An example of a feedback loop that is my curriculum is the exchange of oxygen and carbon dioxide between photosynthesis & cellular respiration. Although autotrophs (organisms that make their food by using light energy) need carbon dioxide to make food through photosynthesis, they need oxygen to break down the glucose that they make in order to use its stored energy. Because they do not use energy to move around, plants, autotrophic bacteria, and algae make more oxygen than they use, which adds oxygen to the atmosphere—creating a positive feedback loop. Heterotrophs use oxygen to release the energy from the glucose that they consume, and give off carbon dioxide which is in turn used by the autotrophs.
Students could use a computer model of a sealed terrarium in which they use sliders to change the amount of autotrophs and heterotrophs in the system. Possibly students could alter the coding in order to balance the oxygen and carbon dioxide ratios in an attempt to keep the system going perpetually.