14.21 Chapter Summary

1. Introduction to Homeostasis

What is Homeostasis?

• Definition: Homeostasis refers to the maintenance of a stable internal environment within an organism despite changes in external conditions.
• Importance in Mammals:
  • Optimal Functioning: Ensures that physiological processes operate within optimal ranges.
  • Survival: Maintains conditions necessary for survival, such as temperature, pH, and glucose levels.
  • Adaptation: Allows mammals to adapt to varying environmental stresses and demands.

2. Principles of Homeostasis

Internal and External Stimuli

• External Stimuli: Changes outside the body (e.g., temperature fluctuations, light, sound).
• Internal Stimuli: Changes within the body (e.g., blood glucose levels, oxygen concentration).

Receptors

• Function: Detect changes (stimuli) and send information to the coordinating center.
• Types:
  • Mechanoreceptors: Detect mechanical changes (e.g., touch, pressure).
  • Chemoreceptors: Detect chemical changes (e.g., pH, glucose levels).

Coordination Systems

1. Nervous System:
   • Structure: Comprises the brain, spinal cord, and nerves.
   • Function: Rapid response to stimuli through electrical impulses.
   • Effectors: Muscles and glands that execute responses.
2. Endocrine System:
   • Structure: Composed of glands (e.g., pituitary, thyroid, adrenal).
   • Function: Slower but longer-lasting responses via hormone secretion.
   • Effectors: Target cells and organs influenced by hormones.

Effectors

• Muscles: Respond to nervous stimuli by contracting or relaxing.
• Glands: Respond to hormonal signals by secreting substances (e.g., enzymes, hormones).

Negative Feedback Mechanisms

• Definition: Processes that counteract changes in the internal environment to maintain stability.
• Mechanism:
  • Stimulus Detection: Receptor detects a deviation from the set point.
  • Response Activation: Coordinating center sends signals to effectors.
  • Action by Effectors: Effectors act to reverse the deviation.
• Example: Regulation of body temperature (sweating to cool down, shivering to generate heat).

3. Nitrogen Excretion

Urea Production

• Location: Liver
• Process: Deamination of excess amino acids results in the formation of ammonia, which is converted to urea.
• Importance: Urea is less toxic than ammonia and is excreted via the kidneys.

4. Structure of the Human Kidney

Overview

• Function: Filter blood to remove waste products and excess substances, forming urine.

Key Structures:

1. Fibrous Capsule:
   • Description: A tough outer layer that encases the kidney.
   • Function: Protects the kidney and maintains its shape.
2. Cortex:
   • Location: Outer region beneath the fibrous capsule.
   • Components: Contains nephrons, glomeruli, and convoluted tubules.
   • Function: Site of ultrafiltration and initial urine formation.
3. Medulla:
   • Location: Inner region containing renal pyramids.
   • Components: Loop of Henle and collecting ducts.
   • Function: Concentrates urine by reabsorbing water and salts.
4. Renal Pelvis:
   • Description: Funnel-shaped cavity that collects urine from the medulla.
   • Function: Channels urine into the ureter.
5. Ureter:
   • Description: A muscular tube connecting the renal pelvis to the urinary bladder.
   • Function: Transports urine for excretion.
6. Blood Vessels:
   • Renal Artery: Supplies oxygenated blood to the kidneys.
   • Renal Vein: Returns deoxygenated blood from the kidneys to the circulatory system.

5. Structure of the Nephron

Overview

• Definition: The functional unit of the kidney responsible for filtering blood and forming urine.

