In many biological situations, the concentration of a particular ion or small molecule inside the cell differs from that outside the cell. Such imbalances are maintained by a protein in the cell membrane that moves the substance against its concentration and/or electrical gradient. Movement of molecules or ions against a gradient is called active transport because it requires an input of energy. Often the energy source is adenosine triphosphate (ATP), which has chemical energy stored in its terminal phosphate bond. In eukaryotes, ATP is produced in the mitochondria. Energy is released in a hydrolysis reaction that breaks the terminal phosphate bond of ATP, converting it to adenosine diphosphate (ADP). We will detail how ATP provides energy to cells in Key Concept 8.2.
The differences between diffusion and active transport are summarized in Table 6.1.
| Simple diffusion | Facilitated diffusion (through channel or carrier) | Active transport | |
|---|---|---|---|
| Cellular energy required? | No | No | Yes |
| Driving force | Concentration gradient | Concentration gradient | ATP hydrolysis (against concentration gradient) |
| Membrane protein required? | No | Yes | Yes |
| Specificity | No | Yes | Yes |
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Active transport requires energy to move a substance against its concentration gradient.
Three kinds of membrane proteins are involved in active transport: uniporters, symporters, and antiporters.
Primary active transport uses ATP hydrolysis directly to provide the energy for transport, whereas secondary active transport uses an ion concentration gradient that was established by ATP hydrolysis.
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Animation 6.2 Active Transport
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