We have the cytoplasm of the cell. And this is the membrane of the er. So this is the inside of the endoplasm reticulum, the lumen of the endoplasm reticulum. Now, the first thing we should notice is if we compare the concentration of calcium ions on the inside the cytoplasm to, let's say, the outside or even the lumen, we'll see that inside the cytoplasm, we have a very low concentration of calcium ions. The question is why? Well, to answer the question, let's recall a bit of general chemistry.