Osmosis & Tonicity (Passive Transport)

Osmosis: the diffusion of water across a semi-permeable membrane

  • Passive Transport

  • water is a small, uncharged, polar molecule ——> only a little bit can pass through the membrane (simple diffusion)

Aquaporins: channel proteins that are specific to water ——> allow for greater volumes of Osmosis to occur (facilitated diffusion)

Solution = solvent + solute

Solvent: does the dissolving

Solute: what’s being dissolved

NET direction of water movement in Osmosis: water moves from a low concentration of solute to a high concentration of solute

  • high concentration of solute = low concentration of solvent (water)

  • low solute concentration = high solvent concentration (water)

Tonicity: ability of a solution to make water move into or out of a cell by Osmosis

Isotonic: same solute concentration ——> A is isotonic to B if A has the same solute concentration as B

  • B is also isotonic to A

  • same solvent concentration too

  • NO NET MOVEMENT of water molecules (same solvent concentration ——> Dynamic Equilibrium)

Hypertonic: higher solute concentration ——> A is hypertonic to B if A has a higher solute concentration compared to B

  • B is hypotonic to A

  • NET MOVEMENT of water molecules into solution A

Hypotonic: lower solute concentration ——> A is hypotonic to B if A has a lower solute concentration compared to B

  • B is hypertonic to A

  • NET MOVEMENT of water molecules into solution B

Water always moves toward the HYPERtonic space

  • Cell Walls make plants behave differently than animal cells

Animal Cells:

  • In isotonic solution: behave normally ——> NO NET MOVEMENT of water, so the cell stays the same size

  • In hypotonic solutions: NET MOVEMENT of water into the cell ——> bc cell is hypertonic compared to the solution ——> cell will swell up/burst: lyse

  • In hypertonic solution: NET MOVEMENT of water out of the cell ——> bc cell is hypotonic compared to the solution ——> cell will shrink/shrivel: crenate

Plant Cells:

  • In isotonic solution: NO NET MOVEMENT of water (flaccid) ——> w/o net diffusion of water into the cell, it will start to wilt ——> bc there is no turgor pressure to keep it upright

  • In hypotonic solution: turgid (normal) bc water diffuses into the cell

    -Turgor Pressure: pressure from cell swelling w/in the cell wall (cell membrane can’t spread bigger than the wall) ——> keeps plants upright/prevents wilting

  • In hypertonic solution: cells will plasmolyze ——> membrane shrivels up & pulls away from the cell wall as the cell loses water

Amoeba and Paramecium:

  • Amoeba & Paramecium are unicellular organisms that live in freshwater (hypotonic environments)

  • Water will diffuse into the cell from the freshwater/hypotonic environment —> -must have some way to remove excess water to avoid bursting

  • Contractile Vacuole: collects & expels water from the cell to maintain an internal osmotic balance

    -relates to the Homeostasis process of life ——> maintaining internal water condition/stable water concentration

Medical Applications of Osmosis:

  • IV Fluids & organ transplant fluids must be isotonic to the human body cells to prevent damage to the patient/organ

  • Must be isotonic: we are animals & don’t have cell walls ——> our cells would be damaged in a hypertonic/hypotonic solution

  • in hyper: our cells would shrivel

  • in hypo: our cells would swell up & burst (lyse)