Osmosis, Plasmolysis, And Turgor Essay, Research Paper

Osmosis, Plasmolysis, and Turgor

Each cell has constantly to cope with osmotic phenomenons. Cells without walls that live in aqueous solutions are usually hypertonic. They are subject to a continuous influx of water that results in a pressure on the inside of the membrane. Some ciliates (like Paramecium) and flagellates (Euglena) have a special organ, a pulsating vacuole, that pumps the water out again and has to be supplied with energy. Red blood cells (erythrocytes) occur normally in isotonic surroundings (blood plasma). The cells burst if the blood is diluted with water since their membranes cannot face the osmotic pressure within the cells. Blood is therefore always diluted with an isotonic (0.9%) NaCl-solution.

The situation in plant cells is different in that they are enclosed by a rigid cell wall. In a hypotonic solution can they only take up water until the inner and the outer water potential are equal. The solution at the inside of the plant cell is subject to an additional hydrostatic pressure. Plant cells store ions, sugars, organic and amino acids and other substances in considerable concentrations in their vacuoles. The solutes cause an influx of water. In this way can plant cells build up a large positive internal pressure, the turgor pressure. It has a decisive influence on the maintenance of the rigidity and stability of plant tissues. Each cell exerts a pressure on its neighbouring cells. The pressures add up to a large tissue tension.

Plants that loose water wilt because their turgor pressure decreases. The stability of the tissues cannot be maintained. As long as the cells are still living can the turgor pressure be fixed again, a phenomenon common in everyday life: wilted plants recover after watering. Osmosis and turgor pressure have been extensively examined at the end of the last century by W. PFEFFER. He constructed a model (an osmometer, the PFEFFER cell) that allows the quantitative determination of the osmotic pressure. He expressed the relation between turgor and osmosis in a formula that describes the osmotic state of the cell:

Sz = Oz – W

where Sz is the suction force of the cell, Oz its osmotic value and W the wall pressure.

If plant cells are given into in a hypertonic solution (a 5% potassium nitrate solution, for example), water is extracted from the protoplasm, it shrinks. This phenomenon is called plasmolysis. The process is reversed as soon as the cells are transferred into a hypotonic solution (deplasmolysis).

Plants use osmosis, or diffusion of water, plasmolysis and Turgor in stomate control and flowring.

Venus-Fly Trap

The Venus Fly Trap has specialized leaves that serve as traps. To a typical insect the color of the leaves looks very good. And to make it even more difficult for them to just fly by without taking a close-up look, the traps secrete a smelly stuff that is just irresistible to insects. When an insect touches one of the six special trigger-hairs in the trap, the hinged leaves fold up and it is trapped. Since the trap is not too fast, it takes about 1/2 second, it seldom catches a fly. Its prey is more likely an ant, small spider or beetle.

The plant then secretes a red digestive sap, that digests the insect body and gives the leave this reddish color.The sap contains different enzymes that are responsible for the digestion.Enzymes are proteins. Some of these enzymes digest the outer hull of insects, made of chitin. Others digest the protein. It is very similar to what happens in a human’s stomach after a meal, though humans cannot digest chitin. The digestion products are then taken up by the plant and used to feed itself. It takes about 10 days before the insect is digested and the trap opens again. After having captured three or four insects, the trap dies. Though the plant does not have to capture insects to survive, but the plants that do capture insects will generally be more healthy.