Jun 5, 2013
Medical Student
Hi guys, got something that's puzzling me a little. This is from my SMP, so it's not exactly an MCAT question but I remember something similar asked by AMCAS anyway

I understand that insulin stimulates glycogen formation/ glycolysis, and glucagon stimulates gluconeogenesis and glycogenolysis.

What confuses me is HOW cahill/cori cycles work

Let's theoretically say that someone is starving and glucagon is released, and there is a very low insulin level. That person should start producing free glucose in the liver, just as the diagram shows. Why however, does the muscle/brain/whatever cell undergo glycolysis. I understand that it NEEDS to, but I don't understand the contradictory regulation of this



5+ Year Member
Jul 29, 2011
OK, so you gotta understand that both muscle and liver have different roles.

Think of the liver as the altruistic organ in the body - it has to produce glucose first and send it off to other organs of the body that TRULY need the glucose.

Think of the muscles and brain as selfish organs in the body - they call dibs on glucose in order to quickly break glucose down into ATP/NADH/FADH2 to produce energy STAT for the body to function during extreme times (heavy exercise, running away from tiger, etc).

When you exercise/starve and are running out of ATP, those active muscles will build up lactate due to lack of O2. It wants to generate NAD+ fast in order to go through glycolysis. It will do this through the reaction aided by lactate dehydrogenase: (pyruvate + NADH <-> lactate + NAD+).

The lactate will move to the liver which undergoes the reverse reaction of lactate dehydrogenase, where it converts lactate to pyruvate & NADH. The liver undergoes gluconeogenesis in order to produce G6P --> glucose, which is sent off to other selfish tissues (like muscles and brain - which need the glucose first in order to break it down into ATP to serve the body).


Active muscles will build up lactate --> go to liver --> convert to pyruvate + NADH --> gluconeogenesis --> G6P --> glucose --> go to other tissues (like muscle).

Does this answer the question?