Diffusion and Osmosis Chapter 3, Section 4 Of your textbook

Passive Transport  Particles are constantly in motion, colliding and scattering. This motion is random. This motion is random.  Does not require the cell to use ATP / energy  The membrane is still semi-permeable, so only certain substances can cross (in or out) by passive transport.

Passive Transport  A concentration gradient is the difference in the concentration of a substance from one location to another  When there is a concentration gradient, the NET movement is DOWN their concentration gradient - from areas of higher concentration to areas of lower concentration ANIMATION

Diffusion  Movement of molecules in a fluid or gas from an area of higher concentration to an area of lower concentration ANIMATION

Equilibrium  Diffusion continues until the solution is at equilibrium (no concentration gradient).  Molecules still move, but there is no NET movement.

Facilitated Diffusion  Some small, polar molecules cannot easily diffuse across a membrane.  They need the help of transport proteins.

Facilitated Diffusion  The process of diffusion being “helped” by transport proteins is called facilitated diffusion.  Still passive transport because molecules are still moving down their concentration gradient  Requires no ATP / energy  ANIMATION ANIMATION

Transport Proteins  In facilitated diffusion, transport proteins, pierce the cell membrane and allow openings for molecules to pass.  These proteins are considered integral proteins because they are fully embedded in the membrane.

Transport Proteins  There are many types of transport proteins.  Most only allow certain ions or molecules to pass.  Some are simple channels or tunnels and some are more complex, shape- changing proteins.

Osmosis  The diffusion of water across a semipermeable membrane is called osmosis.  In a solution, there are water molecules and dissolved particles (the solute).  The more dissolved particles there are, the lower the concentration of water molecules. ANIMATION

Solutions  Water is considered the solvent.  The substance(s) dissolved in water is / are the solute(s).  Together, solvent + solute  solution.

Comparing Solutions  A solution may be desribed as isotonic, hypertonic or hypotonic relative to another solution  These are comparisons; they require a point of reference (ie, my hair is shorter… …than it was last year).  The comparison in biology is usually to the inside of a cell.

Isotonic  A solution is isotonic to a cell if it has the same concentration of dissolved particles as the cell.  This means the water concentration is also the same.  Water molecules move into and out of the cell at an equal rate in an isotonic solution.  The cell size remains the same.

Hypertonic  A hypertonic solution has a higher concentration of dissolved particles than a cell.  This means the water concentration is lower than that of the cell.  Thus, water flows out of the cell – so, the cell will shrivel and eventually die. ANIMATION

Hypotonic  A hypotonic solution has a lower concentration of dissolved particles than a cell.  Therefore the water concentration is higher than that of the cell.  Thus, water diffuses into the cell – causing the cell to expand and potentially burst.

Impact on Cells  In an isotonic solution (center), water enters / exits red blood cells at equal rates.  In a hypertonic solution (like salt water – right), water rushes out and the cell shrivels.  In a hypotonic solution (like distilled water – left), water rushes in and the cell swells / bursts (lysis). Video clips: RBC in isotonic solutionisotonic RBC in hypertonic solutionhypertonic RBC in hypotonic solutionhypotonic

Adaptations - Plants  Plant cells use the cell wall to prevent bursting. At center, the plant cell is in an isotonic solution. Water moves in / out at equal rates (no NET movement) At left, the plant cell is in a hypotonic solution. Water rushes in, filling the vacuole. This cell is turgid / has high turgor pressure. At right, the plant cell is in a hypertonic solution. Water rushes out of the cell, draining the vacuole. This is called plasmolysis. Video: Elodea in isotonic / hyper / hypoisotonichyperhypo

Adaptations - Protists  Paramecia live in freshwater  This makes paramecia hypertonic to their surroundings  Water is constantly rushing into the paramecium  So the paramecium uses a contractile vacuole to pump the water back out (and prevent bursting ) Video: The contractile vacuole in actionThe contractile vacuole in action