Class 11 Biology



Protein is a polymer of amino acids. Based on similar or different monomers repeating in a protein, it is classified as homopolymer and heteropolymer. When same monomer is repeated in the protein, it is called homopolymer. When different monomers are present in the protein, it is called heteropolymer.

Essential Amino Acids

Some amino acids are essential for our health. But our body does not make them and they need to be supplemented through diet. Such amino acids are called essential amino acids. Collagen is the most abundant protein in the animal world. Ribulose biphosphate Carboxylase-Oxygenae (RUBISCO) is the most abundant protein in the whole biosphere.

Some Proteins and their Functions
CollagenIntercellular ground substance
AntibodyFights pathogens
ReceptorSensory reception
GLUT – 4Enables glucose transport into cells


The long chains of sugars are called polysachharides. If a polysaccharide is made up of similar monosaccharides, it is called homopolymer, e.g. cellulose. If a polysaccharide is made up of different monosachharides, it is called heteropolymer.

The right end of a polysaccharide chain is called the reducing end and the left end is called the non-reducing end.

Starch forms helical secondary structures. Starch can hold I2 (iodine) molecules in helical portion. Cellulose does not contain complex helices and hence cannot hold I2 .

Nucelic Acids

A nucleic acid is composed of nucleotide. There are three chemically distinct components in a nucleotide. One of them is a heterocyclic compound, the second is a monosaccharide and the third is phosphoric acid or phosphate.

The heterocyclic compounds, present in nucleic acids are the nitrogenous bases, viz. adenine, guanine, uracil, cytosil and thymine. Adenine and Guanine are substituted purines, while uracil, cytosil and thymine are substituted pyrimidines.

Based on the presence of purine or pyrimidine, the heterocyclic ring is called purine and pyrimidine. Polynucleotides contain either ribose sugar or 2’ deoxyribose sugar. If ribose sugar is present then the nucleic acid is called ribonucleic acid (RNA). If deoxyribose sugar is present then the nucleic acid is called deoxyribose nucleic acid (DNA).

Structure of Protein

Primary Structure

The sequence of amino acids is called the primary structure of a protein. The left end is represented by the first amino acid, while the right end is represented by the last amino acid. The first amino acid is also called N-terminal amino acid. The last amino acid is called C-terminal amino acid.

Secondary Structure

The protein is not a linear chain of amino acids rather the chain would bend at some places and even form helices. Regularly repeating local structures gives secondary structure to protein.

Tertiary Structure

The overall shape of a protein molecule; and the spatial relationship of the secondary structures to one another; is called tertiary structure of protein. In other words, the various folds which give three dimensional appearances to protein form its tertiary structure.

Quaternary Structure

The manner in which the individual folded polypeptides are arranged with respect to each other is called quaternary structure of protein.