What is a "sentient being" in Tibetan Buddhism?

The answer to this question seems to depend on what school one talks about. So my question is about Mahayana Buddhism, more specifically Tibetan Madhyamaka Buddhist schools. I kind of wonder where the line is drawn between beings that are sentient and beings that are not. How about f.ex. corals, algae, amoeba, crill and living creatures like that. As far as I can understand (which is probably not very far) this has got to do with whether the being has consciousness/mind or not. Is that so? Maybe the question should be "what does it mean to say that a being is conscious or has a mind? Is there a generally accepted definition of what a sentient being is? Anywhere in the suttas the Buddha is talking about this?