Biomolecules

There is a wide diversity in living organisms in our biosphere. Now a question that arises in our minds is: Are all living organisms made of the same chemicals, i.e., elements and compounds? You have learnt in chemistry how elemental analysis is performed. If we perform such an analysis on a plant tissue, animal tissue or a microbial paste, we obtain a list of elements like carbon, hydrogen, oxygen and several others and their respective content per unit mass of a living tissue. If the same analysis is performed on a piece of earth’s crust as an example of non-living matter, we obtain a similar list. What are the differences between the two lists? In absolute terms, no such differences could be made out. All the elements present in a sample of earth’s crust are also present in a sample of living tissue. However, a closer examination reveals that the relative abundance of carbon and hydrogen with respect to other elements is higher in any living organism than in earth’s crust (Table 9.1).

9.1 How to Analyse Chemical Composition?

9.2 Primary and Secondary Metabolites

9.3 Biomacromolecules

9.4 Proteins

9.5 Polysaccharides

9.6 Nucleic Acids

9.7 Structure of Proteins

9.8 Nature of Bond Linking Monomers in a Polymer

9.9 Dynamic State of Body Constituents - Concept of Metabolism

9.10 Metabolic Basis for Living

9.11 The Living State

9.12 Enzymes

We can continue asking in the same way, what type of organic compounds are found in living organisms? How does one go about finding the answer? To get an answer, one has to perform a chemical analysis. We can take any living tissue (a vegetable or a piece of liver, etc.) and grind it in trichloroacetic acid (Cl3CCOOH) using a mortar and a pestle. We obtain a thick slurry. If we were to strain this through a cheesecloth or cotton we would obtain two fractions. One is called the filtrate or more technically, the acid-soluble pool, and the second, the retentate or the acid-insoluble fraction. Scientists have found thousands of organic compounds in the acid-soluble pool.

TABLE 9.1 A Comparison of Elements Present in Non-living and Living Matter*

In higher classes you will learn about how to analyse a living tissue sample and identify a particular organic compound. It will suffice to say here that one extracts the compounds, then subjects the extract to various separation techniques till one has separated a compound from all other compounds. In other words, one isolates and purifies a compound. Analytical techniques, when applied to the compound give us an idea of the molecular formula and the probable structure of the compound. All the carbon compounds that we get from living tissues can be called ‘biomolecules. However, living organisms have also got inorganic elements and compounds in them. How do we know this? A slightly different but destructive experiment has to be done. One weighs a small amount of a living tissue (say a leaf or liver and this is called wet weight) and dry it. All the wate, evaporates. The remaining material gives dry weight. 

Q.The major portion of the dry weight of plants comprises of:

1. Nitrogen, phosphorus, and potassium

2. Calcium, magnesium, and sulphur

3. Carbon, nitrogen, and hydrogen

4. Carbon, hydrogen, and oxygen

(AIPMT - 2003)

Now if the tissue is fully burnt, all the carbon compounds are oxidised to gaseous form (CO₂, water vapour) and are removed. What is remaining is called ‘ash’. This ash contains inorganic elements (like calcium, magnesium etc). Inorganic compounds like sulphate, phosphate, etc., are also seen in the acid-soluble fraction. Therefore elemental analysis gives elemental composition of living tissues in the form of hydrogen, oxygen, chlorine, carbon etc. while analysis for compounds gives an idea of the kind of organic (Figure 9.1) and inorganic constituents (Table 9.2) present in living tissues. From a chemistry point of view, one can identify functional groups like aldehydes, ketones, aromatic compounds, etc. But from a biological point of view, we shall classify them into amino acids, nucleotide bases, fatty acids etc.

Table 9.2 A List of Representative Inorganic Constituents of Living Tissues

Amino acids are organic compounds containing an amino group and an acidic group as substituents on the same carbon i.e., the α-carbon. Hence, they are called α-amino acids. They are substituted methanes. There are four substituent groups occupying the four valency positions. These are hydrogen, carboxyl group, amino group and a variable group designated as R group. Based on the nature of R group there are many amino acids. However, those which occur in proteins are only of twenty types. The R group in these proteinaceous amino acids could be a hydrogen (the amino acid is called glycine), a methyl group (alanine), hydroxy methyl (serine), etc.

Q. Identify the statement which is incorrect. (NEET - 2020)

1.	Sulphur is an integral part of cysteine.
2.	Glycine is an example of lipids.
3.	Lecithin contains a phosphorus atom in its structure.
4.	Tyrosine possesses an aromatic ring in its structure.

Three of the twenty are shown in Figure 9.1.

The chemical and physical properties of amino acids are essentially of the amino, carboxyl and the R functional groups. Based on number of amino and carboxyl groups, there are acidic (e.g., glutamic acid), basic (lysine) and neutral (valine) amino acids. Similarly, there are aromatic amino acids (tyrosine, phenylalanine, tryptophan).

Q. Given below are two statements:

Statement I:	Amino acids have a property of ionizable nature of –NH2 and –COOH groups, hence have different structures at different pH.
Statement II:	Amino acids can exist as Zwitterionic form at acidic and basic pH.

In the light of the above statements, choose the most appropriate answer from the options given below:

1.	Both Statement I and Statement II are correct.
2.	Both Statement I and Statement II are Incorrect.
3.	Statement I is correct but Statement II is incorrect.
4.	Statement I is incorrect but Statement II is correct.

A particular property of amino acids is the ionizable nature of –NH2 and –COOH groups. Hence in solutions of different pH, the structure of amino acids changes.

B is called zwitterionic form.

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 (–CH3), or ethyl (–C2H5) or higher number of –CH2 groups (1 carbon to 19 carbons). For example, palmitic acid has 16 carbons including carboxyl carbon.

Q. Following are the statements with reference to 'lipids'. (NEET - 2021)

(a)	Lipids having only single bonds are called unsaturated fatty acids.
(b)	Lecithin is a phospholipid.
(c)	Trihydroxy propane is glycerol.
(d)	Palmitic acid has 20 carbon atoms including carboxyl carbon.
(e)	Arachidonic acid has 16 carbon atoms.

You have to choose the correct statements out of the five given above. Choose the correct answer from the options given below:

1. (b) and (c) only

2. (b) and (e) only

3. (a) and (b) only

4. (c) and (d) only

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).

