It would help if you didn't rely on a grade school science textbook for your scientific learning. I mean, it's a good start and/or useful when you don't want to waste time learning 5000 complicated words and just want the explanation (for example, reading children's books about the four seasons is easier than going through wikipedia for an explanation of orbital tilt and surface area/sunlight correlation)

Finally a dictionary is just a list of words and their definitions. It's not an exhaustive list of a language's lexicon.