why does pencillin have any effect on bacteria and not on human
Answers
An ideal antibiotic is the one which targets metabolic processes or molecules in the pathogens, without having any such target in the human cells. Penicillin affects cell wall formation in bacteria by binding to an enzyme called DD Transpeptidase and inhibiting its activity. Since human cells have no cell wall and don't have DD transpeptidase, there is nothing that penicillin can affect and cause harm. Hence human cells remain virtually unaffected by penicillin.
However, there is hardly any ideal antibiotic. Antibiotics are biologically active molecules and may find unusual targets in human body thus resulting in side effects.
Because penicillin blocks the biochemical pathways essential for bacteria which helps bacteria in the formation of cell wall and as we know that human cells does not have a cell wall so it does not harm human cells an the bacteria is unable to make cell wall and it dies.