🫀 Strand D: Homeostasis

Course: SBI4U — Biology, Grade 12
Focus: How organisms maintain stable internal conditions through feedback mechanisms

1. What is Homeostasis?

Homeostasis is the maintenance of a stable internal environment despite changing external conditions. It relies on feedback mechanisms involving a stimulus, receptor, control centre, and effector.
Negative Feedback Loop Stimulus (change detected) Receptor (senses change) Control Centre (processes info) Effector (responds) REVERSES the change (negative feedback)

2. Key Systems

SystemWhat it RegulatesKey OrgansFeedback Type
ThermoregulationBody temperatureHypothalamus, skin, musclesNegative
Blood glucoseBlood sugar levelsPancreas (insulin/glucagon)Negative
Water balanceOsmolarityKidneys, ADH, hypothalamusNegative
Blood pressurePressure in arteriesHeart, baroreceptors, kidneysNegative
Childbirth (oxytocin)ContractionsUterus, pituitaryPositive

📝 Worksheet — Strand D: Homeostasis

1. [K/U — 3 marks] Describe the four components of a feedback loop (stimulus, receptor, control centre, effector) using the example of blood glucose regulation after eating a meal.

2. [Thinking — 3 marks] Explain why negative feedback is more common than positive feedback in biological systems. What would happen if body temperature were regulated by positive feedback instead?

3. [Communication — 4 marks] Draw a labeled diagram showing the negative feedback loop for blood glucose regulation. Include: pancreas, insulin, glucagon, liver, and blood glucose levels for BOTH high and low blood sugar scenarios.

4. [Application — 4 marks] A person with Type 1 diabetes cannot produce insulin.

a) Explain what happens to their blood glucose after eating carbohydrates.

b) Why must they inject insulin externally?

c) What could happen if they inject too much insulin?

SBI4U — Biology, Grade 12 | Strand D: Homeostasis | Ontario Curriculum 2008