Osmosis

Diffusion

• Definition: Diffusion is the passive movement of molecules from an area of higher concentration to an area of lower concentration.

• Example: Adding food dye to water.

  • Initial state: High concentration of dye where it's dropped.

  • After a while: Dye spreads evenly throughout the water.

• Mechanism: Molecules are constantly in motion, colliding and changing direction until evenly distributed.

• Concentration Gradient:

  • Definition: An unequal distribution where some areas have higher concentration and some lower.

  • Down a Concentration Gradient: Movement from high to low concentration; requires no energy.

  • Against a Concentration Gradient: Movement from low to high concentration; requires energy (e.g., paddling upstream).

Osmosis

• Definition: Osmosis is a specific type of diffusion referring to the passive movement of water through a semipermeable membrane.

• Mechanism:

  • Occurs along the water's concentration gradient.

  • Involves the movement of water, moving from areas of higher water concentration to lower water concentration.

  • A semipermeable membrane allows some particles, like water, to pass through but blocks others, like solutes.

• Example of Osmosis:

  • Beaker filled with water and solute.

  • No solute on one side, so water moves from that side to the solute side to equalize the concentration of solute.

Membrane Permeability

• Water can cross cell membranes, albeit with difficulty due to its polarity and the hydrophobic fatty acid tails of phospholipids.

• Aquaporins: Special protein channels that facilitate the movement of water across the membrane efficiently.

• Cells can regulate the number of aquaporins to control water permeability.

Tonicity

• Definition: Tonicity describes how an extracellular solution can change a cell's volume by affecting osmosis.

  • Hypertonic: Higher solute concentration outside the cell compared to inside.

  • Isotonic: Equal solute concentration inside and outside the cell.

  • Hypotonic: Lower solute concentration outside the cell compared to inside.

• Relative Nature: Tonicity is always relative to the solute concentration inside the cell.

Effects on Cells

• Isotonic Solution: No net movement of water; equilibrium is maintained.

• Hypertonic Solution: Net movement of water out of the cell, causing cell shrinkage (crenation for animal cells).

• Hypotonic Solution: Net movement of water into the cell, which may lead to cell lysis (bursting) in animal cells but turgidity in plant cells due to their cell wall.

• Turgor Pressure: In plant cells, pressure against the cell wall that maintains structural integrity. Loss of this causes wilting.

Specific Cases for Plant vs Animal Cells

• Animal Cells: Most benefit from isotonic solutions; bursting occurs in hypotonic solutions.

• Plant Cells: Prefer hypotonic solutions; maintain turgor pressure, benefitting from being in a hypotonic environment.

  • Hypertonic: Membrane pulls away from cell wall (plasmolysis).

  • Isotonic: Cells are flaccid without pressure.

  • Hypotonic: Cells are turgid from increased interior pressure, promoting health and upright structure.

• Grocery Store Practice: Spraying produce with water to maintain turgor pressure, thereby preserving crispness and freshness.