Final answer:
The sarcoplasmic reticulum reabsorbs calcium ions using the Calcium ATPase pump, which is an active transport mechanism requiring ATP.
Step-by-step explanation:
The sarcoplasmic reticulum constantly takes calcium back up using a mechanism known as the Calcium ATPase pump. This pump actively transports calcium ions into the sarcoplasmic reticulum of muscle cells, which is necessary for the muscle to relax after contraction. This action requires energy in the form of ATP (adenosine triphosphate) to move the calcium ions against their concentration gradient, contrasting with a diffusion process that would allow ions to move passively. The sodium-potassium pump, chloride pump, or any variation thereof, does not play a direct role in this specific process of calcium uptake in the sarcoplasmic reticulum.