The cell membrane is essentially a bi-lipid layer that contains phosoplipids and proteins. The concentration of ions and certain molecules is vastly different between the intracellular and extra cellular concentrations in cells across the cell membrane. This is because the lipid bi-layer in the cell membrane has hydrophobic fatty acid chains from the phospholipids in its interior. These serve as a barrier to the passage of most polar molecules. This allows the cell to maintain a concentration of solutes in its cytosol that is different from that of the extra cellular fluid. This results in the situation whereby the cell is more negative inside than it is outside, thereby creating a negative cell membrane potential. Yet how is this achieved?
Cells have evolved ways of transferring specific water-soluble molecules across membranes. Specialized transmembrane proteins achieve this transport of inorganic ions. Each protein is specialized to transport a particular ion in a particular way across the membrane.
[...] These proteins can transport solutes against their concentration gradient to create an asymmetric distribution of solutes across the membrane, this is known as active transport as it requires energy from the hydrolysis of ATP and these proteins are generally recognised as pumps. Alternatively they can transport solutes down their concentration gradients in facilitated diffusion. This process does not directly require energy, but the concentration gradient required for it to occur does require the energy used in active transport, therefore facilitated diffusion in this case is also known as secondary active transport. [...]
[...] The pump is described as electrogenic as more positive charge is pumped out of the cel than in, thereby creating an electrical difference across the cell membrane between the intracellular and extra cellular fluids. The sodium potassium ion exchange ATPase has many important affects on the cell. The potential energy created by the concentration gradient of sodium across the cell membrane can be utilised in the transport of glucose into the cell. On its own glucose is a membrane impermeable molecule, yet it can be transported across the membrane via a symport carrier with the sodium ions as they leak back into the cell, this is a prime example of secondary active transport. [...]
[...] ION EXTRACELLULAR INTRACELLUALR CONCENTRATION CONCENTRATION cellular but differs according to tissue Therefore sodium ions will try to move into the cell diffusing along there electrochemical gradient into the cell, but the cell is not very permeable to their movement and are therefore in greater concentration outside the cell. Potassium, which is in high concentration inside the cell, will try to move down its concentration gradient out of the cell but its movements are opposed by the charge gradient. Free calcium ions are in much higher concentrations outside the cell and so will try to move down there electrochemical gradient into the cell. [...]
[...] The Nernst equation is therefore a mathematical analysis of the forces acting on each ion across the cell membrane and is a calculation of the magnitude of the equilibrium potential of a certain ion across the cell membrane that takes into account chemical and electrical gradients. V = equilibrium potential in volts Co and Ci = outside and inside concentration of the ion R = gas constant T = absolute temperature F = faradays constant Z = valence of ion Ln = logarithm to the base e These electrochemical gradients are set up by the Donnan effect. [...]
[...] As the internal chloride ion concentration is variable it can be said that the equilibrium potential is close enough to the resting membrane potential of –70mV that no forces other than those represented by the chemical and electrical gradients in the Nernst equation are acting on the chloride ions. Yet the same cannot be said for potassium of sodium ions, where the equilibrium potential for potassium ions is as follows: - This gives a value –20mV lower than the resting membrane potential, and as in this case the concentration gradient is outward and the electrical gradient is inward there are more potassium ions in the cell than can be accounted for by the electrical and chemical gradients alone. [...]
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