Which mechanism would be best to meet the organisms need to gain Ca?
I will say the best mechanism in this case is endocytosis. Endocytosis is also called bulk eatting, when you need large amount of something, the cell use this mechanism to do so. Since you described that “The cells require large amounts of calcium ions”, so why not this? Specifically, endocytosis consists of three sub-mechanisms which are:phagocytosis, pinocytosis, and the receptor endocytosis, thus the one large mechanism can need the requirement no matter what the outside is like, in this case, this is water, thus the cell can use pinocytosis to get the Ca++ ions form the droplets. Look at the other way,simple diffusion is wrong, since ions can go through membrance freely; Facilitated diffusion requires proteins that are embedded in the phophalipids bilayer, an unicellular, how many proteins that they will have to sustain the facilitated diffusion; Active transprt does not only requires proteins to transport, but also they only work when there is chemical or electric input, a unicell
