Homeostasis (A Level Only)

邊玩邊學

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課程筆記

Principles of Homeostasis

  • Homeostasis is the maintenance of the internal environment within restricted limits.
  • Physiological control systems keep internal conditions within narrow limits to ensure optimal conditions for enzyme action and cell function.
  • Receptor cells detect changes inside or outside the body and send information to a coordination system, which communicates with effectors to restore conditions to normal.
  • The nervous system communicates via nerve impulses in neurones; the endocrine system communicates via hormones carried in the blood.
  • Conditions controlled by homeostasis in mammals include core body temperature, blood pH, blood glucose concentration and water potential of the blood.
  • Temperature and pH must be maintained because they affect enzyme activity; even small changes in enzyme activity can significantly impact cells.
  • At low temperatures, molecules have limited kinetic energy so fewer enzyme-substrate complexes form; at high temperatures or extremes of pH, bonds holding the active site break and enzymes denature.

Negative Feedback

  • Negative feedback is a system in which any change is restored to original levels.
  • A negative feedback loop involves: a receptor detects a stimulus (a change from normal levels); signals are sent to a coordination system (nervous or hormonal); signals are sent to an effector to carry out a response that reverses the change.
  • Negative feedback systems have two corrective mechanisms: one for when a factor becomes too low and one for when it becomes too high, increasing the level of control.
  • Negative feedback reduces the effect of the original stimulus to restore original levels; positive feedback enhances the effect of the original stimulus and causes conditions to deviate further from normal.
  • In exams, use A-level language: say receptors detect a stimulus and the brain acts as a coordinator.

Glucose Concentration & Insulin

  • Blood glucose concentration varies due to factors such as consumption of foods containing carbohydrates, exercise, and secretion of hormones that affect blood glucose.
  • Hormones affecting blood glucose: insulin reduces blood glucose; glucagon increases blood glucose; adrenaline increases blood glucose.
  • An increase in blood glucose above normal is detected by cells in the pancreas; β cells secrete insulin.
  • Insulin binds to specific receptors on the membranes of target cells (e.g. liver and muscle cells).
  • Insulin causes more glucose transporter proteins to be added to the cell surface membranes by fusion of vesicles, increasing permeability to glucose.
  • Glucose moves into target cells by facilitated diffusion.
  • Insulin activates enzymes that convert glucose into glycogen; this is glycogenesis.

Glucose Regulation: Glucagon and Adrenaline

  • A decrease in blood glucose below normal is detected by cells in the pancreas; α cells secrete glucagon.
  • Glucagon activates enzymes that carry out glycogenolysis (conversion of glycogen to glucose) and gluconeogenesis (conversion of glycerol and amino acids into glucose).
  • Glucagon does not directly convert molecules into glucose; it activates enzymes that carry out these processes.
  • The second messenger model of glucagon action: glucagon binds to receptors on target cell membranes (e.g. liver cells); this activates a G protein which activates adenylate cyclase.
  • Active adenylate cyclase catalyses the conversion of ATP to cyclic AMP (cAMP); cAMP is the second messenger.
  • cAMP activates protein kinase A, which initiates a cascade of reactions resulting in activation of enzymes that break down glycogen to glucose.
  • Adrenaline increases blood glucose by binding to receptors on cell surface membranes and activating enzymes that convert glycogen to glucose, via the same second messenger model as glucagon.

Glucose Regulation: the Liver

  • The liver plays a vital role in regulating blood glucose concentration.
  • Both insulin and glucagon have specific receptors on the membranes of liver cells.
  • Insulin binding activates glycogenesis: the synthesis of glycogen from glucose; this lowers glucose concentration in liver cells and allows more glucose to be removed from the blood by diffusion.
  • Glucagon binding activates glycogenolysis: the breakdown of glycogen to produce glucose, which diffuses into the blood.
  • Glucagon binding also activates gluconeogenesis: the synthesis of glucose from non-carbohydrate molecules such as fatty acids and amino acids, resulting in release of glucose into the blood.
  • Remember: genesis = creation; lysis = splitting. Do not confuse these with glycolysis, the first stage of respiration.

