You are interested in studying The composition of lipid bilayers and how they ar
ID: 52777 • Letter: Y
Question
You are interested in studying The composition of lipid bilayers and how they are maintained. You discover two uncharacterized phospholipids, which you call PLX and PLZ. You decide to characterize The behavior of PLX and PLZ by labeling The head group of each phospholipid. This label is stable when The lipid resides in The membrane's outer leaflet but unstable when The lipid resides in The membrane's inner leaflet. You incorporate labeled versions of PLX and PLZ into either The inside or The outside of The cell, and monitor The change in signal intensity of these lipids in The plasma membrane. Your data are presented in The graphs in Figure 1. Where in The plasma membrane are PLX and PLZ normally located? Are there flippases in The cell for either of these phospholipids? Why?Explanation / Answer
A) From the data presented above we can determine the following:
In case of PLX, the signal intensity obtained for labeled lipids outside the cell remains constant at 100 % with respect to(w.r.t) time thus indicating its presence on the membrane's outer-leaflet.
The signal intensity however decreases for labeled lipids inside the cell w.r.t time and this is only because the label is unstable in the inner leaflet. This also thus indicates the presence of it in the inner leaflet.
Thus, PLX is present on bothe the outer and inner leaflets of the membrane.
In case of PLZ, we see that the signal intensity remains constant at near 100% for labeled lipids outside the cell thus indicating its presence in the outer leaflet of the membrane.
However its signal intensity also remains constant at near 100% for labeled lipids inside the cell.
This would not have happened if it was actually present on the inner leaflet as the label is unstable inside and intensity should have decreased w.r.t time.
Thus this indicates the presence of PLZ ONLY on the OUTER leaflet of the membrane.
B) Yes there are flippases in the cell but ONLY for PLX.
Flippases are enzymes that help in the movement of phospholipids in between the 2 leaflets of the membrane.
PLX is present in BOTH the leaflets thus indicating that a specific flippase for PLX is distributing it evenly between the 2 leaflets in the membrane.
PLZ is present only on the outer leaflet thus indicating no specific flippase is distributing it evenly on both the leaflets of the membrane.
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