Question

Which of the following metabolic processes only occur in the liver? (select all that apply)

Removal of nitrogen waste through the urea cycle

Glycogen degradation

Ketone body synthesis

Glycogen synthesis

Answer 1

CORRECT OPTION

Ketone Body Synthesis

Ketone bodies are synthesized by breakdown of fatty acids and ketogenic amino acids, through ketogenesis. This is occurred only in mitochondria of liver cells. When blood glucose level is low and glycogen stores are exhausted, ketogenesis occurs in liver cells. This process provides energy. Ketone bodies β-Hydroxybutyrate and acetoacetate produced in this process is an energy source for brain as it can cross blood brain barrier. Ketolysis cannot occur in liver as enzymes for ketone bodies degradation is absent in liver cells.

INCORRECT OPTIONS

Removal of Nitrogen waste through the urea cycle

Urea cycle take place mainly in the liver. Complete enzymes for urea cycle is present only in liver. But some of them are found in other organs also. Urea cycle is composed of series of biochemical reactions which occurs in mitochondria and cytoplasm of hepatocytes (liver cells). The nitrogen waste in the body is removed as highly toxic ammonia which is further converted into less toxic form urea in the urea cycle. The 80 percent of nitrogen waste is excreted as urea. This urea through bloodstream enter into kidneys and finally excreted in urine.

Glycogen degradation

The process of glycogen degradation is called glycogenolysis. It occurs in the muscle cells and liver cells. It can also occur to a lesser extent in the intestines and kidneys. Glycogen is a multi-branched and complex chain of glucose. To use glycogen as energy source by body, first it must be degraded into simpler molecules. In muscle cells, glycogen is degraded into glucose-6-phosphate to provide as substrate for glycolysis to provide energy for muscle contraction. In liver cells, glycogen degradation generates glucose, by first generating glucose-6-phosphate and then remove phosphate by an enzyme which is not present in muscle cells called glucose-6-phosphatase.

Glycogen synthesis

The process of glycogen synthesis is called glycogenesis. This is occurred in liver and skeletal muscle when the enough carbohydrates are available to restore the used one. In liver, glycogenesis is stimulated both by increased glucose availability and insulin level. After exercise of muscles, glycogen synthesis is high due to high blood glucose levels and insulin is available.

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