[Just my small fan fiction on Kerbal biology i will keep updating when i feel like it] Energy: Kerbals get energy in the form of glucose the same as life on earth however they are different from anything on earth because they both use photosynthesis and use their mouths and internal organs to digest food. When food is scarce Kerbals go into a sort of comma lying out in a area with accesses to CO2 water and direct sunlight until they have gathered enough energy to move on. This shows that the green skin is in fact due to chlorophyll as opposed to some form of green metal in the blood stream. However one thing observed is that the surface of the skin is actually the same lichen/ algae inhabiting most of kerbin which explains how they are able to photosynthesize. this lichen does appear to have a symbiotic relationship with the Kerbals.{see effect on Kerbins ecosystem(coming soon)} Physical traits: As stated early Kerbals have a symbiotic relationship with said lichen which is yet to be named and this lichen has been observed to be super resistant to heat and impact. even managing to survive a impact from a interplanetary missile traveling at 4000 m/s without a scratch or burn. This is what makes Kerbals almost indestructible. a high speed impact will still be enough to cause an affect on the Kerbal causing a hidden sack inside of their left forearm toe spew out smoke as a defensive reaction allowing them to escape. Another important physical feature of a Kerbal is that their skulls and feet are made of around 50% of spongium carbonate a very bouncy material meaning if a Kerbal lands on their heads or feet they will merely bounce. Respiration/Breathing: Kerbals use aerobic respiration (6O+C6H12O6-->6CO2+ 6H2O) however this process combined with the photosynthesis allow for a infinite cycle of oxygen production/carbon dioxide consumption so as long as the kerbals get sunlight they respirate. they also lungs but they are not very strong and require this process to supplement getting oxygen into the blood stream.