Summary

The most common molecules in the body are water and simple inorganic molecules mainly sodium, potassium, and chloride ions. The water in the body can be divided into two compartments: the intracellular and extracellular fluids. The barrier between those two compartments is the plasma membrane of the cell, which is a phospholipid bilayer with protein molecules inserted into it. The extracellular fluid is high in sodium and chloride, but low in potassium, while the intracellular fluid is low in sodium and chloride, but high in potassium. This difference is maintained and regulated by control mechanisms residing in the plasma membrane, which acts as a selectively permeable barrier permitting some substances to cross but excluding others.

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