Preview
IB DP Biology — D: Continuity and Change | SL | D3.3
📋 What You Will Learn
- Identify the components of a homeostatic control system: receptor, control centre, effector, and feedback loop
- Explain how negative feedback maintains physiological variables within a narrow set point range
- Analyse data showing blood glucose regulation by insulin and glucagon as a worked example of negative feedback
- Explain thermoregulation in mammals as a second example of negative feedback involving antagonistic effectors
- Apply the principles of homeostasis to unfamiliar physiological contexts and evaluate the consequences of homeostatic failure
🔍 Guiding Question How do organisms maintain a stable internal environment in the face of a constantly changing external world — and what happens when that stability breaks down?
🌍 Why This Matters Type 1 diabetes is a disease of broken homeostasis: the pancreas no longer produces insulin, so blood glucose spirals out of control after every meal. Without external intervention — insulin injections timed to meals — the consequences are life-threatening. Understanding homeostasis is not abstract physiology; it is the biological logic behind every glucose monitor, every insulin pump, and every thermostat in a hospital incubator.
The Problem Every Living Cell Faces
Here is the problem the cell had to solve: enzymes only work within a narrow range of temperature, pH, and substrate concentration. Stray too far from that range and proteins denature, reactions stall, and the cell dies. Yet the external environment is relentlessly variable — temperatures swing, food arrives in bursts, exercise floods the blood with CO₂. Life needed a solution that could detect deviation and correct it automatically.
That solution is homeostasis. It is not passive stability — it is active, continuous correction. The body is never truly at rest; it is always measuring, comparing, and adjusting. Every organ system you will study in IB Biology is, in some sense, a homeostatic machine.
📖 Key Definition — Homeostasis Homeostasis is the maintenance of a relatively stable internal environment despite changes in internal and external conditions. It involves the regulation of variables such as body temperature, blood glucose concentration, blood pH, and water potential within narrow physiological limits.
The Architecture of a Control System
Before you can understand how the body regulates anything, you need to understand the universal architecture it uses. Every homeostatic mechanism — whether controlling glucose, temperature, or blood pressure — is built from the same four components. Learn this architecture once and you can apply it to any physiological system.
Component 1 — The Receptor (Sensor)
The receptor detects a change in the internal environment — a deviation from the set point. It does not correct the change; it simply monitors and reports. For example, thermoreceptors in the hypothalamus detect changes in blood temperature. Chemoreceptors in the islets of Langerhans detect changes in blood glucose concentration.
📖 Key Definition — Receptor A receptor (in the context of homeostasis) is a cell or structure that detects a change in a physiological variable and sends a signal to the control centre. It is distinct from a cell-surface receptor for hormones — context determines meaning.
Component 2 — The Control Centre (Comparator)
The control centre receives the signal from the