Ch 1 · 14.1 Homeostasis in mammals · AS & A Level Biology 9700

Topic 1 of 2 in Homeostasis — AS & A Level Biology 9700 — 98 lessons.

Introduction

Homeostasis keeps the body's internal environment within narrow limits, letting cells function efficiently despite external change. This lesson covers feedback control and two examples: the kidney's control of water content and the pancreas's control of blood glucose concentration, including how glucagon's signal is passed into a liver cell. (Syllabus section 14.1.)

Key ideas — Homeostasis and feedback control

Homeostasis is the maintenance of a constant internal environment (tissue fluid and blood plasma) within narrow limits, despite external or internal change. It matters because enzymes work only within narrow ranges of temperature, pH and solute concentration, and cells are damaged by osmotic stress.

🧑‍🏫 Teacher's explanation

Imagine you are driving a car on a highway. You want to maintain a constant speed of 100 km/h. If you see the speedometer drop to 90 km/h (the stimulus), your brain (coordination system) tells your foot to press the accelerator (effector). As the speed increases back to 100 km/h, you stop pressing the accelerator. This 'reversing' of the change is the essence of negative feedback.

📖 Definitions

🔍 Worked example — Easy

A student is measuring blood glucose using a biosensor. The sensor uses the enzyme glucose oxidase. Complete the sequence of events for the chemical reaction that occurs on the test strip.

🔍 Worked example — Standard

Explain the role of negative feedback in regulating blood glucose concentration when levels are too high.

🔍 Worked example — Exam-style

A patient has a high concentration of amino acids in their blood. Describe the process by which these are converted to urea in the liver.

⚠️ Common errors

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🎯 Exam question

Key ideas — Urea and the kidney

Excess amino acids cannot be stored. In the liver they are deaminated: the amino group is removed as ammonia, which is converted to urea, less toxic, and carried in the blood to the kidneys. The remaining part of the amino acid is respired or converted to glycogen or fat.

🧑‍🏫 Teacher's explanation

Think of your body like a high-end climate-controlled building. If the temperature gets too high or too low, the HVAC system kicks in to bring it back to a set point. This 'self-regulating' process is homeostasis. In biology, we don't just wait for things to go wrong; we use sensors to detect changes and effectors to fix them, always aiming to return to a 'normal' state.

📖 Definitions

🔍 Worked example — Easy

A patient has a blood glucose concentration of 5.5 mmol/L. After a meal, it rises to 8.5 mmol/L. Describe the homeostatic response to return the glucose to 5.5 mmol/L.

🔍 Worked example — Standard

Calculate the percentage decrease in blood glucose if it drops from 7.2 mmol/L to 4.8 mmol/L.

🔍 Worked example — Exam-style

Explain the role of the enzyme cascade in glucagon signaling.

⚠️ Common errors

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🎯 Exam question

(a) Explain how the concentration of glucose in a blood sample can be determined using this biosensor. [3] (b) Describe the role of the enzyme peroxidase in this process. [1] [4 marks]

Key ideas — Structure of the nephron

The nephron is the functional unit of the kidney; each region is structurally adapted to its function.

🧑‍🏫 Teacher's explanation

Think of the kidney as a highly advanced water treatment plant for your body. Instead of just letting all the water flow through, it uses a multi-stage process to ensure that valuable nutrients aren't wasted while toxic waste is removed.

📖 Definitions

🔍 Worked example — Easy

A student observes a diagram of a nephron. Identify the specific structure where ultrafiltration occurs and name the knot of capillaries involved.

🔍 Worked example — Standard

Explain how the structure of the Proximal Convoluted Tubule (PCT) is adapted for the selective reabsorption of glucose.

🔍 Worked example — Exam-style

A patient's urine analysis shows a significant presence of glucose (glycosuria). Based on your knowledge of nephron function, explain the most likely defect in the nephron.

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Key ideas — Ultrafiltration in the Bowman's capsule

🧑‍🏫 Teacher's explanation

Imagine a coffee machine with a very fine filter. You pour a mixture of water, coffee grounds, and sugar into it. The water and sugar pass through the filter into your cup, but the large coffee grounds are trapped and stay in the machine. Ultrafiltration in the kidney works almost exactly like this, but it uses pressure instead of gravity.

📖 Definitions

🔍 Worked example — Easy

A student observes a sample of glomerular filtrate under a microscope. They identify the presence of glucose and urea, but no red blood cells or albumin (a large plasma protein). Explain why this occurs.

🔍 Worked example — Standard

Calculate the percentage change in blood pressure within the glomerulus if the pressure increases from 30 mmHg to 45 mmHg.

🔍 Worked example — Exam-style

Compare the diameter of the afferent arteriole and the efferent arteriole and explain how this relates to ultrafiltration.

