A hydroponic system can be a closed loop provided it receives enough sunlight. Such a model can show the proportions of plants to bacteria to fish to be sustainable. By comparison a mouse would need much more area in order to both uptake its CO2 exhales, provide it with oxygen and feed it. This difference shows what a warm blooded animal requires compared to a cold-blooded one. It also shows the role of plants in feeding, exchanging gases, and uptaking waste.