Diabetes

  • Diabetes is a condition in which the homeostatic control of blood glucose has failed or deteriorated, resulting in high blood glucose and symptoms such as glucose in the urine, dehydration and fatigue.
  • Type I diabetes: the pancreas fails to produce insulin; onset often in childhood, caused by an autoimmune attack on the β cells.
  • Type I diabetes is normally treated with insulin injections calculated on the basis of carbohydrate intake and exercise.
  • Type II diabetes: insulin receptors no longer respond to insulin; development usually occurs in adults and is linked to risk factors such as obesity, a high carbohydrate diet, age and family history.
  • Treatments for type II diabetes include a low carbohydrate diet, exercise, and medications that help cells take up glucose from the blood.
  • Individuals with type II diabetes still produce insulin but their insulin receptors are unable to sense insulin.
  • Public health approaches to type II diabetes include promoting healthy eating and physical activity, reducing intake of processed foods, saturated fats and sugary drinks, and improving nutrition labelling; food industry approaches include reformulating products and continuing to produce and advertise unhealthy foods.

Required Practical: Determining the Concentration of Glucose in Urine

  • Colorimetry can determine the concentration of glucose in a urine sample of unknown concentration.
  • The test uses quantitative Benedict's reagent containing potassium thiocyanate, so it does not produce a red precipitate with glucose.
  • The presence of glucose is measured by the loss of blue colour and formation of a white precipitate, which can be filtered out before analysis.
  • A positive test is indicated along a spectrum from blue = low concentration to colourless = high concentration.
  • A calibration curve is set up by preparing a dilution series of glucose solutions, adding a fixed volume of quantitative Benedict's solution, heating in a water bath at at least 70 °C for 5 minutes, filtering, and adding to labelled cuvettes.
  • The colorimeter is set to red (complementary to blue) and calibrated using a cuvette containing only distilled water (100% transmission).
  • Plot a graph of glucose concentration against % transmission to create the calibration curve; treat the unknown sample in the same way and use its % transmission to read the glucose concentration from the curve.

Nephron Structure and Function

  • The kidneys are responsible for osmoregulation (regulating the water content of the blood) and excretion (removal of metabolic waste and excess substances from the blood).
  • The internal structure of the kidney includes the outer renal cortex, inner renal medulla, renal pelvis where urine is funnelled into the ureter, and thousands of tiny tubes called nephrons.
  • Nephrons contain the glomerulus, Bowman's capsule, proximal convoluted tubule, loop of Henle, distal convoluted tubule and collecting duct.
  • Urine formation occurs in two stages: ultrafiltration (small molecules filtered from blood into Bowman's capsule forming glomerular filtrate) and selective reabsorption (useful molecules taken back from the filtrate into the blood).
  • In ultrafiltration, the afferent arteriole is wider than the efferent arteriole, creating high blood pressure in the glomerulus that forces small molecules into the Bowman's capsule.
  • The glomerular filtrate contains amino acids, water, glucose, urea and inorganic ions (Na⁺, K⁺, Cl⁻); blood cells and large proteins remain in the blood as they are too large to pass out.
  • Features aiding ultrafiltration: capillary endothelium (gaps between cells), basement membrane (mesh of collagen and glycoproteins), and Bowman's capsule epithelium with podocytes (finger-like projections with gaps).

Selective Reabsorption and the Loop of Henle

  • Selective reabsorption occurs as filtrate passes along the nephron; reabsorbed substances include water, salts, glucose and amino acids, with most reabsorption in the proximal convoluted tubule (PCT).
  • PCT epithelial cells are adapted with microvilli (increase surface area), co-transporter proteins (each transports a specific solute, e.g. glucose or an amino acid), and many mitochondria (provide energy for sodium-potassium pumps in basal membranes).
  • Sodium ions are transported from the PCT into surrounding tissues by active transport; chloride ions follow by diffusion due to the electrical gradient; water follows by osmosis; urea moves out by diffusion.
  • Sugars and amino acids are transported by co-transporter proteins that also transport sodium ions: sodium-potassium pumps actively transport Na⁺ out of epithelial cells into the blood; Na⁺ in the filtrate diffuses into epithelial cells down its concentration gradient; Na⁺ moves via co-transporter proteins, transporting another solute at the same time; solutes then diffuse into the blood.
  • In the loop of Henle, sodium and chloride ions are pumped out of the ascending limb (impermeable to water) into the medulla, reducing the water potential of the medulla.
  • The neighbouring descending limb is permeable to water, so water moves out by osmosis into neighbouring capillaries; ions cannot move out of the descending limb due to its low permeability to ions.
  • The loop of Henle generates a low water potential in the renal medulla, causing reabsorption of water from the distal convoluted tubule and collecting duct by osmosis, producing concentrated urine.

