The greater the solute concentration of the environment, the less readily available the water. Some prokaryotes can maintain the availability of water in environments with high solute concentrations hypertonic environments by increasing the solute concentration within the cell.
Microorganisms that can do this and thus tolerate hypertonic environments are osmotolerant. Osmotolerant bacteria, such as Staphylococcus aureus can grow in a wide variety of environments with varying osmotic pressures. In fact this bacteria can be cultured in media containing sodium chloride NaCl concentrations as high as 3M. Some bacteria specifically require an environment with a high concentration of sodium chloride.
Now, there is an electrostatic force repelling chloride out of the intravascular compartment. Consequently, more chloride collects in the interstitial fluid. The same force is attracting sodium back into the intravascular compartment. This competes with the concentration gradient. In order to render the concept easier to understand, the author has resorted to kindergarten-level graphic design, representing the electrochemical gradients with coloured slopes.
One can almost imagine little ions sliding down them. The attractive force of anionic protein for sodium competes with the concentration gradient sucking it back into the interstitial compartment.
At a certain concentration, some sort of equilibrium is reached. Of course, in reality this is not a true equilibrium. There is still unequal particle concentration on both sides of the membrane. An equilibrium between the concentration gradient and the electrostatic gradient is reached, but there is still water to consider. Water is osmotically attracted into the vascular compartment. The movement of water would then dilute the concentration of the ions, and there would be a change in their concentration gradients.
So there is no stable steady state. There is movement of some ions out of the intravascular space, but at Gibbs-Donnan equilibrium there are still more particles in the vascular compartment, exerting an oncotic pressure. The oncotic force sucking water into the capillaries is opposed by the capillary hydrostatic pressure, which is applied by the pumping action of the heart. If this pressure becomes too great eg. Oedema ensues.
The distribution of ions in the interstitial and intravascular compartments can be expressed in terms of a coefficient factor which describes the distribution of the ion in the interstitial fluid as a proportion of its concentration in the plasma.
This is generally referred to as the Gibbs-Donnan Factor. The value of this factor for monovalent cations is 0. For monovalent anions, its 1.
Divalent cations like calcium are partially protein bound, and the Gibbs — Donnan effect only applies to the ionized forms. For them, the factor is 0.
Donnan, Frederick George. A contribution to physical-chemical physiology. Adair, G. Nguyen, Minhtri K. Masuda, Takashi, Geoffrey P. Dobson, and Richard L. Tosteson, D. Wilson, T. Russo, M. Van Rossum, and T. The Gibbs-Donnan effect. Previous chapter: Mechanisms responsible for the cell resting membrane potential Next chapter: Structure and function of the mitochondria.
In some cases, the size of a pore or channel and the size of the ion or molecule passively determine the permeability of a membrane to the substance. In other cases, energy is expended at a channel which then actively pumps a substance in or out of the cell. This process is called active transport. When the neuron is at rest, potassium and chloride are allowed to pass at a moderate rate while sodium channels remain closed. The final mechanism for maintaining the concentration gradient involves active transport of sodium and potassium ions via the sodium-potassium pump.
The sodium-potassium pump ejects three sodium ions for every two potassium ions pumped into the cell; energy is needed for this process. The energy expended by the neuron in maintaining the resting potential is justified because the resting potential increases the neuron's ability to respond rapidly to stimulation.
This neuronal response to stimulation, the action potential, will be described in Tutorial 6. The neuronal membrane is composed of two fat layers phospholipids with protein molecules embedded between and within.
The phospholipid molecules have a water-attracting head which form the outer and inner boundaries of the membrane where contact is made with extra-cellular fluids and cytoplasm and two water-repelling tails which form the internal layer of the membrane.
The membrane is approximately eight nanometers thick, or less than 0. This molecular structure provides the neuron with an adequately firm, yet flexible boundary that can control the movement of substances into and out of the cell.
See Tutorial 2 for additional information on the neuronal cell membrane. Arms, K. Keynes, R.
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