Osmosis

• Osmosis

  • the movement of water molecules
  • across a selectively permeable membrane
  • from a region of higher water concentration
  • to a region of lower water concentration
• Partially permeable

  • also called semi-permeable
  • allows water and other small molecules to pass through
  • not larger molecules such as starch
• • when the concentration of water is the same on both sides of the membrane, the movement of water molecules will be the same in both directions
  • there will be no net movement of water molecules
• Ion

  • electrically charged particle
  • formed when an atom or molecule gains or loses electrons
• Amino acid

  • the building blocks
  • make up a protein molecule
• Osmosis across living cells

  • cells contain dilute solutions of ions, sugars and amino acids
  • water may move into and out of cells by osmosis
• Root hair cell

  • a specialised cell
  • increases the surface area of the root epidermis
  • to improve the uptake of water and minerals
• Cellulose

  • a carbohydrate
  • forms the cell wall in plant cells
• Plant cells

  • isolated plant cells placed in a dilute solution or water will take in water by osmosis
  • root hair cells, if the soil is wet or moist, will also take up water by osmosis
  • plant cells have a strong cellulose cell wall outside the cell membrane
  • the cell wall is fully permeable to all molecule and supports the cell and stops it bursting when it gains water by osmosis
• Pure water

  • in pure water, the cell contents (the cytoplasm and vacuole) push against the cell wall
  • the cell becomes turgid
  • fully turgid cells support the stems of non-woody plants
• Flaccid

  • lacking turgor
  • lacking in stiffness or strength
  • soft and floppy
• Plasmolysed

  • description of plant cell
  • the vacuole has shrunk
  • the membrane has pulled away from the wall
  • due to water loss by osmosis
• Concentrated solution

  • in a more concentrated solution, the cell contents lose water by osmosis
  • they shrink and pull away from the cell wall
  • the cell becomes flaccid
  • it is becoming plasmolysed
• Highly-concentrated solution

  • the cell undergoes full plasmolysis as the cells lose more water
  • plants would be exposed to higher concentrations of solutes if there was less water in the soil
  • plant cells would then lose water by osmosis
  • aquatic, freshwater plants placed in the sea, or a seaweed in a rock pool where the water evaporated in the Sun, would also lose water by osmosis
• Animal cells

  • take in and lose water by osmosis
  • do not have a cell wall
  • will change size and shape when put into solutions that are at a different concentration to the cell contents
  • in animals, the concentration of body fluids - blood plasma and tissue fluid - must be kept within strict limits
  • if cells lose or gain too much water by osmosis, they do not function efficiently
• Red blood cells could:

  • lose water and shrink in concentrated solutions
  • gain water, swell and burst in a more dilute solution
• Blood plasma

  • the liquid part of the blood
  • contains useful things like glucose, amino acids, minerals, vitamins (nutrients) and hormones, as well as waste materials such as urea
• Tissue fluid

  • fluid which is derived from blood plasma
  • passes through the walls of capillaries