Q. Match List -I with List - II.(NEET - 2021)

List-I	List-Ii
(a) Protein (b) Unsaturated fatty acid  (c) Nucleic acid (d)Polysaccharides	I. C-C double bonds II. Phosphodiester bonds III. Glycosidic bonds IV. Peptide bonds

Choose the correct answer from the options given below:

Options:	(a)	(b)	(c)	(d)
1.	ii	i	iv	iii
2.	iv	iii	i	ii
3.	iv	i	ii	iii
4.	i	iv	iii	ii

Another simple lipid is glycerol which is trihydroxy propane.

Q. Following are the statements with reference to 'lipids'.

(a)	Lipids having only single bonds are called unsaturated fatty acids.
(b)	Lecithin is a phospholipid.
(c)	Trihydroxy propane is glycerol.
(d)	Palmitic acid has 20 carbon atoms including carboxyl carbon.
(e)	Arachidonic acid has 16 carbon atoms.

You have to choose the correct statements out of the five given above. Choose the correct answer from the options given below:

1. (b) and (c) only

2. (b) and (e) only

3. (a) and (b) only

4. (c) and (d) only

Many lipids have both glycerol and fatty acids. Here the fatty acids are found esterified with glycerol.

Q. A phosphoglyceride is always made up of: (AIPMT - 2013)

1.	only an unsaturated fatty acid esterified to a glycerol molecule to which a phosphate group is also attached.
2.	a saturated or unsaturated fatty acid esterified to a glycerol molecule to which a phosphate group is also attached.
3.	a saturated or unsaturated fatty acid esterified to a phosphate group which is also attached to a glycerol molecule.
4.	only a saturated fatty acid is esterified to a glycerol molecule to which a phosphate group is also attached.

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., gingelly oil) and hence remain as oil in winters. Can you identify a fat from the market? Some lipids have phosphorous and a phosphorylated organic compound in them. These are phospholipids.

Q. Following are the statements with reference to 'lipids'. (NEET - 2021)

(a)	Lipids having only single bonds are called unsaturated fatty acids.
(b)	Lecithin is a phospholipid.
(c)	Trihydroxy propane is glycerol.
(d)	Palmitic acid has 20 carbon atoms including carboxyl carbon.
(e)	Arachidonic acid has 16 carbon atoms.

You have to choose the correct statements out of the five given above. Choose the correct answer from the options given below:

1. (b) and (c) only

2. (b) and (e) only

3. (a) and (b) only

4. (c) and (d) only

They are found in cell membrane.

Q. Which one of the following biomolecules is correctly characterised? (AIPMT - 2012)

1.	Palmitic acid - an unsaturated fatty acid with 18 carbon atoms
2.	Adenylic acid - adenosine with a glucose phosphate molecule
3.	Alanine amino acid - Contains an amino group and an acidic group anywhere in the molecule
4.	Lecithin - a phosphorylated glyceride found in the cell membrane

Lecithin is one example.

Q. Following are the statements with reference to 'lipids'. (NEET - 2021)

(a)	Lipids having only single bonds are called unsaturated fatty acids.
(b)	Lecithin is a phospholipid.
(c)	Trihydroxy propane is glycerol.
(d)	Palmitic acid has 20 carbon atoms including carboxyl carbon.
(e)	Arachidonic acid has 16 carbon atoms.

You have to choose the correct statements out of the five given above. Choose the correct answer from the options given below:

1. (b) and (c) only

2. (b) and (e) only

3. (a) and (b) only

4. (c) and (d) only

Some tissues especially the neural tissues have lipids with more complex structures.

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.

Q. Nucleotides are building blocks of nucleic acids. Each nucleotide is a composite molecule formed by -

1. Base-sugar-OH

2. Base-sugar-phosphate

3. Sugar-phosphate

4. (Base-sugar-phosphate)_n

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.

Q. Uridine, present only in RNA is: (NEET - 2013)
1. nucleoside
2. nucleotide
3. purine
4. pyrimidine

Nucleic acids like DNA and RNA consist of nucleotides only. DNA and RNA function as genetic material.

Figure 9.1 Diagrammatic representation of small molecular weight organic compounds in living tissues

Q. Which of the following organic compounds is the main constituent of Lecithin? (NEET - 2019)

1. Arachidonic acid

2. Phospholipid

3. Cholesterol

4. Phosphoprotein

9.2 Primary and Secondary Metabolites

The most exciting aspect of chemistry deals with isolating thousands of compounds, small and big, from living organisms, determining their structure and if possible synthesising them.

Table 9.3 Some Secondary Metabolites

Q. Which of the following are not secondary metabolites in plants? (NEET - 2021)

1. Vinblastin, curcumin

2. Rubber, gums

3. Morphine, codeine

4. Amino acids, glucose

If one were to make a list of biomolecules, such a list would have thousands of organic compounds including amino acids, sugars, etc. For reasons that are given in section 9.10, we can call these biomolecules as ‘metabolites’. In animal tissues, one notices the presence of all such categories of compounds shown in Figure 9.1. These are called primary metabolites. However, when one analyses plant, fungal and microbial cells, one would see thousands of compounds other than these called primary metabolites, e.g. alkaloids, flavonoids, rubber, essential oils, antibiotics, coloured pigments, scents, gums, spices.2 These are called secondary metabolites (Table 9.3). While primary metabolites have identifiable functions and play known roles in normal physiologial processes, we do not at the moment, understand the role or functions of all the ‘secondary metabolites’ in host organisms. However, many of them are useful to ‘human welfare’ (e.g., rubber, drugs, spices, scents and pigments). Some secondary metabolites have ecological importance. In the later chapters and years you will learn more about this.

9.3 Biomacromolecules

There is one feature common to all those compounds found in the acid soluble pool. They have molecular weights ranging from 18 to around 800 daltons (Da) approximately.

The acid insoluble fraction, has only four types of organic compounds i.e., proteins, nucleic acids, polysaccharides and lipids. These classes of compounds with the exception of lipids, have molecular weights in the range of ten thousand daltons and above. For this very reason, biomolecules, i.e., chemical compounds found in living organisms are of two types. One, those which have molecular weights less than one thousand dalton and are usually referred to as micromolecules or simply biomolecules while those which are found in the acid insoluble fraction are called macromolecules or biomacromolecules.

The molecules in the insoluble fraction with the exception of lipids are polymeric substances. 

Q. Which of the following is not a polymeric substance? (NEET - 2017)

(1) Nucleic acid               

(2) Proteins

(3) Polysaccharides           

(4) Lipids

Then why do lipids, whose molecular weights do not exceed 800 Da, come under acid insoluble fraction, i.e., macromolecular fraction? Lipids are indeed small molecular weight compounds and are present not only as such but also arranged into structures like cell membrane and other membranes. When we grind a tissue, we are disrupting the cell structure. Cell membrane and other membranes are broken into pieces, and form vesicles which are not water soluble. Therefore, these membrane fragments in the form of vesicles get separated along with the acid insoluble pool and hence in the macromolecular fraction. Lipids are not strictly macromolecules.

