What is ATP, coupling?

Example 8.17 (Why ATP hydrolysis releases so much)

Three reasons: the four negative charges of ATP’s phosphates repel one another and are relieved by cleavage; the products $\mathrm{ADP}$ and $\mathrm{P_i}$ are better stabilised by resonance and by hydration than the anhydride; and the cell keeps ATP a thousand times above its equilibrium with ADP and phosphate. The last is the largest term: with ATP at $5\,\mathrm{mmol}/\mathrm{L}$, ADP $1\,\mathrm{mmol}/\mathrm{L}$, $\mathrm{P_i}$ $10\,\mathrm{mmol}/\mathrm{L}$, $\Delta G = -30.5 + 2.58\ln\frac{10^{-3}\times 10^{-2}}{5\times 10^{-3}} = -30.5 - 16 = -46.5\,\mathrm{kJ}/\mathrm{mol}$.

Example 8.18 (Coupling)

Glucose $+\ \mathrm{P_i} \to$ glucose-6-phosphate $+\ \mathrm{H_2O}$ has $\Delta G^{\circ\prime} = +13.8\,\mathrm{kJ}/\mathrm{mol}$: at equilibrium almost no glucose would be phosphorylated. Hexokinase instead transfers the phosphate straight from ATP: glucose $+$ ATP $\to$ glucose-6-phosphate $+$ ADP, $\Delta G^{\circ\prime} = 13.8 - 30.5 = -16.7\,\mathrm{kJ}/\mathrm{mol}$, equilibrium constant $800$, reaction essentially complete. The phosphate never appears free: the two reactions are one.

Example 8.14 (The cell’s inventory)

Of a bacterium’s dry mass, proteins are $55\,\%$, RNA $20\,\%$, DNA $3\,\%$, lipids $9\,\%$, polysaccharides $5\,\%$, small molecules and ions the rest. By number of molecules the picture inverts: a thousand kinds of small metabolite and ion, at concentrations from nanomolar to a tenth of a mole per litre, make up most of the molecules that are not water. Potassium is at $150\,\mathrm{mmol}/\mathrm{L}$, glutamate $100\,\mathrm{mmol}/\mathrm{L}$, ATP $5\,\mathrm{mmol}/\mathrm{L}$, a transcription factor at a few molecules per cell.