Why must a container or system be closed for equilibrium to be established?

If you're dealing with gases, some of them could escape the container. Or, if you're dealing with liquids, there could be some outside factors playing in. In general, you need a closed system to observe a reaction accurately.

answer: cellular respiration takes in food and uses it to create atp, a chemical which the cell uses for energy. usually, this process uses oxygen, and is called aerobic respiration. it has four stages known as glycolysis, link reaction, the krebs cycle, and the electron transport chain.