ATP-dependent mechanics of red blood cells

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ATP-dependent mechanics of red blood cells, Timo Betz, Martin Lenz, Jean-Francois Joanny, and Cecile Sykes., PNAS vol.106 no.36 (2009) [1]


The fluctuations of the membrane of red blood cells is studied using an optical deflection method similar to that used to detect the position of trapped colloids in an optical trap. Membrane fluctuations observed in the absence of ATP are regarded as occurring passively - that is they are thermal in nature. The frequency dependence of these thermal fluctiations is compared with membrane fluctuations occurring in the presence of APT, and a significant deviation from equilibrium statistics os observed.

Soft Matter

Figure 1

The basic experimental setup for this study is shown is Figure 1. A tightly focused laser beam is incident on the edge of a red blood cell immobilized on a wall of a well formed by two cover slips. The scattered light is picked up (in "transmission mode") with a quadrant photodiode (QPD). This is same manner in which the position of optically trapped microparticles is usually measured. The QPD consists of 4 separate photodiodes, but really only 2 are needed for this experiment. The relative amount of signal in one "side" of the QPD is proportional to the position of the scattering object, within some linear regime which is found from a simple calibration. Though the authors do not say explicitly that they take the differential signal of the two sides of the QPD, this is how it is generally done in an optical tweezer setup. It is possible that the QPD signal used is simply a sum over all four quadrants. Whatever the case, the important result is that the position of the membrane can be measured to sub-nanometer precision and with temporal resolution down to 10 <math>\mu s</math>.

Figure 2

The power spectral density (PSD = <math>|\tilde{x}|^2</math>, where <math>\tilde{x}</math> in the fourier transform of a time series of membrane fluctuations) of the membrane fluctuations is the basic type of data obtained using this setup. An example of such a curve is shown in figure 2.