The acid soluble pool represents roughly the cytoplasmic composition. The macromolecules from cytoplasm and organelles become the acid insoluble fraction. Together they represent the entire chemical composition of living tissues or organisms.

Table 9.4 Average Composition of Cells

In summary if we represent the chemical composition of living tissue from abundance point of view and arrange them class-wise, we observe that water is the most abundant chemical in living organisms (Table 9.4).

9.4 Proteins

Proteins are polypeptides. They are linear chains of amino acids linked by peptide bonds as shown in Figure 9.3.

Table 9.5 Some Proteins and their Functions2

Q. Identify the substances having a glycosidic bond and peptide bond, respectively in their structure:(NEET - 2020)

1.  Glycerol, trypsin

2.  Cellulose, lecithin

3.  Inulin, insulin

4.  Chitin, cholesterol 

Each protein is a polymer of amino acids. As there are 20 types of amino acids (e.g., alanine, cysteine, proline, tryptophan, lysine, etc.), a protein is a heteropolymer and not a homopolymer. A homopolymer has only one type of monomer repeating ‘n’ number of times. This information about the amino acid content is important as later in your nutrition lessons, you will learn that certain amino acids are essential for our health and they have to be supplied through our diet. Hence, dietary proteins are the source of essential amino acids. Therefore, amino acids can be essential or non-essential. The latter are those which our body can make, while we get essential amino acids through our diet/food. Proteins carry out many functions in living organisms, some transport nutrients across cell membrane, some fight infectious organisms, some are hormones, some are enzymes, etc. (Table 9.5). Collagen is the most abundant protein in animal world and Ribulose bisphosphate Carboxylase-Oxygenase (RuBisCO) is the most abundant protein in the whole of the biosphere.

Q. Match the following: (NEET - 2020)

(a)	Inhibitor of catalytic activity of succinate dehydrogenase	(i)	Ricin
(b)	Possess peptide bonds	(ii)	Malonate
(c)	Cell wall material in fungi	(iii)	Chitin
(d)	Secondary metabolite	(iv)	Collagen

Choose the correct option from the following:

	(a)	(b)	(c)	(d)
1.	(iii)	(i)	(iv)	(ii)
2.	(iii)	(iv)	(i)	(ii)
3.	(ii)	(iii)	(i)	(iv)
4.	(ii)	(iv)	(iii)	(i)

9.5 Polysaccharides

The acid insoluble pellet also has polysaccharides (carbohydrates) as another class of macromolecules.

Q. The chitinous exoskeleton of arthropods is formed by the polymerization of?(NEET - 2015)

1. Keratine sulphate and chondroitin sulphate

2. D-glucosamine

3. N-acetyl glucosamine

4. Lipoglycans

 Polysaccharides are long chains of sugars. They are threads (literally a cotton thread) containing different monosaccharides as building blocks. For example, cellulose is a polymeric polysaccharide consisting of only one type of monosaccharide i.e., glucose.

Q. Given below are two statements: 

Statement I	Cellulose is a polymeric polysaccharide
Statement II	The building blocks of cellulose are glucose molecules

In the light of the above statements , choose the correct answer from the options given below: 

1.	Statements-I is correct but statements-II is incorrect.
2.	Statements-I is incorrect but statements-II is correct.
3.	Both statements-I and statements-II are correct.
4.	Both statements-I and Statements -II are false.

 Cellulose is a homopolymer. Starch is a variant of this but present as a store house of energy in plant tissues. Animals have another variant called glycogen. Inulin is a polymer of fructose. 

Q. Identify the substances having a glycosidic bond and peptide bond, respectively in their structure:

1.  Glycerol, trypsin

2.  Cellulose, lecithin

3.  Inulin, insulin

4.  Chitin, cholesterol 

In a polysaccharide chain (say glycogen), the right end is called the reducing end and the left end is called the non-reducing end. It has branches as shown in the form of a cartoon (Figure 9.2). Starch forms helical secondary structures. 

Q. Cellulose does not form blue colour with Iodine because

1.	It breaks down when iodine reacts with it.
2.	It is a disaccharide.
3.	It is a helical molecule.
4.	It does not contain complex helices and hence cannot hold iodine molecules.

In fact, starch can hold I₂ molecules in the helical portion. The starch-I₂ is blue in colour. Cellulose does not contain complex helices and hence cannot hold I₂.

Figure 9.2 Diagrammatic representation of a portion of glycogen

Plant cell walls are made of cellulose. Paper made from plant pulp and cotton fibre is cellulosic. There are more complex polysaccharides in nature. They have as building blocks, amino-sugars and chemically modified sugars (e.g., glucosamine, N-acetyl galactosamine, etc.). Exoskeletons of arthropods, for example, have a complex polysaccharide called chitin.4 These complex polysaccharides are mostly homopolymers.

9.6 Nucleic Acids

The other type of macromolecule that one would find in the acid insoluble fraction of any living tissue is the nucleic acid. These are polynucleotides. Together with polysaccharides and polypeptides these comprise the true macromolecular fraction of any living tissue or cell. For nucleic acids, the building block is a nucleotide. A nucleotide has three chemically distinct components. One is a heterocyclic compound, the second is a monosaccharide and the third a phosphoric acid or phosphate.

As you notice in Figure 9.1, the heterocyclic compounds in nucleic acids are the nitrogenous bases named adenine, guanine, uracil, cytosine, and thymine. Adenine and Guanine are substituted purines while the rest are substituted pyrimidines. The skeletal heterocyclic ring is called as purine and pyrimidine respectively. 

Match List-I with List -II: (NEET - 2022)

List- I	List-II
(A) Adenine	(I) Pigment
(B) Anthocyanin	(II) Polysaccharide
(C) Chitin	(III) Alkaloid
(D) Codeine	(IV) Purine

Choose the correct answer from the options given below :

	(A)	(B)	(C)	(D)
1.	(IV)	(I)	(II)	(III)
2.	(IV)	(III)	(II)	(I)
3.	(III)	(I)	(IV)	(II)
4.	(I)	(IV)	(III)	(II)

The sugar found in polynucleotides is either ribose (a monosaccharide pentose) or 2’ deoxyribose. A nucleic acid containing deoxyribose is called deoxyribonucleic acid (DNA) while that which contains ribose is called ribonucleic acid (RNA).

