Active transport is the mediated process of moving molecules and other substances across membranes.

Active transport differs from passive transport in that it utilizes chemical energy in the form of adenosine triphosphate, or ATP, to move molecules against the concentration gradient -- from an area of lower concentration to an area of higher concentration.

There are two types of active transport:

Primary active transport directly utilizes chemical energy to move molecules through a membrane.

The sodium-potassium pump, present in almost all animal cells, is an example of the use of primary active transport. It expends ATP energy to move sodium ions out of the cell and replace them with potassium ions.

This exchange leaves the interior of the cell more positively charged than the cell's exterior. This means the sodium-potassium pump must overcome not only a concentration gradient but also an electrochemical gradient as well.

In secondary active transport, molecules are moved through a membrane as the direct result of the diffusion of another substance.

The sodium-calcium exchanger, or antiporter, uses the normal diffusion of sodium ions into the cell to power the transport of calcium out of the cell (and across a higher concentration gradient).

In another example, the glucose symporter uses the normal diffusion of sodium ions into the cell to piggyback the transport of glucose into the cell as well.

Check out these resources for more information about cell membranes:

• Active Transport
• Passive Transport
• Carrier Proteins
• Membrane Proteins
• Lipid Bilayer
• Endocytosis
• Exocytosis
• Diffusion Through a Membrane
• Osmosis in a Plant Cell
• Osmosis in an Animal Cell
• Plasma Membrane - Plant Cell
• Plasma Membrane - Animal Cell
• My Cell Membranes flashcards
• Biology Quiz: Cell Membranes