Energy Metabolism and Hormonal Regulation

Overview

This lecture explains how the body maintains energy balance, especially focusing on glucose metabolism and its regulation by hormones, with a discussion of diabetes as a related disease.

Energy Balance and Mass Balance

• The body follows the law of mass balance: total energy = current energy + intake – energy used.
• Energy comes mainly from dietary fats, carbohydrates (especially glucose), and proteins.
• Glucose is essential for energy but must stay within a narrow concentration range to avoid hypoglycemia (low energy) or hyperglycemia (organ damage).
• Fats are a more efficient long-term energy storage form than carbohydrates.

Cellular Metabolism of Energy

• Glucose in cells is converted to glucose-6-phosphate, beginning glycolysis.
• Glycolysis (anaerobic) yields 2 ATP per glucose and produces pyruvate.
• Without oxygen, pyruvate is converted to lactic acid; with oxygen, it becomes acetyl CoA.
• Acetyl CoA enters the citric acid cycle, yielding 2 ATP, CO₂, and electron carriers.
• The electron transport system (aerobic, needs O₂) yields about 26 ATP.
• Total ATP from aerobic glucose metabolism: 30–32; from fatty acids: 28–30.
• Cells use both glucose and fatty acids for energy; proteins can contribute via amino acids.

Energy Storage and Use

• Glucose is stored as glycogen in liver and muscles; excess is converted to fat (adipose tissue).
• Glycogen storage is limited but can be increased by exercise.
• Proteins mainly serve structural roles but can be broken down for energy if needed.
• When energy is needed, glycogen, fats, and proteins can be broken down to supply glucose or fatty acids.

Hormonal Control of Blood Glucose

• Pancreatic beta cells secrete insulin (promotes glucose storage); alpha cells secrete glucagon (promotes glucose release).
• Insulin release is triggered by high blood glucose after meals, driving storage and decreasing blood glucose.
• Glucagon increases during fasting, stimulating glucose release from the liver.
• The relative levels of insulin and glucagon determine energy storage or mobilization.
• Insulin mainly impacts the liver, muscles, and adipose tissue to promote glucose uptake and storage.

Diabetes Mellitus

• Type 1 diabetes: Autoimmune destruction of beta cells, loss of insulin, results in high blood glucose, treated with insulin injections.
• Type 2 diabetes: Reduced sensitivity to insulin, often due to obesity and inactivity, managed with lifestyle changes and medication.

Key Terms & Definitions

• Glucose — Main blood sugar used for energy in cells.
• Glycogen — Storage form of glucose in liver and muscle.
• Glycolysis — Anaerobic process breaking down glucose for ATP.
• Citric Acid Cycle — Aerobic process generating ATP, CO₂, and electron carriers from acetyl CoA.
• Insulin — Hormone from pancreatic beta cells promoting glucose storage.
• Glucagon — Hormone from pancreatic alpha cells promoting glucose release.
• Hyperglycemia — Excessively high blood glucose.
• Hypoglycemia — Dangerously low blood glucose.

Action Items / Next Steps

• Review biochemical pathways of glycolysis, citric acid cycle, and electron transport.
• Understand insulin and glucagon actions and their impact on metabolism.
• Study differences between type 1 and type 2 diabetes.