Final answer:
Ca₂⁺-pumps in the plasma membrane and endoplasmic reticulum primarily function to prevent Ca₂⁺ from altering molecules in the cytosol, thus maintaining calcium homeostasis essential for cell function and signaling.
Step-by-step explanation:
Ca₂⁺-pumps in the plasma membrane and endoplasmic reticulum are essential for b) preventing Ca₂⁺ from altering the activity of molecules in the cytosol. These pumps help to maintain low levels of cytosolic calcium, which is crucial since elevated intracellular calcium levels can activate a variety of enzymes and influence cellular processes in a way that can be detrimental to cell function. The primary active transport mediated by these pumps moves calcium ions out of the cytosol back into the ER or extracellular space against their concentration gradient.
This action of Ca₂⁺-pumps is crucial not just for preventing the inadvertent activation of molecules but also plays a vital role in signal transduction pathways, where the influx of calcium ions from external or internal stores leads to cellular responses. This mechanism ensures that the cell maintains calcium homeostasis, which is essential for proper cellular function and survival.