9.7 Structure of Proteins

Proteins, as mentioned earlier, are heteropolymers containing strings of amino acids. Structure of molecules means different things in different contexts. In inorganic chemistry, the structure invariably refers to the molecular formulae (e.g., NaCl, MgCl₂, etc.). Organic chemists always write a two dimensional view of the molecules while representing the structure of the molecules (e.g., benzene, naphthalene, etc.). Physicists conjure up the three dimensional views of molecular structures while biologists describe the protein structure at four levels. The sequence of amino acids i.e., the positional information in a protein – which is the first amino acid, which is second, and so on – is called the primary structure (Figure 9.3) of a protein.

Q. Primary proteins are also known as polypeptides, because: (NEET - 2022)
1. they are linear chains.
2. they are polymers of peptide monomers.
3. successive amino acids are joined by peptide bonds.
4. they can assume many conformations.

 A protein is imagined as a line, the left end represented by the first amino acid and the right end represented by the last amino acid. The first amino acid is also called as N-terminal amino acid. The last amino acid is called the C-terminal amino acid.

Q. Given below are two statements:

Statement I:	A protein is imagined as a line, the left end represented by first amino acid (C-terminal) and the right end represented by last amino acid (N-terminal).
Statement II:	Adult human haemoglobin, consists of 4 subunits (two subunits of α� type and two subunits of ẞ type.)

In the light of the above statements, choose the correct answer from the options given below: 

1.	Statement I is false but Statement II is true.
2.	Both Statement I and Statement II are true.
3.	Both Statement I and Statement II are false.
4.	Statement I is true but Statement II is false.

 A protein thread does not exist throughout as an extended rigid rod. The thread is folded in the form of a helix (similar to a revolving staircase). Of course, only some portions of the protein thread are arranged in the form of a helix. In proteins, only right handed helices are observed. Other regions of the protein thread are folded into other forms in what is called the secondary structure. In addition, the long protein chain is also folded upon itself like a hollow woolen ball, giving rise to the tertiary structure (Figure 9.4 a, b). This gives us a 3-dimensional view of a protein. Tertiary structure is absolutely necessary for the many biological activities of proteins.

Figure 9.3 Various levels of Protein Structure

Figure 9.4 Cartoon showing: (a) A secondary structure and (b) A tertiary structure of proteins

Some proteins are an assembly of more than one polypeptide or subunits. The manner in which these individual folded polypeptides or subunits are arranged with respect to each other (e.g. linear string of spheres, spheres arranged one upon each other in the form of a cube or plate etc.) is the architecture of a protein otherwise called the quaternary structure of a protein. Adult human haemoglobin consists of 4 subunits. Two of these are identical to each other. Hence, two subunits of α type and two subunits of β type together constitute the human haemoglobin (Hb).

Q. Given below are two statements: (NEET - 2023)

Statement I:	A protein is imagined as a line, the left end represented by first amino acid (C-terminal) and the right end represented by last amino acid (N-terminal).
Statement II:	Adult human haemoglobin, consists of 4 subunits (two subunits of α type and two subunits of ẞ type.)

In the light of the above statements, choose the correct answer from the options given below: 

1.	Statement I is false but Statement II is true.
2.	Both Statement I and Statement II are true.
3.	Both Statement I and Statement II are false.
4.	Statement I is true but Statement II is false.

9.8 Nature of Bond Linking Monomers in a Polymer

In a polypeptide or a protein, amino acids are linked by a peptide bond which is formed when the carboxyl (-COOH) group of one amino acid reacts with the amino (-NH₂) group of the next amino acid with the elimination of a water moiety (the process is called dehydration).2 In a polysaccharide the individual monosaccharides are linked by a glycosidic bond.2 This bond is also formed by dehydration. 

Match List -I with List - II.

List-I	List-Ii
(a) Protein (b) Unsaturated fatty acid  (c) Nucleic acid (d)Polysaccharides	I. C-C double bonds II. Phosphodiester bonds III. Glycosidic bonds IV. Peptide bonds

Choose the correct answer from the options given below:

Options:	(a)	(b)	(c)	(d)
1.	ii	i	iv	iii
2.	iv	iii	i	ii
3.	iv	i	ii	iii
4.	i	iv	iii	ii

This bond is formed between two carbon atoms of two adjacent monosaccharides. In a nucleic acid a phosphate moiety links the 3’-carbon of one sugar of one nucleotide to the 5’-carbon of the sugar of the succeeding nucleotide. The bond between the phosphate and hydroxyl group of sugar is an ester bond. As there is one such ester bond on either side, it is called phosphodiester bond (Figure 9.5).

Q. Match List -I with List - II. (NEET - 2021)

List-I	List-Ii
(a) Protein (b) Unsaturated fatty acid  (c) Nucleic acid (d)Polysaccharides	I. C-C double bonds II. Phosphodiester bonds III. Glycosidic bonds IV. Peptide bonds

Choose the correct answer from the options given below:

Options:	(a)	(b)	(c)	(d)
1.	ii	i	iv	iii
2.	iv	iii	i	ii
3.	iv	i	ii	iii
4.	i	iv	iii	ii

Figure 9.5  Diagram indicating secondary structure of DNA

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. 

Q. ATP is:

1.	Nucleotide	2.	Nucleoside
3.	Nucleic acid	4.	Vitamin

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. 

Q. If A and C make 30% and 20% of DNA, respectively, what will be the percentage composition of T and G ?

1. T : 20%, G : 30%
2. T : 30%, G : 20%
3. T : 30%, G : 30%
4. T : 20%, G : 20%

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°. One full turn of the helical strand would involve ten steps or ten base pairs. Attempt drawing a line diagram. The pitch would be 34Å. The rise per base pair would be 3.4Å. This form of DNA with the above mentioned salient features is called B-DNA. In higher classes, you will be told that there are more than a dozen forms of DNA named after English alphabets with unique structural features.

