The cell membrane acts as a gatekeeper, allowing some substances to enter the cell, but excluding others. In other words, the membrane is selectively permeable. This selective permeability is an essential feature of the membranes of all living cells, because it provides them with the power to control their internal environments. The cell may use channels, carriers, or pumps to move substances from one side of a membrane to the other.
In this animation, we examine a type of transport across a membrane that requires no energy for a cell to perform. It is called passive transport. In passive transport, a substance moves across a membrane from a region of higher concentration to a region of lower concentration. Active transport (not shown here) is the opposite: it requires energy, moving a substance from a lower concentration to a higher concentration.
A cell continually exchanges molecules and ions with its environment. When a cell takes in substances that are in greater concentration outside the cell, the cell does not need to expend energy. These transport processes are called passive transport, and they occur by either simple diffusion (the substance slips through the lipid bilayer on its own) or facilitated diffusion (a channel or carrier provides a passageway). Substances spontaneously flow down their concentration gradients.
In some cases (not shown here), the cell brings in substances that are already in greater concentration inside the cell than outside. This process is called active transport and requires that the cell expend energy.
Textbook Reference: Concept 5.2 Passive Transport across Membranes Requires No Input of Energy