To answer questions from the last posts, せいぶつかがく　is biochemistry (my major). Or so says でんし　じしょ,　which is an online Japanese-English dictionary I found. You can type in the word in English, and it will give you the japanese in hiragana and kanji, or in katakana if it's a loan word. You can also type in the hiragana of a japanese word that you want to know and it will give you a list of related words and the English translations.

つかれた　is indeed a colloquial expression for "I'm tired." I picked it up, along with a motley collection of curses, exclamations, and imperatives (basically things that are short and frequently used), from watching subbed anime and J-drama. Most of what I know isn't useful for class, tho, since we're learning formal speech patterns.