9.9 Dynamic State of Body Constituents – Concept of Metabolism

What we have learnt till now is that living organisms, be it a simple bacterial cell, a protozoan, a plant or an animal, contain thousands of organic compounds. These compounds or biomolecules are present in certain concentrations (expressed as mols/cell or mols/litre etc.). One of the greatest discoveries ever made was the observation that all these biomolecules have a turn over. This means that they are constantly being changed into some other biomolecules and also made from some other biomolecules. This breaking and making is through chemical reactions constantly occuring in living organisms. Together all these chemical reactions are called metabolism. Each of the metabolic reactions results in the transformation of biomolecules. A few examples for such metabolic transformations are: removal of CO₂ from amino acids making an amino acid into an amine, removal of amino group in a nucleotide base; hydrolysis of a glycosidic bond in a disaccharide, etc. We can list tens and thousands of such examples. Majority of these metabolic reactions do not occur in isolation but are always linked to some other reactions. In other words, metabolites are converted into each other in a series of linked reactions called metabolic pathways. These metabolic pathways are similar to the automobile traffic in a city. These pathways are either linear or circular. These pathways criss-cross each other, i.e., there are traffic junctions. Flow of metabolites through metabolic pathway has a definite rate and direction like automobile traffic. This metabolite flow is called the dynamic state of body constituents. What is most important is that this interlinked metabolic traffic is very smooth and without a single reported mishap for healthy conditions. Another feature of these metabolic reactions is that every chemical reaction is a catalysed reaction. There is no uncatalysed metabolic conversion in living systems. Even CO₂ dissolving in water, a physical process, is a catalysed reaction in living systems. The catalysts which hasten the rate of a given metabolic conversation are also proteins. These proteins with catalytic power are named enzymes.

9.10 Metabolic Basis for Living

Metabolic pathways can lead to a more complex structure from a simpler structure (for example, acetic acid becomes cholesterol) or lead to a simpler structure from a complex structure (for example, glucose becomes lactic acid in our skeletal muscle). The former cases are called biosynthetic pathways or anabolic pathways. The latter constitute degradation and hence are called catabolic pathways. Anabolic pathways, as expected, consume energy. Assembly of a protein from amino acids requires energy input. On the other hand, catabolic pathways lead to the release of energy. For example, when glucose is degraded to lactic acid in our skeletal muscle, energy is liberated. This metabolic pathway from glucose to lactic acid which occurs in 10 metabolic steps is called glycolysis. Living organisms have learnt to trap this energy liberated during degradation and store it in the form of chemical bonds. As and when needed, this bond energy is utilised for biosynthetic, osmotic and mechanical work that we perform. The most important form of energy currency in living systems is the bond energy in a chemical called adenosine triphosphate (ATP).

How do living organisms derive their energy? What strategies have they evolved? How do they store this energy and in what form? How do they convert this energy into work? You will study and understand all this under a sub-discipline called ‘Bioenergetics’ later in your higher classes.

9.11 The Living State

At this level, you must understand that the tens and thousands of chemical compounds in a living organism, otherwise called metabolites, or biomolecules, are present at concentrations characteristic of each of them. For example, the blood concentration of glucose in a normal healthy individual is 4.2 mmol/L–6.1 mmol/L, while that of hormones would be nanograms/mL. The most important fact of biological systems is that all living organisms exist in a steady-state characterised by concentrations of each of these biomolecules. These biomolecules are in a metabolic flux. Any chemical or physical process moves spontaneously to equilibrium. The steady state is a non-equilibrium state. One should remember from physics that systems at equilibrium cannot perform work. As living organisms work continuously, they cannot afford to reach equilibrium. Hence the living state is a non-equilibrium steady-state to be able to perform work; living process is a constant effort to prevent falling into equilibrium. This is achieved by energy input. Metabolism provides a mechanism for the production of energy. Hence the living state and metabolism are synonymous. Without metabolism there cannot be a living state.

9.12 Enzymes

Almost all enzymes are proteins. 

Q Which of the following statements about enzymes is wrong?

1.	Enzymes are denatured at high temperatures
2.	Enzymes are mostly proteins but some are lipids also
3.	Enzymes are highly specific.
4.	Enzymes require optimum pH and temperature for maximum activity.

There are some nucleic acids that behave like enzymes. 

Q. Three of the following statements about enzymes are correct and one is wrong. Which one is wrong?
 

1.	Enzymes are denatured at high temperatures but in certain exceptional organisms they are effective even at temperatures 80º - 90ºC
2.	Enzymes are highly specific
3.	Most enzymes are proteins but some are lipids
4.	Enzymes require optimum pH for maximal activity

These are called ribozymes. 

One can depict an enzyme by a line diagram. An enzyme like any protein has a primary structure, i.e., amino acid sequence of the protein. An enzyme like any protein has the secondary and the tertiary structure. When you look at a tertiary structure (Figure 9.4 b) you will notice that the backbone of the protein chain folds upon itself, the chain criss-crosses itself and hence, many crevices or pockets are made. One such pocket is the ‘active site’. An active site of an enzyme is a crevice or pocket into which the substrate fits. Thus enzymes, through their active site, catalyse reactions at a high rate. Enzyme catalysts differ from inorganic catalysts in many ways, but one major difference needs mention. Inorganic catalysts work efficiently at high temperatures and high pressures, while enzymes get damaged at high temperatures (say above 40°C). However, enzymes isolated from organisms who normally live under extremely high temperatures (e.g., hot vents and sulphur springs), are stable and retain their catalytic power even at high temperatures (upto 80°-90°C). Thermal stability is thus an important quality of such enzymes isolated from thermophilic organisms.

9.12.1 Chemical Reactions

How do we understand these enzymes? Let us first understand a chemical reaction. Chemical compounds undergo two types of changes. A physical change simply refers to a change in shape without breaking of bonds. This is a physical process. Another physical process is a change in state of matter: when ice melts into water, or when water becomes a vapour. These are also physical processes. However, when bonds are broken and new bonds are formed during transformation, this will be called a chemical reaction. For example:

Ba(OH)₂ + H₂SO4 →  BaSO4 + 2H₂O

is an inorganic chemical reaction. Similarly, hydrolysis of starch into glucose is an organic chemical reaction. Rate of a physical or chemical process refers to the amount of product formed per unit time. It can be expressed as:

rate =

Rate can also be called velocity if the direction is specified. Rates of physical and chemical processes are influenced by temperature among other factors. A general rule of thumb is that rate doubles or decreases by half for every 10°C change in either direction. Catalysed reactions proceed at rates vastly higher than that of uncatalysed ones. When enzyme catalysed reactions are observed, the rate would be vastly higher than the same but uncatalysed reaction. For example

CO₂ + H₂O 

carbon dioxide water

H₂CO₃

 carbonic acid

In the absence of any enzyme this reaction is very slow, with about 200 molecules of H₂CO₃ being formed in an hour. However, by using the enzyme present within the cytoplasm called carbonic anhydrase, the reaction speeds dramatically with about 600,000 molecules being formed every second. The enzyme has accelerated the reaction rate by about 10 million times. The power of enzymes is incredible indeed!

