Can someone please check my answer to this question?
**Q: To send a signal, a neuron may carry out exocytosis of chemical signals that are recognized by a second neuron. In some cases, the first neuron ends the signal by taking up the signaling molecules by endocytosis. Would you expect this to occur by pinocytosis or by receptor-mediated endocytosis? Explain. (The answer is receptor-mediated)
My Answer: Receptor-mediated endocytosis. The first neuron that ends the signal knows how much of the chemical signal abound. Also, receptor-mediated endocytosis ensures that all chemicals released from the previous neuron are caught and brought into this first neuron via endocytosis, making sure that no leftover wandering chemicals will reach the next neuron.
Explain why the sodium-potassium pump would not be considered a cotransporter.
My Answer: The sodium-potassium pump pumps both ions against the electrochemical gradient. The sodium ions and potassium ions both use ATP to be transported across the plasma membrane. In order to be a cotransporter, however, one pump should pump a substance against the concentration gradient only to create potential energy that will be used to transport another substance. As the first substance flows back down the concentration gradient, the second substance will be transported in the desired direction. This is not true for the sodium-potassium pump.
Following was the answer I was given by the textbook which confounded me.
->"Each ion is being transported against its electrochemical gradient. If either ion were flowing down its electrochemical gradient, this would be considered cotransport."
Well, an example of cotransport is the sucrose-H+ cotransporter in plant cells and I am 100% sure that the sucrose is NOT flowing down its concentration gradient. The cell uses the potential energy from H+'s flow back into the cell to transport need sucrose into the cell, which is an active transport. So is the last line saying that the H+ ions are EVENTUALLY flowing down their electrochemical gradient?