I took some time to read your statement. It's telling. Anyway, I talked with the chair of our department today and asked about his opinion on what he expects one to be. He gave me a few points:
   First, you need to show why you're interested in biology. It could be just a brief history, a class or something that turned on your interest. But don't give the impression of being too romantic. He told me about a bad statement, in which a Chinese student told the story of when he was 2 years old and his mother showed him a tadpole and he's been interested in biology ever since then and bla bla bla...
   Second, tell what kind of field you would want to work in. Don't be too specific because if there's no one in the department doing such a thing, the committee will think you won't be satisfied when you get here. So, you may check first to make sure that the research area you want in represented in the department.
   Third, tell why you want to be a Ph.D. This is generally showing that you're interested in research.
   I told them you have been in the lab. 'Them' means the chair and his wife. They said you should stress that in your statement. I think you can write about what your lab is doing, what kind of experimental techniques are you using and what your role is in there.
   They also said that they are looking for someone with genuine interest in biology research and they expect this expressed in the statement. Since you've been working so long, you can sound more mature or formal when describing your lab experience. And, don't make the statement too long, they get tired out before reaching the end. You can have your script read by some other teachers and have some feedback on what they think of it.