Osmosis in Cells (HL IB Biology)

• The direction of the net movement of water will depend on whether a cell is placed in a hypertonic or hypotonic solution
  • In a hypertonic solution there will be a net movement of water out of the cell, as the cytoplasm is more dilute than the outside solution
  • In a hypotonic solution there will be a net movement of water into the cell because now the outside solution is more dilute than the cytoplasm
• In an isotonic solution, the movement of water into the cell will be balanced out by the movement of water out of the cell
  • There will therefore be no net movement of water into or out of the cell
  • The cell is now in dynamic equilibrium with the isotonic solution
  • It is especially important for animal cells to maintain their osmotic concentration as any deviation from this equilibrium may either cause the cell to shrink or burst

• Animal cells lose and gain water as a result of osmosis
• As animal cells do not have a supporting cellulose cell wall, the results on the cell are more severe than on plant cells
• If an animal cell is placed into a hypertonic solution (more concentrated than the cytoplasm of the cell), it will lose water by osmosis and become crenated (shrivelled up)
  • This may lead to the formation of blood clots as crenated red blood cells may become stuck while moving through capillaries
• If an animal cell is placed into a hypotonic solution (more dilute than the cytoplasm of the cell), it will gain water by osmosis and, as it has no cell wall to create turgor pressure, will continue to do so until the cell membrane is stretched too far and it bursts
• Multicellular organisms must therefore maintain isotonic tissue fluid around their cells to prevent these harmful changes from happening

Osmosis in animal cells diagram

The effects of water movement on animal cells

• Some unicellular organisms, such as the protozoan Amoeba, live in freshwater aquatic habitats that is hypotonic to their cytoplasm.
  • There will be a constant net influx of water into the organism by osmosis, which increases the internal pressure 
• To prevent these organisms from bursting, they contain structures called contractile vacuoles in their cytoplasm
  • Excess water will be continuously collected in the contractile vacuole and pumped out of the organism to maintain the osmotic concentration of the cytoplasm

Osmosis in an amoeba diagram

The contractile vacuole is responsible for removing excess water from Amoeba to prevent them from bursting

• If a plant cell is placed in a hypotonic solution, water will enter the plant cell through its partially permeable cell surface membrane by osmosis, as the solution has a lower solute concentration than the plant cell
• As water enters the vacuole of the plant cell, the volume of the plant cell increases
• The expanding protoplast (living part of the cell inside the cell wall) pushes against the cell wall and pressure builds up inside the cell
  • This pressure is known as turgor pressure
  • The inelastic cell wall prevents the cell from bursting
• The pressure created by the cell wall also stops too much water entering and this also helps to prevent the cell from bursting
• When a plant cell is fully inflated with water and has become rigid and firm, it is described as fully turgid
• This turgidity 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

The net movement of water into a plant cell will increase the turgor pressure and result in a turgid cell

• If a plant cell is placed in a more concentrated solution, water will leave the plant cell through its partially permeable cell surface membrane by osmosis
• As water leaves the vacuole of the plant cell, the volume of the plant cell decreases
• The protoplast gradually shrinks and no longer exerts pressure on the cell wall
• As the protoplast continues to shrink, it begins to pull away from the cell wall
• This process is known as plasmolysis – the plant cell becomes flaccid and is said to be plasmolysed

Plasmolysis of a plant cell that has been placed in a solution with a lower water potential than the cell itself

• In some cases, patients may require an intravenous (IV) drip to treat dehydration or to deliver medicine directly into the bloodstream
• It is important that the solution in the IV drip is isotonic in relation to blood plasma
  • The solution is usually a 0.9% sterile saline solution (saltwater) 
  • If the solution was hypotonic then there would be a net movement of water into red blood cells causing them to burst
    • This would result in a decrease in the oxygen carrying capacity of blood
  • A hypertonic IV solution would result in a net movement of water out of the red blood cells causing them to shrivel and become crenated
    • This would increase the risk of blood clots forming as these red blood cells cannot move freely through capillaries
• Another important medical application of isotonic solutions is in the preparation of donated human organs for transplant surgery
  • These organs must be kept in an isotonic saline solution to prevent damage to the cells due to the net movement of water by osmosis