Biomedical Engineering Reference
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FIgurE 5.5 Insertion geometry of amphiphilic dye into the plasma membrane. The dye molecule is perfused
extracellularly (top side) leading to insertion of the dye molecule into the outer lipid leaflet. Notice the alignment
of the embedded molecules about the axes perpendicular to the membrane plane. (With kind permission from
Springer Science+Business Media: Cell Biochem Biophys , Cell imaging and manipulation by nonlinear optical
microscopy, 2006b, 45 , 289-302, Sacconi, L. et al.)
Significant energy is required to flip a molecule from one leaflet of the bilayer to the other, since such
passage requires traversing of the polar head group through the nonpolar membrane interior. Such a
process, although kinetically slow, does occur and is known as “flip-flop.” Figure 5.6 shows two-photon
fluorescence (TPF) and SHG images of a cell stained with a membrane HRS dye (RH 237). It is apparent
from the TPF image that some dye molecules have been internalized into the cell cytoplasm (due to
flip-flop, followed by diffusion of the dye inside the cell). The internalized dye molecules are randomly
(a)
(b)
FIgurE 5.6 Membrane contrast of SHG signal in living cell. SY5Y cell labeled with RH237 membrane dye. (a)
Two-photon fluorescence (TPF) image. (b) SHG image. (With kind permission from Springer Science+Business
Media: Cell Biochem Biophys , Cell imaging and manipulation by nonlinear optical microscopy, 2006b, 45 , 289-302,
Sacconi, L. et al.)
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