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What is the type of membrane transport in which substances move across the membrane along their concentration gradient until equilibrium is reached?

Small neutral and soluble substances are able to travel across the membrane by the process of simple diffusion. During simple diffusion, substances are able to pass unimpeded through the membrane along a concentration gradient. A concentration gradient occurs when there is more of the substance on one side of the membrane than the other.


When substances travel along their concentration gradient, they travel from the side of the membrane where they are present in a higher concentration to the side of the membrane where they are present in a lower concentration. Transport across the membrane continues until the concentration of the substances is the same on both sides of the membrane. Examples: oxygen, carbon dioxide, most lipids.


When water is transported across the membrane by simple diffusion, it is called osmosis.


There are several other processes that are used to transport substances across the membrane. These include:



  • Facilitated Diffusion: Substances travel across the membrane through protein channels.


  • Active Transport: Substances travel across the membrane against their concentration gradient. (i.e. from the side of the membrane with a lower concentration of the substance to the side of the membrane with the higher concentration of the substance). This process requires the input of energy from the breakdown of the molecule ATP.


  • Endocytosis and Exocytosis: Substances travel in and out of the cell by being engulfed by a portion of the cell membrane. Movement into the cell by this process is called endocytosis. Movement out of the cell by this process is called exocytosis.

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