Question

How can you predict whether it will be thermodynamically favorable for an uncharged substance to move from one side of a membrane to the other?

Short answer

By following the concentration gradient. Therefore, the opposite is true if the concentration of [A] inside is higher than outside, and the net flow of A inside can only occur if it is related to ATP hydrolysis, which is an exergonic activity.

Step-by-step solution

Introduction

When particle concentrations are higher in one location than in another, a concentration gradient arises. Particles diffuse down a concentration gradient, from higher to lower concentration locations, until they are uniformly distributed in passive transport.

 Thermodynamically favourable for an uncharged substance

Because the free energy change for transporting a substance across a membrane is dependent on the concentrations on both sides of the membrane, we can predict whether it will be thermodynamically favourable for an uncharged substance to move from one side of the membrane to the other by checking the concentration gradient. When the concentrations of a material on two sides of a membrane differ, a chemical potentialdifference occurs.

Substance move from one side to another

$∆\mathrm{GA}={\mathrm{G}}_{\mathrm{A}\left(\mathrm{in}\right)}-{\mathrm{G}}_{\mathrm{A}\left(\mathrm{out}\right)}=\mathrm{RTln}\left(\frac{{\left[\mathrm{A}\right]}_{\mathrm{in}}}{{\left[\mathrm{A}\right]}_{\mathrm{out}}}\right)$

If the concentration [A] outside the membrane is larger than the concentration [A] inside the membrane, the GA for transport from outside to inside will be negative, and A will flow freely inside the membrane.

The opposite is true if the concentration of [A] inside is higher than outside, and the net flow of A inside can only occur if it is related to ATP hydrolysis, which is an exergonic activity.

Conclusion

Using the concentration gradient as a guide. If the concentration of A outside the membrane is higher than inside, the transfer of A from outside to inside will be negative, resulting in an inward spontaneous net flow of A.