Components:

1. Glomerulus:
   • Description: A network of capillaries.
   • Function: Filters blood to form glomerular filtrate.
2. Bowman’s Capsule:
   • Description: A double-walled capsule surrounding the glomerulus.
   • Function: Collects the filtrate from the glomerulus.
3. Proximal Convoluted Tubule (PCT):
   • Location: Immediately follows Bowman’s capsule.
   • Function: Reabsorbs essential nutrients, ions, and water from the filtrate.
4. Loop of Henle:
   • Structure: Descending and ascending limbs.
   • Function: Creates a concentration gradient in the medulla, facilitating water and salt reabsorption.
5. Distal Convoluted Tubule (DCT):
   • Location: Follows the loop of Henle.
   • Function: Regulates ion balance and pH through selective reabsorption and secretion.
6. Collecting Duct:
   • Description: Receives filtrate from multiple nephrons.
   • Function: Final concentration of urine by reabsorbing water under hormonal control.

Associated Blood Vessels:

• Peritubular Capillaries: Surround the nephron tubules, facilitating exchange of substances.
• Vasa Recta: Capillary networks adjacent to the loop of Henle, maintaining the concentration gradient.

6. Formation of Urine in the Nephron

1. Glomerular Filtration (Ultrafiltration)

• Location: Bowman’s capsule and glomerulus.
• Process:
  • Blood Pressure: Forces water and small solutes out of the blood into Bowman’s capsule.
  • Filtrate Composition: Contains water, glucose, ions, and waste products (e.g., urea).

2. Selective Reabsorption in the Proximal Convoluted Tubule (PCT)

• Process:
  • Reabsorption of Nutrients: Glucose, amino acids, and vitamins are reabsorbed into the blood.
  • Ion Transport: Sodium, potassium, and chloride ions are selectively reabsorbed.
  • Water Reabsorption: Follows osmotic gradients created by ion reabsorption.
• Outcome: Essential substances are returned to the bloodstream, reducing the volume of filtrate.

7. Structure-Function Relationship in Urine Formation

Bowman’s Capsule

• Structure:
  • Double-Walled: Outer layer of simple squamous epithelium and inner visceral layer with podocytes.
  • Podocytes: Specialized cells with foot-like projections forming filtration slits.
• Function:
  • Filtration Barrier: Allows passage of water and small solutes while retaining larger molecules like proteins and blood cells.
  • Protection: Prevents loss of essential proteins from blood into filtrate.

Proximal Convoluted Tubule (PCT)

• Structure:
  • Folding: Highly convoluted with microvilli (brush border) increasing surface area.
  • Cells: Rich in mitochondria to provide energy for active transport.
• Function:
  • Active Transport: Pumps sodium ions out of the tubule cells into the blood, facilitating reabsorption.
  • Passive Transport: Allows diffusion of reabsorbed substances into the blood.
  • High Reabsorption Efficiency: Approximately 65% of filtered sodium and water are reabsorbed here.

8. Osmoregulation Mechanisms

Key Components:

1. Hypothalamus:
   • Function: Detects blood osmolarity and regulates ADH release.
2. Posterior Pituitary Gland:
   • Function: Stores and releases antidiuretic hormone (ADH) based on signals from the hypothalamus.
3. Antidiuretic Hormone (ADH):
   • Function: Increases water permeability of the collecting ducts.
   • Mechanism: Binds to receptors on collecting duct cells, triggering insertion of aquaporin channels.
4. Aquaporins:
   • Function: Water channels that facilitate passive water movement from the filtrate into the interstitial fluid.
5. Collecting Ducts:
   • Function: Reabsorb water from filtrate under the influence of ADH, concentrating the urine.

Process of Osmoregulation:

• High Blood Osmolarity:
  • Detection: Hypothalamus senses increased osmolarity.
  • Response: ADH is released, promoting water reabsorption in collecting ducts.
  • Outcome: Dilution of blood plasma, reduced osmolarity.
• Low Blood Osmolarity:
  • Detection: Hypothalamus senses decreased osmolarity.
  • Response: ADH secretion is inhibited, reducing water reabsorption.
  • Outcome: Excretion of dilute urine, increased blood osmolarity.

