Biology (9700) | Homeostasis

Welcome to Homeostasis: Keeping Life Balanced!

Have you ever wondered how your body manages to maintain a stable internal temperature, even when it’s freezing outside or boiling hot? That incredible stability is thanks to Homeostasis.

This A-Level chapter is fundamental because it connects many biological systems—from membranes and transport to hormones and excretion. We will break down the complex control mechanisms used by mammals and even look at how plants achieve balance. Don't worry if the detailed processes seem overwhelming at first; we will use clear steps and analogies to make them stick!

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14.1 Homeostasis in Mammals: The Principles of Internal Control

What is Homeostasis and Why is it Important?

Homeostasis is the maintenance of a constant internal environment (e.g., core temperature, pH, water potential, blood glucose concentration) within narrow limits, despite changes in the external or internal conditions.

Analogy: Think of a thermostat in your house. You set the temperature (the optimum condition), and the thermostat (the control system) works constantly to keep the temperature close to that setting, regardless of the weather outside.

Keeping conditions optimal is vital because:

• Enzymes require a specific temperature and pH to function efficiently, ensuring metabolism runs smoothly.
• Changes in water potential can cause cells to shrink or burst, damaging tissues.
• Stable conditions ensure maximum efficiency for all cellular functions.

The Homeostatic Control System (The Loop)

All homeostatic mechanisms follow a standard control loop. You must know the role of each component:

1. Stimulus: A change away from the optimum level (e.g., blood glucose concentration rises).
2. Receptor: Specialized cells (e.g., nerve endings or endocrine cells) detect the change (e.g., receptors in the pancreas detect high glucose).
3. Co-ordination System: Systems that communicate the message. These are the Nervous System (fast electrical impulses) and the Endocrine System (slower chemical hormones, like ADH or Insulin).
4. Effector: Muscles or glands that carry out the response (e.g., liver cells or sweat glands).
5. Response: The change brought about by the effector, which aims to return the body back to the optimum level.

Negative Feedback: The Core Principle

Homeostasis primarily uses Negative Feedback. This mechanism detects a deviation from the set point and initiates corrective actions that return the variable back toward the set point.

Key point: The response negates or reverses the initial stimulus.

If blood temperature rises, the response (sweating, vasodilation) causes the temperature to fall, reversing the initial rise.

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14.1.2: Osmoregulation – The Role of the Kidney

Osmoregulation is the homeostatic control of water potential in the blood and tissue fluid. The kidney is the master organ responsible for this, as well as for the excretion of metabolic waste, like urea.

Urea Formation

Urea is the primary nitrogenous waste product in mammals.
Urea is produced in the liver from the deamination of excess amino acids. Since amino acids cannot be stored, if they are in excess, their amine group must be removed and converted to less toxic urea for transport and excretion.

Structure of the Human Kidney (Limited Scope)

You need to recognize the main regions of the kidney:

• Fibrous Capsule: Protective outer layer.
• Cortex: Outer region, containing the Bowman’s capsules, proximal and distal convoluted tubules (PCTs/DCTs).
• Medulla: Inner region, containing the Loops of Henle and collecting ducts.
• Renal Pelvis: Collects urine before it passes down the ureter.
• Blood Supply: Renal artery branches deliver blood, and renal vein branches carry filtered blood away.

The Nephron: Functional Unit of the Kidney

Each kidney contains millions of nephrons. The nephron's main job is to filter blood and selectively reabsorb useful substances before the remainder is excreted as urine.

Key Nephron Structures & Associated Vessels:

(Remember these are microscopic structures you must recognize in diagrams/micrographs):

• Glomerulus: Network of capillaries.
• Bowman's Capsule (Renal Capsule): Cup-shaped structure surrounding the glomerulus.
• Proximal Convoluted Tubule (PCT)
• Loop of Henle
• Distal Convoluted Tubule (DCT)
• Collecting Duct: Leads down through the medulla to the renal pelvis.

Step 1: Ultrafiltration in the Bowman's Capsule

Blood arrives at the glomerulus via the afferent arteriole and leaves via the narrower efferent arteriole, creating high hydrostatic pressure.