Control of Blood Water Potential

  • Osmoregulation is the control of the water potential of body fluids; it is an example of homeostasis.
  • Osmoreceptors are specialised sensory neurones located in the hypothalamus of the brain that monitor the water potential of the blood.
  • When blood water content decreases: osmoreceptors detect a decrease in water potential; water moves out of osmoreceptor cells causing them to shrink; nerve impulses are sent to the posterior pituitary gland; ADH is released into the blood.
  • ADH increases the water permeability of the collecting ducts, so the kidneys reabsorb more water; water moves from high water potential in the collecting duct to low water potential in the medulla; a small volume of concentrated urine is produced.
  • ADH action: ADH molecules bind to receptors on cells lining the collecting duct; vesicles containing aquaporins fuse with the membranes; the number of aquaporins increases, increasing the membrane's permeability to water.
  • When blood water content increases: osmoreceptors detect an increase in water potential; nerve impulses cause the posterior pituitary gland to release less ADH; water permeability of collecting ducts decreases; the kidneys reabsorb less water; a large volume of dilute urine is produced.

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  1. 1.Which of the following is the best definition of homeostasis?

    Easy
    • AMaintaining the internal environment within restricted limits
    • BKeeping the internal environment completely constant at all times
    • CAllowing the internal environment to fluctuate freely with external conditions
    • DRemoving all waste products from the body

  2. 2.The endocrine system communicates within the body using which type of signal?

    Easy
    • AHormones carried in the blood
    • BNerve impulses along neurones
    • CElectrical synapses between adjacent cells
    • DMechanical vibrations through tissues

  3. 3.Which pair of hormones both act to increase blood glucose concentration?

    Medium
    • AGlucagon and adrenaline
    • BInsulin and glucagon
    • CInsulin and adrenaline
    • DInsulin and thyroxine

  4. 4.In the second messenger model of glucagon action, which molecule acts as the second messenger?

    Medium
    • ACyclic AMP (cAMP)
    • BAdenylate cyclase
    • CProtein kinase A
    • DATP

  5. 5.Which of the following correctly describes the difference between glycogenesis and glycogenolysis?

    Medium
    • AGlycogenesis is the synthesis of glycogen from glucose, while glycogenolysis is the breakdown of glycogen to glucose
    • BGlycogenesis is the breakdown of glycogen to glucose, while glycogenolysis is the synthesis of glycogen from glucose
    • CGlycogenesis is the synthesis of glucose from non-carbohydrates, while glycogenolysis is the breakdown of glycogen
    • DGlycogenesis is the breakdown of glucose during respiration, while glycogenolysis is the synthesis of glycogen

  6. 6.Which of the following is a feature of type I diabetes but NOT type II diabetes?

    Medium
    • AThe pancreas fails to produce insulin
    • BInsulin receptors no longer respond to insulin
    • COnset is usually linked to obesity in adulthood
    • DIt can be managed with a low carbohydrate diet alone

  7. 7.In the required practical using quantitative Benedict's reagent and a colorimeter, why is the colorimeter set to red light?

    Medium
    • ARed is the complementary colour to blue, so a blue solution absorbs red light
    • BRed light passes through the solution without being absorbed, giving a baseline reading
    • CRed light reacts with the white precipitate to produce a colour change
    • DRed light denatures the glucose so it can be measured

  8. 8.Which structure of the nephron is the site of ultrafiltration?

    Medium
    • AGlomerulus and Bowman's capsule
    • BProximal convoluted tubule
    • CLoop of Henle
    • DCollecting duct

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