Question

1. Liver metabolism during prolonged fasting.

A) What happens when your glucose stored as liver glycogen is gone. What options does the liver have to provide fuel to the brain?

B) During starvation muscle tissue is at first used, but then there is a shift to another fuel source. This saves muscle tissue necessary for life (such as muscles of the diaphragm needed for breathing). What is this alternative fuel source?

Answer 1

A) When the glucose stored as liver glycogen is gone, body strats to conserve the supply of sugar to most essiential tissues such as brain, red blodd cells and kidney which performs the critical functions within the body. However, the individual starts feeling tired.

When the glycogen is depleted, liver utilizes multiple other ways to make essential sugars, some of them are given below,

1) Use of glycerol and glucogenic amino acids into glucose for gluconeogenesis ( synthesis of glucose).

2) long-chain fatty acids and Triglycerides can not cross the brain cells due to their hydrophobicity. Hence liver starts producing ketones from fats, the process called ketogenesis. Ketogenesis occurs in the mitochndria of the liver cells. Ketogenesis makes acetone, acetoacetate, and beta-hydroxybutyrate molecules by breaking down fatty acids. These ketones are released into the blood stream and utilized by brain. β-ketoacyl-CoA transferase enzyme plays major role for the utilization of ketone bodies in the brain. Acetoacetate and β-hydroxybutyrate can be reconverted to acetyl-CoA. For each acetoacetate molecule that is oxidized in the mitochondria of the brain cells, 2 GTP and 22 ATP molecules are produced.

B) The another major fule sources during stratvation are following

1) Triglycerides which are mostly stored in the adipose tissues. By lipolysis these triglycerides are broken down to fatty acids and glycerol. Glycerol is further used for gluconeogenesis.

2) Liberated fatty acids undergo beta oxidation in the mitochondria of skeletel muscles, liver cells and cardiac muscles. Beta ixidation produce acetyl-CoA that enters the TCA cycle. moreover, it undergoes oxidative phosphorylation to produce ATP. These ATPs can be further used to produce sugars required by the body.