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 (a polymer of glucose), myoglobin, a red-colored protein necessary for binding oxygen molecules that diffuse into muscle fibers, and mitochondria. The calcium ion concentration in sarcoplasma 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.