Diffusion and Osmosis

Diffusion The passive movement of molecules or particles along a concentration gradient from areas of high concentration to low concentration due to the random thermal energy of those molecules or particles.

Remember... Individual molecules move randomly even if The overall flow has direction. Equilibrium does not mean that all motion ceases, but that there is an equal flow in each direction

Figure 5.9 Molecules of dye Net diffusion WATER (a) Diffusion of one solute Net diffusion Equilibrium (b) Diffusion of two solutes Equilibrium Membrane (cross section)

Many membranes, including the cell membrane, are semipermeable. That is, that allow some things to cross as they undergo random movement but not others.

Let’s look at an example: Dialysis tubing In tubing – Permeable to: -water, starch and Glucose glucose Water In Beaker IKI - water and IKI – Impermeable to starch Indicators: – Starch-IKI – Glucose- Testape

What will happen? Draw the system Indicate with arrows which direction each substance will flow.

Osmosis A type of diffusion The diffusion of free water across a semipermeable membrane This movement and the balance of water between the cell and its environment... crucial.

Osmosis is passive and proceeds from an area of high concentration to one of low concentration. Be careful! The concentration gradient refers to the concentration of free water, NOT solute. It is water that is moving.

In a solution with high solute concentration, water concentration is naturally lower than in a solution with low solute concentration. More solute = less water Less solute = more water Amplified by the fact that ions in solution are hydrophilic and water clusters around them making the water molecules less free to move about.

Figure 5.10 Sugar molecule Lower concentration of solute (sugar) Higher concentration of solute H2OH2O Selectively permeable membrane More similar concen- trations of solute Osmosis

Try these Which direction will water flow?

Try these Which direction will water flow?

Try these Which direction will water flow? 2 M sucrose1 M Sucrose

Try these Which direction will water flow? 2 M sucrose1 M Sucrose

Why is osmosis important to cells? Cells that do not have rigid cell walls depend on maintaining a balance of fluid in/fluid out. Tonicity = The ability of a surrounding solution to cause a cell to gain or lose water. Tonicity depends on both the concentration of the nonpenetrating solutes in the surrounding solution and the permeability of the cell membrane

Terms to know Remember, each is a relative term. An extracellular solution is: – Isotonic, if the concentration of nonpenetrating solutes is the same as inside the cell. There is no net movement of water across the membrane. – Hypertonic, if there is a greater concentration outside the cell. Net movement of water out of the cell. – Hypotonic, if there is a lower concentration outside the cell. Net movement is into the cell.

Figure 5.11 Turgid (normal) Flaccid Lysed Cell wall Normal H2OH2O H2OH2O H2OH2O H2OH2O H2OH2O H2OH2O Plasmolyzed Shriveled H2OH2O H2OH2O HypertonicIsotonic Hypotonic Animal cell Plant cell

Try these: Which direction will water flow? Is the surrounding solution iso-, hyper-, or hypotonic to the cell? 2M 1M

Try these: Which direction will water flow? Is the surrounding solution iso-, hyper-, or hypotonic to the cell? 1M 2M

Try these: Which direction will water flow? Is the surrounding solution iso-, hyper-, or hypotonic to the cell? 1M