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🎯 Exam question

Key ideas — Selective reabsorption in the proximal convoluted tubule

Around 85% of the filtrate, including all glucose and amino acids, is reabsorbed in the PCT.

🧑‍🏫 Teacher's explanation

Imagine a busy airport security checkpoint. Most people (water, salts, glucose) are allowed to pass through easily, but some people (waste products) are kept in the queue. However, there are also 'VIP passengers' (glucose and amino acids) that the airport staff must actively grab from the crowd and pull through to the departures lounge so they aren't lost.

📖 Definitions

🔍 Worked example — Easy

Identify the two specific structural adaptations of the PCT epithelial cells that allow for efficient selective reabsorption.

🔍 Worked example — Standard

Calculate the amount of glucose remaining in the filtrate if the initial concentration in the glomerular filtrate is 5.0 mmol/L and the PCT reabsorbs 99\% of the glucose.

🔍 Worked example — Exam-style

Explain the role of the sodium-potassium pump in facilitating the reabsorption of glucose in the PCT.

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🎯 Exam question

(a) Describe how the structure of the PCT cell is adapted to its function in selective reabsorption. [3] (b) Explain how the concentration of glucose in the glomerular filtrate is reduced during its passage through the PCT. [3] [6 marks]

Key ideas — Osmoregulation: the loop of Henle, DCT, collecting duct and ADH

🧑‍🏫 Teacher's explanation

Think of osmoregulation like a smart thermostat in a house. If the air gets too dry, the humidifier turns on to add moisture; if it gets too humid, it turns off. In your body, the 'thermostat' is the hypothalamus, and the 'humidifier' is the collecting duct.

📖 Definitions

🔍 Worked example — Easy

A student is asked to identify the primary mechanism by which the loop of Henle creates an osmotic gradient in the renal medulla.

🔍 Worked example — Standard

Calculate the percentage change in urine volume if a person's urine volume decreases from 2.5 L/day to 1.2 L/day due to high ADH secretion.

🔍 Worked example — Exam-style

Explain the role of ADH in osmoregulation when blood water potential decreases.

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🎯 Exam question

Key ideas — Control of blood glucose concentration

🧑‍🏫 Teacher's explanation

Imagine your body is like a high-tech climate control system in a house. If the temperature gets too high, the air conditioning turns on to bring it back down; if it gets too cold, the heater kicks in. This 'self-regulating' process is called homeostasis. In your blood, glucose is the fuel. If it's too high, you're like a room with too much heat; if it's too low, you're freezing.

📖 Definitions

🔍 Worked example — Easy

A patient's blood glucose concentration is 5.5 mmol/L. After a carbohydrate-rich meal, the concentration rises to 8.5 mmol/L. Identify the hormone released and the primary target organs.

🔍 Worked example — Standard

Explain the role of glucagon in maintaining blood glucose levels during a period of fasting.

🔍 Worked example — Exam-style

A researcher is studying the cell signalling pathway of glucagon. The concentration of cAMP increases by a factor of 10^3 following hormone binding. If the initial concentration of cAMP was 2.0× 10^-9 M, calculate the final concentration.

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🎯 Exam question

(a) Explain how the concentration of blood glucose is maintained during this period. [3] (b) Describe the role of the enzyme glucose oxidase in a biosensor used to monitor blood glucose. [2] [5 marks]

Key ideas — Cell signalling: how glucagon works

Glucagon cannot cross the membrane, so its message is passed inside by a chain of events:

🧑‍🏫 Teacher's explanation

Imagine you are at a massive music festival. A VIP guest (the hormone) arrives at the front gate but cannot enter the crowd. Instead, they hand a special wristband (the signal) to a security guard (the receptor) at the gate. That guard taps a radio (the G-protein), which alerts a coordinator (adenylyl cyclase). The coordinator then hands out hundreds of identical glowing sticks (cAMP) to the crowd. Each person with a stick then grabs ten more people, spreading the message rapidly through the crowd. Eventually, the entire crowd starts dancing (the cellular response).

📖 Definitions

🔍 Worked example — Easy

Identify the specific role of cyclic AMP (cAMP) in the glucagon signalling pathway.

🔍 Worked example — Standard

Explain why a small concentration of glucagon can lead to a large release of glucose from the liver.

🔍 Worked example — Exam-style

A researcher observes that a mutation in a liver cell prevents the activation of adenylyl cyclase even when glucagon is present. Predict the effect of this mutation on blood glucose levels during fasting.

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🎯 Exam question

Key ideas — Measuring blood and urine glucose

🧑‍🏫 Teacher's explanation

Imagine you are driving a car on a highway. To stay in your lane, you constantly make tiny steering adjustments based on how close you are to the edge. This constant monitoring and correcting is exactly how your body maintains 'homeostasis'—keeping internal conditions like blood glucose stable despite external changes.

Ch 1 · 14.1 Homeostasis in mammals · AS & A Level Biology 9700

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