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Step 3: Get the Salts Out


If you had a bag of cell contents containing proteins, salts, and water, can you think of a way to get some of the salt removed? What if I tell you that the bag is plastic and that very small molecules, like salts, can get out?

In this case, what would happen if you set your plastic bag in a tub of pure water that contained no salts? These ideas are discussed also in the unit on cell membranes. The salts inside the bag would move outside the bag until a balance ("equilibrium") was reached between the amount of salt molecules inside the bag and the amount of salt in the water outside the bag? This is the process known as osmosis 

But, we only have half of the salt out. How would we get some more out? We just put the bag in a fresh tub of pure water and repeat the process over and over until we get the protein pure enough for our purposes.