What happens to proteins as they pass through the golgi apparatus?

Proteins are modified by having sugars attached or removed

The Golgi apparatus consists of a stack of flattened cisternae and associated vesicles. ... The modified proteins, lipids, and polysaccharides then move to the trans Golgi network, which acts as a sorting and distribution center, directing molecular traffic to lysosomes, the plasma membrane, or the cell exterior.

Well, the "system" is basically a group of factors that are related in some way. So, ideally you'd be able to think of a individual organism as a living system.