The theology of the issue has been popularly explained as such. 1)"The wages of sin are death" and "there is no remission of sin without the shedding of blood". The whole idea of Christs death is the the kinsmen redeemer had to pay the debt owed for sin for all mankind to God. It is a repugnant thing. In the Garden of Eden there was no death until Adam and Eve sinned ushering in death as God killed the animals to get their skin...the first shedding of blood to cover sin. The 'covering' of sin is temporal until the sacrifice of Christ which is eternal. So there was no death before sin, Adam and Eve sinned, temporary covering of sin by animals death, the first death. Plants have no blood so are not considered alive for this theological purpose. Also, it is a logical fallacy, an argument from silence to say that Adam did not know what death was. He did not have to experience it to know it and the supposition that he would have to presupposes and understanding of Adam's mind which was a perfect, pre-fall man's mind. We do not understand what the conditions were in the Garden of Eden and cannot extrapolate what Adam did an didnt understand except for the fact the God warned him means he must have understood what death was.
"...but according to their own desires, because they have itching ears, they will heap up for themselves teachers; and they will turn their ears away from the truth, and be turned aside to fables"
™ Version 4.2
