Physical Biochemistry of Free Energy Transfer for Membrane Transport of Molecules and Ions.Dr Dan Guerra 19 Feb 2021

•Free energy change(chemical potential difference) for transporting 1 mole of a substance from region where its concentration is C1 (e.g., Cout) to region where its concentration is C2 (e.g., Cin):

∆G= RTln(C2/C1)

(favorable with ∆G< 0 if C2< C1)

•Transport of ionsacross membrane (must consider electrical potential in addition to concentration difference):

∆G= RTln(C2/C1) + ZF ∆Y

(Z=charge of ion, F=Faraday’s constant, ∆Y=membrane  electrical potential in volts)

•Coupled transport (active transport):

∆G= RTln(C2/C1) + ∆G´

(∆G´ of coupled process, such as ATP hydrolysis, may be  negative enough to compensate for unfavorable transport  against concentration gradient when RT ln (C2/C1) > 0)

•Diffusional transport: movement of substance from high to low concentration across membrane (down concentration gradient)

–Non-facilitated diffusion across lipid bilayer (slow for most biological substances)

–Facilitated diffusion (accelerated diffusion by making membrane more permeable to specific transported substance, e.g., channels and carriers)

•Active transport: Actively driven (generally directly or indirectly coupled to ATP hydrolysis) transport against concentration gradient from low to high concentration across membrane (e.g., pumps)