|MadSci Network: Biochemistry|
I checked with an expert in Paramecium and ion channels to make sure I was correct. The answer is almost certainly both. His reply is below. We're pretty sure that Paramecium has both voltage-gated and ligand-gated channels, but that hasn't been proven with 100% certainty as yet - at least with respect to the ligand-gated channels. We are sure that they have voltage-gated channels. They have voltage- gated Ca2+ channels that open when the cell bumps into something. The Ca2+ makes the cells go backwards. These are the channels that get activated by electric current. They also have voltage-gated K+ channels that open shortly after the Ca2+ channels open. They help turn off the Ca2+ channels and allow the cell to start swimming forward again by repolarizing the membrane. The ligand-gated channel story is more complicated. Cells do depolarize when placed in potassium chloride and calcium chloride but not because they interact with ligand binding sites. Potassium and calcium ions carry positive charges that are attracted to negative charges on the cell surface. This causes local charge disturbances that open the voltage gated channels in much the same way that an electrical stimulus does. But the cells are clearly able to respond to many chemicals in the environment. Some of these signal that there is food nearby (the Paramecium equivalent of "smell") and some signal a threat. Surprisingly, Paramecium is very sensitive to many of the chemicals that we associate with multicellular organisms such as humans. These include glutamate, aspartate, acetylcholine, and insulin, to name a few. Their genome also contains sequences that encode receptors for these ligands, so its very likely that there are indeed ligand-gated channels in Paramecium. We just need to find a few good researchers with an interest in Paramecium to prove it!
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