What does the part labeled '$X$' represent in the structural formula shown below?

Which structure represents the three-dimensional form of a protein?

Which bond connects two amino acids?

Which of the following groups consists of amino acids with non-polar $R$ groups?

Match the following columns correctly.

Column $I$	Column $II$
$(a)$ Actin	$(i)$ Transport of respiratory gases
$(b)$ Immunoglobulin	$(ii)$ Immunity
$(c)$ Melanin	$(iii)$ Pigmentation of the body
$(d)$ Hemoglobin	$(iv)$ Muscle movement

Which of the following statement$(s)$ is/are true?
$(i)$ All amino acids contain one chiral centre.
$(ii)$ Some amino acids contain one, while some contain more chiral centres or even no chiral centre.
$(iii)$ All amino acids in proteins have $L$-configuration.
$(iv)$ All amino acids found in proteins have $1^{\circ}$ amino group.