Lec 2 - Membrane Transport Flashcards
What is the difference between oxygen transmissibility and oxygen permeability of a lens?
Oxygen permeability = diffusivity x solubility
Oxygen Transmissibility =Lens quality and corneal oxygen deprivation is better
determined by oxygen transmissibility
-Thicker lens have greater resistance to oxygen flow despite same permeability
-Oxygen transmissibility: permeability/ thickness of lens (Dk/ t)
Why partition coefficient of the white matter is greater than partition coefficient of the gray matter?
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How will tonicity of oral fluid change if an isotonic saline is applied into the oral cavity? onto the eye?
“isotonic” saline = .9% NaCl or 150 mM NaCl
- Saliva(80-130 mOsm) is hypotonic to blood plasma (285-295 mOsm). So placing .9% NaCl (which is isotonic to blood plasma) will increase the tonicity of oral fluid (saliva + saline soln –> hypertonic)
- Tears are isotonic to blood plasma so isotonic saline will not change the tonicity of the eye.
How is tonicity different form osmolarity?
Osmolarity of a solution reflects its chemical concentration, while tonicity of a solution reflects its effect on the volume of submerged cells.
Osmolarity = chemical concentration Tonicity = cell volume
If saliva is hypotonic, why do we normally not observe swelling of the oral mucosa?
The epithelial cells (stratified squamous) are water impermeable and therefore present a barrier to osmosis
What are the differences between active and passive transmembrane transport?
- Passive transmembrane transportdoes not require ATP to transport molecules. Instead, the molecules are carried across the membrane down/with their concentration gradient
- Active transportrequires ATP to transport molecules across the membrane against their concentration gradient. It is unidirectional and there are two types - primary and secondary systems
What are the properties of carrier mediated transport?
- Specificity
- Competition (if substrates are structurally similar)
- Saturation (when carriers are bounded by substrate-Tmax)
If cocaine has partition coefficient 4.5 will it be more potent as local anesthetic then procain? bupivacaine?
Substance Partition Coefficient(Ki)
Glucose 0
Procaine 0.6
Lidocaine 2.9
Bupivacaine 27.5
Cocaine is a more potent local anesthetic than procaine but less potent that bupivacaine
Oil/gas partition coefficient of halothane, an inhalation anesthetic, is similar to the oil/water partition coefficient of lidocain (2.9). Will the blood/gas partition coefficient of halothane be greater or smaller than 2.9?
- Smaller
- Blood/gas partition coefficient will be less than oil/gas partition coefficient for halothane because if halothane penetrates oil easily, then it will not penetrate blood as easily.
Gases diffuse according to their partial pressures (not according to their concentration). If Pf and Pb are partial pressures of oxygen at the front and back surfaces of the lens, Flick’s law for oxygen diffusion through a unit of area of a lens can be written as:
a) J= (Pf - Pb) x oxygen permeability
b) J= (Pf - Pb) x oxygen transmissibility
c) J= (Pb - Pf) x oxygen permeability
d) J= (Pb - Pf) x oxygen transmissibility
b) J= (Pf - Pb) x oxygen transmissibility
A patient still responds with pain to an initiation of a dental procedure 20 minutes after injection of a regular dose of a fresh local anesthetic. What would be the most likely explanation and the best action?
a) The area for diffusion of the anesthetic is insufficient, try to apply more anesthetic on the surface around the tooth.
b) the gradient of concentration of the anesthetic is not sufficient for its diffusion in tissue, a dose of more concentrated anesthetic is needed
c) The patient is not sensitive to local anesthetics, be ready to apply an inhalation narcosis
d) The diffusion distance from the focus of injection to an active nerve fiver is too long, apply another shot of the drug into the tissue nearby
e) perfusion of the area of needle insertion is compromised by local vasoconstriction.
d) The diffusion distance from the focus of injection to an active nerve fiver is too long, apply another shot of the drug into the tissue nearby
What is the importance of the distance between the site of injection of local anesthetics and the location of the sensory nerves (alveolar n., mental n, etc.)?
Diffusion slows very fast with distance. If the injection is put 1 cell away from the other, the time it will take is 1 ms, whereas if it was 100 cells away it would take 10,000 ms. This is why injections should be placed very close to the nerve of interest.
Ficks Law is J= net flux = Di (Delta C/ Delta x) x A. What is each variable?
Di = diffusion coefficient (proportional to temperature and inversely proportional to size of a molecule MW)
Delta C = magnitude of the concentration gradient
Delta x = thickness of the barrier (inversely proportional)
A = area of diffusion
In addition to solubility in lipids, affects local anesthetics?