9. Cell Signalling in Blood Glucose Regulation

Control of Blood Glucose by Glucagon

Signalling Pathway:

1. Hormone Binding:
   • Glucagon: Secreted by alpha cells of the pancreas.
   • Receptor: Binds to glucagon receptors on liver cell surfaces.
2. Conformational Change:
   • Effect: Binding induces a change in the receptor’s shape, activating the associated G-protein.
3. G-Protein Activation:
   • Function: Activates adenylyl cyclase enzyme.
4. Adenylyl Cyclase Activation:
   • Function: Converts ATP to cyclic AMP (cAMP), acting as a second messenger.
5. cAMP Role:
   • Function: Activates protein kinase A (PKA).
6. Protein Kinase A Activation:
   • Function: Initiates an enzyme cascade through phosphorylation of target enzymes.
7. Enzyme Cascade:
   • Effect: Amplifies the signal by activating multiple enzymes sequentially.
8. Cellular Response:
   • Outcome: Final enzyme catalyzes the breakdown of glycogen into glucose (glycogenolysis), increasing blood glucose levels.

Signal Amplification:

• Mechanism: Each activated PKA molecule can activate multiple enzymes, exponentially increasing the cellular response to a single glucagon molecule.

10. Negative Feedback Control of Blood Glucose

Regulation Mechanisms:

1. Insulin Action:
   • Secreted by: Beta cells of the pancreas.
   • Function on Muscle Cells:
     • Facilitates Glucose Uptake: Promotes the transport of glucose into muscle cells for energy or storage as glycogen.
   • Function on Liver Cells:
     • Enhances Glycogenesis: Stimulates conversion of glucose to glycogen for storage.
     • Inhibits Gluconeogenesis: Reduces production of new glucose molecules.
2. Glucagon Action:
   • Secreted by: Alpha cells of the pancreas.
   • Function on Liver Cells:
     • Stimulates Glycogenolysis: Promotes breakdown of glycogen to release glucose into the bloodstream.
     • Enhances Gluconeogenesis: Increases production of glucose from non-carbohydrate sources.

Negative Feedback Loop:

• High Blood Glucose:
  • Response: Increased insulin secretion.
  • Effect: Lowers blood glucose by promoting uptake and storage.
• Low Blood Glucose:
  • Response: Increased glucagon secretion.
  • Effect: Raises blood glucose by stimulating release from liver stores.

Outcome:

• Homeostasis Maintenance: Blood glucose levels are kept within a narrow, optimal range, preventing hyperglycemia and hypoglycemia.

11. Measurement of Glucose Concentration

Test Strips and Biosensors

Principles of Operation:

1. Enzymatic Reaction:
   • Glucose Oxidase:
     • Function: Catalyzes the oxidation of glucose to gluconic acid and hydrogen peroxide.
     • Reaction: Glucose + O₂ → Gluconic acid + H₂O₂
   • Peroxidase:
     • Function: Uses hydrogen peroxide to oxidize a colorless dye to a colored product.
     • Reaction: H₂O₂ + Dye (colorless) → H₂O + Dye (colored)
2. Signal Detection:
   • Color Change: The intensity of the color produced is proportional to the glucose concentration.
   • Measurement: The biosensor quantifies the color intensity, translating it into a glucose concentration reading.

Components:

• Test Strip:
  • Layers: Contains reagents (glucose oxidase, peroxidase, dye) that react with glucose.
  • Electrodes: Detect the electrical signal generated by the reaction.
• Biosensor Device:
  • Function: Analyzes the electrical signal from the test strip to determine glucose levels.
  • Display: Provides a digital readout of glucose concentration in blood or urine.

Advantages:

• Rapid Results: Quick measurement of glucose levels.
• Convenience: Portable and easy to use for regular monitoring.
• Accuracy: High specificity for glucose due to enzyme specificity.

Applications:

• Diabetes Management: Helps patients monitor blood glucose levels to manage insulin therapy.
• Medical Diagnostics: Used in clinical settings for glucose testing in blood and urine samples.

12. Stomatal Responses to Environmental Changes

• Stomata are microscopic pores found on the surfaces of leaves and stems that facilitate gas exchange. They play a crucial role in maintaining homeostasis by balancing two primary needs:

• Carbon Dioxide (CO₂) Uptake: Essential for photosynthesis.
• Water Loss Minimization: Prevented through transpiration.

