Chapter 9 Biomolecules

NCERT Notes For Biology Class 11 Download In PDF

POINTS TO REMEMBER

Biomolecules : All the carbon compounds that we get from living tissues.

Micromolecules : Molecules which have molecular weights less than one thou- sand dalton.

Amino acids : Organic compounds containing an amino group and one car- boxyl group (acid group) and both these groups are attached to the same carbon atom called α α α α α carbon.

Lipids :
• Water insoluble, containing C, H, O.
• Fats on hydrolysis yield fatty acids.
• Fatty acid has a carboxyl group attached to an R group (contains 1 to 19 carbons).
• Fatty

Acids : Saturated With single bonds in carbon chain. e.g., Palmitic acid, butyric acid.

Unsaturated : With one or more double bonds. e.g., oleic acid, linoleic acid.

Glycerol : A simple lipid, is trihydroxy propane.

• Some lipids have fatty acids esterified with glycerol.
• They can be monoglycerides, diglycerides and triglycerides.

• Phospholipids are compound lipids with phosphorus and a phosphory- lated organic compound e.g., Lecithin .

Nucleoside : Nitrogenous base + Sugar e.g., Adenosine, guanosine.

Nucleotide : Nitrogenous base + Sugar + Phosphate group. e.g., Adenylic acid, thymidylic acid.

Nucleic acid : Polymer of nucleotides - DNA and RNA.

Biomacromolecules : Biomolecules with molecular weights in the range of ten thousand daltons and above; found in acid insoluble fraction. Lipids are not strictly macromolecules as their molecular weights do not exceed 800 Da but form a part of the acid insoluble pool.

Proteins :
• Are polymers of aminoacids linked by peptide bond.
• Is a heteropolymer.
• For functions of proteins

(Refer Table 9.5, Page no. 147, NCERT, Text Book of Biology for Class XI. Structure of Proteins)

(a) Primary structure : Is found in the form of linear sequence of amino acids. First amino acid is called N-terminal amino acid and last amino acid is called C-terminal amino acid.

(b) Secondary structure : Polypeptide chain undergoes folding or coiling which is stabilized by hydrogen bonding. Right handed helices are observed. e.g., fibrous protein in hair, nails.

(c) Tertiary structure : Long protein chain is folded upon itself like a hollow wollen ball. Gives a 3-dimensional view of protein, e.g., myosin.

(d) Quaternary structure : Two or more polypeptides with their foldings and coilings are arranged with respect to each other. e.g., Human haemoglobin molecule has 4 peptide chains - 2a and 2b subunits.

Peptide bond : Formed between the carboxyl (-COOH) group of one amino acid and the amino (-NH ) group of the next amino acid with the elimination of water moeity.

2 Polysaccharides : Are long chain of sugars.

(a) Starch : Store house of energy in plant tissues. Forms helical second- ary structures.
(b) Cellulose : Polymer of glucose.
(c) Glycogen : Is a branched homopolymer, found as storage polysaccharide in animals.
(d) Insulin : Is a polymer of fructose.
(e) Chitin : Chemically modified sugar (amino-sugars) N-acetyl galac- tosamine. Form exoskeleton of arthropods.

Anabolic pathways : Lead to formation of more complex structure from a sim- pler structure with the consumption of energy. e.g., Protein from amino acids.

Catabolic pathway : Lead to formation of simpler structure from a complex structure. e.g., Glucose → Lactic Acid.

Enzymes : Are biocatalysts.
• Almost all enzymes are proteins.
• Ribozomes - Nucleic acids that behave like enzymes.
• Has primary, secondary and tertiary structure.
• Active site of an enzyme is a crevice or pocket into which substrate fits.
• Enzymes get damaged at high temperatures.
• Enzymes isolated from thermophilic organisms (live under high temperatures) are thermostable.
• Enzymes accelerate the reactions many folds.
• Enzymes lower the activation energy of reactions.

(Fig. 9.6, Page no. 156, NCERT Text Book of Biology for Class XI).

• ES ES EP EP +→→+
where E = Enzyme, S = Substrate, P = Product.

Factors affecting enzyme activity :
(a) Temperature : Show highest activity at optimum temperature. Activity declines above and below the optimum value.
(b) pH : Enzymes function in a narrow range of pH. Highest activity at optimum pH.

(Fig. 9.7, Page no. 157, NCERT, Text Book of Biology for Class XI)

(c) Concentration of substrate : The velocity of enzymatic reaction rises with increase in substrate concentration till it reaches maximum ve- ). Further increase of substrate does not increase the rate of reaction as no free enzyme molecules are available to find with additional substrate. locity

V max Enzyme inhibition : When the binding of a chemical shuts off enzyme activity, the process is called inhibition and chemical is called inhibitor.

Competitive inhibition : Inhibitor closely resembles the substrate in its molecular structure and inhibits the enzyme activity. E.g., inhibition of succinic dehydrogenase by malonate.

Classification of enzymes :

Oxidoreductase/dehydrogenases : Catalyse oxidoreduction between 2 sub- strates.

Transferases : Catalyse transfer of a group between a pair of substrates.

Hydrolases : Catalyse hydrolysis of ester, ether, peptide, glycosidic, C-C, P-N bonds.

Lyases : Catalyse removal of groups from substrates by mechanisms other than hydrolysis.

Isomerases : Catalyse inter-conversion of optical, geometric or positional isomers.

Ligases : Catalyse linking together of 2 compounds. Cofactors : Non-protein constituents found to the enzyme to make it cata- lytically active. Protein portion of enzyme is called apoenzyme.

Cofactors :
• Prosthetic groups : Are organic compounds tightly bound to apoenzyme. E.g., haem in peroxydase and catalase.
• Co-enzymes : Organic compounds which has transient association with enzyme. E.g., NAD, NADP.
• Metal ions : Required for enzyme activity. Form coordination bond with side chains at active site and with substrate.

E.g., zinc is a co-factor for enzyme carboxypeptidase.

18. Nucleic acids : Deoxyribonucleic acid (DNA) and ribonucleic acid (RNA). DNA structure (Watson and Crick Model) : DNA is a right handed, double helix of two polynucleotide chains, having a major and minor groove. The two chains are antiparallel, and held together by hydrogen bonds (two between A and T and three between C and G). The backbone is formed by sugar-phosphatesugar chain. The nitrogen bases are projected more or less perpendicular to this backbone and face inside.

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