Diffusion, Osmosis, and Tonicity Notes

Membrane Transport

The plasma membrane acts as a selective gateway, regulating the entry and exit of substances. It's primarily composed of a phospholipid bilayer with:

  • Hydrophilic polar heads facing the external and cytoplasmic surfaces.
  • Hydrophobic fatty-acid tails sandwiched in between.

This structure makes the membrane selectively permeable, allowing only certain substances to pass through easily. Transport occurs via:

  • Passive processes (diffusion, osmosis, filtration): driven by concentration/pressure gradients; require no ATP.
  • Active processes (active transport, vesicular transport): driven by energy (ATP).

Membrane Permeability

Membrane permeability depends on a solute's:

  • Size
  • Polarity (charge)
  • Lipid solubility

Small, uncharged, nonpolar, lipid-soluble molecules pass most easily through the membrane's lipid core.

Diffusion

Diffusion is the random movement of molecules from an area of higher concentration to an area of lower concentration (down the concentration gradient). Examples include the smell of perfume diffusing in the air and a teabag diffusing in hot water.

Factors affecting diffusion rate:

  • Temperature: Higher temperature, higher motion of particles, higher rate.
  • Pressure: Higher pressure, higher rate.
  • Molecular weight: Larger molecules move slower.
  • Steepness of concentration gradient: Greater difference, higher rate.
  • Membrane surface area: Greater area, higher rate.
  • Membrane permeability: Higher permeability, higher rate.

Osmosis

Osmosis is a special case of diffusion focusing on solvent (water) movement. It occurs when water molecules diffuse through a selectively permeable membrane from an area of higher water concentration to an area of lower water concentration. Water diffuses both ways, but solute cannot.

  • Osmotic pressure: The amount of hydrostatic pressure required to stop osmosis.
  • Osmolarity: Number of osmoles of solute per liter of solution. Physiological solutions are expressed in milliosmoles per liter (mOsm/L). Blood plasma = 300300 mOsm/L.

Tonicity

Tonicity is the ability of an extracellular solution to make water move into or out of a cell by osmosis. It's related to osmolarity (total solute concentration).

  • Isotonic: Same solute concentration as the cell. No net water movement.
  • Hypotonic: Lower solute concentration than the cell. Water moves into the cell.
  • Hypertonic: Higher solute concentration than the cell. Water moves out of the cell.
Effects of Tonicity on Red Blood Cells (RBCs)
  • Hypotonic solution: Cell gains water and ruptures (hemolysis).
  • Hypertonic solution: Cell loses water and shrinks (crenation).
  • Isotonic solution: Cell retains normal size and shape.

0.9%0.9\% normal saline solution is isotonic to human cells.

Tonicity in Plant vs. Animal Cells
  • Animal Cells
    • Isotonic: No net water movement.
    • Hypotonic: Water enters, cell may burst (lysis).
    • Hypertonic: Water leaves, cell shrivels (crenation).
  • Plant Cells
    • Isotonic: No net water movement.
    • Hypotonic: Vacuoles fill with water, turgor pressure develops.
    • Hypertonic: Vacuoles lose water, cytoplasm shrinks (plasmolysis).