Mechanism of Balance:

• Opening Stomata:
  • When to Open: During daylight when photosynthesis occurs.
  • Purpose: To allow CO₂ to diffuse into the leaf for photosynthesis.
  • Process: Guard cells absorb water, become turgid, and bend to open the stomatal pore.
• Closing Stomata:
  • When to Close: During high temperatures, drought conditions, or at night.
  • Purpose: To reduce water loss via transpiration.
  • Process: Guard cells lose water, become flaccid, and cause the stomatal pore to close.

Factors Influencing Stomatal Behavior:

• Light Intensity: Higher light promotes stomatal opening.
• Carbon Dioxide Concentration: High internal CO₂ can trigger stomatal closure.
• Humidity: Low humidity increases transpiration rates, prompting stomatal closure.
• Temperature: High temperatures can increase transpiration, leading to stomatal closure.
• Water Availability: Drought conditions induce stomatal closure to conserve water.

Balancing Act:

• Plants must optimize stomatal aperture to maximize CO₂ intake for photosynthesis while minimizing water loss. This balance is critical for plant survival, especially in varying environmental conditions.

13. Daily Rhythms of Stomatal Opening and Closing

• Stomatal behavior follows a diurnal (daily) rhythm, influenced by the day-night cycle and environmental cues.

Stomatal Opening:

• Time: Typically during daylight hours.
• Reason: Availability of light triggers photosynthesis, necessitating CO₂ uptake.
• Process:
  • Light activates proton pumps in guard cells.
  • Proton pumps expel H⁺ ions, creating an electrochemical gradient.
  • K⁺ ions enter guard cells via channels to balance charge.
  • Water follows ion movement osmotically, causing guard cells to swell and stomata to open.

Stomatal Closing:

• Time: Generally at night or during adverse conditions (e.g., drought).
• Reason: Photosynthesis ceases at night, reducing the need for CO₂ intake, and to conserve water.
• Process:
  • Lack of light causes proton pumps to deactivate.
  • K⁺ ions exit guard cells, reducing osmotic pressure.
  • Water exits guard cells, leading to loss of turgor pressure and stomatal closure.

Circadian Rhythms:

• Stomatal opening and closing are also regulated by internal circadian clocks, which anticipate daily environmental changes, ensuring stomata are optimally open or closed even before external signals change.

14. Structure and Function of Guard Cells and Stomatal Mechanism

Guard Cells:

• Location: Paired cells surrounding each stomatal pore on leaf epidermis.
• Shape: Typically kidney-shaped in dicots and dumbbell-shaped in monocots.
• Structure:
  • Thin Ends: Perpendicular to the stomatal pore, allowing easy shape changes.
  • Thick Ends: Parallel to the pore, providing structural support.
  • Plasmodesmata: Channels connecting guard cells to each other for coordinated function.

Function:

• Guard cells regulate the opening and closing of stomata by altering their turgor pressure in response to environmental and internal signals.

Mechanism of Stomatal Opening:

1. Signal Reception: Light, CO₂ levels, and internal signals (e.g., hormones) initiate the process.
2. Ion Transport:
   • K⁺ Influx: Potassium ions enter guard cells via specific channels.
   • Anion Efflux: Anions (e.g., Cl⁻, malate²⁻) exit guard cells to maintain charge balance.
3. Osmotic Adjustment:
   • The accumulation of K⁺ and anions increases the osmotic potential inside guard cells.
   • Water Movement: Water enters guard cells osmotically, increasing turgor pressure.
4. Cell Wall Flexibility: The unequal thickening of the guard cell walls allows them to swell and bow apart, opening the stomatal pore.

