The Graph B best depicts the effect of frequency of muscle cell stimulation on concentration of Ca2+ in the sarcoplasmic reticulum.
The sarcoplasmic reticulum (SR) is a network of tubules that stores Ca2+ ions in muscle cells. When a muscle cell is stimulated, Ca2+ ions are released from the SR into the cytoplasm.
The Ca2+ ions then bind to troponin, which initiates muscle contraction.
The frequency of muscle cell stimulation affects the amount of Ca2+ that is released from the SR.
At low frequencies of stimulation, there is enough time for the SR to refill with Ca2+ ions between contractions. However, at high frequencies of stimulation, the SR does not have enough time to refill, and the amount of Ca2+ released from the SR decreases.
Graph B shows that the concentration of Ca2+ in the SR increases with increasing frequency of stimulation, reaching a maximum at a frequency of 10 Hz. This is because the SR is able to keep up with the demand for Ca2+ ions at low frequencies of stimulation.