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MCAT TOPIC REVIEWS & RESOURCES

Biological and Biochemical Foundations of Living Systems

BC
Hormonal Regulation and Integration of Metabolism

Higher level integration of hormone structure and function

Hormones are signals, generally globular protein or specialized lipid in structure, produced in specific tissues (the endocrine system) and transported globally via circulation. To receive the signal, cells will produce receptors for a specific hormone of interest, present on the cell's membrane for polar hormones that do not cross the cell membrane, or present in the cytosol or nucleus for small, nonpolar hormones that are able to diffuse across the cell membrane.

Tissue specific metabolism

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:

  • glycogenesis (synthesis of glycogen, glucose storage)
  • glycogenolysis (breakdown of glycogen; glucose release)
  • gluconeogenesis (synthesis of glucose from non-carbohydrate sources)
  • ketogenesis (production of ketone bodies from acetyl-CoA in absence of insulin)
  • beta-oxidation (breakdown of fatty acids)
  • pentose phosphate pathway (lipid synthesis, ribose synthesis)
  • conversion of excess carbohydrates and proteins into fatty acids for storage
  • deamination of amino acids for synthesis of glucose or lipids
  • non-essential amino acid synthesis
  • urea synthesis (removes 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 a steady availability of blood glucose. It can also make use of ketone bodies produced in the liver from acetyl-CoA via ketogenesis.

Hormonal regulation of fuel metabolism

Insulin and glucagon are two hormones released from the pancreas that impact blood glucose levels. An increase in blood glucose levels triggers the release of insulin which in turn promotes glycolysis, glycogen synthesis in liver and skeletal muscle, fatty acid synthesis in the liver, and fatty acid storage in adipose tissue. When blood glucose levels are decreased, glucagon is released to promote beta-oxidation, glycogen breakdown in liver and skeletal muscle, gluconeogenesis in the liver, and release of fatty acids from adipose tissue.

Epinephrine and cortisol are two hormones release from the adrenal glands in response to stress. Epinephrine triggers glycogen breakdown, and cortisol promotes gluconeogenesis.

Obesity and regulation of body mass

Obesity is a prevalent metabolic disorder characterized by a body mass index (weight to height in kg/m2) higher than 30. It is associated with numerous factors that impact metabolic processes and regulation including caloric surpluses (diet and activity levels), and changing levels of hormones such as insulin (impacting energy storage and utilization) and leptin (impacting satiety).

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