Revision: Class 11 >> Biomolecules NEET (UG) Biomolecules

Chemically proteins are polyamides which are high molecular weight polymers of the monomer units, i.e., α-amino acids. OR It can also be defined as proteins are the biopolymers of a large number of α-amino acids and they are naturally occurring polymeric nitrogenous organic compounds containing 16% nitrogen and peptide linkages (-CO-NH-)

Chemically, proteins are polyamides, which are high molecular weight polymers of the monomer units called \[\alpha\]-amino acids.

Bifunctional organic compounds containing a carboxylic and an amino group either at the same carbon atom or at nearby carbon atoms are called amino acids.

Enzymes are biological catalysts that speed up chemical reactions in living cells without being consumed in the process.

α-Amino acids are carboxylic acids having an amino (–NH₂) group bonded to the α-carbon, that is, the carbon next to the carboxyl (–COOH) group.

The bond that connects α-amino acids to each other is called a peptide bond.

The unit formed by joining the anomeric carbon of the furanose (sugar) with a nitrogen of a base is called nucleoside.

RNA is a single-stranded nucleic acid that helps in protein synthesis and information transfer.

A nucleotide is the basic structural unit of nucleic acids, composed of a nitrogenous base, a pentose sugar, and a phosphate group.

A nucleoside consists of a nitrogenous base linked to a pentose sugar without a phosphate group.

A nitrogenous base is an organic molecule (purine or pyrimidine) that carries genetic information in nucleic acids.

Nucleic acids are large biological macromolecules that store and transmit genetic information in living organisms.

DNA is a double-stranded nucleic acid that stores and transmits hereditary information and can replicate itself.

Biomolecules are essential substances produced by our body which are necessary for life.

Metabolism is the sum of the chemical reactions that take place within each cell of a living organism and provide energy for vital processes and for synthesizing new organic material.

Nucleic acids are macromolecules composed of many small units or monomers called nucleotides.

Chemically, peptide linkage is an amide formed between the –COOH group and –NH₂ group. The reaction between two molecules of similar or different amino acids proceeds through the combination of the amino group of one molecule with the carboxyl group of the other. This results in the elimination of a water molecule and the formation of a peptide bond –CO–NH–. The product of the reaction is called a dipeptide because it is made up of two amino acids.

For example, when the carboxyl group of glycine combines with the amino group of alanine, we get a dipeptide, glycylalanine.

• Lipids are esters of fatty acids with a hydrogen-to-oxygen ratio greater than 2:1.
• They are classified into simple lipids (fats and waxes), compound lipids (phospholipids, glycolipids, lipoproteins) and sterols (derived lipids).
• Simple lipids are esters of fatty acids with various alcohols, while compound lipids typically contain 1 glycerol, 2 fatty acids and either 1 phosphate group (phospholipid) or 1 simple sugar (glycolipid).
• Glycolipids, also called cerebrosides, are abundant in the myelin sheath of nerve cells.
• In plants, sterols occur as phytosterols; the yam plant (Dioscorea) yields the sterol diosgenin, used to manufacture birth‑control pills.

• Origin of the name: The word "protein" is derived from the Greek word 'proteios', meaning "primary" or "of prime importance", reflecting proteins' essential role in living organisms.
• Chemical nature: Proteins are polyamides — high molecular weight polymers made up of monomer units called α-amino acids (general formula: RCH(NH₂)COOH).
• Nutritional sources: Key dietary sources of protein include milk, pulses, nuts, fish, and meat.
• Fibrous proteins have thread-like structures where polypeptide chains run parallel, held by hydrogen and disulphide bonds; they are insoluble in water — e.g., Keratin (hair, skin, nails) and Myosin (muscles).
• Globular proteins have a spherical shape where polypeptide chains coil around; they are soluble in water — e.g., Insulin, Egg albumin, and Legumelin.

• Starch — A polymer of α-D-glucopyranose. It has two components: amylose (α-1,4-glycosidic linkage) and amylopectin (both α-1,4 and α-1,6-glycosidic linkages).
• Cellulose — A polymer of β-glucopyranose units linked by β-1,4-glycosidic bonds.
• Glycogen — A polymer of glucose units.
• Linkage Comparison —

  Polysaccharide	Monomer	Linkage
  Amylose (Starch)	α-D-glucopyranose	α-1,4
  Amylopectin (Starch)	α-D-glucopyranose	α-1,4 and α-1,6
  Cellulose	β-glucopyranose	β-1,4
  Glycogen	Glucose	—

   
• Key Distinction — Starch and Cellulose are both glucose polymers but differ in linkage type: Starch has α-glycosidic bonds (digestible by humans), while Cellulose has β-glycosidic bonds (not digestible by humans).

• The primary structure of proteins shows the sequence of amino acids in a polypeptide chain.
• Secondary structure is formed by hydrogen bonding and mainly includes α-helix and β-pleated sheet.
• In an α-helix, the chain coils into a right-handed spiral stabilised by hydrogen bonds.
• In a β-pleated sheet, chains are stretched and arranged side by side, held by intermolecular hydrogen bonds.
• Tertiary structure is the overall 3D folding of the chain due to interactions like hydrogen bonds, disulfide bonds, and van der Waals forces, while quaternary structure is the arrangement of multiple polypeptide chains.

• Increasing substrate concentration raises enzyme activity only up to a maximum, after which the rate levels off because all active sites become saturated.
• Increasing enzyme concentration generally increases the reaction rate, as more active sites are available for substrate binding.
• Enzyme activity is highest at an optimum temperature; high temperatures denature enzymes (destroy higher‑level structure), while low temperatures reduce their activity.
• Each enzyme has its own optimum pH range; outside this range, activity falls sharply and the enzyme may not function.
• Co‑enzymes, activators and inhibitors also affect enzyme activity: activators (often inorganic ions) enhance activity, inhibitors decrease it, and many enzymes function as a combination of apoenzyme plus co‑enzyme.