Describe the primary, secondary, and tertiary structures of proteins. $OR$ Describe the structure of proteins.

The structure of molecules means different things in different contexts.
$\rightarrow$ In inorganic chemistry, structure invariably refers to molecular formulae (e.g., $NaCl$, $MgCl_2$, etc.).
$\rightarrow$ Organic chemistry represents molecular structure in a two-dimensional form (e.g., benzene, naphthalene, etc.).
$\rightarrow$ Physicists visualize molecules in three dimensions, while biologists describe protein structure at four levels.
(a) Primary Structure:  
The sequence of amino acids in a protein (i.e., positional information such as which amino acid is first, second, and so on) is called the primary structure.
(b) Secondary Structure:  
A protein is imagined as a line where the left end represents the first amino acid and the right end represents the last amino acid.
The first amino acid is called the $N$-terminal amino acid, and the last amino acid is called the $C$-terminal amino acid.
The protein chain is folded into a helical structure (similar to a revolving staircase). In proteins, only right-handed helices are observed. Other regions of the chain are folded into different forms, which constitute the secondary structure.
(c) Tertiary Structure:  
The long protein chain is further folded upon itself like a hollow woolen ball, giving rise to the tertiary structure. It provides a three-dimensional view of the protein. The tertiary structure is essential for many biological activities of proteins.
(d) Quaternary Structure:  
Some proteins consist of more than one polypeptide chain or subunit. These folded subunits are arranged relative to each other (e.g., linear string of spheres, cubes, etc.). This arrangement is called quaternary structure.
Haemoglobin: Human haemoglobin consists of four subunits. Two are identical; hence, there are two $\alpha$-type and two $\beta$-type subunits.