Il potenziale di membrana

The composition of the intracellular fluid differs significantly from that of the extracellular fluid. Proteins are more concentrated in the intracellular fluid, where they dissociate to form anions. Potassium, the main cation, is also found in the intracellular fluid. In contrast, sodium is the main cation in the extracellular fluid, while chloride is the main anion. While proteins cannot cross the cell membrane at all, ions can do so by facilitated diffusion or active transport. However, the membrane is not equally permeable to all ions. In fact, the ease with which an ion can cross it depends on the number of channels open for that ion. The greater the number of channels open for an ion, the higher the conductance for that ion. At rest, the sodium channels on the membrane are mostly closed, so the conductance for sodium is low. However, there are numerous passive channels for potassium, which are always open. Therefore, the conductance for potassium is higher, between 50 and 100 times higher than that of sodium. The few sodium ions that manage to pass down the gradient through the very few open channels are immediately pumped out of the cell thanks to the sodium-potassium atipase pump we saw in the previous lesson. This pump, if you recall, carries three sodium ions out and two potassium ions into the cell at the cost of one ATP molecule. The escape of potassium down the chemical gradient is halted by the electrical gradient. The strong presence of negative charges inside the cell, mainly due to proteins that cannot cross the membrane, and the presence of positive charges outside the cell, prevent potassium from following its chemical gradient. This creates an ELECTROCHEMICAL EQUILIBRIUM dictated by both the electrical gradient and the concentration gradient, which determines a difference in electrical potential between the two sides of the membrane. The extracellular side of the membrane will be positively charged, while the intracellular side will be negatively charged. This is called the membrane potential and varies between -50 mV and -90 mV depending on the cell type, because the composition of the intracellular fluid obviously differs slightly depending on the cell type. Some cells, however, are able to modify their membrane potential (i.e., become "excited," as they say) and thus perform very important functions in the body. These are neurons and muscle cells. 00:00 Distribution of ions between the LIC and the ECF 01:46 Chemical and electrical gradient 04:02 Nernst equation 04:57 Membrane potential 06:50 Goldman equation