which are not involved in basic primary metabolic processes and have no direct function in growth and development of organisms, are called secondary metabolites.
Examples: Alkaloids, terpenoides, essential oils, toxins, lectins, drugs, polymeric substances etc.

Properties of rubber: Rubber is a secondary metabolite (a polymeric substance).

• It is extracted from the rubber tree (Hevea brasiliensis).
• It is the polymer of isoprene units.
• It has the following properties:
• High tensile strength
• Elasticity

These properties make it useful for making a variety of useful items like erasers, toys, gloves etc. It can be modified to have a rigid structure by addition of sulphur in a process called vulcanization. The modified rubber is used in production of tyres of vehicles.

Final answer: Secondary metabolite

Q3. Schematically represent primary, secondary and tertiary structures of a hypothetical polymer say for example a protein.

Answer:Proteins are the large-sized, heteropolymeric macromolecules having on or more polypeptides (chains of amino acid). Primary structure The primary structure of a protein is the linear sequence of amino acid structural units and partially comprises its overall biomolecular structures. The amino acids are linked together in a sequence by peptide bonds.

Secondary structure It is a three dimensional form of local segments of bipolymers such as proteins. Secondary struture of proteins is defined by hydrogen bonds between backbone amino and carboxyl groups. Mainly secondary structure in proteins possess two forms, i.e., \(\alpha\)-helix and \(\beta\)-pleated sheet.
\(\alpha\)-helix is a polypeptide chain spirally coiled to form a righ handed helix. This helix may be coiled regularly at places and at some places randomly coiled. The helix is stabilised by many hydrogen bonds which are formed between CO of one anino acid and – NH group of next fourth amono acid.
\(\beta\) – Pleated sheets two or more polypeptide chains are joined together by intermolecular hydrogen bonds to produce a sheet like structure instead of fibre as in a-helix.The polypeptide strands in a sheet may run parallel in same direction, e.g., keratin or in opposite direction called antiparalel \(\beta\)-sheet, e.g., fibroin.

Tertiary structure involves interactions that are caused by the bending and folding of \(\alpha\)-helix or \(\beta\)-sheets leading to the formation of rods, spheres of fibres. Such interactions are typically conferred by H -bonds, ionic bonds, covalent bonds. van der Waat’s interactions and hydrophobic interactions or disulphide linkages. It gives the protein a three dimensional conformation.

Q4. Nucleic acids exhibit secondary structure, justify with example.

Answer: Nucleic acids exhibit a wide variety of secondary structures. For example, one of the secondary structures exhibited by DNA is the famous Watson-Crick model. This model says that DNA exists as a double helix. The two strands of polynucleotides are antiparallel, i.e. run in the opposite direction. The backbone is formed by the sugar-phosphate-sugar chain. The nitrogen bases are projected more or less perpendicular to this backbone but face inside. A and G of one strand compulsorily base pairs with T and C , respectively, on the other strand. There are two hydrogen bonds between \(A\) and \(T\) and three hydrogen bonds between G and C . Each strand appears like a helical staircase. Each step of ascent is represented by a pair of bases. At each step of ascent, the strand turns \(36^{\circ}\). One full turn of the helical strand would involve ten steps or ten base pairs. Attempt drawing a line diagram. The pitch would be 34 A . The rise per base pair would be 3.4 A . This form of DNA with the above mentioned salient features is called B-DNA.

LONG ANSWER TYPE QUESTIONS

Q1. Formation of enzyme-substrate complex (ES) is the first step in catalysed reactions. Describe the other steps till the formation of product.

Answer:  The catalytic cycle of an enzyme action can be described in the following steps:
(1) First, the substrate binds to the active site of the enzyme, fitting into the active site.
(2) The binding of the substrate induces the enzyme to alter its shape, fitting more tightly around the substrate.
(3) The active site of the enzyme, now in close proximity of the substrate breaks the chemical bonds of the substrate and the new enzyme-product complex is formed.
(4) The enzyme releases the products of the reaction and the free enzyme is ready to bind to another molecule of the substrate and run through the catalytic cycle once again.

