osmosis

• each cell is surrounded by a cell membrane; separates the contents of the cells from the outside
• the cell membranes is described as being partially permeable membrane
• osmosis: net movement of water molecules from a region of high water potential to low water potential through a semi permeable membrane
• the tiny holes in the membrane allow small water molecules to pass through, but large molecules are too big to pass through
• osmosis may be affected by pressure exerted on water in plant cells by the cell wall or how much water is available
• as water enters, it makes the cells swell up, the water pushes against the cell wall developing turgor pressure

  • the cell wall prevents the cell in plants from bursting
  • this is called "turgid"
• turgid cells give the plant support; keep stems of plants upright

  • cells of stems are held up by turgor pressure
  • the water pressure within the cells acts against the inelastic cell walls, keeping the cells turgid and firm
  • when these cells lose water, they are no longer firm and turgid; thus the leaves will start wilting as water is lost
• plasmolysis:

  • when plant cells are placed into a concentrated sugar or salt solution, water passes out of the cells through osmosis
  • as the water passes out the cell sap begins to shrink; these cells are no longer firm
  • as more water leaves the cells, the cytoplasm starts to move away from the cell wall
  • the cells are now plasmolysed
• osmosis in animal cells

  • in hypotonic solution: bursts/swells
  • in isotonic solution: flaccid
  • in hypertonic solution: crenated