During the process of continuous starvation for a long period of time, the primary source of energy in body is hepatic gluconeogenesis which is responsible for producing endogenous glucose. Also lipolysis takes place in the adipose tissue to release fatty acid which is then converted into ketone bodies through though mitochondrial β oxidation and ketogenesis. The liver communicates with extrahepatic tissues, including adipose tissue and skeletal muscle and deliver glucose and ketone bodies to muscle and other extrahepatic tissues as fuels.
It is a cycle because oxaloacetic acid (oxaloacetate) is the exact molecule needed to accept an acetyl-CoA molecule and start another turn of the cycle
it's a summary of the number of individuals of each age in the population. Age structure is useful in understanding and predicting population growth. An understanding of population age structure is critically important to industries that harvest living organisms.