Nerve fiber size, type, and myelination
Vasodilator/vasoconstrictor properties
pH (an acidic environment will antagonize the effect)
Local anesthetics are weak _____ and in a solution exist in 2 forms: Neutral and ionized.
Bases
Which form permeates faster across the cell membrane/
Neutral lipid form
Which pH will delay the action of local anesthetics?
Low pH (acidic)
Which substance was used as the first local anesthetic?
Cocaine
What is osmosis?
Movement of H20 across a semipermeable membrane (ex: permeable to solvent but not solute) from a region of low solute concentration to a region of high solute concentration.
Explain the different types of solutions.
Isosmotic - equal osmotic pressure pi
Hyperosmotic - higher pi, pulls water
Hyposmotic - lower pi, gives water
Osmotic pressure is directly proportional to the _______________ of solution and solute concentration.
temperature
What are osmoles?
number of moles of a substance times number of particles into which it dissociates.
Which of the following would produce the same osmotic pressure as a solution of 150 mM NaCl?
a) 300 mM glucose
b) 150 mM CaCl2
c) 300 mM KCl
d) 300 mM MgCl2
e) 150 mM urea
a) 300 mM glucose
Glucose doesn’t dissolve in water. Since the osmolality of 150 x 2 particles = 300 mM and glucose doesn’t dissolve, then both would have the same osmotic pressure = 300 mM.
What is osmotic pressure of a solution determined by?
The number of particles in solution
- ) If a solute does not dissociate in solution then one gram molecular weight of the substance is one osmole
- ) If a solute dissociates in solution then the number of osmoles produced by one gram molecular weight (1 mole) of the solute will be equal to the number of particles into which the solute dissociates ex: 1 mol NaCl = 2 osmoles and 1 mole CaCl2 = 3 osmoles
What is tonicity?
Ability of a solution to change the cell volume or “tone” (ex: RBCs). It depends on the number and type of solutes (only non-penetrating solutes count) and determined by the volume of the cell upon achievement of “new” osmotic equilibrium (cell volume is stable)
What is an isotonic solution and give examples?
285-300 mOsm of non-penetrating solutes
It is about the same as blood plasma
Ex: Isotonic saline = 0.9% NaCl or 150 mM NaCl
There is no change in cell volume
What is a hypotonic solution and give examples?
Contains less than 300 mOsm of non-penetrating solutes.
Ex: 100 mM NaCl, 0.15 M sucrose
Hemolysis occurs at osmolarities <200 mOsm
Increase in cell volume - water rushing into cell causes swelling
What is a hypertonic solution and give examples?
Contains more than 300 mOsm of non-penetrating solutes
Ex: 200 mM NaCl
Decrease in cell volume - water rushes out of cell into surrounding environment, causes cell crenation
What is the tonicity of sweat?
Hypotonic
What is the tonicity of tears?
Isotonic
What is the tonicity of saliva?
Very hypotonic (80-130 mOsm)
What is the tonicity of gastric juice?
Isotonic (300 mOsm)
What is the tonicity of Urine
50-1200 mOsm ?
Why do the epithelial cells of our oral mucosa not swell?
The epithelial cells (stratified squamous) are water impermeable and therefore present a barrier to osmosis .
Why does our skin not swell when we bathe in rivers?
Skin is water impermeable so water won’t make us swell and lyse.
Is sea water hypertonic?
Yes, Sea water is hypertonic. Sea water is 3.0% NaCl, and isotonic solutions are 0.9% NaCl.
What is the percent of NaCl in sea water?
3.0% NaCl in sea water.
Why does our skin and cornea not shrink in the ocean water?
Skin and some epithelial cells are water impermeable and present a barrier for osmosis. Chlorinated water of swimming pools can transiently impair corneal impermeability. Swimmers can see rainbows and/or halos around lights indicating corneal edema (fluid excess)
Why can contact lens wearers have an increased incidence of “dry eyes”?
Contact lenses can cause an elevated tear film osmolarity, leading to hypertonicity and shrinking or crenation of cells. This increased osmolarity is a result of decreased tear production or increased evaporation.
Rehydration of the cornea requires a therapeutic solution with (higher/lower) osmolarity?
Lower - the eye drops should be hypotonic to the tear film of the dry eyes
What are some causes for increased tear osmolarity?
- Decreased secretion - due to lacrimal gland disease or decreased corneal sensation
- Increased evaporation - large papebral fissure width or meibomian gland dysfunction
What is Fuchs’ endothelial corneal dystrophy and what is used to treat it?
It is a slowly progressing disease where there is edema of the cornea. It is treated by using hypertonic solution of NaCl (5% NaCl ophthalmic drops) - salt water is usually 3% - so these drops are highly hypertonic
