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Transcript
OSMOSIS Osmosis is the movement of 1 molecules from a 2 concentration of water molecules ( 3 solution) into a region of 4 concentration of water molecules ( 5 solution) through a 6 permeable membrane. Therefore osmosis is simply another example of 7 but the term is specifically used for the movement of 8 molecules. In pure water the water molecules move about 9 within the volume of the liquid. This ability to move is known as the 10 ( - psi) and for pure water it is given a value of 11 . If any 12 molecules are dissolved in the water then they bind to the water molecules and reduce their ability to move about. The more solute present (ie. more 13 the solution) then the more 14 the 15 (WP) due to the interference with the movement of the water molecules. The whole solution therefore has a lower 16 ie. more negative value of the WP. The contents of the cell cytoplasm are a complex mixture of dissolved solutes, giving a certain water potential. If the cell is placed in a solution of equal concentration then as much water will diffuse in as out and the two solutions are 17 . If a cell is placed in a more dilute solution then the solution outside the cell is 18 the cell contents and water will pass 19 the cell by 20 since the cell contents have a 21 WP. Placing a cell in a more concentrated solution ie. 22 23 by osmosis since the surrounding solution now has the ) water potential. Ref. Bailey and Hirst pg. 30 for diagrams. to will cause water to pass 24 (more negative The 25 of water movement depends upon the WP of the cell contents and the bathing solution. The 26 of movement depends upon the numerical difference of the water potentials ie. the diffusion (osmotic) 27 . Blood cells placed in water will 28 as water flows into them and they soon burst. When in 29 solution animal cells 30 as water flows 31 by osmosis. In plant cells the movement of water into cells is also influenced by the presence of the 32 cell wall. This is fully 33 but, as the cytoplasm swells against it , the wall presses 34 to create a constraining pressure potential. This is always a positive value and so squeezes the cell and influences the rate at which water can enter the cell. As water flows into a plant cell placed in a 35 solution the cytoplasm and cell vacuole increase in volume pushing outwards against the cell wall. The latter stretches and applies a 36 . Eventually the cell wall will stretch no more and the cell is now fully 37 . The water potential of the cell is now 38 because the pressure potential of the wall is just 39 to the water potential of the solution within the cell. If a plant cell is placed in a hypertonic solution (lower WP), then water will flow 40 by osmosis. The cytoplasm and vacuole will 41 in volume, shrinking the cell contents away from the cell wall. 42 drops to zero and the cytoplasm pulls away from the cell wall – this cell is now said to be showing 43 . Ref. Bailey and Hirst p.32