We will examine these processes in this section. Before we begin, however, I want to stress the importance of the plasma membrane that surrounds cells. To get in or out of a cell, material must pass through this membrane. It is the membrane that is the gatekeeper of cells. Passive transport .
Many materials move into cells without requiring the expenditure of cellular energy.
The transport mechanism in such cases is considered passive or inactive because the force moving these molecules or ions is not external to them. They have this motion because they have heat. The amount of molecular movement depends upon the temperature and physical properties of a substance. But if we cool its molecules down enough, the air can be converted to a liquid and at even colder temperatures it will become a solid. We think of oxygen as a gas because that is its state at normal temperatures, but we have heard about the liquid oxygen that is used to fuel rockets.