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Passive transport is the movement of molecules through a permeable membrane without expending chemical energy. It plays a key role in a number of biological processes by allowing the body to move nutrients and waste materials in and out of cells without having to use energy to do so. Diffusion, osmosis, and facilitated diffusion are examples of passive transport people can see in action in various environments throughout the body.
Diffusion reflects the tendency for molecules to spread out, if they have room to do so. In a classic example, oxygen molecules flow from the oxygen-rich environment outside cells to the oxygen-poor environment inside, diffusing across the cell membrane. Diffusion allows molecules to move from areas where their numbers are high to locations where their numbers are low. Cells do not have to expend any energy to move the molecules, as they push through the membrane on their own.
In facilitated diffusion, structures in the cell membrane help the process along. Some molecules may not naturally fit through the membrane. They could travel down an ion channel, a structure in the membrane of the cell that allows larger molecules and ions to pass through. They may also connect to carrier proteins. These proteins lock on and act like keys to open the cell membrane and allow a molecule to enter.
Osmosis is another form of passive transport. It is closely related to diffusion, but refers to water specifically, rather than any molecules. Water will move across a membrane if the balance of salts across the membrane is unequal. The water moves from the side with a low concentration of salts to the side with a high concentration, in the goal of equalizing the distribution by diluting the hypertonic, or more salty, side. Once the two sides of the membrane are isotonic, with similar concentrations of salt, the water will stop moving.
Passive transport can have pitfalls for the body. Dangerous molecules may be able to pass through the cell membrane because they are small enough or they resemble chemicals the cell has a use for. Once inside, they may damage the cell, hijack it and force it to produce rogue proteins, or cause other problems.
People can see passive transport at work outside the body as well as within it. For example, diffusion is commonly used for techniques like making agar plates to research microorganisms. The researcher can deliver antibiotics to one area of the plate and rely on them to diffuse throughout the gel, moving away from the area where they are highly concentrated. Likewise, osmosis can play a role in scientific experiments where people need to move water across a membrane by changing the balance of salts on either side.