Cellular Tonicity and Osmosis

Diffusion

  • Diffusion is the movement of molecules from an area of high concentration to an area of low concentration.
  • Cells do not expend energy (ATP) for diffusion; it relies on natural kinetic energy.
  • Diffusion is a type of passive transport.

Osmosis

  • Osmosis is the diffusion of water across a selectively permeable membrane.
  • Water moves from an area of high water concentration to an area of low water concentration.

Tonicity

  • Tonicity indicates the effect of a solution on the osmotic movement of water.
  • It's an indicator of the solute concentration in a solution.
  • The terms used describe solutions relative to each other.

Solution Types Described by Tonicity

Isotonic

  • "Iso" means equal.
  • Isotonic solutions have equal solute concentrations.

Hypotonic

  • "Hypo" means low concentration.
  • A hypotonic solution has a lower solute concentration compared to another solution.

Hypertonic

  • "Hyper" means high concentration.
  • A hypertonic solution has a greater solute concentration compared to another solution.
  • When you're hyper, you're high energy. So hypertonic.

Hypertonic Solutions

  • Hypertonic solutions have a greater solute concentration compared to another solution.
  • Example: A beaker with 5% NaClNaCl (sodium chloride) and 95% water compared to a red blood cell with 3% NaClNaCl and 97% water. The beaker solution is hypertonic to the red blood cell.
  • Water moves from high to low concentration; in this case, water leaves the cell.
  • In a hypertonic solution, the cell loses water and shrinks (crenates).

Hypotonic Solutions

  • Hypotonic solutions have a lower solute concentration compared to another solution.
  • Example: A beaker of 1% NaClNaCl and 99% water compared to a red blood cell with 3% NaClNaCl and 97% water. The beaker solution is hypotonic to the red blood cell.
  • Water moves into the cell because the water concentration is higher outside the cell.
  • The cell swells and may burst (lyse).
  • Drinking excessive water can lead to a hypotonic state, causing red blood cells to burst. Be cautious when drinking more than four liters a day and balance electrolytes.

Isotonic Solutions

  • Isotonic solutions have equal solute concentrations compared to another solution.
  • Example: A beaker with 3% NaClNaCl and 97% water, and a red blood cell with the same concentration.
  • Water moves in and out at the same rate; there is constant flux, but no net change.

Effects on Animal Cells

  • Hypotonic Solution: Animal cells lyse (explode).
  • Hypertonic Solution: Animal cells crenate (shrivel).
  • Hemolysis: The lysis of red blood cells, specifically the lysis of hemoglobin.

Visual Example with Red Blood Cells

  • Hypotonic Environment (distilled water): Cells swell and burst.
  • Hypertonic Environment (concentrated salt solution): Cells shrink and shrivel (crenate).

Implications for Animal Cells

  • Hypotonic Solution: Cells swell with water.
  • Isotonic Solution: Equal movement of water in and out.
  • Hypertonic Solution: Water is pulled out, causing the cell to shrink.

Plant Cells

  • Plant cells thrive in hypotonic solutions.
  • Turgid: When plant cells are full of water; the preferred, normal state; cell vacuole is full and firm.
  • Flaccid: Equal movement in and out; not crisp but not limp.
  • Plasmolyzed: In a hypertonic solution, the cell wilts.

Beaker Examples

  • Three beakers: distilled water, 10% sucrose, and 2% sucrose.
  • A bag of 2% sucrose solution is placed into each beaker.
    • Distilled water: The bag swells because water moves from high to low concentration.
    • 10% Sucrose: The bag loses water to the hypertonic solution.
    • 2% Sucrose: Movement in equals movement out.

Plant Cell Details

  • Plant cells have a rigid cell wall.
  • Turgor Pressure: Firmness or tension due to the vacuole being full in a hypotonic environment.
  • Plasmolysis: The plasma membrane pulls away from the cell wall in a hypertonic environment due to water loss.

Practice Examples

  • High water concentration/low sugar (hypotonic) vs. sugar (hypertonic): Water moves from hypotonic to hypertonic.
  • Hypertonic Medium: Water moves out of the cell.
  • Hypotonic Medium: Water moves into the cell, causing it to swell.