Look up and Look on

Look up verb - To go in search of.

Look on and look up are semantically related in take a look topic. In some cases you can use "Look on" instead a verb "Look up".

Look on verb - Observe with attention.

Look up and look on are semantically related in visit topic. Sometimes you can use "Look up" instead a verb "Look on", if it concerns topics such as take a look, visit at residence or business, be a guest of.