Metabolism: general characteristics. Carbohydrate metabolism. Regulation of blood glucose level.
General Characteristics of Metabolism-
- Metabolism encompasses all reactions within the body, including anabolism (formation, e.g., glycogenesis) and catabolism (breakdown, e.g., glycolysis).
- All metabolic processes are driven by enzymes, whose activity is regulated to maintain metabolic control.
- Regulation of metabolism occurs via:
- Nervous regulation: The central nervous system signals endocrine glands to modify secretions.
- Humoral regulation: Hormones regulate enzyme synthesis/breakdown and alter cell membrane permeability.
- Feedback regulation: Enzymes can be inhibited by their products.
Carbohydrate Metabolism.
- Carbohydrates are broken down in the GI tract into simpler molecules for metabolism.
- Glucose is the most important carbohydrate, serving as the primary energy source.
- Metabolic fates of glucose:
- Used to produce ATP.
- Stored as glycogen in the liver or muscles.
- Converted into fatty acids and stored as triglycerides in adipose tissue.
- Key processes in carbohydrate metabolism:
- Glycolysis: Glucose oxidation to generate ATP and pyruvate; pyruvate enters the Krebs cycle in mitochondria for further energy production.
- Pentose Phosphate Pathway: Converts hexoses to pentoses (for nucleotide synthesis) and NADPH (for lipid synthesis).
- Glycogenesis: Excess glucose stored as glycogen.
- Glycogenolysis: Breakdown of glycogen into glucose.
- Gluconeogenesis: Synthesis of glucose from non-carbohydrate sources (e.g., amino acids).
- Summary of Glucose Metabolism: Pyruvate oxidation leads to acetyl coenzyme A, which enters the Krebs cycle to generate energy carriers (NADH, FADH) and GTP for ATP synthesis.
Regulation of Blood Glucose Level.
- Blood glucose levels indicate the amount of glucose circulating in the blood.
- Humoral regulation of blood glucose:
- High blood glucose: Pancreatic β-cells release insulin, promoting glucose conversion into glycogen (storage).
- Low blood glucose (Hypoglycemia): Pancreatic α-cells release glucagon, while epinephrine, cortisol, and growth hormone are also released to raise blood sugar.
- Neural regulation of blood glucose:
- Sympathetic stimulation: Promotes carbohydrate catabolism (breakdown).
This is done via the Hypothalamus sending signals (via hormones) to Pituitary gland and the pituitary to Thyroid gland and Adrenal gland (epinephrine & cortisol). (e.g. epinephrine activates Protein kinase A which breaks down Glycogen → Glucose)
- Parasympathetic stimulation: Promotes carbohydrate anabolism (formation of stores).
Recap Questions
- What are the two main categories of metabolic reactions, and what are some examples of each?
- Describe the three main metabolic fates of glucose.
- Explain the key processes involved in carbohydrate metabolism.
- How do insulin and glucagon regulate blood glucose levels?
- What are the roles of the sympathetic and parasympathetic nervous systems in blood glucose regulation?