DK

Cell Tonicity, Membrane Permeability, and Water Intoxication

Tonicity and Cell Environments

Review of Tonicity

  • Isotonic Solution:

    • Equal concentration of solute both inside and outside the cell.

    • The cell maintains its normal shape, and there is an equal amount of water moving in and out.

  • Hypertonic Solution:

    • Higher concentration of solute outside the cell than inside.

    • Water, acting as a solvent, moves from an area of lower solute concentration (inside the cell) to an area of higher solute concentration (outside the cell).

    • Cells lose water and crenate (shrink).

  • Hypotonic Solution:

    • Higher concentration of solute inside the cell than outside.

    • Water moves from an area of lower solute concentration (outside the cell) to an area of higher solute concentration (inside the cell).

    • Cells take on water, get very large, and if the solution is very hypotonic, they can lyse (explode) like a balloon. A mnemonic to remember this is that in a "Hypotonic" solution, cells get "real big" like a balloon.

Membrane Permeability Through a Phospholipid Bilayer

  • Question: Which molecules can move through a phospholipid membrane with the least difficulty?

  • Factors for Easy Permeability: To easily pass through a phospholipid membrane without assistance, molecules must be:

    • Small

    • Nimble

    • Nonpolar

  • Examples of Molecules and Their Permeability:

    • Glucose: Large, difficult to pass.

    • Amino Acid: Large, difficult to pass.

    • Sodium ion ( \text{Na}^+ ): Small but charged, making it difficult to pass.

    • Oxygen ( \text{O}_2 ): A nonpolar gas, highly permeable.

    • Water ( \text{H}_2\text{O} ):

      • Composed of oxygen and two hydrogen atoms. Oxygen is highly electronegative, leading to electrons spending more time around it. This creates a slight negative charge near the oxygen and slight positive charges near the hydrogens, making water a polar molecule.

      • Despite being small and ubiquitous, water has only moderate permeability through the phospholipid bilayer due to its polarity. It is not as easy for water to pass directly through as nonpolar gases.

  • Highly Permeable Molecules (Small, Nonpolar): Examples include carbon dioxide ( \text{CO}2 ), nitrogen ( \text{N}2 ), oxygen ( \text{O}_2 ), and ethanol.

Britney's Case: Ecstasy, Water, and Hyponatremia

  • Background: Britney consumed ecstasy and drank a significant amount of water. Normally, kidneys would respond to high water intake by producing large amounts of dilute urine to expel excess water.

  • Effect of Ecstasy: Ecstasy acts as an antidiuretic.

    • It forces the kidneys to produce concentrated urine instead of dilute urine.

    • This prevents the body from effectively getting rid of excess water.

    • The inability to excrete excess water causes the body's electrolyte concentrations, particularly sodium, to fall.

  • Impact on Britney's Cells:

    • Drinking a lot of water results in a lower solute (salt) concentration outside her cells compared to inside the cells.

    • Therefore, Britney's cells are in a hypotonic environment.

  • Water Movement into Cells:

    • While water has moderate permeability through the lipid bilayer, the majority of water molecules enter cells via specialized transport proteins called aquaporins.

    • Aquaporins: These are channel proteins specifically designed for water transport. The discovery of aquaporins by Peter Agre earned him a Nobel Prize.

    • This movement through aquaporins is a form of facilitated diffusion.

    • Facilitated Diffusion: Water still moves down its concentration gradient (from an area of lower solute concentration to higher solute concentration), but it requires a transport protein (aquaporin) to "facilitate" its passage across the membrane. It is not active transport; it does not require energy input against the gradient.

  • Expected Outcome for Britney's Cells:

    • In a hypotonic environment, with aquaporins actively transporting water, the overall net movement of water will be into the cells.

    • There is still dynamic equilibrium, meaning water moves in both directions, but more water moves into the cell than out.

  • Consequences in the Body: Water Intoxication and Hyponatremia

    • When cells in the body are exposed to a hypotonic environment, they swell and can lyse, which is detrimental.

    • Hyponatremia: This is the clinical term for low sodium ( \text{Na} ) concentration in the blood (hypo = low, Na = sodium, emia = in the blood). It signifies a hypotonic environment for the cells.

    • Treatment for Hyponatremia: Involves administering intravenous (IV) fluids with normal or slightly higher sodium concentrations to correct the salt imbalance in the tissues.

    • Acute Hyponatremia and Water Intoxication: Rapid onset hyponatremia can lead to water intoxication, a real and dangerous condition where tissue swelling occurs.

    • Vulnerability of the Brain: The brain is particularly susceptible because it is enclosed within the rigid skull and has very little room to swell. If brain cells swell in a hypotonic environment, the increased pressure can damage brain tissue.

  • Britney's Tragic Fate:

    • Britney died in 2001 after taking ecstasy and drinking large amounts of water. The treatment she received was insufficient.

    • Her initial lab results showed a slightly elevated heart rate and blood pressure, a slightly high temperature, and normal glucose levels, suggesting cellular issues rather than overt systemic organ failure.

  • Other Cases:

    • This phenomenon of ecstasy-related hyponatremia and water intoxication has led to other deaths, including an Australian individual.

    • Matthew Carrington died in February 2005 during a fraternity hazing incident, likely due to forced water consumption.

    • Jennifer Strange died in Sacramento after participating in a water-drinking contest, demonstrating the real-world danger of water intoxication.