Cell membrane capacitance

Neher, E. and Marty, A. Discrete changes of cell membrane capacitance observed under conditions of enhanced secretion in bovine adrenal chromaffin cells. Proc. Natl Acad. Sci. U.S.A. 79 6712-6716,1982. 

An electric double layer covers die wetted outer cell membrane surface. The total cell has a net charge revealed by its electrophoretic mobility. The cell membrane capacitance with the thickness of approximately 7 nm is on the order of 1 pF/cm. The frequency dependence of the membrane capacitance has been a subject of dispute (Cole, 1972), but the BLM as such is often considered to have a frequency-independent membrane capacitance. 

Therefore, measurement of both the crossover frequency and cell radius can be used to calculate an average membrane capacitance. Values of cell membrane capacitances per unit area range from 0.7 to over 2 pF/cm indicating that the DEP crossover frequencies can differ substantially - one of the reasons why DEP has been used to fractionate different cell types. Measurement of the cell capacitance is also used to predict the DEP response of different cell populations and aid in the design and operation of DEP-based cell separation devices [10]. 

Impedimetric biosensors for whole cells have demonstrated two mechanisms in response. Considering the overall impedance of a biological cell as including the resistance and the capacitance of the cell membrane, the presence of intact cell membranes on the electrodes would contribute to the sensor s capacitance and/or resistance, and would determine the current flow and thus the sensor signal. However, when cells are attached to the electrode surface, they are usually separated by a gap of 10 -20 run (up to several hundred nanometers). This aqueous gap between the cell membrane and the electrode surface prevents a direct influence of the cell membrane capacitance on the impedance of the electrode. Therefore, the cell membrane resistances of these attached cells act as resistors on the IME surface and affect the interfacial resistance. The interfacial resistance is best represented as electron transfer resistance in the presence of a redox probe (e. g., [Fe(CN)6] / ) and can be sensitively monitored. Figure 10a presents a representative group of Nyquist plots of the impedance spectroscopic responses of an IME-based biosensor to different cell numbers of E. coli 0157 H7 at 10a antibodies, 10b 4.36 x 10 CFTJ/ml,... 

For typical parameters values at 25 °C, = cell membrane capacitance... 

The electrophysiological technique used to measure changes in membrane capacitance is the patch clamp [5,6] in the whole-cell recording mode, where the plasma membrane patch in the pipet is ruptured. In another configuration of the patch clamp, the plasma membrane patch is maintained intact. In this case, small currents due to the opening of individual channels can be measured in the membrane patch. The whole-cell patch clamp... 

Rat peritoneal mast-cell exocytosis (as monitored by membrane capacitance measurements) in response to either antigenic stimulation or to the intracellular perfusion with guanine nucleotides (for example, GTP[AS]), occurs after a measurable lag period which has been suggested to be due to the involvement of a GTP-binding regulatory protein [202]. In contrast, stimula-... 

Ong W, Jiang B, Tang N, Ling S, Yeo J et al. (2006) Differential effects of polyunsaturated fatty acids on membrane capacitance and exocytosis in rat pheochromocytoma-12 cells. Neurochem Res 31 41-8... 

On the air side calibrated thermocouples to measure temperature at different locations in the cell two capacitive hygrometers on the evaporator a humidity controller designed to deliver a maximum vapor flow rate of lOkg/h have been installed. A liquid flowmeter to measure the volumetric flow rate of the aqueous desiccant solution at the membrane contactor outlet was used. All instruments are connected to a data logger. 

In these equations C and G are capacitance and conductance per cm2 of the cell membrane, R "is the cell radius, p the cellular volume fraction, O = 1/p, and O = 1/p are the conductivities of the cell interior and suspending medium. The equations apply for small volume fractions p and assume that the radius of the cell is very large compared with the membrane thickness. More elaborate closed form expressions have been developed for cases when these assumptions are no longer valid (23, 24) and an exact representation of the suspension dielectric properties as a sum of two dispersions is available (25). If, as is usually the case, the membrane conductance is sufficiently low, equations (2)-(5) reduce to the simple forms to the right of the arrows. 

If f<total potential difference applied across the cell is developed across the membrane capacitance. In this limit, the induced membrane potential AV across a spherical cell is AV = 1.5 ER, where E represents the applied external field. Thus the cell samples the external field strength over its dimensions and delivers this integrated voltage to the membranes, which is a few mV at these low frequencies for cells larger than 10 ym and external fields of about 1 V/cm. These transmembrane potentials can be biologically significant. 

Here, w = m, n, and S. V represents the membrane potential, n is the opening probability of the potassium channels, and S accounts for the presence of a slow dynamics in the system. Ic and Ik are the calcium and potassium currents, gca = 3.6 and gx = 10.0 are the associated conductances, and Vca = 25 mV and Vk = -75 mV are the respective Nernst (or reversal) potentials. The ratio r/r s defines the relation between the fast (V and n) and the slow (S) time scales. The time constant for the membrane potential is determined by the capacitance and typical conductance of the cell membrane. With r = 0.02 s and ts = 35 s, the ratio ks = r/r s is quite small, and the cell model is numerically stiff. The calcium current Ica is assumed to adjust immediately to variations in V. For fixed values of the membrane potential, the gating variables n and S relax exponentially towards the voltage-dependent steady-state values noo (V) and S00 (V). Together with the ratio ks of the fast to the slow time constant, Vs is used as the main bifurcation parameter. This parameter determines the membrane potential at which the steady-state value for the gating variable S attains one-half of its maximum value. The other parameters are assumed to take the following values gs = 4.0, Vm = -20 mV, Vn = -16 mV, 9m = 12 mV, 9n = 5.6 mV, 9s = 10 mV, and a = 0.85. These values are all adjusted to fit experimentally observed relationships. In accordance with the formulation used by Sherman et al. [53], there is no capacitance in Eq. (6), and all the conductances are dimensionless. To eliminate any dependence on the cell size, all conductances are scaled with the typical conductance. Hence, we may consider the model to represent a cluster of closely coupled / -cells that share the combined capacity and conductance of the entire membrane area. 

Channel activity is best studied electrochemically as charged species cross a cell membrane or artificial lipid bilayer. There is a difference in electrical potential between the interior and exterior of a cell leading to the membrane itself having a resting potential between -50 and -100 mV. This can be determined by placing a microelectrode inside the cell and measuring the potential difference between it and a reference electrode placed in the extracellular solution. Subsequent changes in electrical current or capacitance are indicative of a transmembrane flux of ions. 

To apply Equation 3.3 to a specific situation, let us consider a spherical cell with a radius (r) of 30 pm and an electrical potential difference (A ) across the membrane of —100 mV (inside negative), a value close to that occurring for many cells. If the membrane capacitance per unit area (Cr) has a typical value of 10 mF m-2 (10-2 C V-1 m-2), to what net charge concentration in the cell does this electrical potential difference correspond Using Equation 3.3, we obtain... 

In the 1920s, impedance was applied to biological systems, including the resistance and capacitance of cells of vegetables and the dielectric response of blood suspensions. ° Impedance was also applied to muscle fibers, skin tissues, and other biological membranes. " The capacitance of the cell membranes was found to be a function of frequency, and Fricke observed a relationship between the frequency exponent of the impedance and the observed constant phase angle. In 1941, brothers Cole and Cole showed that the frequency-dependent complex... 

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