Anatomy & Physiology

Anatomy & Physiology Study Guide by ion. Both passive and active forces act across the plasma membrane to determine the transmembrane potential at any moment. Passive Forces Acting across the Plasma Membrane The passive forces acting across the plasma membrane are both chemical and electrical in disposition. Chemical Gradients Because the intracellular concentration of potassium ions is relatively large, these ions tend to transport out of the cell through open potassium channels. The transportation is driven by a concentration gradient or chemical gradient. Similarly, a chemical gradient for sodium ions tends to drive those ions into the cell. Electrical Gradients Because the plasma membrane is much more permeable to potassium than to sodium, potassium ions exit the cytoplasmmore quickly than sodium ions enter. As a result, the cytosol along the interior of the membrane displays a net loss of positive charges, leaving an overflow of negatively charged proteins. At the same time, the extracellular fluid near the outer surface of the plasma membrane shows a net gain of positive charges. The positive and negative charges are divided by the plasma membrane, which limits the free movement of ions. Whenever positive and negative ions are held apart, a potential difference arises. The size of a potential difference is measured in volts (V) or millivolts (mV; thousandths of a volt). The resting potential varies widely, depending on the type of cell, but averages about 0.07 V (- 70mV) for many cells, including most neurons. The minus sign signifies that the inner surface of the plasma membrane is negatively charged with respect to the exterior. The Electrochemical Gradient The electrochemical gradient for a specific ion is the amount of the chemical and electrical forces acting on that ion across the plasma membrane. The electrochemical gradients for K+ and Na+ are the main factors disturbing the resting potential of most cells, including neurons. The intracellular concentration of potassium ions is relatively large, whereas the extracellular concentration is very small. Therefore, the chemical gradient for potassium ions tends to move them out of the cell. However, the electrical gradient counters this movement because K+ ions inside and outside of the cell are drawn to the negative charges on the inside of the plasma membrane and deterred by the positive charges on the outside of the plasma membrane. The chemical gradient is strong enough to overpower the electrical gradient, but this weakens the force driving K+ out of the cell. Active Forces across the Membrane The Sodium–Potassium Exchange Pump At the usual resting potential, the cell must bail out sodium ions that leak in and recapture potassium ions that escape. The activity occurs through the movement of an exchange pump powered by ATP. The ion pump involved is the carrier protein sodium–potassium ATPase. This pump exchanges three intracellular sodium ions for two extracellular potassium ions. At the normal resting potential, this pump’s primary significance is that it ejects sodium ions as quickly as they enter the cell. Thus, the Achieve Page 149 of 368 ©2018