A2 Level Biology Chapter 14: Homeostasis Study Notes

Introduction to Homeostasis

  • Definition of Homeostasis: The maintenance of a constant internal environment. It involves keeping a set-point stable within narrow limits in the body, irrespective of changes in the external environment.

  • Importance of Maintenance: Factors such as temperature, water potential, and glucose concentration must be maintained in the tissue fluid. This is crucial because fluctuations affect cell function.

  • Stability and Function: Homeostasis allows the internal environment to remain stable and function optimally.

  • Control Systems: Most control systems in the body work via the negative feedback mechanism.

The Negative Feedback Mechanism

  • General Principle: Continuous monitoring of a factor affecting the internal environment results in many "corrective actions." This causes the factor to fluctuate around the norm or set-point rather than staying perfectly static.

  • Components of the Feedback Loop:

    • Stimulus: Refers to an internal or external change in a factor away from the norm/set-point.

    • Receptor: Cells, tissues, or organs that detect the stimulus.

    • Control Center (Coordinating Centre): Consists of tissue that receives and processes messages from the receptors. These messages are in the form of hormones or nerve impulses. The center determines the appropriate response.

    • Threshold: Messages are only sent to an effector if the stimulus reaches a certain threshold or is strong enough.

    • Effector: Tissues or organs that receive messages from the coordinating center and carry out a corrective reaction.

    • Response: The reaction carried out by the effectors. In negative feedback, the response counteracts the original stimulus to return the factor to the set-point/norm.

Positive Feedback vs. Negative Feedback

  • Negative Feedback:

    • Action: Response counteracts the original stimulus.

    • Occurrence: Very common in the body.

    • Purpose: To maintain homeostasis and a stable internal environment.

    • Examples: Maintaining blood glucose levels, body temperature, oxygen levels, and water content in the blood.

  • Positive Feedback:

    • Action: Response reinforces the original stimulus.

    • Occurrence: Very uncommon.

    • Result: The response worsens or intensifies the initial change.

    • Examples: Labor pains (childbirth), the ripening of fruit, and the inhalation of CO2CO_2.

Excretion and Nitrogenous Waste Products

  • Definition of Excretion: The removal of unwanted products of metabolism which are toxic or poisonous and would cause tissue damage if allowed to accumulate.

  • Main Excretory Products:

    • Carbon Dioxide (CO2CO_2): Produced during aerobic respiration. It is excreted via the bloodstream and the lungs.

    • Urea: The main nitrogenous waste product produced in the liver from excess amino acids. It is excreted via the kidneys.

    • Creatinine: Produced in the liver from certain amino acids. While most is used for energy storage in muscles, small amounts are excreted by the kidneys.

    • Uric Acid: Produced in the liver from the breakdown of excess purines from nucleotides. It is excreted via the kidneys.

Urea Production: Deamination and the Urea Cycle

  • Step 1: Deamination: Occurs in liver cells. The amine group and a hydrogen atom are removed from an excess amino acid. This produces ammonia (NH3NH_3), which is highly toxic if it accumulates.

  • Step 2: Urea Cycle (Ornithine Cycle): Ammonia is combined with carbon dioxide to produce urea:

    • NH3+CO2→ureaNH_3 + CO_2 \rightarrow \text{urea}

  • Keto Acid: The remaining part of the amino acid (the keto acid) is either respired or converted into glucose, glycogen, or fat.

Anatomy of the Urinary System and Kidney

  • Blood Vessels:

    • Renal Artery: Carries blood into the kidney.

    • Renal Vein: Carries filtered blood out of the kidney.

  • Excretory Tubes:

    • Ureter: Carries urine from the kidney to the urinary bladder.

    • Urethra: Carries urine out from the urinary bladder to the exterior.

  • Kidney Structure:

    • Capsule: A tough, protective outer layer.

    • Cortex: The outer region of the kidney.

    • Medulla: The middle region, containing renal pyramids.

    • Pelvis: The central collecting region leading to the ureter.

    • Calyces: Extensions of the pelvis that channel urine from the medulla.

Structure of the Nephron

  • Nephrons are tiny tubes located across the cortex and medulla.

  • Major Parts:

    1. Bowman’s Capsule (Renal Capsule): Located in the cortex.

    2. Proximal Convoluted Tubule (PCT): Located in the cortex.

    3. Loop of Henle: Located in the medulla. It consists of the descending limb and the ascending limb.

    4. Distal Convoluted Tubule (DCT): Located in the cortex.

    5. Collecting Duct: Located in the medulla, leading to the ureter in the pelvis.

  • Vascular Components:

    • Afferent Arteriole: A branch of the renal artery leading into the glomerulus.

    • Glomerulus: A tangle of capillaries sitting within the cup of the Bowman's capsule.

    • Efferent Arteriole: Carries blood away from the glomerulus. It has a narrower lumen than the afferent arteriole.