There are thousands of types of enzymes each catalysing a unique chemical or metabolic reaction. A multistep chemical reaction, when each of the steps is catalysed by the same enzyme complex or different enzymes, is called a metabolic pathway. For example,

Glucose  → 2 Pyruvic acid

C₆H₁₂O₆ + O₂ → 2C₃H₄ O₃ + 2H₂O

is actually a metabolic pathway in which glucose becomes pyruvic acid through ten different enzyme catalysed metabolic reactions. When you study respiration in Chapter 14 you will study these reactions. At this stage you should know that this very metabolic pathway with one or two additional reactions gives rise to a variety of metabolic end products. In our skeletal muscle, under anaerobic conditions, lactic acid is formed. Under normal aerobic conditions, pyruvic acid is formed. In yeast, during fermentation, the same pathway leads to the production of ethanol (alcohol). Hence, in different conditions different products are possible.

9.12.2 How do Enzymes bring about such High Rates of Chemical Conversions?

To understand this we should study enzymes a little more. We have already understood the idea of an ‘active site’. The chemical or metabolic conversion refers to a reaction. The chemical which is converted into a product is called a ‘substrate’. Hence enzymes, i.e. proteins with three dimensional structures including an ‘active site’, convert a substrate (S) into a product (P). Symbolically, this can be depicted as: 

S  → P

It is now understood that the substrate ‘S’ has to bind the enzyme at its ‘active site’ within a given cleft or pocket. The substrate has to diffuse towards the ‘active site’. There is thus, an obligatory formation of an ‘ES’ complex. E stands for enzyme. This complex formation is a transient phenomenon. During the state where substrate is bound to the enzyme active site, a new structure of the substrate called transition state structure is formed. Very soon, after the expected bond breaking/making is completed, the product is released from the active site. In other words, the structure of substrate gets transformed into the structure of product(s). The pathway of this transformation must go through the so-called transition state structure. There could be many more ‘altered structural states’ between the stable substrate and the product. Implicit in this statement is the fact that all other intermediate structural states are unstable. Stability is something related to energy status of the molecule or the structure. Hence, when we look at this pictorially through a graph it looks like something as in Figure 9.6.

Figure 9.6 Concept of activation energy

Q. Which of the following describes the graph correctly?

(1) Endothermic reaction with energy A in the presence of enzyme and B in the absence of enzyme

(2) Exothermic reaction with energy A in the presence of enzyme and B in the absence of enzyme

(3) Endothermic reaction with energy A in the absence of enzyme and B in the presence of enzyme

(4) Exothermic reaction with energy A in the absence of enzyme and B in the presence of enzyme

The y-axis represents the potential energy content. The x-axis represents the progression of the structural transformation or states through the ‘transition state’. You would notice two things. The energy level difference between S and P. If ‘P’ is at a lower level than ‘S’, the reaction is an exothermic reaction. One need not supply energy (by heating) in order to form the product. However, whether it is an exothermic or spontaneous reaction or an endothermic or energy requiring reaction, the ‘S’ has to go through a much higher energy state or transition state. The difference in average energy content of ‘S’ from that of this transition state is called ‘activation energy’

Q. In a reaction catalyzed by an enzyme, which of the following statements is correct ?

1.	Enzymes decrease the activation energy for formation of transition state.
2.	Enzymes make transition form substrate to product more difficult.
3.	Enzymes increase the activation energy for formation of transition state.
4.	Enzyme-substrate complex formed during a reaction lasts for a very long time.

Enzymes eventually bring down this energy barrier making the transition of ‘S’ to ‘P’ more easy.

Role of enzyme in reactions is to:

(AIPMT - 2000)
1. Decrease activation energy
2. Increase activation energy
3. Act as an inorganic catalyst
4. None of the above

9.12.3 Nature of Enzyme Action

Each enzyme (E) has a substrate (S) binding site in its molecule so that a highly reactive enzyme-substrate complex (ES) is produced. This complex is short-lived and dissociates into its product(s) P and the unchanged enzyme with an intermediate formation of the enzyme-product complex (EP).

The formation of the ES complex is essential for catalysis.

E + S <—> ES → EP → E + P

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.

9.12.4 Factors Affecting Enzyme Activity

The activity of an enzyme can be affected by a change in the conditions which can alter the tertiary structure of the protein. These include temperature, pH, change in substrate concentration or binding of specific chemicals that regulate its activity.

Figure 9.7 Effect of change in : (a) pH (b) Temperature and (c) Concentration of substrate on enzyme activity

Q.The curve given below shows enzymatic activity with relation to three conditions (pH, temperature and substrate concentration) what do the two axis (X and Y) represent? 

	X-axis	Y-axis
(a)	Temperature	Enzyme activity
(b)	Substrate concentration	Enzyme activity
(c)	Enzyme activity	Temperature
(d)	Enzyme activity	pH

1.  (a)
2.  (b)
3.  (c)
4.  (d)

Temperature and pH

Enzymes generally function in a narrow range of temperature and pH (Figure 9.7). Each enzyme shows its highest activity at a particular temperature and pH called the optimum temperature and optimum pH. Activity declines both below and above the optimum value. Low temperature preserves the enzyme in a temporarily inactive state whereas high temperature destroys enzymatic activity because proteins are denatured by heat.

Given below are two statements: (NEET - 2023)

Statement I:	Low temperature preserves the enzyme in a temporarily inactive state whereas high temperature destroys enzymatic activity because proteins are denatured by heat.
Statement II:	When the inhibitor closely resembles the substrate in its molecular structure and inhibits the activity of the enzyme, it is known as competitive inhibitor

In the light of the above statements, choose the correct answer from the options given below:
1. Statement I is false but Statement II is true.
2. Both Statement I and Statement II are true.
3. Both Statement I and Statement II are false.
4. Statement I is true but Statement II is false.

Concentration of Substrate

With the increase in substrate concentration, the velocity of the enzymatic reaction rises at first. The reaction ultimately reaches a maximum velocity (Vmax) which is not exceeded by any further rise in concentration of the substrate. This is because the enzyme molecules are fewer than the substrate molecules and after saturation of these molecules, there are no free enzyme molecules to bind with the additional substrate molecules (Figure 9.7).

The activity of an enzyme is also sensitive to the presence of specific chemicals that bind to the enzyme. When the binding of the chemical shuts off enzyme activity, the process is called inhibition and the chemical is called an inhibitor.

