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Cell transport is a vital function of cells. It allows the cell to bring molecules and individual atoms into the cell and send unwanted molecules and atoms out of the cell. The barrier between the interior of the cell and the outer environment is the cell membrane. It regulates what can come into the cell. Cell transport can be a passive process which doesn't require energy, or it can be an active process that requires energy.
Passive transport doesn't require energy from the cell. It utilizes a concentration gradient to facilitate the passage of atoms or molecules through the cell membrane. In this type of cell transport, the interior of the cell has a low concentration of a particular molecule, and the exterior environment has a high concentration of the molecule. Transport occurs to balance the concentrations inside and out to reach an equilibrium.
Simple diffusion is one type of passive transport. Individual molecules travel down the concentration gradient, from high to low, passing through the cell membrane without any assistance. Osmosis is a type of simple diffusion that specifically describes how water molecules pass through the cell membrane.
Another type of passive transport is called facilitated diffusion. This type of cell transport utilizes a protein within the cell membrane to carry molecules into the cell. Glucose is one type of molecule that requires a membrane protein to pass through the cell membrane.
One type of cell transport that requires energy is called active transport. It describes transport processes that go against the concentration gradient, meaning from low concentration to high concentration. Membrane proteins are required for this type of transport.
An example of active transport is the sodium-potassium pump in nerve cells. Normally, the concentration of sodium is low in the cell and high outside the cell while potassium concentration is typically high inside the cell and low outside the cell. When a nerve impulse is propagated, sodium and potassium pass through the cell membrane along the concentration gradient. To return the concentration gradient to its initial state, the membrane protein must continually pump sodium out of the cell and bring potassium into the cell. This requires energy from the cell.
Other types of transport that require energy involve vesicles. Vesicles are small sacs that are produced from the cell's Golgi apparatus. They collect waste products, which they expel from the cell by fusing with the cell membrane. Vesicles can also be used to collect molecules which are outside the cell. In a process called endocytosis, the cell membrane surrounds the molecule and incorporates it into a vesicle, which detaches from the membrane and enters the cell. The contents of the vesicle are then digested and released into the cell.