    • Peritubular Capillaries: A network of blood capillaries surrounding the rest of the nephron, eventually joining the renal vein.

Mechanism of Excretion: 1. Ultrafiltration

  • Definition: The filtering of small molecules out of the blood in the glomerulus into the lumen (space) of the Bowman’s capsule under high pressure.

  • Structural Layers of the Filtration Barrier:

    1. Endothelium of Glomerular Capillaries: Contains many gaps or fenestrations allowing substances to pass.

    2. Basement Membrane: A mesh of collagen and glycoprotein fibers. It acts as the main selective barrier or filter.

    3. Epithelial Cells of Bowman’s Capsule (Podocytes): These cells have finger-like projections called foot processes that wrap around the capillaries, leaving gaps known as filtration slits.

  • Pressure Dynamics: The diameter of the afferent arteriole is wider than the efferent arteriole. This creates high hydrostatic pressure in the glomerulus, forcing fluid out into the capsule.

  • Molecular Selectivity: Small molecules pass through, but the basement membrane prevents the passage of Red Blood Cells (RBCs), White Blood Cells (WBCs), and large plasma proteins with a Relative Molecular Mass (RMM) greater than 68,000 Da68,000\,Da.

  • Glomerular Filtrate Composition: Contains water, amino acids, glucose, urea, inorganic ions (Na1Na^1, K1K^1, Cl−Cl^{-}), uric acid, creatinine, and vitamins.

Mechanism of Excretion: 2. Selective Reabsorption

  • Purpose: To reabsorb essential substances from the glomerular filtrate back into the blood.

  • Process in the Proximal Convoluted Tubule (PCT):

    • Glucose and Amino Acids: 100%100\% are reabsorbed via active transport.

    • Water: Approximately 65%65\% is reabsorbed passively via osmosis.

    • Ions: Na1Na^1, Cl−Cl^-, and vitamins are reabsorbed.

    • Urea: Reabsorbed passively.

    • Waste: Uric acid and creatinine are not reabsorbed; creatinine is actively secreted into the lumen.

Cellular Mechanism of Reabsorption in the PCT

  1. Sodium-Potassium Pump: In the basal/basolateral membrane (facing the blood), Na1Na^1 ions are actively transported out of the PCT cells into the blood, and K1K^1 ions are moved in. This creates a low concentration of Na1Na^1 inside the cell.

  2. Co-transport: Na1Na^1 ions in the PCT lumen diffuse down their concentration gradient into the PCT cells through co-transporter proteins in the apical membrane (microvilli). They carry glucose, amino acids, vitamins, or Cl−Cl^- ions with them via facilitated diffusion.

  3. Facilitated Diffusion to Blood: Glucose and other solutes then diffuse out of the PCT cells into the blood through specific transport proteins.

  • Adaptations of PCT Cells:

    • Microvilli: Increase surface area for absorption.

    • Transport Proteins: Co-transporters, ion pumps, and aquaporins.

    • Basal Membrane Infolding: Increases surface area for transport into blood.

    • Mitochondria: High density provides ATPATP for active transport.

    • Tight Junctions: Hold cells together to ensure substances pass through cells, not between them, and to separate apical and basal membrane proteins.

Selective Reabsorption in the Loop of Henle (LOH)

  • Descending Limb: Permeable to water, Na1Na^1, and Cl−Cl^- ions.

  • Ascending Limb: Impermeable to water but permeable to Na1Na^1 and Cl−Cl^- ions.

  • The Process:

    1. In the ascending limb, Na1Na^1 and Cl−Cl^- are actively transported out into the medulla tissue fluid.

    2. This increases the solute concentration in the medulla, making the renal fluid in the tubule more dilute (higher water potential).

    3. In the descending limb, water moves out into the medulla by osmosis and is reabsorbed into the blood.

    4. Na1Na^1 and Cl−Cl^- diffuse into the descending limb. Consequently, the filtrate becomes most concentrated at the base of the loop.

  • Length Factor: A longer Loop of Henle allows for a higher concentration of solutes in the medulla, leading to more water reabsorption and more concentrated urine.

Selective Reabsorption in the DCT and Collecting Duct

  • Distal Convoluted Tubule (DCT):

    • First part acts like the ascending limb (reabsorbing Na1Na^1 and Cl−Cl^-).

    • Second part acts like the collecting duct (water reabsorption and secretion of K1K^1, H+H^+ ions, and urea into the lumen).

  • Collecting Duct: The main site for osmoregulation. Water reabsorption here is regulated by Antidiuretic Hormone (ADH).

Osmoregulation and the Role of ADH

  • Definition of Osmoregulation: The control of the water potential of body fluids.

