Biology (9610) | Homeostasis and negative feedback

Welcome to the World of Homeostasis!

Hello Biologists! This chapter is incredibly important because it explains how your body (and all mammals) manages to stay perfectly stable inside, even when the outside world is changing dramatically. Think of it as your body's essential self-regulation system.

Understanding Homeostasis is key to understanding health and disease, and the core concept of Negative Feedback is a recurring theme throughout A-Level Biology. Let’s dive in and master how organisms maintain internal harmony!

Section 1: The Principles of Homeostasis (3.4.6.1)

What is Homeostasis?

Homeostasis is the maintenance of a stable internal environment within restricted limits, despite changes in the external environment.

Analogy: Think of a pilot flying an aeroplane. They constantly adjust the controls (rudder, ailerons) to keep the plane flying straight and level, even if they hit turbulence or crosswinds. Homeostasis is your body’s control tower, ensuring crucial variables don't drift too far off course.

Why Stability is Crucial: The Role of Enzymes

Maintaining a stable internal environment is vital because many biological reactions are controlled by enzymes. Enzymes are highly sensitive proteins whose shape (and therefore function) is determined by temperature and pH.

• Stable Core Temperature: If the core temperature becomes too high, enzymes (like those controlling metabolism) will denature (lose their specific 3D structure). If it is too low, the rate of enzyme-controlled reactions slows down significantly. Maintaining optimum temperature ensures maximum reaction rates.
• Stable Blood pH: Changes in pH alter the concentration of hydrogen ions (\(H^+\)). These ions disrupt the hydrogen bonds and ionic bonds that hold the enzyme’s tertiary structure, especially in the active site. A stable blood pH is necessary to keep enzyme structure intact and working efficiently.

Key Takeaway: Homeostasis ensures optimal conditions for enzyme activity, which is essential for life.

Section 2: The Role of Feedback Mechanisms (3.4.6.2)

Negative Feedback: The Restorative Mechanism

Negative feedback is the primary method used in homeostasis. It works to restore a system to its original level after a change has been detected.

Analogy: Imagine a simple shower mixer. If the water gets too hot (stimulus/change), you turn the cold tap up (response), which brings the temperature back down to the set point (restoration). The response negates the original stimulus.

Why We Need Separate Mechanisms

The syllabus highlights that the possession of separate mechanisms controlling departures in different directions gives a greater degree of control.

For example, in temperature control:

• If body temperature rises, we use mechanisms like sweating and vasodilation (to cool down).
• If body temperature falls, we use mechanisms like shivering and vasoconstriction (to warm up).

Having two distinct pathways ensures the body can react efficiently whether the variable is too high or too low, offering precise control around the set point.

Positive Feedback: The Amplifying Mechanism

Positive feedback occurs when the initial change causes a response that amplifies the original change, resulting in greater departures from the original set point.

This is generally *not* associated with stable control systems, but with processes that need a rapid conclusion.

• Example 1 (Functional): Contractions during childbirth. The stretching of the cervix causes the release of the hormone oxytocin. Oxytocin increases the strength of contractions, which stretches the cervix even more, leading to more oxytocin release, until the baby is born.
• Example 2 (Breakdown of Control): High Fever. If body temperature rises critically high (e.g., above 42°C), metabolic reactions accelerate out of control. This produces even more heat, leading to further temperature rise, often causing serious damage or death.

Memory Aid: Negative = Negates the change (restores). Positive = Pushes the change further (amplifies).

Section 3: Hormones and the Control of Blood Glucose Concentration (3.4.7)

Blood glucose control is the most important example of negative feedback you will study. Maintaining stable blood glucose is critical for two reasons:

1. It provides a constant supply of energy (glucose) for respiration, particularly for the brain.
2. It maintains the correct water potential of the blood. If glucose concentration is too high, the water potential of the blood is lowered, potentially causing water to leave cells by osmosis, leading to cell damage (dehydration).

Factors Influencing Blood Glucose

Blood glucose concentration changes frequently due to:

• Diet: Eating carbohydrates increases glucose concentration (absorption from the gut).
• Exercise/Respiration: Increased respiration decreases glucose concentration (used up by cells).
• Hormones: Insulin, Glucagon, and Adrenaline.

