Biology XI: 11 Transport in plants(3 Osmosis)

Osmosis, Plasmolysis and Imbibition Osmosis Diffusion of water across a semi-permeable membrane Direction and rate of osmosis depends upon pressure gradient and concentration gradient. Water diffuses from region of higher chemical potential to lower chemical potential until equilibrium is reached. Osmotic pressure − External pressure applied to prevent the diffusion of waterIt depends upon solute concentration.Osmotic pressure  Solute concentration Numerically: osmotic pressure = osmotic potential Osmotic pressure has positive sign.Osmotic potential has negative sign Behaviour of Plant Cell Depending Upon the Surrounding Solution Isotonic solution When concentration of external solution = Concentration in cytoplasm No change in cell size Hypotonic solution When concentration of external solution < Concentration in cytoplasm Cells swell. Hypertonic solution When concentration of external solutions > Concentration in cytoplasm Cells shrink Plasmolysis Occurs when cell is placed in hypertonic solution Water moves out, first from cytoplasm and then from vacuole. Cell membrane shrinks away from the cell wall. In isotonic solution, water flowing into the cell = water flowing out of the cell Cell is said to be flaccid. In hypotonic solution, water diffuses into the cell and cells enlarge and extension growth occurs. As water diffuses in, cytoplasm builds up a pressure against the cell wall. This pressure is called turgor pressure. Because of rigidity of cell wall, the cell wall does not rupture but cells enlarge. Imbibition Diffusion in which water is absorbed by solids i.e. colloids, causing them to enormously increase in volume Substances are absorbed without forming a solution. Diffusion is along the concentration gradient and depends upon affinity between adsorbent and liquid being adsorbed. Example − Imbibition of water by seeds that causes seeding to emerge out of soil