When the inhibitor closely resembles the substrate in its molecular structure and inhibits the activity of the enzyme, it is known as competitive inhibitor.

Q. Given below are two statements: (NEET - 2023)

Statement I:	Low temperature preserves the enzyme in a temporarily inactive state whereas high temperature destroys enzymatic activity because proteins are denatured by heat.
Statement II:	When the inhibitor closely resembles the substrate in its molecular structure and inhibits the activity of the enzyme, it is known as competitive inhibitor

In the light of the above statements, choose the correct answer from the options given below:
1. Statement I is false but Statement II is true.
2. Both Statement I and Statement II are true.
3. Both Statement I and Statement II are false.
4. Statement I is true but Statement II is false.

Due to its close structural similarity with the substrate, the inhibitor competes with the substrate for the substrate-binding site of the enzyme. Consequently, the substrate cannot bind and as a result, the enzyme action declines, e.g., inhibition of succinic dehydrogenase by malonate which closely resembles the substrate succinate in structure.4 Such competitive inhibitors are often used in the control of bacterial pathogens.

9.12.5 Classification and Nomenclature of Enzymes

Thousands of enzymes have been discovered, isolated and studied. Most of these enzymes have been classified into different groups based on the type of reactions they catalyse. 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’ e.g.,

S reduced + S’ oxidised  →  S oxidised + S’ reduced.

Transferases: Enzymes catalysing a transfer of a group, G (other than hydrogen) between a pair of substrate S and S’ e.g.,

S - G + S’  →  S + S’ - G

Hydrolases: Enzymes catalysing hydrolysis of ester, ether, peptide, glycosidic, C-C, C-halide or P-N bonds.

Q. Hydrolytic enzymes that act on low pH are called:

1. Protease

2. α-Amylase

3. Hydrolases

4. Peroxidase

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.

9.12.6 Co-factors

Enzymes are composed of one or several polypeptide chains. However, there are a number of cases in which non-protein constituents called co-factors are bound to the the enzyme to make the enzyme catalytically active. In these instances, the protein portion of the enzymes is called the apoenzyme. 

Q. Consider the following statements:(NEET - 2019)

A:	A coenzyme or metal ion that is tightly bound to enzyme protein is called prosthetic group
B:	A complete catalytic enzyme with its bound prosthetic group is called apoenzyme

Select the correct option

1. (A) is false but (B) is true

2. Both (A) and (B) are true

3. (A) is true but (B) is false

4. Both (A) and (B) are false

Three kinds of cofactors may be identified: prosthetic groups, co-enzymes and metal ions.

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.

Co-enzymes are also organic compounds but their association with the apoenzyme is only transient, usually occurring during the course of catalysis. 

Q. An organic substance bound to an enzyme and essential for its activity is called:

1. coenzyme

2. holoenzyme

3. apoenzyme

4. isoemzyme

Furthermore, co-enzymes serve as co-factors in a number of different enzyme catalyzed reactions. The essential chemical components of many coenzymes are vitamins, e.g., coenzyme nicotinamide adenine dinucleotide (NAD) and NADP contain the vitamin niacin.

Q. The essential chemical components of many coenzymes are:

1. Nucleic acid
2. Carbohydrates
3. Vitamins
4. Proteins

A number of enzymes require metal ions for their activity which form coordination bonds with side chains at the active site and at the same time form one or more cordination bonds with the substrate, e.g., zinc is a cofactor for the proteolytic enzyme carboxypeptidase.

Q. For its activity, carboxypeptidase requires:

1. Iron
2. Niacin
3. Copper
4. Zinc

Catalytic activity is lost when the co-factor is removed from the enzyme which testifies that they play a crucial role in the catalytic activity of the enzyme.

Summary

Although there is a bewildering diversity of living organisms, their chemical composition and metabolic reactions appear to be remarkably similar. The elemental composition of living tissues and non-living matter appear also to be similar when analysed qualitatively. However, a closer examination reveals that the relative abundance of carbon, hydrogen and oxygen is higher in living systems when compared to inanimate matter. The most abundant chemical in living organisms is water. There are thousands of small molecular weight (<1000 Da) biomolecules. Amino acids, monosaccharide and disaccharide sugars, fatty acids, glycerol, nucleotides, nucleosides and nitrogen bases are some of the organic compounds seen in living organisms. There are 20 types of amino acids and 5 types of nucleotides. Fats and oils are glycerides in which fatty acids are esterified to glycerol. Phospholipids contain, in addition, a phosphorylated nitrogenous compound.

Only three types of macromolecules, i.e., proteins, nucleic acids and polysaccharides are found in living systems. Lipids, because of their association with membranes separate in the macromolecular fraction. Biomacromolecules are polymers. They are made of building blocks which are different. Proteins are heteropolymers made of amino acids. Nucleic acids (RNA and DNA) are composed of nucleotides. Biomacromolecules have a hierarchy of structures – primary, secondary, tertiary and quaternary. Nucleic acids serve as genetic material. Polysaccharides are components of cell wall in plants, fungi and also of the exoskeleton of arthropods. They also are storage forms of energy (e.g., starch and glycogen). Proteins serve a variety of cellular functions. Many of them are enzymes, some are antibodies, some are receptors, some are hormones and some others are structural proteins. Collagen is the most abundant protein in animal world and Ribulose bisphosphate Carboxylase-Oxygenase (RuBisCO) is the most abundant protein in the whole of the biosphere.

(a)	Inhibitor of catalytic activity of succinate dehydrogenase	(i)	Ricin
(b)	Possess peptide bonds	(ii)	Malonate
(c)	Cell wall material in fungi	(iii)	Chitin
(d)	Secondary metabolite	(iv)	Collagen

Choose the correct option from the following:

	(a)	(b)	(c)	(d)
1.	(iii)	(i)	(iv)	(ii)
2.	(iii)	(iv)	(i)	(ii)
3.	(ii)	(iii)	(i)	(iv)
4.	(ii)	(iv)	(iii)	(i)

Enzymes are proteins which catalyse biochemical reactions in the cells. Ribozymes are nucleic acids with catalytic power. 

Q. A non-proteinaceous enzyme is (NEET - 2016)

(1) lysozyme           

(2) ribozyme

(3) ligase               

(4) deoxyribonuclease

Proteinaceous enzymes exhibit substrate specificity, require optimum temperature and pH for maximal activity. They are denatured at high temperatures. Enzymes lower activation energy of reactions and enhance greatly the rate of the reactions. 

