QLS Seminar Series - Charles S. Peskin
Control of Cell Volume with Space-Charge Layers and Bulk Electroneutrality
Charles S. Peskin, New York University
Tuesday September 12, 12-1pm
Zoom Link:听
In Person: 550 Sherbrooke, Room 189
Abstract:听Animal cells do not have cell walls, and their membranes cannot withstand any significant pressure difference. Macromolecules that are confined to the cell contain numerous charged groups, and the counterions of these charges have a substantial osmotic effect, pulling water into the cell. The cell therefore faces a severe osmotic challenge to avoid having its volume increase until the cell bursts. This challenge is met by pumping ions, specifically by continuous action of the Na+/K+ exchange mechanism, which repeatedly expels three Na+ ions from the cell in exchange for two K+ ions from the external environment. This allows the cell to reach an out-of-equilibrium steady state, which can only be maintained by the constant expenditure of energy. The energy source is ATP hydrolysis.
A by-product of the above mechanism is an electrical potential difference across the cell membrane. All animal cells have such a resting potential, on the order of 70 mV negative inside, and neurons (among some other cell types) have learned the trick of transiently modulating this potential as a signaling mechanism. Thus, the control of cell volume is the foundation of electrophysiology.
A typical animal cell is therefore charged. The charge is not distributed uniformly over the cell, but instead is concentrated in a very thin space-charge layer adjacent to the inner face of the cell membrane. A similar layer with charge of opposite sign is formed adjacent to the outer face. We use the linearized Poisson-Boltzmann equation for the description the space-charge layers, and in this way we are able to evaluate their ionic content. This is needed for the formulation of the ODE that describe the control of cell volume, especially to obtain results in which volume changes interact with electrical signaling, so that the capacitance of the cell membrane cannot be neglected.