The Process of Osmosis and its Importance to Living Organisms.

Many marine organisms are highly affected by changes in salinity. This is because of a process called osmosis which is the ability of water to move in and out of living cells, in response to a concentration of a dissolved material, until equilibrium is reached. In general the dissolved material does not easily cross the cell membrane so the water flows by osmosis to form equilibrium. Marine organisms respond to this as either being osmotic conformers (also called poikilosmotic) or osmotic regulators (or homeosmotic).

Osmotic conformers have no mechanism to control osmosis and their cells are the same salt content as the liquid environment in which they are found. If a marine osmotic conformer were put in fresh water, osmosis would cause water to enter its cells to form equilibrium, eventually causing the cells to pop. If a marine osmotic conformer were put in super salty water then osmosis would cause the water inside the cells to move out, eventually causing the cells to dehydrate.

Osmotic regulators have a variety of mechanisms to control osmosis and the salt content of their cells varies. It does not matter what the salt content is of the water surrounding a marine osmotic regulator, their mechanisms will prevent any drastic changes to the living cells. Marine osmotic regulators include most of the fish, reptiles, birds and mammals. These are the organisms that are most likely to migrate long distances where they may encounter changes in salinity. An excellent example of this is the salmon fish. The fish is about 18 o/oo salt so in seawater it tends to dehydrate and constantly drinks the seawater. Special cells on the gills called chloride cells excrete the salt so the fish can replace its lost water. When a salmon migrates to fresh water its cells start to take on water so the salmon stops drinking and its kidneys start working to produce large amounts of urine to expel the water.

Plants gain water through osmosis in their roots from the soil. Without a water potential gradient, water will be loss from the roots. Plant cells contain vacuoles, which, if not full with water, will cause the cell to become flaccid. If all the cells in a leaf become flaccid, the whole leaf will become flaccid, hence causing the plant to wilt. Plant cells therefore need water to remain turgid and keep firm. If an animal cell surrounded with a high water potential, osmosis will take place, and if the water is not expelled some way or another, the cell will burst. This is because an animal cell doesn’t have a cell wall to keep it strong. If an animal cell is surrounded with low water potential, the water in the cytoplasm will diffuse outwards, causing the cell to shrink.