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Phophotidyl choline in a lipid monolayer with its ph=3.5 (below its pKa) leads to: (AIIMS May 2008)

A Increases surface tention

B Decreases surface tention

C Decreases dipole movement

D Zero dipole movement

Ans. B Decreases surface tention

a. Lysobisphosphatidic acid (LBPA) can be regarded to represent a unique derivative of phosphatidylglycerol.

b. This lipid is highly enriched in late endosomes where it can comprise up to 10–15 mol% of all lipids and in these membranes, LBPA appears to be segregated into microdomains.

c. We studied the thermotropic behavior of pure dioleoyl-LBPA mono- and bilayers using Langmuir-lipid monolayers, electron microscopy, differential scanning calorimetry (DSC), and fluorescence spectroscopy.

d. LBPA formed metastable, liquid-expanded monolayers at an air/buffer interface, and its compression isotherms lacked any indication for structural phase transitions.

e. Neat LBPA formed multilamellar vesicles with no structural transitions or phase transitions between 10 and 80 °C at a pH range of 3.0–7.4.

f. We then proceeded to study mixed LBPA/dipalmitoylphosphatidylcholine (DPPC) bilayers by DSC and fluorescence spectroscopy. Incorporating increasing amounts of LBPA (up to XLBPA (molar fraction) = 0.10) decreased the co-operativity of the main transition for DPPC, and a decrease in the main phase transition as well as pretransition temperature of DPPC was observed yet with no effect on the enthalpy of this transition.

g. In keeping with the DSC data for DPPC, 1-palmitoyl-2-oleoyl-phosphatidylcholine (POPC)/LBPA mixed bilayers were more fluid, and no evidence for lateral phase segregation was observed.

h. These results were confirmed using fluorescence microscopy of Langmuir-lipid films composed of POPC and LBPA up to XLBPA = 0.50 with no evidence for lateral phase separation.

i. As late endosomes are eminently acidic, we examined the effect of lowering pH on lateral organization of mixed PC/LBPA bilayers by DSC and fluorescence spectroscopy.

j. Even at pH 3.0, we find no evidence of LBPA-induced microdomain formation at LBPA contents found in cellular organelles.


a. Surfactant is a complex substance containing phospholipids and a number of apoproteins. This essential fluid is produced by the Type II alveolar cells, and lines the alveoli and smallest bronchioles.

b. Surfactant reduces surface tension throughout the lung, thereby contributing to its general compliance. It is also important because it stabilizes the alveoli.

c. Laplace’s Law states that the pressure within a spherical structure with surface tension, such as the alveolus, is inversely proportional to the radius of the sphere (P=4T/r for a sphere with two liquid-gas interfaces, like a soap bubble, and P=2T/r for a sphere with one liquid-gas interface, like an alveolus: P=pressure, T=surface tension, and r=radius).

d. That is, at a constant surface tension, small alveoli will generate bigger pressures within them than will large alveoli.

e. Smaller alveoli would therefore be expected to empty into larger alveoli as lung volume decreases.

f. This does not occur, however, because surfactant differentially reduces surface tension, more at lower volumes and less at higher volumes, leading to alveolar stability and reducing the likelihood of alveolar collapse.

g. Surfactant acts to break the surface tension (cohesiveness) of water which would cause collapse of the alveoli during expiration.

h. The problem is the surface tension on the inner surface of the alveoli due to the layer of water there.

Without surfactant to counteract the surface tension a great deal of energy is required for the breathing process. (In expanding the alveoli, energy must be used to break the hydrogen bonds of water.). Surfactant lines the respiratory membrane of the alveoli and works by breaking up and interfering with the hydrogen bonds of water and thus interferes with the cohesiveness of water molecules to reduce the surface tension of the alveolar fluid.

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