coupled reactions

Meaning: Cells drive an unfavorable reaction by linking it to a favorable one, most often ATP hydrolysis. The two steps share a common intermediate, typically a phosphorylated one, so an enzyme transfers a phosphate group from ATP to the substrate. Free energy is a state function, so the ΔG values simply add. Example: glucose + Pi → glucose-6-phosphate has a ΔG°′ of about +14 kJ/mol, and ATP hydrolysis has about −30 kJ/mol, giving a net of about −16 kJ/mol.

Equation: ΔGnet = ΔG₁ + ΔG₂, where ΔG₁ and ΔG₂ are the free energy changes of the individual reactions and ΔGnet is the overall change, all in J/mol. Equilibrium constants of coupled steps multiply: Knet = K₁ × K₂ (unitless).

On the MCAT: A passage gives ΔG values for two reactions and asks whether the pair is spontaneous, or asks you to find the net ΔG. You may also be asked to name the shared intermediate, or to explain why an enzyme that binds both reactions is needed. Expect questions on why ATP hydrolysis can power biosynthesis and active transport.

Common confusion: Coupling does not make the unfavorable reaction's own ΔG negative. That step stays positive, and only the sum is negative. Also, ATP hydrolysis happening somewhere in the cell does not drive a reaction. The two must be physically linked, through a shared intermediate or one enzyme.