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IB DP Biology — C — Interaction and Interdependence | SL | C2.1 Chemical Signalling
📋 What You Will Learn
- Identify the major endocrine glands and the hormones they secrete, including their target tissues
- Explain how hormones travel through the bloodstream and bind to specific receptors to produce a cellular response
- Distinguish between the mechanisms of action of peptide hormones and steroid hormones at the cellular level
- Analyse data showing hormonal feedback loops, including the regulation of blood glucose by insulin and glucagon
- Apply understanding of negative feedback to evaluate how the endocrine system maintains homeostasis
- Evaluate the consequences of endocrine disruption, including Type 1 and Type 2 diabetes mellitus
🔍 Guiding Question How does chemical signalling allow cells and organs to coordinate their activities across the whole organism, and what happens when this coordination breaks down?
🌍 Why This Matters Over 537 million adults worldwide live with diabetes mellitus — a condition caused by the failure of just two hormones, insulin and glucagon, to maintain blood glucose within a narrow range. Understanding how hormones work is not abstract biochemistry: it is the difference between a functioning pancreas and a life-threatening metabolic crisis.
The Problem of Long-Distance Communication
A single nerve impulse travels at up to 120 metres per second — fast, precise, and local. But what if a cell in your liver needs to know what your stomach ate three hours ago? What if your kidneys need to respond to a drop in blood pressure that happened in your heart? The nervous system is too fast and too targeted for this kind of whole-body coordination. Evolution solved this problem with a second signalling system: the endocrine system.
Rather than sending electrical signals along dedicated wires, the endocrine system releases chemical messengers directly into the bloodstream. These messengers travel everywhere — but only cells carrying the right receptor will respond. It is a broadcast, not a telephone call. The elegance of this system lies in its specificity: the same molecule can reach every cell in the body, yet only trigger a response in exactly the right tissue.
📖 Key Definition — Hormone A hormone is a chemical messenger secreted by an endocrine gland directly into the blood, which is transported to a target organ where it produces a specific response by binding to a complementary receptor.
📖 Key Definition — Endocrine Gland An endocrine gland is a ductless gland that secretes hormones directly into the bloodstream, as opposed to exocrine glands which secrete substances through ducts to a surface or cavity.
📖 Key Definition — Target Cell A target cell is a cell that possesses specific receptor proteins complementary in shape to a particular hormone, allowing it to detect and respond to that hormone even at very low concentrations.
The Major Endocrine Glands
The endocrine system is not a single organ — it is a network of glands distributed throughout the body, each producing hormones that regulate different physiological processes. Understanding which gland produces which hormone, and what that hormone does, is the foundation of this entire topic.
The Hypothalamus and Pituitary Gland — The Master Controllers
The hypothalamus sits at the base of the brain and acts as the bridge between the nervous system and the endocrine system. It detects changes in blood composition (temperature, osmolarity, hormone levels) and responds by releasing or inhibiting releasing hormones that control the pituitary gland directly below it.
The pituitary gland is often called the "master gland" because its hormones regulate the activity of many other endocrine glands. The anterior pituitary releases hormones such as thyroid-stimulating hormone (TSH), adrenocorticotropic hormone (ACTH), and follicle-stimulating hormone (FSH). The posterior pituitary releases antidiuretic hormone (ADH) and oxytocin — both actually produced in the hypothalamus and stored in the posterior pituitary for release.
The Thyroid Gland
Located in the neck, the thyroid gland produces thyroxine (T4) and