Ch 4 · Homeostasis · O Level Biology 5090
Topic 4 of 5 in Coordination and Response — O Level Biology 5090 — 58 lessons.
Introduction
Cells and their enzymes work best at a particular temperature, water content and glucose concentration. Homeostasis is the set of mechanisms that hold these steady while the outside world changes. This lesson covers blood glucose, body temperature and the structures of the skin.
Key ideas — Homeostasis and negative feedback
Homeostasis is the maintenance of a constant internal environment. The internal environment means the conditions around the cells, chiefly the temperature, water content and glucose concentration of the blood and tissue fluid, for example core temperature held close to 37^circC.
🧑🏫 Teacher's explanation
Imagine your home heating system. If the temperature drops below 20^circC, the boiler turns on to heat the house. As the temperature rises back to 20^circC, the boiler turns off. It does not keep heating until the house is 30^circC; it stops exactly when the target is reached. This is negative feedback: the response opposes the change that triggered it.
📖 Definitions
🔍 Worked example — Easy
A student runs a marathon in hot weather. Their core temperature rises to 38.5^circC. Describe the sequence of events that returns their temperature to 37^circC, explicitly naming the control mechanism.
🔍 Worked example — Standard
After a large meal, blood glucose concentration rises to 9.5 mmol/dm^3. Explain how the body returns this to the normal range of 4.0 mmol/dm^3, specifying the hormone involved and its target organ.
🔍 Worked example — Exam-style
Explain why a person with Type 1 diabetes requires regular insulin injections, referring to the role of the pancreas and the concept of negative feedback.
⚠️ Common errors
🧠 Quick check 1
🧠 Quick check 2
🎯 Exam question
Key ideas — Control of blood glucose concentration
insulin and glucagon. Blood glucose is monitored by the pancreas, which acts both as the receptor and as the endocrine gland that responds. The liver is the main effector, storing glucose as the insoluble polysaccharide glycogen.
🧑🏫 Teacher's explanation
Think of your blood glucose level like the temperature in a room with a smart thermostat. The thermostat is the pancreas, which constantly checks the temperature (glucose level). If the room gets too hot (glucose rises after a meal), it turns on the air conditioning (insulin) to cool it down. If the room gets too cold (glucose falls during fasting), it turns on the heater (glucagon) to warm it up. The goal is not to make the room freezing or boiling, but to keep it steady at a comfortable set point.
📖 Definitions
🔍 Worked example — Easy
A student eats a large meal of carbohydrates. Describe the sequence of events that returns blood glucose concentration to its normal set point.
🔍 Worked example — Standard
Explain why insulin and glucagon are described as antagonistic hormones, referencing their effects on the liver.
🔍 Worked example — Exam-style
A patient with Type 1 diabetes has high blood glucose levels. Explain how the lack of insulin leads to this symptom and describe the treatment.
⚠️ Common errors
🧠 Quick check 1
🧠 Quick check 2
🎯 Exam question
(a) Describe the change in blood glucose concentration between time 0 and time 2. (b) Explain the role of insulin in returning blood glucose to its set point between time 2 and time 4. (c) Explain why blood glucose concentration rises between time 4 and time 6. (d) State the name of the hormone responsible for the rise in blood glucose between time 4 and time 6. (e) Explain why the blood glucose concentration does not fall to zero during the fasting period. [10 marks]
Key ideas — Type 1 diabetes
outline of treatment. In Type 1 diabetes the pancreas produces little or no insulin, so blood glucose rises to dangerous levels after meals. The signs are an increased blood glucose concentration and glucose in the urine. Treatment, the administration of insulin, replaces the missing control:
🧑🏫 Teacher's explanation
Imagine your blood glucose level is the temperature in a room with a faulty thermostat. In a healthy person, the pancreas acts as the thermostat: if the temperature (glucose) rises after a meal, it releases insulin to cool it down; if it drops, it releases glucagon to warm it up. This is negative feedback, keeping the system stable.
📖 Definitions
🔍 Worked example — Easy
Explain why insulin must be injected rather than swallowed in the treatment of Type 1 diabetes.
🔍 Worked example — Standard
A patient with Type 1 diabetes has a blood glucose concentration of 12 mmol/dm^3 after a meal. Describe the physiological response that would occur in a healthy person and explain why this response is absent in the patient.
🔍 Worked example — Exam-style
A student with Type 1 diabetes eats a large meal rich in carbohydrates. Explain how the student should manage their blood glucose concentration to prevent it from rising to dangerous levels, referencing the concept of negative feedback.
⚠️ Common errors
🧠 Quick check 1
🧠 Quick check 2
🎯 Exam question
Key ideas — Control of body temperature
Mammals keep a constant body temperature so that enzymes work at their optimum. The brain contains the temperature control centre: it monitors the temperature of the blood flowing through it, receives impulses from temperature receptors in the skin, and coordinates the responses of the skin and muscles.
🧑🏫 Teacher's explanation
Imagine your body is a house with a thermostat. The brain is that thermostat, constantly checking the temperature of the blood flowing through it. If the temperature drifts away from the set point (around 37^circC), the brain triggers responses to bring it back. This is homeostasis: the maintenance of a constant internal environment despite external changes.
📖 Definitions
🔍 Worked example — Easy
Explain how the skin helps to cool the body when the external temperature is high.
🔍 Worked example — Standard
Explain how the body maintains a constant temperature when the external temperature drops significantly.
🔍 Worked example — Exam-style
A student is exercising on a hot day. Their body temperature rises. Describe the sequence of events that returns their temperature to 37^circC, specifically mentioning the role of the brain and two skin structures.
⚠️ Common errors
🧠 Quick check 1
🧠 Quick check 2
🎯 Exam question
Key ideas — Structure of the skin
On a section through the skin, identify:
🧑🏫 Teacher's explanation
Think of your skin as a high-tech thermostat and radiator combined. It is not just a wrapper; it is a dynamic control system that constantly adjusts to keep your internal environment stable. When you are hot, the skin acts like a radiator that can be turned down or flushed with water; when you are cold, it insulates you like a duvet.
📖 Definitions
🔍 Worked example — Easy
Identify the function of the fatty tissue layer in the skin and explain how it contributes to homeostasis.
🔍 Worked example — Standard
Explain how the skin helps a mammal cool down when its body temperature rises above the set point.
🔍 Worked example — Exam-style
A student is shivering in a cold environment. Explain the role of the hair erector muscles and blood vessels in this response, and why shivering is necessary.
⚠️ Common errors
🧠 Quick check 1
🧠 Quick check 2
🎯 Exam question
Worked example
🔍 Worked example — Easy
A person eats a meal high in carbohydrates. Explain, in terms of negative feedback, how the blood glucose concentration is returned to its normal set point.
🔍 Worked example — Standard
Compare the roles of insulin and glucagon in the control of blood glucose concentration.
🔍 Worked example — Exam-style
A patient with Type 1 diabetes is given an injection of insulin. Explain why this treatment is necessary and how it restores homeostasis after a meal.
⚠️ Common errors
Common mistakes
Practice
Answers