OK, here is a very brief metabolism that we can already start using. I decided to go with the barest minimum for two reasons:
1. Don't want to intimidate people
2. We can always add stuff later as a way to introduce balance/diversity.
As a result, what you'll see below is only about 0.1% of all reactions that are happening in our bodies. I assume that we'll have carbs/protein/fat system. These turn up in bot's stomach, and then are digested. What follows below is description of how bots can use them to extract energy. Theoretically all these reactions are reversible, meaning that they can be used to create carbs/protein/fat, but the energy required to make stuff is always more than you get when you destroy stuff. It is especially true for proteins, which require 10 times more energy to make them, than energy that is released when you destroy them.
Another note: I'll use ATP to keep track of how much energy is generated. Simply assume that ATPs will be converted to nrg with some coefficient.
So here it goes:
CARBS
' Note: carb degradation is the major route of energy production. Fats and proteins join in as small streams join big river.
carb+7 H20=8 glucose
glucose = 2 pyruvate + 2 H2O + 8 ATP
pyruvate + 1.5 O2 = AcCoA + CO2 + 3H20 +6 ATP
AcCoA + 2 O2 = 2 CO2 +12 ATP
FAT
fat + ATP = fatCoA
fatCoA +7 O2=8 AcCoA + 42 H2O + 35 ATP
(see above for fate of AcCoA)
PROTEIN
protein = 8 am.acids
am.acid = pyruvate + NH4
(see above for fate of pyruvate, see below for fate of NH4)
NH4
'Note: This is very-very toxic. If bot does not get rid of it, it gets really messed up. It cannot be directly thrown out, it has to be converted to urea.
2 NH4 + CO2 + 3 ATP + H2O = urea
(urea is basically "waste", it can be discarded as such).
Final summary of how much energy you get from each type of food:
1 fat = -1+35+8x12=130
1 protein = 8x6+8x12-4x3=116
1 carb = 8x36=288
P.S: As I said, this is only the barest possible basics. We can add more steps everywhere. Two possible reasons to do that: a) make sim more complicated, leading to bigger diversity and b) balancing issues - adding more steps means that it takes longer to digest and requires more enzymes.