Q. In a reaction catalyzed by an enzyme, which of the following statements is correct ?(NEET - 2022)

1.	Enzymes decrease the activation energy for formation of transition state.
2.	Enzymes make transition form substrate to product more difficult.
3.	Enzymes increase the activation energy for formation of transition state.
4.	Enzyme-substrate complex formed during a reaction lasts for a very long time.

Nucleic acids carry hereditary information and are passed on from parental generation to progeny.

Exercises

1. What are macromolecules? Give examples.

2. Illustrate a glycosidic, peptide and a phospho-diester bond.

3. What is meant by tertiary structure of proteins?

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.

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?

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

7. Explain the composition of triglyceride.

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

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

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

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

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

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!

15. Describe the important properties of enzymes.

14.1: What are monosaccharides?

14.2: What are reducing sugars?

14.3: Write two main functions of carbohydrates in plants.

14.4: Classify the following into monosaccharides and disaccharides. Ribose, 2-deoxyribose, maltose, galactose, fructose and lactose  

14.5: What do you understand by the term glycosidic linkage?

 14.6: What is glycogen? How is it different from starch?

14.7: What are the hydrolysis products of (i) sucrose and (ii) lactose?

14.8: What is the basic structural difference between starch and cellulose?

14.9:What happens when D-glucose is treated with the following reagents?
(i) HI (ii) Bromine water (iii) HNO3

14.10: Enumerate the reactions of D-glucose which cannot be explained by its open chain structure.

14.11: What are essential and non-essential amino acids? Give two examples of each type.

14.12: Define the following as related to proteins

(i) Peptide linkage (ii) Primary structure (iii) Denaturation.

14.13: What are the common types of secondary structure of proteins?

14.14: What type of bonding helps in stabilising the -helix structure of proteins?

14.15: Differentiate between globular and fibrous protein?

14.16: How do you explain the amphoteric behaviour of amino acids?

14.17: What are enzymes?

14.18: What is the effect of denaturation on the structure of proteins?

14.19: How are vitamins classified? Name the vitamin responsible for the coagulation of blood.

14.20: Why are vitamin A and vitamin C essential to us? Give their important sources.

14.21: What are nucleic acids? Mention their two important functions.

14.22: What is the difference between a nucleoside and a nucleotide?

14.23: The two strands in DNA are not identical but are complementary. Explain.

14.24: Write the important structural and functional differences between DNA and RNA. 

EXEMPLAR QUESTION

Multiple Choice Questions (MCQs)

1. It is said that elemental composition of living organisms and that of inanimate objects (like earth’s crust) are similar in the sense that all the major elements are present in both. Then what would be the difference between these two groups? Choose a correct answer from among the following.

(a) Living organisms have more gold in them than inanimate objects

(b) Living organisms have more water in their body than inanimate objects

(c) Living organisms have more carbon, oxygen and hydrogen per unit mass than inanimate objects

(d) Living organisms have more calcium in them than inanimate objects

2. Many elements are found in living organisms either free or in the form of compounds. One of the following is not found in living organisms.

(a) Silicon

(b) Magnesium

(c) Iron

(d) Sodium

3. Amino acids have both an amino group and a carboxyl group in their structure. Which amongst the following is an amino acid?

(a) Formic acid

(b) Glycerol

(c) Glyolic acid

(d) Glycine

4. An amino acid under certain conditions have both positive and negative charges simultaneously in the same molecule. Such a form of amino acid is called

(a) acidic form

(b) basic form

(c) aromatic form

(d) zwitterionic form

5. Which of the following sugars howe the same number of carbon as present in glucose?

(a) Fructose

(b) Erythrose

(c) Ribulose

(d) Ribose

7. When we homogenise any tissue in an acid, the acid soluble pool represents

(a) cytoplasm 

(b) cell membrane

(c) nucleus

(d) mitochondria

8. The most abundant chemical in living organisms could be

(a) protein 

(b) water

(c) sugar

(d) nucleic acid

EXEMPLAR QUESTION

9. A homopolymer has only one type of building block called monomer repeated ‘n’ number of times. A heteropolymer has more than one type of monomer. Proteins are heteropolymers usually made of

(a) 20 types of monomers

(b) 40 types of monomers

(c) 30 types of monomers

(d) only one type of monomer

10. Proteins perform many physiological functions. For example, some functions as enzymes. One of the following represents an additional function that some proteins discharge

(a) Antibiotics

(b) Pigment conferring colour to skin

(c) Pigments making coloursof flowers

(d) Hormones

11. Glycogen is a homopolymer made of

(a) glucose units

(b) galactose units

(c) ribose units

(d) amino acids

12. The number of ‘ends’ in a glycogen molecule would be

(a) Equal to the number of branches plus one

(b) Equal to the number of branch points

(c) One

(d) Two, one on the left side and another on the right side

13. The primary structure of a protein molecule has

(a) two ends

(b) one end

(c) three ends

(d) no ends

14. Which of the following reactions is not enzyme mediated in biological system?

(a) Dissolving CO₂ in water

(b) Unwinding the two strands of DNA

(c) Hydrolysis of sucrose

(d) Formation of peptide bond

Very Short Answer Type Questions

15. 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) Sulphonamide

(c) Vitamin-C

(d) Growth hormone

16. 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

EXEMPLAR QUESTION

17. Write the name of any one amino acid, sugar, nucleotide and fatty acid.

18. Reaction given below is catalysed by oxidoreductase between two substrates A and A’, complete the reaction.

A reduced + A’ oxidised ——>

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

20. Glycine and alanine are different with respect to one substituent on the α�-carbon. What are the other common substituent groups?

21. 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                           ................

Match List-I with List -II: 

List- I	List-II
(A) Adenine	(I) Pigment
(B) Anthocyanin	(II) Polysaccharide
(C) Chitin	(III) Alkaloid
(D) Codeine	(IV) Purine

Choose the correct answer from the options given below :

	(A)	(B)	(C)	(D)
1.	(IV)	(I)	(II)	(III)
2.	(IV)	(III)	(II)	(I)
3.	(III)	(I)	(IV)	(II)
4.	(I)	(IV)	(III)	(II)

Short Answer Type Questions

22. Enzymes are proteins. Proteins are long chains of amino acids linked to each other by peptide bonds. 

Amino acids 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 ionisation 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.

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

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

EXEMPLAR QUESTION

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

26. Comment on the statement ‘living state is a non-equilibrium steady state to be able to perform work’.

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

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

29. Nucleic acids exhibit secondary structure. Describe through Watson-Crick model.

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

31. Describe various forms of lipid with a few examples.