Was just thinking, such a system could actually be quite easy to display to the player. We could have a “Metabolism” tab in the Organism Editor, that shows what the organism depends on for survival. Then we could have little Metabolism Trees that show the different food, gasses, and fluids that the organism needs for different functions.
This was just a super quick mockup, but for example an oxygen breathing carnivore would have this as their tree. Eat meat, fat, and bones. Breathe oxygen. And drink water. And your body will slowly turn these into energy.
Then you can click on Growth to see what compounds your body uses to grow you (from childhood to adulthood). Typically these will be the exact same.
Other Metabolism trees could include Poison/Venom/Toxins, Electricity, Pheromones, etc. The game would basically add them in for whatever extra metabolic functions you have.
Based on what the cells of that organism evolved to eat in the Microbe Stage, these trees will reflect different combinations of compounds needed to eat, breathe, or drink.
Lastly, we could have an energy balance bar (perhaps at the top of this panel) to show how much energy is produced each second by this organism’s metabolism, and how much of that is being spent on maintaining different parts or activities. The Energy Balance Bar, like the ATP Balance Bar, assumes you have all raw materials available. Obviously, if one of these compounds runs out, your organism stops producing energy.
EDIT: I think visualizing it like this greatly helps to imagine how all these systems will work together and be presented to the player. I may go back and create a more detailed visual concept of what the Metabolism tab could look like to represent the Metabolic Rate System.