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Cell Biology
The World of Cells
Getting Substances
In and Out of Cells
Diffusion plays an important role in moving small molecules short distances within and around cells. However, for large molecules and many others diffusion is too slow and nonselective If they are to obtain vital ingredients in a timely manner, cells must have a way of speeding up the movement of molecules into and out of their cytoplasm.

Also, the cell membrane is impermeable to many of the larger substances needed by the cells. Sugars, amino acids, etc. cannot simply diffuse from one side of the membrane to the other. Cells, therefore, transport these needed molecules across the membrane using special carrier proteins.

These proteins are located in the membrane, are very specific and attach only to certain types of molecules. In this way, a cell can carefully regulate the amount and types of molecules that pass into and out of it.

Passive transport
Carrier proteins can only move the molecules in the same direction they would normally move by diffusion. This process, called facilitated diffusion, allows the cell to control what materials are transported across the membrane and in what quantity. No expenditure of energy is required.

As its names suggests, diffusion is still important in this mechanism. Molecules, like sugars, reach the carrier proteins in the membrane by diffusion and are then moved across the membrane from a region of high concentration to a region of low concentration.

For example, if a cell is placed in a sugar solution the sugar molecules will diffuse and make contact with the carrier proteins located in the membrane. Carrier proteins will then transport these sugars across the membrane and into the cell, but only as long as the concentration of sugar is greater outside the cell than inside.

Active transport
There are many instances, however, in which the cell must move substances from regions of low concentration to regions that are already highly concentrated. For example, a particular sugar may be in dilute solution outside the cell as well as in high concentration inside the cell, but it would be beneficial for the cell to accumulate more of this valuable resource.

Built into the membrane are carrier proteins that attach to the sugar molecules outside the cell and then use energy to "pump" these molecules against the normal direction of diffusion and into the cell. This system is known as active transport. Energy is required for active transport because work is being done against the natural force of diffusion.

© 2001, Professor John Blamire