Naked mRNA is not readily taken up by cells, but needs to be encased inside a protective envelope to gain entry.
In this sentence, I'm confused by the phrase taken up. After searching a dictionary, I'm even more bemused. I just couldn't find a meaning that matches this sentence appropriately. But I do have a hypothesis that according to one of the items of the meaning of "take up" in the dictionary, which is to begin a friendly or romantic relationship with (someone), I guess that maybe taken up is used as a figurative way to suggest that cells and mRNA can't coordinate well enough to function normally. If I'm wrong, please correct me.
By the way, I noticed that but is used in this sentence for conveying logical meaning. But it's more logical for me to use so. Can anyone explain why but is used as such?