  • The ADH Mechanism (Low Water Potential):

    1. Stimulus: Low water potential of blood (dehydration).

    2. Receptor: Osmoreceptors in the hypothalamus detect the low water potential.

    3. Effector: The hypothalamus sends nerve impulses to the posterior pituitary gland.

    4. Release: ADH is released from the posterior pituitary into the bloodstream.

    5. Binding: ADH binds to receptors on the plasma membrane of DCT and collecting duct cells.

    6. Enzyme Cascade: This activates an enzyme cascade (including active phosphorylase).

    7. Aquaporins: Vesicles containing aquaporins fuse with the cell surface membrane on the lumen side.

    8. Result: Membrane permeability to water increases. Water moves out of the tubule into the blood by osmosis. A small volume of concentrated urine is produced.

  • High Water Potential Response: Osmoreceptors are not stimulated. ADH secretion stops. Aquaporins are moved from the membrane back into cytoplasmic vesicles. The collecting duct becomes less permeable to water, resulting in a large volume of dilute urine.

Control of Blood Glucose Concentration

  • The Pancreas as a Dual Gland:

    • Exocrine: Secretes pancreatic juice via the pancreatic duct to the duodenum.

    • Endocrine: Secretes hormones from the Islets of Langerhans directly into the blood.

      • Alpha (\alpha) Cells: Secrete Glucagon.

      • Beta (\beta) Cells: Secrete Insulin.

  • Hormone Characteristics: Small molecules, chemical messengers needed in small quantities, specific to target cell receptors, and quickly broken down.

Insulin Mechanism (Decreasing Blood Glucose)

  • Stimulus: Increase in blood glucose levels.

  • Action:

    1. Insulin binds to receptors on liver, muscle, and adipose (fat) cells.

    2. Vesicles with glucose transporter proteins (GLUT proteins) fuse with the plasma membrane.

    3. Facilitated diffusion of glucose into cells increases.

    4. Glucokinase: Stimulates this enzyme to phosphorylate glucose, trapping it in the cell.

    5. Glycogenesis: Increases conversion of glucose to glycogen (via phosphofructokinase and glycogen synthetase).

    6. Inhibition: Inhibits glucagon secretion, glycogenolysis (breakdown of glycogen), and gluconeogenesis (production of glucose from non-carbohydrates).

Glucagon Mechanism (Increasing Blood Glucose)

  • Stimulus: Decrease in blood glucose levels.

  • Action:

    1. Glucagon binds to receptors on liver cells only.

    2. G-Protein Activation: Activates G-proteins which then activate adenyl cyclase.

    3. Second Messenger: Adenyl cyclase converts ATPATP to cyclic AMP (cAMP).

    4. Signal Amplification: cAMP acts as a second messenger, triggering an enzyme cascade.

    5. Glycogenolysis: cAMP activates glycogen phosphorylase, breaking down glycogen to glucose.

    6. Gluconeogenesis: Increases production of glucose from fats and proteins.

    7. Glucose diffuses out of the liver into the blood via GLUT proteins.

Adrenaline and Glycemic Control

  • Produced by the adrenal gland during stress or exercise ("fight or flight").

  • Functions similarly to glucagon to increase blood glucose levels, providing muscles with substrate for aerobic or anaerobic respiration to produce ATPATP.

Diabetes Mellitus

  • Type I (Insulin-Dependent):

    • Early onset (childhood).

    • Cause: Autoimmune destruction of β\beta-cells; insufficient insulin production.

    • Treatment: Insulin injections.

  • Type II (Non-Insulin-Dependent):

    • Late onset (adulthood).

    • Cause: Down-regulation or desensitization of insulin receptors; cells fail to respond to insulin.

    • Factors: Managed by diet and lifestyle.

  • General Symptoms:

    • High glucose in blood and urine (reabsorption threshold exceeded).

    • Feeling thirsty and dehydration (glucose in blood lowers water potential, drawing water out of cells).

    • Weight loss (fats/proteins used for respiration).

    • Ketoacidosis: Build-up of keto acids/ketones lowers blood pH, potentially leading to coma.

Diagnostic Tests: Dipsticks and Biosensors

  • Dipsticks (Urine Analysis):

    • Pad contains immobilized glucose oxidase and peroxidase.

    • Lowered into urine; a color change occurs.

    • Darker color indicates a higher concentration of glucose.

  • Biosensors (Blood Analysis):

    • Directly measures blood glucose; reusable and more precise.

    • Glucose oxidase on a pad reacts with blood, creating a small electric current.

    • The current is amplified and provides a numerical value. Greater current signifies more glucose.

Homeostasis in Plants

  • Stomata: Pores in the leaves for gas exchange.

  • Guard Cells: Control the opening and closing of stomata.

  • Abscisic Acid (ABA): A plant hormone that acts during water stress to cause stomatal closure.

  • Calcium Ions (Ca2+Ca^{2+}): Act as a second messenger in the mechanism of stomatal closure during water stress.