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

(N/A) $ \Rightarrow $ Structure of molecules means different things in different contexts.
$ \Rightarrow $ In inorganic chemistry, the structure refers to the molecular formulae (e.g., $ NaCl, MgCl_2 $, etc.).
$ \Rightarrow $ Organic chemistry uses a two-dimensional view of molecules to represent their structure (e.g., benzene, naphthalene, etc.).
$ \Rightarrow $ Physicists visualize three-dimensional views of molecular structures, while biologists describe protein structure at four levels.
$ \Rightarrow $ $(a)$ Primary Structure: 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.
$ \Rightarrow $ $(b)$ Secondary Structure: $ A $ protein is imagined as a line, where the left end is the first amino acid ($ N $-terminal) and the right end is the last amino acid ($ C $-terminal). The protein thread is folded in the form of a helix (similar to a revolving staircase). In proteins, only right-handed helices are observed. Other regions of the protein thread are folded into other forms, which is called the secondary structure.
$ \Rightarrow $ $(c)$ Tertiary Structure: The long protein chain is folded upon itself like a hollow woolen ball, giving rise to the tertiary structure. It provides a $ 3 $-dimensional view of a protein and is essential for many biological activities.
$ \Rightarrow $ $(d)$ Quaternary Structure: Some proteins are an assembly of more than one polypeptide or subunit. These individual folded polypeptides are arranged with respect to each other (e.g., human haemoglobin consists of $ 4 $ subunits: two $ \alpha $ type and two $ \beta $ type).