This pressure forces small molecules (water, glucose, urea, mineral ions) out of the capillaries and into the Bowman's capsule space, forming the glomerular filtrate.

Detailed Structure-Function Link (14.1.7):

• The capillaries of the glomerulus have large pores (fenestrations).
• The basement membrane acts as the primary filter, preventing large proteins and blood cells from entering the filtrate.
• The Bowman’s capsule contains podocytes, specialized cells with finger-like extensions that wrap around the capillaries, leaving small gaps called filtration slits, allowing easy movement of filtrate.

Step 2: Selective Reabsorption in the Proximal Convoluted Tubule (PCT)

Most useful materials (glucose, amino acids, 80-85% of water and salt) are returned to the blood here.

Detailed Structure-Function Link (14.1.7):

• The cells lining the PCT have microvilli on the surface facing the lumen, greatly increasing the surface area for reabsorption.
• The cells are rich in mitochondria, providing ATP for active transport (e.g., pumping glucose, amino acids, and ions out of the tubule).
• The PCT walls are thin, creating a short diffusion distance.
• Water reabsorption occurs by osmosis, following the movement of actively transported solutes.

Remember: If everything useful was not reabsorbed here, you would lose liters of essential nutrients and water every hour!

The Mechanism of Osmoregulation (ADH)

The final water content of the urine is determined by hormonal control in the collecting duct and DCT.

This process controls blood water potential, involving three key components:

1. Receptors: Osmoreceptors in the hypothalamus detect changes in blood water potential (WP). If WP is low (blood is concentrated), they are stimulated.
2. Co-ordinator/Effector: The hypothalamus stimulates the posterior pituitary gland to release more Antidiuretic Hormone (ADH) into the blood.
3. Response: ADH travels to the collecting ducts (and DCTs).

The Role of ADH, Aquaporins, and Collecting Ducts (The Key Mechanism)

ADH makes the walls of the collecting duct more permeable to water. How?

1. ADH binds to receptor proteins on the collecting duct cell membrane.
2. This binding triggers a sequence of events within the cell.
3. It causes vesicles containing water channel proteins, called aquaporins, to fuse with the cell surface membrane.
4. The increased number of aquaporins allows more water to move out of the collecting duct lumen and into the tissue fluid (which is very concentrated due to the loops of Henle).
5. Water is thus returned to the blood, increasing blood water potential.

When blood water potential returns to normal, the osmoreceptors are less stimulated, the posterior pituitary releases less ADH, aquaporins are recycled back into vesicles, and less water is reabsorbed (Negative Feedback).

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14.1.3: Homeostasis of Blood Glucose Concentration

Blood glucose must be kept stable (around 90 mg per 100 cm³) to ensure a constant supply of substrate for cellular respiration. This is primarily controlled by two hormones produced by the pancreas: Insulin (lowers glucose) and Glucagon (raises glucose).

Control by Negative Feedback (Insulin and Glucagon)

When blood glucose rises (after a meal):

• Receptors: Beta cells in the Islets of Langerhans (pancreas) detect the rise.
• Response: Beta cells release Insulin.
• Effectors: Insulin targets liver, muscle, and fat cells. It increases the permeability of cell membranes to glucose (by causing the insertion of glucose carrier proteins) and stimulates the conversion of glucose into insoluble storage glycogen (glycogenesis) in the liver and muscles. This lowers the blood glucose.

When blood glucose falls (during exercise or fasting):

• Receptors: Alpha cells in the Islets of Langerhans (pancreas) detect the drop.
• Response: Alpha cells release Glucagon.
• Effectors: Glucagon targets liver cells, stimulating the breakdown of stored glycogen into glucose (glycogenolysis) and the synthesis of glucose from non-carbohydrate sources (gluconeogenesis). This raises the blood glucose.

The Detailed Glucagon Cell Signalling Cascade (Crucial for A2)

When glucagon binds to a liver cell, it doesn't enter the cell. Instead, it triggers an amplification cascade using a second messenger inside the cell.

(This process is an excellent example of cell signalling – converting an extracellular signal into an intracellular response).

