Cell Transport Methods
A cell membrane is a semi-permeable structure consisting of proteins and fats that keeps foreign substances out and the contents of the cell in. There are certain materials that a cell membrane will let in through passive transport, active transport and simple diffusion.
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Passive Transport
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Passive transport does not involve any energy on the cell’s part. Passive transport, or diffusion, occurs when there is a high concentration of a substance outside the cell and a low concentration of the substance inside of the cell. The substance diffuses into the cell through the cellular membrane in an effort to balance the concentration of the substance. One example of passive transport is when oxygen and carbon dioxide are diffused across alveolar-capillary membranes in lung cells. Passive transport requires a protein carrier.
Active Transport
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Active transport requires the cell to use energy–typically in the form of ATP–because it is passing a molecule against the concentration gradient. To make these molecules go against the current, the cell uses adenosine triphosphate (ATP) to activate the carrier proteins that help the cellular membrane pass the molecules through. One example of active transport is human intestinal cells taking in glucose.
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Simple Diffusion
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Simple diffusion is very similar to passive transport but without the protein carrier. Molecules flow freely from an area of high concentration to an area of low concentration through the cell’s semi-permeable membrane. An example of this process is osmosis, in which water flows into a cell to balance out the concentration of water in the cell’s environment. Animal cells that are placed in too much water will burst from osmosis, but plant cells do not burst due to the stabilization of the plant cell’s cell wall.
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References
- Photo Credit glass with water image by Alexander Ivanov from Fotolia.com