The Liver’s Crucial Role (3.4.7.1)

The liver is the central regulator of blood glucose, performing three key processes:

1. Glycogenesis: Conversion of glucose into the storage molecule glycogen. (Happens when glucose is HIGH).
2. Glycogenolysis: Breakdown of glycogen back into glucose. (Happens when glucose is LOW).
3. Gluconeogenesis: Synthesis of glucose from non-carbohydrate sources (like glycerol and amino acids). (Happens when glucose is VERY LOW, during starvation or intense exercise).

The Hormonal Control System

The pancreas, specifically the Islets of Langerhans, detects changes and releases hormones:

Hormone 1: Insulin (To Lower Glucose) (3.4.7.2)

Released by Beta cells when blood glucose concentration rises (e.g., after a meal). Insulin is a protein hormone that acts primarily on liver, muscle, and fat cells.

Insulin lowers blood glucose by:

1. Attaching to receptors on the surfaces of target cells.
2. Controlling the uptake of glucose: This signal causes the regulation/inclusion of channel proteins (specifically glucose transporters) into the cell surface membrane, allowing more glucose to enter the cell via facilitated diffusion.
3. Activating enzymes involved in the conversion of glucose to glycogen (Glycogenesis).

Hormone 2: Glucagon (To Raise Glucose) (3.4.7.3)

Released by Alpha cells when blood glucose concentration falls (e.g., during fasting).

Glucagon raises blood glucose by:

1. Attaching to receptors on the surfaces of target cells (primarily liver cells).
2. Activating enzymes involved in the conversion of glycogen to glucose (Glycogenolysis).
3. Activating enzymes involved in the conversion of glycerol and amino acids to glucose (Gluconeogenesis).

Hormone 3: Adrenaline (The Emergency Hormone) (3.4.7.4)

Released during stress or excitement (fight or flight). Adrenaline works similarly to glucagon to rapidly increase blood glucose for immediate energy use.

Adrenaline acts by:

1. Attaching to receptors on the surfaces of target cells.
2. Activating enzymes involved in the conversion of glycogen to glucose (Glycogenolysis).

The Second Messenger Model (3.4.7.4)

Glucagon and Adrenaline are non-steroid hormones (peptide hormones), meaning they cannot pass through the cell membrane. They use an indirect mechanism known as the second messenger model:

1. The hormone (first messenger) binds to a complementary receptor on the target cell surface.
2. The receptor-hormone complex activates an enzyme inside the cell, often adenyl cyclase.
3. Adenyl cyclase converts ATP into cyclic AMP (cAMP), which acts as the second messenger.
4. cAMP activates specific protein kinase enzymes.
5. These protein kinases initiate a cascade of reactions (phosphorylation), leading to the final effect (e.g., glycogenolysis).

Did you know? This cascade effect means that one single hormone molecule binding to the receptor can trigger the release of millions of glucose molecules inside the cell, providing massive amplification!

Diabetes Mellitus (3.4.7.1)

Diabetes is a condition where the body cannot effectively control blood glucose concentration.

Type 1 Diabetes (Insulin Dependent)

• Cause: An autoimmune response destroys the Beta cells in the pancreas. This means the body produces little or no insulin.
• Symptoms: High blood glucose (hyperglycaemia) and glucose often appears in the urine (glycosuria). Excessive thirst and weight loss are common symptoms, as the high glucose lowers the blood water potential, causing water loss via the kidneys.
• Control: Requires insulin injections (often multiple times daily) and strict diet manipulation.

Type 2 Diabetes (Non-Insulin Dependent)

• Cause: Either the body does not produce enough insulin, or the target cells do not respond effectively to the insulin (often due to fewer or less sensitive receptors on the target cell surfaces – insulin resistance).
• Risk Factors: Obesity, poor diet, lack of exercise, and genetic predisposition significantly increase the risk.
• Control: Often controlled primarily by diet manipulation, weight loss, and exercise. Medications may also be used to improve insulin sensitivity or stimulate insulin production.