Because membranes are freely permeable to water, it moves into and out of cells by osmosis, depending on relative solute concentrations. Thus, extracellular salt in excess of that in the cytosol (hypertonicity) causes a net movement of water out of cells, whereas hypotonicity causes a net movement of water into cells. The cytosol is rich in charged metabolites and protein species, which attract a large number of counterions that tend to increase the intracellular osmolarity. As a consequence, to prevent overhydration cells must constantly pump out small inorganic ions (e.g., Na+)—typically through the activity of membrane ion-exchanging ATPases. Loss of the ability to generate energy (e.g., in a cell injured by toxins or ischemia) therefore results in osmotic swelling and eventual rupture of cells. Similar transport mechanisms also regulate intracellular and intraorganellar pH; most cytosolic enzymes prefer to work at pH 7.4, whereas lysosomal enzymes function best at pH 5 or less.
Uptake of fluids or macromolecules by the cell, called endocytosis, occurs by two fundamental mechanisms (
Fig. 1.8 ). Certain small molecules—including some vitamins—are taken up by invaginations of the plasma membrane called caveolae. For larger molecules, uptake occurs after binding to specific cell-surface receptors; internalization occurs through a membrane invagination process driven by an intracellular matrix of clathrin proteins. Clathrin is a hexamer of proteins that spontaneously assembles into a basketlike lattice to drive the invagination process. We shall come back to these later.
The process by which large molecules are exported from cells is called exocytosis. In this process, proteins synthesized and packaged within the RER and Golgi apparatus http://ebooksmedicine.net