I didn't know what a mitochondrion was until I looked it up in a dictionary.
If you look up a fact or a piece of information, you find it out by looking in something such as a reference book or a list. I looked your address up in the personnel file Many people have to look up the meaning of this word in the dictionary
If you look someone up, you visit them after not having seen them for a long time. I'll try to look him up, ask him a few questions She looked up some friends of bygone years. = visit
If a situation is looking up, it is improving. Things could be looking up in the computer industry. = improve
seek information from; "You should consult the dictionary"; "refer to your notes"