If a pickle were an animal, what wouild be the function of the exterior surface

The function of the exterior surface would be to prevent predation.

Most pickle are made from the cucumber. The outer layer of cucumber has substance that cause a bitter taste and cytotoxic effect that could harm the predator. The harmful effect will make the predator hesitate to eat the cucumber when they could find other food. It might also kill the predator and prevent their number growing from eating the pickle.

it’s lava came from an explosive eruption

the solute can move "downhill," from regions of higher to lower concentration, relying on the specificity of the protein carrier to pass through the membrane. this process is called passive transport or facilitated diffusion, and does not require energy.