The sequence and number of amino acids is different (each protein has a unique sequence and arrangement of amino acids).
Actually a living thing is not a system. Every part of our body, like our lungs brain or heart, can be seen as a separate system. Therefore we are giant arrays of systems working together to keep you alive
Weight is dependent on the force of the gravity. Mass is the amount of matter a object contains. So if you would go to the moon your weight would be different but your mass would be the same