1. Binding of hormone: Glucagon (the ligand/first messenger) binds to a specific cell surface receptor on the liver cell membrane. This causes the receptor to undergo a conformational change.
2. G-protein activation: The activated receptor stimulates the associated G-protein.
3. Adenylyl Cyclase: The active G-protein stimulates the membrane-bound enzyme, adenylyl cyclase.
4. Second Messenger Formation: Adenylyl cyclase catalyses the formation of cyclic AMP (cAMP) from ATP. cAMP acts as the second messenger inside the cell.
5. Enzyme Cascade Initiation: cAMP activates an enzyme called Protein Kinase A. This initiates a phosphorylation enzyme cascade (a series of enzyme activations via phosphorylation).
6. Cellular Response & Amplification: The enzyme cascade activates the final enzyme in the pathway (glycogen phosphorylase), which catalyses the breakdown of glycogen into glucose (glycogenolysis). The cascade ensures the signal is massively amplified.

Measuring Glucose: Test Strips and Biosensors (14.1.11)

To diagnose and monitor diabetes, glucose concentration must be measured accurately.

A simple test strip or a modern biosensor uses specific enzymes embedded in a small strip to detect glucose.

The principle relies on two enzymes:

1. Glucose Oxidase: Catalyses the oxidation of glucose.
2. Peroxidase: Uses the product of the first reaction to catalyse a reaction involving a dye, which causes a measurable colour change (test strip) or an electrical current change (biosensor). The intensity of the colour or current relates directly to the glucose concentration.

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14.2 Homeostasis in Plants: Stomatal Control

Plants also carry out homeostasis, particularly balancing the need for carbon dioxide uptake (for photosynthesis) against the need to minimize water loss (by transpiration) through the stomata.

Stomatal Response to Environmental Conditions

Stomata respond dynamically to changes in the environment:

• They generally open during the day to allow CO₂ uptake for photosynthesis.
• They exhibit natural daily rhythms of opening and closing, even in constant light, suggesting an internal clock controls them.
• They close in response to water stress or high temperatures to conserve water.

Mechanism of Stomatal Opening and Closing (Guard Cells)

Stomata are controlled by a pair of specialized cells called guard cells. Their structure is related to their function:

• They have unevenly thickened cell walls (thicker on the side next to the stoma).
• When guard cells swell, the thin outer wall stretches more easily than the thick inner wall, forcing the cells to curve outwards, opening the stoma.

How Guard Cells Open (Turgor increases):

1. Light stimulates guard cells to activate proton pumps (H⁺/K⁺ pumps) in their membrane.
2. Proton pumps actively transport H⁺ ions out of the guard cells, creating a large H⁺ gradient and acidifying the cell walls.
3. The electrical potential created drives K⁺ ions (and often Cl⁻ ions) into the guard cell via facilitated diffusion/channels.
4. The influx of solutes (K⁺ ions) lowers the water potential inside the guard cell.
5. Water enters the guard cell by osmosis, increasing turgor pressure, causing the guard cells to swell and the stoma to open.

The Role of Abscisic Acid (ABA) in Water Stress

During periods of water stress (drought), a plant releases the plant hormone Abscisic Acid (ABA). ABA acts as a stress signal, promoting stomatal closure to minimize water loss.

ABA Closure Mechanism:

1. ABA binds to receptors on the guard cell membrane.
2. This binding triggers the release of calcium ions (Ca²⁺) into the guard cell cytoplasm (Ca²⁺ acts as a second messenger here).
3. The presence of Ca²⁺ inhibits the uptake of K⁺ ions and stimulates the outflow of K⁺ ions and other solutes from the guard cell.
4. The loss of solutes increases the water potential inside the guard cell.
5. Water moves out of the guard cell by osmosis, turgor pressure decreases, and the guard cell becomes flaccid, causing the stoma to close.

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Key Takeaways for Homeostasis

Homeostasis is about maintaining dynamic equilibrium using negative feedback. For your exam, focus especially on the specific roles of the nephron sections in urine formation (ultrafiltration/selective reabsorption), the mechanism of ADH (aquaporins), and the detailed cell signalling cascade for glucagon (cAMP and enzyme phosphorylation).

Did you know? The human kidney filters about 180 liters of fluid per day, but only produces about 1.5 liters of urine. That's how efficient selective reabsorption is!