The major sites of metabolic activity in the body are the tissues in liver, skeletal and cardiac muscles, brain, and adipocytes; these tissues are involved in the synthesis and degradation of carbohydrate, lipid, and protein molecules.

The liver plays a significant role in metabolism, responding to changes in metabolite concentrations, hormone regulation, and systemic metabolic needs. It is involved in carbohydrate, lipid, and protein metabolism.

Carbohydrates metabolism maintains a constant level of blood glucose under a wide range of conditions. It includes the following processes:

1. Glycogenesis is the synthesis of glycogen from glucose.
2. Glycogenolysis is the breakdown of glycogen to glucose.
3. Gluconeogenesis is the synthesis of glucose from non-carbohydrate sources.

Lipid metabolism synthesizes and degrades lipid cells. It involves the following processes:

1. Ketogenesis is the production of ketone bodies from acetyl-CoA in the absence of insulin.
2. Beta-oxidation is the breakdown of fatty acids to generate acetyl-CoA.
3. Pentose phosphate pathway is the lipid and ribose synthesis.
4. Conversion of excess carbohydrates and proteins into fatty acids for storage

Protein metabolism is the synthesis and breakdown of proteins and amino acids. It includes

1. Deamination of amino acids for synthesis of glucose or lipids.
2. Synthesis of non-essential amino acid.
3. Urea synthesis or removal of ammonia.

Adipose tissue plays the role of long-term energy storage in the form of fatty acid storage (triglycerides) and release (free fatty acids). It is also a major site of the pentose phosphate pathway.

Skeletal muscle, like all cells, undergoes glycolysis but also utilizes local glycogen storage, employing glycogenesis and glycogenolysis.

Brain tissue has high metabolic needs and relies on the steady availability of blood glucose. It can also make use of ketone bodies produced in the liver from acetyl-CoA via ketogenesis.

Practice Questions

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MCAT Official Prep (AAMC)

Practice Exam 1 B/B Section Passage 10 Question 55

Key Points

• The liver plays a significant role in metabolism, responding to changes in metabolite concentrations, hormone regulation, and systemic metabolic needs; it is involved in carbohydrate, lipid, and protein metabolism.

• Adipose tissue plays the role of long-term energy storage in the form of fatty acid storage and release; it is also a major site of the pentose phosphate pathway.

• Skeletal muscle undergoes glycolysis but also utilizes local glycogen storage, employing glycogenesis and glycogenolysis.

• Brain tissue has high metabolic needs and relies on the steady availability of blood glucose; it can also make use of ketone bodies produced in the liver from acetyl-CoA via ketogenesis.

Key Terms

acetyl-CoA: a molecule that participates in many biochemical reactions in protein, carbohydrate and lipid metabolism

adipose tissue: a term for loose connective tissue composed of adipocytes; its main role is to store energy in the form of fat

glycolysis: the metabolic pathway that converts glucose into pyruvate

ketone bodies: water-soluble molecules containing the ketone group produced by the liver from fatty acids