M3: Integration of Pathways L23

Question 1

What happens when you were fasting and then start eating? (intestine, liver, muscle, adipose)

Answer 1

1. Intesitne: Were in a fasted state and now you start eating. Glucose and TG uptake from food by transporters all the way to the blood. TGs are lypolized, then reincorporated into chylomicrons and then circulate in the blood.
2. Liver: Glucose is taken up into liver, FAs are also taken up into the liver. The liver needs to store this energy either in the form of glycogen or as TGs.
3. Muscle: FAs can also be stored in muscle in the form of TG and glucose as glycogen
4. Adipose tissue: glucose and FA are also taken up. In both the muscle and adipose tissues, glucose uptake is modulated by insulin. Adipose turns glucose into acetyl-CoA which is converted to citrate which activates the synthesis of FAs because the CAC is shut down. The FAs are converted to TGs right away.

Question 2

What happens in the fasting state? (intestine, liver, muscle, brain & heart, adipose)

Answer 2

1. Intestines: We dont have this input of energy coming from foods (glucose and TAGs).
2. We have our storage of energy in the liver, muscle, and plenty of fatty acids in adipose tissue. What happens when we start exercising?
3. Muscle: First, ATP pool broken down, creatine phosphate, free glucose, glycogen, anaerobic, TGs, then proteins. So: Glycolysis, then beta oxidation then proteolysis. This is going to generate some energy for the muscle to use in situ (in muscle). Also makes lactate (garbage) from anaerobic glycolysis which will return to the liver to be further converted back to glucose.
4. Adipose tissue: FAs mobilized as a source of energy for the transport of these FAs to the muscle. Start Beta oxidation in adipose tissue to transport FAs to muscle by Albumin.
5. Liver: Low blood glucose triggers the breakdown of glycogen and TGs in the liver. Eventually, when you run out of glycogen, gluconeogenesis is turned on. Ketone bodies also start being produced.
6. Ketone bodies circulate in blood and are taken up by brain and heart to generate energy. The brain and the heart have NO FUEL RESERVES. This is why they are so dependent on circulating glucose, FAs (heart), or ketone bodies.

Question 3

Explain the opposing roles of insulin and glucagon.

Answer 3

In the fed state, insulin is produced and released from pancreas due to high blood glucose. This triggers the synthesis of glycogen in the liver. In contrast, in the fasting state, glucagon is released from pancreas, breaks down glycogen in liver to regenerate glucose to make sure that the blood glucose rises again.