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L12 Cell Volume Reg 2011

L12 Cell Volume Reg 2011 - COPYRIGHT Mammalian Physiology...

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1/10 COPYRIGHT Prof. Beyenbach Mammalian Physiology BIOAP 4580 2011 MECHANISMS OF CELL VOLUME REGULATION 1. A cell volume constant with time. a) Diffusion distance. Single cells do not grow into giant cells because of the limits of diffusion. Once solute and water have crossed the cell membrane, transport through the cytoplasm depends largely on diffusion. Thus, intracellular diffusion becomes a rate- limiting step of metabolism as cells take on size. An optimum (or perhaps limiting) cell size is reached when diffusion adequately supports transport to the very center of the cell. There is an additional restriction to cell size: Volume (size) increases by the power of 3 whereas surface area increases only by the power of 2 (eq. 1, 2). Thus, as cell size (volume, V) increases it will soon outgrow the transport capacity of the cell membrane, unless the cell surface (S) is amplified via microvilli or membrane infoldings, as in many epithelial cells. (eq.1) where L is length (eq.2) b) Preventing cell swelling and lysis in isotonic media. “The critical event that may best be called the origin of life was the enclosure of several self-replicating polymers within a semi- permeable membrane.” (Haldane, 1954) The enclosure of charged impermeant molecules (proteins, enzymes) in a distensible membrane, that is permeable to small charged solutes and water, inevitably results in solute entry, swelling, and ultimately lysis (Fig. 1). This was first recognized by Josiah Willard Gibbs (1839-1903) and later by Frederick George Donnan (1870-1906). Fig. 1. The problem of negatively charged intracellular macromolecules to which the cell membrane is impermeable. Since the membrane is permeable to diffusible cations and anions, they will enter the cell, setting up an oncotic pressure that eventually will bring about lysis (lest cell swelling is opposed by a rigid cell wall as in plant cells, or water is pumped out again).
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2/10 Even though the cell may be in isosmotic media it will swell as diffusible ions move to reach Gibbs-Donnan equilibrium. However, equilibrium is not reached in the normal cell which actively extrudes solute and hence water from the cell. Particularly useful in this regard is the stoichiometry of the Na/K pump that moves 3Na + ions out of the cell for each 2K + ions taken up into the cell. With each cycle of the pump, it moves one osmolyte out of the cell. Since this osmolyte is charged, an intracellular anion follows the Na + out of the cell, and water follows by osmosis. Thus, the maintenance of cell volume is in part an active transport process that is ATP-dependent. The maintenance of cell volume is a steady state phenomenon where the passive influx of solute and water is matched by an active efflux of solute (and hence water)S. c) Restoring cell volume in hyper-osmotic (hypertonic) media.
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