What defines passive transport across a membrane?

Passive transport is characterized by the movement of particles across a membrane from an area of higher concentration to an area of lower concentration, and this process occurs without the use of cellular energy. This means that substances will naturally diffuse down their concentration gradient until equilibrium is reached.

For example, if you have a concentrated solution on one side of a membrane and a dilute solution on the other, the particles will spontaneously move toward the area of lower concentration until they are evenly distributed. This ability to move freely in response to concentration differences is a fundamental principle of passive transport.

In contrast, processes that require energy from the cell, such as active transport, oppose the concentration gradient and do not apply to passive transport. Similarly, passive transport can occur in various environments, not just aquatic ones, and while it can involve specific transport proteins (like facilitated diffusion through channel proteins), these proteins are not universally required for all forms of passive transport. Simple diffusion, for example, does not require any specific transport proteins at all.