Sarcoplasm is the cytoplasm of a muscle cell. It is comparable to the cytoplasm of other cells, but it contains unusually large amounts of glycogen, myoglobin, a red-colored protein necessary for binding oxygen molecules that diffuse into muscle fibers, and mitochondria. The calcium ion concentration in sarcoplasm is also a special element of the muscle fiber; it is the means by which muscle contractions take place and are regulated. The sarcoplasm plays a critical role in muscle contraction as an increase in Ca2+ concentration in the sarcoplasm begins the process of filament sliding. The decrease in Ca2+ in the sarcoplasm subsequently ceases filament sliding. The sarcoplasm also aids in pH and ion balance within muscle cells.