look up和look for的区别

look up和look for的区别
从这个例句可以体现：could you tell me what time the train leaves for beijing?
sorry,i don't know.you can look for it /look it up on the internet.我想 look up不是仅限于查字典吗,于是就填了look for it.但看了答案是look it up,求具体用法及其分别.（答案有可能是错的）