A membrane-spanning protein that uses ATP actively to remove calcium ions from cells to maintain a low internal calcium ion concentration. Calcium pumps are especially numerous in the membrane of the sarcoplasmic reticulum of muscle cells, where they rapidly restore the resting state following the influx of calcium ions that triggers contraction of muscle fibres; hence the calcium pumps allow the fibres to relax in readiness for a further contraction. For each molecule of ATP hydrolysed to ADP, a calcium pump transfers two calcium ions outwards, in exchange for two or three hydrogen ions moving in the opposite direction.