"Merely to offer relevant text ads?" Why not? It's how they make money.
"Google is conducting research in the fields of molecular biology and genetics." How the fuck would they do that!? They are an IT company, not a pharmaceutical company! They don't have the access or resources to do it. They could try, but that's like a 2 year old programing an Apache Server.
"Does Google really worry about our privacy?" It's not their job to worry about our privacy, it's their job to follow their privacy policy and our job to not be idiots.
If you are worried about Google getting your emails, use RSA.