Osmosis (Cellular Transport)

• Osmosis:

  Is the diffusion of water from a dilute solution to a concentrated solution through a partially permeable membrane.
• Dilute solutions contain a high concentration of water, on the left. However, concentrated solutions contain a low concentration of water, to the right. Like this...

  
• The water now diffuses from the left hand side to the right hand side. Osmosis is simply the diffusion of water. Like this...

  
• Partially permeable membranes allow some molecules to pass through but not all molecules. We've got a higher concentration of water on the left hand side and a lower concentration of water on the right hand side. Like this...

  
• The cytoplasm of cells is a relatively concentrated solution. It contains a relatively low concentration of water. Like this...

  
• If we place this cell in water, then osmosis will take place. Water will move by osmosis from outside the cell to inside the cell. Like this...

  
• In the case of an animal cell, the water moving in will cause the cell to expand. The cell could even burst. Like this...

  
• If we place an animal cell in a very concentrated solution, then water will move out of the cell by osmosis and the cell will shrink. Like this...

  
• If we place a plant cell in water, then water will move into the cell by osmosis and the cell will expand. Like this...

  
• The cell wall prevents the plant cell from bursting. Instead, the cell becomes swollen, this is called turgid.
• If we place the plant cell into a concentrated solution, then water moves out of the plant cell by osmosis. This causes the cell to shrink, scientists say that the cell has become flaccid. Like this...

  
• Like, diffusion, osmosis is a form of passive transport (does not require energy) but it only applies to water.

  • Water can move in and out of cells by osmosis and will move down its concentration gradient
• Osmosis and the partially permeable membrane:

  
• Osmosis in cells:

  
• Instead, we can talk about osmosis in terms of water potential.

  • Water potential is the potential (likelihood) of water molecules to diffuse out of or into a solution.
  • A concentrated solution (of sugar) has a low concentration of water molecules and a low water potential.
  • It is highly likely for water molecules to move into the concentrated sugar solution.
• A dilute solution (of sugar) has a high concentration of water molecules and a high water potential.

  • It is less likely for water molecules to move into the dilute sugar solution.
  • It helps to remember that pure water has the highest water potential of any solution.
• How osmosis works:

  
• Osmosis in animal cells (Part 1):

  • Animal cells lose and gain water as a result of osmosis.
  • As animal cells do not have a supporting cell wall, the results of osmosis can be severe.
  • If an animal cell is placed into a strong sugar solution (with a lower water potential than the cell), it will lose water by osmosis and become crenated (shrivelled up).
• Osmosis in animal cells (part 2):

  • If an animal cell is placed into distilled water (with a higher water potential than the cell), it will gain water by osmosis as it has no cell wall to create turgor pressure.
  • It will continue to gain water until the cell membrane is stretched too far and it bursts.
• Effect of osmosis on animal cells:

  
• Osmosis in plant cells (part 1):

  • Plant cells lose or gain water as a result of osmosis.
  • Unlike animal cells, plant cells have a supporting cell wall so are protected from cell lysis (bursting of the cell).
  • If a plant cell is placed into a strong sugar solution (with a lower water potential than the cell), it will lose water by osmosis.
  • The vacuole gets smaller and the cell membrane shrivels away from the cell wall.
  • It becomes flaccid (shrivelled up).
• Osmosis in plant cells (part 2):

  • If a plant cell is placed into distilled water (with a higher water potential than the cell), it will gain water by osmosis.
  • The vacuole gets bigger, pushing the cell membrane against the cell wall.
  • The plant cell is described as being turgid or as containing a high turgor pressure (the pressure of the cytoplasm pushing against the cell wall).
• The effect of osmosis on plant cells. Hypertonic solutions contain less water, hypotonic solutions contain more water:

  
• Water entering the cell by osmosis makes the cell rigid and firm.

  • This is important for plants as the effect of all the cells in a plant being firm is to provide support and strength for the plant - making the plant stand upright with its leaves held out to catch sunlight.
  • If plants do not receive enough water the cells cannot remain rigid and firm (turgid) and the plant wilts.