Detailed Notes on Water Movement (Osmosis)

The Plasma Membrane: Composition

  • The plasma membrane has a lipid bilayer acting as a highly impermeable barrier for most charged (polar) & non-lipid soluble substances.

  • Integral proteins function as pores, channels, or carriers, facilitate the transport of these substances across the membrane.

Movement of Water: Osmosis

  • Water molecules cross the membrane either via diffusion thru the lipid bilayer or through aquaporins → specialised trans-membrane proteins that serve as water channels.

  • Osmosis → "the movement of water from a low solute concentration to a high solute concentration across a semi-permeable membrane."

Driving Forces of Osmosis

  • The driving forces behind the movement of water are discussed in terms of the concentration of solutes rather than the water concentration.

    • Water, serving as the solvent, has a very high concentration (about 56 mols).

  • "osmotic pressure" = the solution's drawing power to attract water. Water naturally moves towards the solution with the highest osmotic pressure.

  • therefor, osmosis = "the movement of water from a low solute concentration to a high solute concentration across a semi-permeable membrane."

Key Points on Osmosis

  • Osmosis constitutes a net movement of water through a selectively permeable membrane.

  • This occurs only if the membrane is permeable to water but not to specific solutes, highlighting the importance of the osmotic pressure exerted by solute particles that cannot cross the membrane.

  • Osmotic pressure is proportionate to the number of osmotically active particles in a solution.

Tonicity

  • Tonicity is a term that describes a solution's ability to change cell volume by affecting water content.

    • Isotonic solution: No net movement of water; cells retain their normal shape.

    • Hypotonic solution: Cells gain water, risking swelling and potential bursting (hemolysis).

    • Hypertonic solution: Cells lose water, risking shrinkage and dehydration (crenation).

  • Clinical relevance: Isotonic, hypotonic, and hypertonic solutions are essential for medical applications, particularly in IV fluids.

Demonstrations of Tonicity Using Red Blood Cells

  • When red blood cells are placed in different saline solutions:

    • In an isotonic solution, they maintain normal shape.

    • In a hypotonic solution, they undergo hemolysis.

    • In a hypertonic solution, they undergo crenation.