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(sry have to make an answer 20 characters /!:!27837)
Both plant and animal cells have membranes so whatever chemical reactions they have going inside the cell, it is regulated and maintained. Membranes help separate the internal environment from the external environment.
Hope this helps - I'm currently in cell biology so this question is interesting
Due to the need to have completely controlled experiments to test a hypothesis, science can not prove everything. For example, the scientific method cannot alone say that global warming is bad or harmful to the world, as it can only study the objective causes and consequences.