One thousand Ca2+ channels open in the plasma membrane of a cell that is 1000 μm3 in size and has a cytosolic Ca2+ concentration of 100 nM. for how long would the channels need to stay open in order for the cytosolic Ca2+ concentration to rise to 5 μm? There is virtually unlimited Ca2+ available in the outside medium (the extracellular Ca2+ concentration in which most animal cells live is a few millimolar), and each channel passes 106 Ca2+ ions per second.

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Osmosis

In simple words, osmosis is the slow, gradual movement of a liquid through a membrane. But there are nuances to this process. For instance, there has to be an imbalance in the distribution of solute in the liquid on either side of the membrane, which has to be semipermeable. The liquid in the region with a lower solute concentration moves to the region with a higher concentration, thereby displacing the solution in the process. The objective is to equalize the concentration of molecules on both sides of the membrane; the displacement of liquid can be viewed as a byproduct of this equalization. Since there is no energy required to carry osmosis, it is considered a passive process.