Letters from TA's usually do not carry the weight of those from Professors. I am not sure about MS versus PhD professors, but usually a PhD is a requirement to be a Professor in the sciences. Different schools have different requirements, and you may have to send different combinations of letters to different schools. You may not need all letters for all schools. But if you can, I would try to locate:
2 science LOR's (At least one life-science, but also physical science if possible). 1 humanities LOR. 1+ Others that you can think of from major activities (volunteering, research, etc ...). If you attended grad school, you will want to try and get one from there as well since many schools require a letter from your grad school advisor or committee member. Also, if your school has a pre-med commitee, some schools will require a letter from that committee (sometimes as a replacement for the individual letters, sometimes in addition to them).
Again, you won't need all of these for each school. But if you have them, it makes your life a lot easier. Everyone is different though, and most schools will work with you and circumstances. Bottom line: LOR's can be a royal pain in the buttocks.