![]() ![]() Although water is a polar molecule, it is able to penetrate the nonpolar lipid region of membranes through a group of transmembrane channel proteins called aquaporins (AQPs), which form channels through which water can readily diffuse. Most cell membranes are semipermeable, that is, relatively impermeable to most solutes but highly permeable to water. The process of osmosis also explains the movement of water across cell membranes. This pressure is called the osmotic pressure. As shown in diagram III, application of a pressure can prevent osmotic movement of water across the semipermeable membrane. Because the volume of compartment B increased, there were no significant changes in hydrostatic pressure in the compartments. In this example, the volume of compartment B increased, whereas the volume of compartment A decreased. The movement of water across the semipermeable membrane leads to a change in the initial volumes at equilibrium. There is net movement of water from A to B until the solutions in the two compartments are iso-osmotic, as shown in diagram II. In diagram I, compartments A and B are shown filled with equal volumes of solution, but the solution in compartment A is hypo-osmotic with reference to the solution in compartment B. This is achieved by removing excess heat from the body by evaporative water loss from the skin.įIGURE 35-2 Osmosis and osmotic pressure can be illustrated by two compartments separated by a semipermeable membrane, permeable to water but not to solutes (circles). Another important physiological function of water is its role in the regulation of body temperature. The intracellular water establishes the physicochemical medium that allows various metabolic processes to take place. The extracellular water bathing the cells serves as a medium for the transport of nutrients and oxygen to the cells and for removing wastes from the cells, which will be eliminated by the liver and kidneys. It is the principal fluid medium in which nutrients, minerals, gases, and enzymes are dissolved. Water performs several functions that are essential to life. The volume of the intracellular fluid provides turgor to the tissues, which is important for the tissue or organ form and ultimately the body form. ![]() It lubricates joints and moistens tissues such as those in the eyes, nose, and mouth. In the body, water is present inside and around the cells and within all blood vessels. Water is an essential nutrient vital to the existence of both animals and plants. ![]()
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