Lesson Notes By Weeks and Term v5 - Grade 12

Lesson Video

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Performance objectives

• Describe how steroid and non-steroid hormones act on cells.
• Explain the role of the adrenal glands in stress and electrolyte balance.
• Explain how blood glucose levels are regulated by negative feedback.
• Describe the causes and symptoms of diabetes.
• Outline the role of the thyroid gland in metabolism.

Lesson summary

The endocrine system is a critical communication network within our bodies. It works in conjunction with the nervous system to maintain homeostasis, the stable internal environment essential for survival. This is particularly important in the context of South Africa, where access to resources and varying environmental conditions can significantly impact physiological processes. Understanding how our bodies regulate everything from blood sugar to water balance is vital for making informed choices about our health and well-being. This week, we will delve deeper into the mechanisms of hormone action and focus on specific endocrine glands and their roles in homeostasis.

Lesson notes

2.1 Mechanisms of Hormone Action Hormones, chemical messengers produced by endocrine glands, travel through the bloodstream to target cells. Their effect depends on the presence of specific receptors on or within these target cells. There are two main types of hormone action: Steroid Hormones: These hormones (e.g., cortisol, aldosterone, testosterone, estrogen) are lipid-soluble. They can diffuse directly across the cell membrane and bind to receptors located in the cytoplasm or nucleus. The hormone-receptor complex then binds to specific DNA sequences, influencing gene transcription and ultimately protein synthesis. This leads to a slower but more sustained effect.

Example: Cortisol, released by the adrenal cortex, can influence glucose metabolism by increasing the production of enzymes involved in gluconeogenesis (the formation of glucose from non-carbohydrate sources) in liver cells. Non-Steroid Hormones (Protein and Peptide Hormones): These hormones (e.g., insulin, glucagon, adrenaline, growth hormone) are water-soluble and cannot directly enter the cell. They bind to receptors on the cell membrane. This binding triggers a cascade of events within the cell, often involving second messengers.

Second Messenger Systems: Common second messengers include cyclic AMP (cAMP) and calcium ions (Ca 2+ ). The hormone-receptor complex activates an enzyme (e.g., adenylate cyclase) that produces the second messenger. The second messenger then activates other enzymes within the cell, leading to a cellular response.

Example: Adrenaline binds to receptors on liver cells, activating adenylate cyclase, which increases cAMP levels. cAMP activates protein kinases, which phosphorylate other enzymes, ultimately leading to the breakdown of glycogen into glucose (glycogenolysis) and increasing blood glucose levels. 2.2 The Adrenal Glands: Stress, Electrolytes, and Blood Pressure The adrenal glands, located on top of the kidneys, have two main regions: the adrenal cortex and the adrenal medulla.

Adrenal Cortex: This outer layer produces steroid hormones.

Cortisol (Glucocorticoid): Regulates glucose metabolism, increases blood glucose levels (gluconeogenesis), suppresses the immune system, and helps the body cope with long-term stress. In South Africa, chronic stress due to socio-economic factors can lead to elevated cortisol levels, contributing to conditions like insulin resistance and increased risk of cardiovascular disease.

Aldosterone (Mineralocorticoid): Regulates electrolyte balance, specifically sodium (Na + ) and potassium (K + ) levels. It acts on the kidneys to increase Na + reabsorption and K + excretion. This also leads to water retention, increasing blood volume and blood pressure. In communities with limited access to clean water, maintaining proper electrolyte balance is crucial for survival.

Adrenal Medulla: This inner region produces catecholamines (adrenaline and noradrenaline). Adrenaline (Epinephrine) and Noradrenaline (Norepinephrine): These hormones prepare the body for "fight or flight" responses to immediate stress. They increase heart rate, blood pressure, breathing rate, and blood glucose levels (by stimulating glycogenolysis in the liver and muscles). They also divert blood flow to the muscles and brain. 2.3 Blood Glucose Regulation Maintaining stable blood glucose levels is crucial for providing energy to cells. The pancreas, liver, and adrenal glands play key roles in this process.

Pancreas: Contains the islets of Langerhans, which secrete two main hormones: Insulin (Beta cells): Decreases blood glucose levels by: Stimulating glucose uptake by cells (especially muscle and adipose tissue). Promoting glycogenesis (the conversion of glucose to glycogen in the liver and muscles). Inhibiting gluconeogenesis.

Glucagon (Alpha cells): Increases blood glucose levels by: Stimulating glycogenolysis (the breakdown of glycogen into glucose in the liver). Stimulating gluconeogenesis.

Liver: The liver is the primary site for glycogenesis, glycogenolysis, and gluconeogenesis, acting as a glucose buffer in the blood.

Adrenal Glands (Adrenaline & Cortisol): Adrenaline and Cortisol also contribute to raising blood glucose levels, especially during stress. Negative Feedback Loop (Blood Glucose Regulation): High Blood Glucose: Pancreas releases insulin. Insulin stimulates glucose uptake by cells and glycogenesis in the liver, decreasing blood glucose.

Low Blood Glucose: Pancreas releases glucagon. Glucagon stimulates glycogenolysis and gluconeogenesis in the liver, increasing blood glucose. 2.4 Diabetes Mellitus Diabetes mellitus is a chronic metabolic disorder characterized by hyperglycemia (high blood glucose levels).

There are two main types: Type 1 Diabetes (Insulin-Dependent): An autoimmune disorder where the body's immune system destroys the beta cells in the pancreas, leading to insulin deficiency. Requires lifelong insulin injections or pump therapy.