Cell transport is the movement of substances across the cell membrane. Probably the most important feature of a cell's phospholipid membranes is that they are selectively permeable. A membrane that is selectively permeable, or semipermeable, has control over what molecules or ions can enter or leave the cell. This feature allows a cell to control the transport of materials, as dictated by the cell's function. The permeability of a membrane is dependent on the organization and characteristics of the membrane lipids and proteins. In this way, cell membranes help maintain a state of homeostasis within cells (and tissues, organs, and organ systems) so that an organism can stay alive and healthy.There are four main ways that molecules can pass through a phospholipid membrane. The firstway requires no energy input by the cell and is called simple diffusion. This type of transport includes passive diffusion and osmosis. No assistance by a transport is necessary in simple diffusion. Facilitated diffusion, does involve the assistance of transport proteins. The third way, called active transport, requires that the cell uses energy to pull in or pump out certain molecules and ions. Active transport involves proteins known as pumps. The fourth way is through vesicle transport, in which large molecules are moved across the membrane in bubble-like sacks that are made from pieces of the membrane. Vesicular transport includes exocytosis and endocytosis.