Diffusion

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Diffusion

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• The passive movement of molecules from an HIGHER to a lower equilibrium is reached

• Each substance will diffuse down its own [ ] gradient unaffected by the [ ] of other substances

• Applies to small non-charged molecules --> CO2 & O2 (gases) H2O and alchohol (liquids)

Example

a) When dye crystals (=solute) are placed in H2O (=solvent) they are [ ]'d in 1 area

b) the crystals then diffuse to an area of [lower]

c) Equilibrium is reached when a solute is evenly distributed throughout the solution (sol'n)

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Osmosis

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• The diffusion of H2O across a differentially permeable membrane

• Osmosis direction is determined by [solute] differences of solutions on either side of the membrane

• The osmotic pressure, (OP) gradient forces H2O to move from an area of low [solute] to an area of High [solute]

• Eg. H2O gets absorbed by kidneys and taken up by capillaries in tissues due to OP

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Osmosis in Cell Isotonic solution

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I) Isotonic solution: the [solute] in cells is normally the same as the [Solute] in the extracellular fluid

• The cell is (three dots) isotonic to its surroundings and vice versa

iso =same as

tonicity = the OP or [solute] of the sol'n

• there is NO net osmosis occurring as equilibrium is reached; cell maintain its size

eg. Tonicity ([NaCl]) of RBC's is ~0.9%

Intravenous solutions administrated at hospitals have 0.9% tonicity

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Osmosis in Cell: Hypertonic Solution

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• Hypo = less than

• Animal Cell: when placed in a hypotonic solution H2O will enter faster than it leaves, & the cell will swell & cytosis (bursting) occurs; hemolysis refers to bursting of RBC's

• Fresh H2O spp. Need to prevent insides from hypotonic, eg. Fish kidneys will excrete large volume dilute urine; fish need to take on salt through the gills

• Plant cell: when placed in hypotonic sol'n H2O will enter through osmosis, filling the central vacuole and pushing the PM against the CW (=turgor pressure)

• Bursting is prevented by the elastic CE, which exerts a back-pressure on the cell, offsetting further H2O uptake

• healthy plants are turgid and can stand erect Surroundings are ideally hypotonic to the cells

• if plants are surroundings are isotonic, plants will Wilt

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Transport by Carrier Proteins

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• Recall: Carrier proteins are specialized to bind with a specific solute
• Both facilitated transport & active transport require carrier proteins

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Facilitated Transfer/Diffusion

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• Does not require energy because substances follow there gradient
• Polar molecules ((aa's & glucose) can then diffuse into the cell
• The carrier protein alternates between 2 conformations/shapes
• Solutes can be transported in either direction as long as movement follows the gradient
• Facil diffusion can occur 100 times/sec
• Glucose can be transported 100's of times faster than other sugars
• Speed can be slowed down by imposter molecules that mimic the solute/substrate; these temporarily bind to the carrier protein., inhibiting the actual solute from binding.

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Hypertonic solution

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refers to a sol'n that has a higher [solute] than that of the cell
• hyper = more than
• Animal Cell: when placed in a hypertonic solution will shrivel or crenate; the cell will lose H2O to its envt
• Eg an increase in lake salinity can kill all of the aquatic organisms
• Eg. Putting salt on a slug will cause cremation of its cells
• Marine sharks will excrete excess salt across their gills; marine turtles will "cry" to excrete salt
• Plant cell when placed in a hypertonic sol'n will lose H2O to its surrounding and shrink; as H2O leaves the central large vacuole and cytoplasm, it's PM pulls away from the CE (= plasmolysis)
• Plants plasmolyze in hypertonic sol'ns as their cytoplasm shrinks