Plasma Membrane and Transport
The sodium-potassium (Na+/K+) pump is a critical element for many processes of the cell. For example, it is responsible for a neuron’s ability to transmit messages. The pump transports sodium out of the cell using energy from ATP (the extracellular matrix has a higher concentration of sodium than the intracellular space). The pump also imports potassium, which is more highly concentrated inside the cell than outside. The pump is an example of a cotransport system. Sodium binds to the transport protein and is exported. Once the sodium ion is released, a potassium ion binds and is transported into the cell (actually, for every three sodium ions pumped out of the cell, two potassium ions are pumped in). Thus, a sodium and potassium gradient are established and maintained across the membrane. Since the ions are being transported in opposite directions, this type of pump is called antiport.
The sodium gradient established by the Na+/K+ pump can be used in many ways. Another cotransport system, the Na+/glucose pump, is active in human gut epithelial cells, and results in the uptake of glucose. Due to its concentration gradient, sodium diffuses into the cells through the Na+/glucose pump and transports glucose with it (this is called symport). The glucose is processed by the cells and can be used to generate energy.
If a sodium gradient is formed, whereby there is a vast excess of sodium outside the cell, which is the most likely result?
|A||An electrical gradient is also formed.|
|B||A magnetic gradient is also formed.|
|C||Other positively charged ions will enter the cell.|
|D||Negatively charged ions will enter the cell.|
The only logical answer is that an electrical gradient will form. In general other ions, be they positive or negative, will not be transported based on sodium concentration. And a magnetic gradient? What’s that?