Mechanism of Stomatal Closing:

1. Signal Reception: Darkness, high internal CO₂, water stress, or abscisic acid (ABA) presence trigger closure.
2. Ion Transport:
   • K⁺ Efflux: Potassium ions exit guard cells.
   • Anion Influx or Retention: Anions may be retained or further effluxed depending on the signal.
3. Osmotic Adjustment:
   • Loss of K⁺ and anions decreases the osmotic potential inside guard cells.
   • Water Movement: Water exits guard cells osmotically, decreasing turgor pressure.
4. Cell Wall Flexibility: Reduced turgor pressure causes guard cells to become flaccid and bend inward, closing the stomatal pore.

Role of Structural Features:

• Plasmodesmata: Allow for the coordinated movement of ions and water between guard cells, ensuring synchronous stomatal responses.
• Shape of Guard Cells: The kidney or dumbbell shape facilitates effective movement during opening and closing.

15. Role of Abscisic Acid (ABA) and Calcium Ions in Stomatal Closure During Water Stress

Abscisic Acid (ABA):

• Nature: A plant hormone involved in stress responses, particularly drought.
• Production: Synthesized in roots and leaves during water stress conditions.

Role in Stomatal Closure:

1. ABA Accumulation: Under water stress, ABA levels increase in plant tissues.
2. Signal Transduction:
   • ABA is transported to guard cells where it binds to specific receptors.
3. Activation of Ion Channels:
   • ABA triggers the opening of anion channels (e.g., SLAC1) allowing anion efflux.
   • This causes membrane depolarization, activating outward-rectifying K⁺ channels, leading to K⁺ efflux.
4. Osmotic Adjustment: The efflux of anions and K⁺ reduces the osmotic potential inside guard cells.
5. Water Loss: Water exits guard cells, decreasing turgor pressure and leading to stomatal closure.

Calcium Ions (Ca²⁺) as Second Messengers:

• Role: Serve as intracellular signaling molecules that amplify and propagate the ABA signal within guard cells.

Mechanism Involving Ca²⁺:

1. ABA Binding: ABA binding to its receptors in guard cells initiates a signaling cascade.
2. Ca²⁺ Elevation: ABA triggers the release of Ca²⁺ from intracellular stores or the influx from the extracellular space.
3. Activation of Kinases: Increased Ca²⁺ activates protein kinases (e.g., CPKs – Calcium-Dependent Protein Kinases).
4. Phosphorylation of Targets: Kinases phosphorylate ion channels, enhancing their activity.
   • Anion Channels: Phosphorylated anion channels (e.g., SLAC1) open, facilitating anion efflux.
   • K⁺ Channels: Activation of K⁺ channels leads to K⁺ efflux.
5. Amplification of the Signal: Ca²⁺ acts to amplify the initial ABA signal, ensuring a robust response leading to stomatal closure.

Summary of ABA and Ca²⁺ Interaction:

• ABA binds to receptors → increases cytosolic Ca²⁺ → activates kinases → phosphorylates ion channels → leads to ion efflux and water loss from guard cells → stomatal closure.

Importance in Homeostasis:

• Water Conservation: Prevents excessive water loss during drought conditions, maintaining cellular hydration and overall plant homeostasis.
• Stress Response Coordination: Integrates environmental signals with physiological responses to optimize plant survival.

Key Terms and Concepts

• Stomata (plural of stoma): Pores on plant surfaces for gas exchange.
• Guard Cells: Specialized cells surrounding each stomatal pore, controlling its opening and closing.
• Transpiration: The process of water movement through a plant and its evaporation from aerial parts.
• Turgor Pressure: The pressure of the cell contents against the cell wall, crucial for maintaining cell rigidity.
• Abscisic Acid (ABA): A plant hormone involved in stress responses and stomatal regulation.
• Calcium Ions (Ca²⁺): Intracellular messengers involved in various signaling pathways.
• Osmosis: The movement of water across a selectively permeable membrane from a region of lower solute concentration to higher solute concentration.
• Ion Channels: Protein structures that allow ions to pass through the cell membrane.
• Second Messenger: A molecule that transmits signals from receptors to target molecules inside the cell.