Diffusion, Osmosis & TonicityRemon Wahba, MD

DiffusionIs random motion of moleculesNet movement is from regions of high concentration to regions of low concentration

ORMovement down the concentration gradient

Diffusion

DiffusionConcentration Number of molecules in a given unit of volume

Gradient Physical difference between two regions

Diffusion (continued)Rate of diffusion depends on:Magnitude of the concentration gradient

Permeability of the membrane

Temperature

Surface area of the membrane6-11

Osmosis6-12

OsmosisIs net diffusion of H20 across a selectively permeable membraneH20 diffuses down its concentration gradientH20 is more concentrated where there are less solutesSolutes have to be Osmotically Active i.e. cannot move freely across the membrane

Osmosis

H20 diffuses Down its Concentration Gradient until its concentration is equal on both sides of membrane

Osmosis continued6-14

Osmotic PressureIs the Force that needs to be exerted to stop osmosisIndicates how strongly H20 wants to diffuse

Osmotic PressureIs proportional to Solute ConcentrationThe more the concentration of the solute, the more is the Osmotic Pressure

Molarity & Molality

Molarity & MolalityThe Molecular weight of a molecule is the sum of the Atomic Weights of its atoms

Molecular WeightsNaCl:Na = 23.0Cl = 35.5 = 58.5Glucose:C6 = 12x6 = 72H12 = 1x12 =12O6 = 16x6 = 96 =180

moleAn amount of any compound equal to its molecular weight in grams is called a mole and it contains a fixed number of molecules.

Avogadros number:Number of molecules present in a moleIt is equal to 6.02 X 1023

moleSo one mole of Nacl contains the same number of molecules as one mole of Glucose

(They are different in weight but they contain the same number of molecules).

= Avogadros number

Molarity & MolalityOne molar solution (1.0M) = One mole of solute dissolved in water to make 1L of solutionDoesn't specify exact amount of H20One molal solution (1.0m) = One mole of solute dissolved in 1 L of H2oMeasurement of concentration of solutes in solutionsOsmolality (Osm) is total Molality of a solutionIs the total solute particle concentration

6-16

Molarity & MolalityOsmolality (Osm) is total Molality of a solution Depending on the number of molecules or particles)

NaCl dissociates into Na+ & Cl-

So1.0 molal solution of NaCl yields a 2 Osm solution ( has double the osmolality of 1 molal solution of glucose

Molarity & Molality

TonicityIs the effect of a Solution on the Osmotic Movement of H20

TonicityIsotonic solutions Have Same osmotic pressure as PlasmaE.g. 5% Dextrose & 0.9% NaClHypertonic solutions Have Higher osmotic pressure than PlasmaWater moves to the outside of CellsHypotonic solutionsHave Lower osmotic pressure than PlasmaWater moves to the inside of Cells

Effects of tonicity on RBCsFig 6.11shrink6-19

Regulation of Blood OsmolalityBlood Osmolality is maintained in a narrow range around 300m Osm

In cases of dehydration, Osmoreceptors in Hypothalamus are stimulated leading to:ADH ReleaseWhich stimulates kidneys to absorb H20ThirstTo increase water intake 6-20

ExperimentsSolubility of compounds in Polar and Nonpolar Solvents

Osmosis Across an artificial Semipermeable Membrane

Concentration and Tonicity

Solubility of Compounds in Polar and Nonpolar Solvents

Add 2 ml of water and 2 ml of oil in a test tubeShake and record your observation

Drop few crystals of Potassium Permanganate in the tubeShake and record your observation

Add 1 ml of vegetable oil to the tube Shake and record your observation

Solubility of Compounds in Polar and Nonpolar SolventsAdd some of the lab detergent to the tubeShake and record your observation

Osmosis Across An Artificial Semipermeable Membrane

We will use a dialysis tubing as an example of an artificial semipermeable membrane.

Cut about 2.5 inches of the dialysis tubing

2. Soak it in water until the two layers separate.

3. Open it to create a rectangular sheet

Osmosis Across An Artificial Semipermeable Membrane

4. Place the rectangular sheet on the mouth of a thistle tube and secure it tightly with a rubber band.

5. Fill the thistle tube with a molasses solution

6. Place the thistle tube in a beaker containing plain water

Osmosis Across An Artificial Semipermeable Membrane

7. Mark the upper level of the molasses in the thistle tube

8. Record the change in the level of the molasses in the tube every 15 minutes

9. Record your findings

Concentration and Tonicity

Add a drop of blood to each of the test tubes that contain different concentration of NaCl.

Place a drop of the solution on a glass slide and examine under the microscope

Record your observations

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