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The organic complex adenosine triphosphate powers a variety of biological functions in living cells, including muscular contraction, nerve impulse transmission, and chemical synthesis.

The three phosphate groups in ATP behave similarly to the phosphate groups in phosphoric acid.The following are the $\text{pKa}$ values for phosphoric acid obtained from the table 2-4 :

$$\begin{gathered} {\text{pKa}}_{\text{1}}\text{= 2.15,} \\ {\text{pKa}}_{\text{2}}\text{= 6.82,} \\ {\text{pKa}}_{\text{3}}\text{= 12.38} \end{gathered}$$

Since each phosphoryl group has a pK value of 2.15, ionisation of the first proton on each one is complete at physiological pH (7.4), while ionisation of the second proton, if present, is more than halfway complete (since pH 7.4 is greater than the pK value of 6.82). Consequently, an ATP molecule has a net charge of between 3.5 and 4.

Hence, the net charge of ATP is between -3.5 and -4 .