1.8

• Osmosis

  The movement of water through a partially permeable membrane down a water concentration gradient
• Effect of osmosis on animal cells

  • Can be dramatic
  • Animals have many complex ways of controlling the concentrations of their body solutions to prevent cell damage
• Osmosis in plants

  • Key to their whole way of life
  • Plants rely on osmosis to support their stems and leaves
  • Water moves into plant cells by osmosis
  • Causes the vacuole to swell, which presses the cytoplasm against the plant cell wall
  • Pressure builds up until no more water can physically enter the cell - this pressure is known as turgor
  • Turgor pressure makes the cells hard and rigid, which in turn keeps the leaves and stems of the plant rigid and firm
• Hypotonic

  The fluid surrounding the plant cells has a lower concentration of solutes and a higher concentration of water than the plant cells themselves
• Hypertonic

  The solution surrounding the plant cells is more concentrated than the cell contents
• Effect of hypertonic solution on plant cells

  1. Water will leave the cells by osmosis

  2. Cells will no longer be firm and swollen - they become flaccid (soft) as there is no pressure on the cell walls

  3. Plant wilts as turgor no longer supports the plant tissues
• Plasmolysis

  The cell membrane pulls away from the cell wall due to excessive water loss by osmosis
• Plasmolysis is usually only seen in laboratory experiments
• Plasmolysed cells die quickly unless the osmotic balance is restored
• Micrographs of red onion cells in hypertonic and hypotonic solutions

  • Show the effect of osmosis on the contents of the cell
• Osmometer

  A way of measuring osmosis using plant tissue that reacts strongly to the concentration of the external solution
• There are lots of ways to investigate the effect of osmosis on plant tissue, each with advantages and disadvantages
• Scientists have discovered ways of measuring the turgor pressure inside individual cells using very tiny probes. The pressures inside the root or leaf cell of a plant are far higher than human blood pressure, or even the pressure in a car tyre.