Last time we defined life, and talked a little about some of the problems specific to a multicellular organism. At the center of our definition of life were metabolism and reproduction. The other characteristics of life were really things that are required for an organism to continue to metabolize and reproduce, so the reason an organism must acquire energy and building blocks, transport materials, sense and respond to the environment, and communicate within an organism is to allow metabolism to continue. Death is the end of metabolism.
Extinction is the end of reproduction.
The rate of metabolism is a key feature of life. It s the rate at which cells can burn fuel and do work. That can be a good thing, or a bad thing, because too little heat will kill you, and so will too much heat. Endotherms use metabolism to regulate the heat Of their bodies, so that at below body temperature the metabolism is high, and in the comfortable range the metabolic rate stays low, while at high temperature the metabolic rate increases because the animal can t stop the rate increase. The reason they can t stop the rate increase is that chemical reactions happen faster and faster with increasing heat until the heat gets so high that the enzymes that catalyze it denature.