Four samples of cells have membranes confining a uniform distribution of a fluor

Question

Four samples of cells have membranes confining a uniform distribution of a fluorescently labeled Phospholipid (sample 1) and three fluorescently labeled membrane proteins X, Y, and Z(samples 2-4), respectively. An intense beam of light is shone on each sample. the light beam destroys the fluorescent label in the illuminated area, but the intensity of fluorescence in the area recovers over time, as bleached molecules diffuse away and unbleached molecules diffuse in. The results of this experiment are shown in below. Which of the following interpretations is INCONSISTENT with these results. A, X is a small freely diffusing protein. B, Y is a large freely diffusing protein. C

Explanation / Answer

Choice D is correct.

Reason: Fluorescent excitation of some molecules is a phenomenon where incident light activates the molecule and it attains an excitation state due to gain of energy. As this energy is lost, the molecule bleaches out of fluorescent energy which can be visualized/recorded by the means of loss of fluorescent signal. However, when the molecule recovers its ground state, it must revert back to its orignical state of fluoresnce/light energy. According to the information, fluorescence bleaching of cellular biomolecules is taking place in this example and as the incident beam is switched off, the fluorophore re-gains its energy and re-acquires its fluorescence energy. The graphs 1, 2 and 3 are highly consistent with each other and clearly suggest that these molecules represent cell-membrane bound phopholipids or phospho-proteins which can move freely about the lipid bilayer of the plasma membrane. However, the bio-molecule in the graph 4 does not regain its fluorescence. Although there is no re-attainment in the fluorescent energy of the bio-moleule of graph 4, it does not correspond to the explanation that is might be a cytoskeletal protein or cortical protein by nature. Infact, the trend of fluorescnce diffusion does not correspond to the diffusivity of the molecule but it relates to its photo-active properties which make it undergo excitation and emmit fluorescence. Thus, it is very less likely that the bio-molecule 4 is an un-diffusible type.

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