I don't understand. Does this mean that parasites living on a more complex host are not alive? I am not trying to trivialize this question. Life on earth is interconnected, and it makes no sense to me to assert that an entity such as a virus or prion is not alive, simply because it depends on a host for metabolism.
That's because I wasn't being clear. Is the matter/energy directly feeding the metabolism significantly less complex? Take the case of your parasite, say an intestinal parasite. Although it lives within a complex host, it feeds on matter of a very simple nature. In fact the host is optional - place the parasite in a broth of shredded proteins and other nutrients and it would do as well. And similarly would its host if the broth were directly delivered to the gut.