Natrium-Kalium-Ionenpumpe [Pumpe, Natrium-Kalium-ATPase] - [Neurobiologie, Oberstufe]

This video is about the sodium-potassium pump – we explain its structure and its function. The sodium-potassium pump is an ion pump. The plasma membrane of a nerve cell, with its lipid bilayer, is actually impermeable to ions – meaning that positively charged ions such as Na+ or K+ cannot easily pass through the plasma membrane. However, there are proteins embedded in the membrane that function as ion pumps and ion channels, enabling the transport of these ions. While ion channels transport ions along their concentration gradient without consuming energy, i.e., from the location of higher concentration to the location of lower concentration, the transport mechanism via ion pumps requires energy. By consuming the energy-rich molecule ATP, a specific ion can be transported against its concentration gradient – ​​from the location of lower concentration to the location of higher concentration. This makes it clear that if the sodium-potassium pump is an ion pump, then this pump will transport ions – namely sodium and potassium – against their concentration gradient, using energy. If we consider the distribution of potassium and sodium ions on both sides of the cell membrane, we notice that while there are significantly more potassium ions in the intracellular space, i.e., inside the cell, than in the extracellular space (the area outside the cell), the opposite is true for sodium ions: a high concentration of sodium ions outside the cell is counteracted by only a few Na ions inside. The fact that ion pumps transport ions against their concentration gradient is revealed in the case of the sodium-potassium ion pump: It actively transports sodium+ ions from the cell interior to the external environment, where sodium is more concentrated, and simultaneously transports K+ ions from the outside to the inside of the cell. Especially in the context of neurobiology, the sodium-potassium ion pump plays a key role in maintaining the resting membrane potential – a detailed description of which is included in the video shown above right. Let's take a closer look: There are many potassium ion channels along the membrane of a nerve cell. In the resting state, these ion channels are open, allowing potassium to pass through the membrane easily. Remember: ion channels transport ions along their concentration gradient without expending energy. Since there are more potassium ions inside the cell, they flow with the chemical concentration gradient from the location of the higher concentration to the location of the lower concentration, i.e. out of the cell. Actually, both sides of the membrane are electrically neutral because the positively charged Ka + Na – ions are balanced out by other, negatively charged ions. With each potassium proton, a positive charge leaves the cell's interior. Because the negative charge, which would otherwise cancel the positive charge, doesn't diffuse out of the cell, the cell's interior is negatively charged compared to the cell's exterior. A membrane potential builds up, which varies depending on the cell type, but is usually around -70 mV. In addition to the potassium ion channels, there are also a few sodium ion channels, which are closed in the resting state. They are not shown here, as sodium cannot be transported through these ion channels when closed. Nevertheless, sodium can also slowly flow through the membrane itself, into the cell, where sodium is less concentrated. This somewhat contradicts the impermeability of the membrane to ions mentioned at the beginning. Nevertheless, these so-called leakage currents exist – like a leak in a ship, sodium flows in and potassium flows out. What would happen if there were no sodium-potassium ion pump? Due to the leakage currents, the concentration differences between the two ion types would continue to decrease. Sodium would flow into the cell – albeit slowly – until the concentration on both sides of the membrane is equal. Potassium ions, on the other hand, would flow out of the cell until a concentration equilibrium was established on both sides. The result would be a charge equalization and thus a collapse of the membrane potential. Another thing to consider: With the help of the sodium-potassium ion pump, three sodium ions are transported out of the cell and two potassium ions into the cell per cycle. The cell interior therefore loses one positively charged particle per cycle and becomes more negatively charged than the area outside the cell – this component would also be missing without the ion pump. So you see: The sodium-potassium pump ensures that the membrane potential is maintained. Without it, it would slowly approach zero, and the nerve cells would no longer be excitable.