Transportation Within and Between Cells
Within and around cells, materials are constantly being shipped one way or another across membranes. At any given moment, a particular substance may exist in higher or lower concentrations on one side of the membrane versus the other. Such a difference in concentration is called a gradient. The particular type of transport used for a particular substance depends on the direction of its concentration gradient and on its size.
The major types of cellular transport are
- Diffusion: Diffusion is the movement of a substance from an area of higher concentration to one of lower concentration (down a gradient). It occurs on its own without the input of energy. Many waste materials exit the cell through diffusion because that is their natural direction of movement as they build up within the cell.
- Osmosis: Osmosis is simply the diffusion of water; it has its own name because water is so important to life. Solutions with higher amounts of dissolved substances have lower concentrations of water, and water will diffuse across membranes in order to minimize the difference in concentration.
- Facilitated diffusion: Most kinds of materials can't easily diffuse across the plasma membrane unless an opening is provided for them. Cells can support and control the diffusion of these materials by means of membrane proteins. Specific membrane proteins allow passage across the membrane to specific ions or molecules, but not to others. As long as the appropriate membrane protein is present and open, a given ion or molecule can diffuse through the protein, across the membrane, along its concentration gradient. Although facilitated diffusion requires a protein tunnel through the membrane, it is still diffusion, and requires no added energy.
- Active transport: Sometimes, much-needed nutrients or harmful substances must be transported across the membrane against a concentration gradient. In these cases, the cell must provide energy in order to move the material against the direction of diffusion. This kind of energy-requiring transport is called active transport and, like facilitated diffusion, uses membrane proteins. Often, these proteins cleave ATP in order to obtain the needed energy. Other proteins use the energy released from the diffusion of one substance to power the active transport of another substance.
- Exocytosis: Membrane proteins are fine for channeling the movement of ions and small molecules, but for transporting large molecules, a different strategy is required. When cells need to send large molecules (like proteins) outside their plasma membrane borders, they turn to exocytosis. Sizable cargo is loaded into spherical membrane vesicles. These vesicles move toward the plasma membrane and fuse with it, exposing the vesicle interior to the outside of the cell and releasing its contents.
- Endocytosis: Sometimes cells have cause to import large molecules. For this challenging task, the solution is endocytosis, which is essentially exocytosis in reverse. Molecules to be imported contact the exterior surface of the plasma membrane, triggering the membrane to fold inward, enveloping them. The infolded membrane pinches off into a vesicle containing the imported molecules, which can be further transported to their eventual destination within the cell.