$\mathrm{H}_{3} \mathrm{O}^{+}+\mathrm{A}^{-} \rightleftharpoons \mathrm{H}_{2} \mathrm{O}+\mathrm{HA}$
$\mathrm{OH^-}+\mathrm{HA} \rightarrow \mathrm{H}_{2} \mathrm{O}+\mathrm{A}^{-}$
If acid is added, it reacts with $A^-$ to form undissociated $HA$. Similarly, if base/alkali is added, $OH^-$ combines with $HA$ to give $H_2O$ and Aand thus, maintains the acidity/alkalinity of buffer solution.
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$\begin{array}{*{20}{c}}
{C{H_3} - CH - CH - C{H_2} - COOH} \\
{|\,\,\,\,\,\,\,\,\,\,\,\,\,|\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,} \\
{\,\,OH\,\,\,\,\,\,\,\,Cl\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,}
\end{array}$