Active membrane transport. Illustration showing a cell membrane (phospholipid bilayer, blue) and a membrane channel (orange), relating to the mechanism of primary active transport across the membrane. A dissolved molecule (solute) of an ion such as calcium (Ca2+) is being moved through the channel against the natural electrochemical gradient. The energy for this ion transport comes from hydrolysis of molecules of ATP (adenosine triphosphate). The calcium ion is attached to the ATP enzyme ATPase. For this artwork with labels, see image C023/8775.