Q2. What are different classes of enzymes? Explain any two with the type of reaction they catalyse.

Answer: Enzymes are divided into 6 classes each with 4—13 subclasses and named accordingly by a four-digit number.
Oxidoreductases/dehydrogenases: Enzymes which catalyse oxidoreduction between two substrates \(S\) and \(S ^{\prime}\). E.g.,
\(
S \text { reduced }+S^{\prime} \text { reduced } \longrightarrow S \text { oxidised }+S^{\prime} \text { oxidised }
\)
Transferases: Enzymes catalysing a transfer of a group. G (other than hydrogen) between a pair of substrate \(S\) and \(S^{\prime}\), e.g.,
\(
S-G+S^{\prime} \longrightarrow S+S^{\prime}-G
\)
Hydrolases: Enzymes catalysing hydrolysis of ester, ether, peptide, glycosidic, \(C – C\). C -halide or \(P – N\) bonds.
Lyases: Enzymes that catalyse removal of groups from substrates by mechanisms other than hydrolysis leaving double bonds.

Isomerases: Includes all enzymes catalysing inter-conversion of optical, geometric or positional isomers.
Ligases: Enzymes catalysing the linking together of 2 compounds, e.g., enzymes which catalyse joining of \(C – O , C – S , C – N , P – O\) etc. bonds.

Q3. Nucleic acids exhibit secondary structure. Describe through Watson Crick Model. 

Answer: Nucleic acids exhibit a wide .variety of secondary structures. For example, one of the secondary structures exhibited by DNA is the famous Watson- Crick model. This model says that DNA exists as a double helix. The two strands of polynucleotides are antiparallel, i.e. run in the opposite direction. The backbone is formed by the sugar-phosphate-sugar chain. The nitrogen bases are projected more or less perpendicular to this backbone but face inside. A and G of one strand compulsorily base pairs with T and C , respectively, on the other strand. There are two hydrogen bonds between \(A\) and \(T\) and three hydrogen bonds between G and C . Each strand appears like a helical staircase. Each step of ascent is represented by a pair of bases. At each step of ascent, the strand turns \(36^{\circ}\). One full turn of the helical strand would involve ten steps or ten base pairs. Attempt drawing a line diagram. The pitch would be 34 A . The rise per base pair would be 3.4 A . This form of DNA with the above mentioned salient features is called B-DNA.

Q4. What is the difference between a nucleotide and nucleoside? Give two examples of each with their structure.

Answer:  Living organisms have a number of carbon compounds in which heterocyclic rings can be found. Some of these are nitrogen bases-adenine, guanine, cytosine, uracil and thymine. When found attached to a sugar, they are called nucleosides. If a phosphate group is also found esterified to the sugar they are called nucleotides. Adenosine, guanosine, thymidine, uridine and cytidine are nucleosides. Adenylic acid, thymidylic acid, guanylic acid, uridylic acid and cytidylic acid are nucleotides.

Q5. Describe various forms of lipid with a few examples

Answer: Lipids are generally water insoluble. They could be simple fatty acids. A fatty acid has a carboxyl group attached to an R-group. The R-group could be a methyl \((- CH 3)\), or ethyl \((- C 2 H 5)\) or higher number of- CH 2 groups ( 1 carbon to 19 carbons). For example, palmitic acid has 16 carbons including carboxyl carbon. Arachidonic acid has 20 carbon atoms including the carboxyl carbon. Fatty acids could be saturated (without double bond) or unsaturated (with one or more \(C = C\) double bonds). Another simple lipid is glycerol which is trihydroxy propane.

• Many lipids have both glycerol and fatty acids. Here the fatty acids are found esterified with glycerol. They can be then monoglycerides, diglycerides and triglycerides. These are also called fats and oils based on melting point. Oils have lower melting point (e.g., gingely oil) and hence remain as oil in winters.
• Some lipids have phosphorous and a phosphorylated organic compound in them. These are phospholipids. They are found in cell membrane. Lecithin is one example. Some tissues especially the neural tissues have lipids with more complex structures.