Chap 14: Homeostasis

• Definition of Homeostasis: the process of maintaining a constant internal body, ensures the maintenance of optimum conditions for enzyme action and cell function.
• Description of negative feedback

  1. Change in factor away from the set point
  2. Detected by the receptor
  3. Hormone release or nerve impulses sent
  4. Hormone/impulse reaches the target cell 
  5. The effector performs corrective action
  6. Factor returns back to set point (response)
• Outline the formation of urea from excess amino acids by liver cells.

  1. Deamination of excess amino acid
  2. Removal of amine group 
  3. Ammonia is formed
  4. Ammonia combines with carbon dioxide to form urea
• Describe the formation of urine in the nephron, relating to ultrafiltration 

  1. Afferent arteriole has a wider lumen/larger diameter than efferent arteriole
  2. Causes high blood/hydrostatic pressure in the from entering glomerulus 
  3. pores/gaps/fenestrations in capillary endothelium
  4. Basement membrane acts as a filter
  5. Passes through epithelium of bowman’s capsule - podocytes (finger-like structures)
  6. Basement membrane stops blood cells/large proteins from entering //allow urea, glucose, amino acids, ions and water to enter
  7. Network of capillaries
• Describe the formation of urine in the nephron, relating to selective reabsorption

  1. Active transport of Na+ out of the cell into the blood/tissue fluid
  2. By sodium-potassium pumps in basal membrane
  3. Na+ concentration decreases inside the cell - Na+ concentration gradient set up
  4. Na+ enters epithelial cells from lumen
  5. By facilitated diffusion
  6. Na+ co-transports glucose into cell
  7. This is secondary active transport
  8. Facilitated diffusion of glucose out of the cells into the blood
• Adaptations of proximal convoluted tubule (PCT) for selective reabsorption:

  • Microvilli → increase surface area
  • Many mitochondria → ATP for proton pumps
  • Tight junctions
  • Folded basal membrane
  • Many transport proteins/cotransporters/pumps
  • Many aquaporins
  • More ER for protein synthesis
• Outline the effect of ADH on the collecting ducts.

  1. ADH binds to receptors on CSM of collecting duct
  2. Activating a signalling cascade
  3. Phosphorylase enzyme to phosphorylate aquaporins
  4. Forming a vesicle-containing aquaporins 
  5. Vesicle move and fuse to CSM of collecting duct
  6. Increases permeability of the membrane to water - increasing the no. of aquaporins
  7. Osmosis - water moves into the blood - down water potential gradient
  8. Increasing concentration of urea, decreasing volume of urine
• Outline the action of ADH on the kidney.

  1. Osmoreceptors in hypothalamus detects change in WP of blood
  2. Osmoreceptors shrink when less water in the blood
  3. Impulse sent to posterior pituitary gland
  4. ADH secreted from posterior pituitary
  5. ADH travels in the blood and binds to receptors in CSM of collecting duct
  6. Aquaporins
  7. ADH increases permeability of distal convoluted tubule/collecting duct
  8. ADH causes more water reabsorption
• Describe how the effect of adrenaline on liver cells results in an increase in blood glucose concentration.

  1. Adrenaline receptor shape changes
  2. G-protein activated
  3. Adenylyl cyclase activated
  4. Cyclic AMP (cAMP) made
  5. cAMP is second messenger
  6. Activates or phosphorylates kinase
  7. Enzyme cascade
  8. Glycogenolysis stimulated - break down of glycogen to glucose
  9. Gluconeogenesis - amino acids/lipids to glucose (formation of glucose from a.a. or lipids)
  10. GLUT proteins
• Outline what happens when blood glucose concentration decreases 

  • Alpha cells secretes glucagon, Beta cells stop secretion of insulin

  1. Glucagon binds to receptors in CSM of liver cells
  2. Conformational change - activating G-protein
  3. G protein activates cyclase adenylyl 
  4. Enzyme cyclase adenylyl converts ATP to cAMP (cyclic AMP) - 2nd messenger
  5. cAMP binds to protein kinase A enzyme - enzyme cascade
  6. Activates phosphorylase kinase enzyme 
  7. Activates phosphorylase enzyme
  8. Glycogenolysis → break down of glycogen to glucose
  9. Gluconeogenesis → a.a. and lipid form glucose
• Outline what happens when blood concentration increases - effect of insulin

  • Beta cells secretes insulin, alpha cells stop secretion of glucagon

  1. Binding of insulin to receptors on target cells 
  2. Stimulates cells to add more glucose transport proteins (GLUT protein) = increase permeability of cell to glucose
  3. Rate of facilitated diffusion of glucose increases
• Outline how a glucose biosensor works.

  1. Sample of glucose (Insert blood sample) placed in biosensor containing glucose oxidase
  2. Catalyses conversion of glucose → gluconic acid + hydrogen peroxide
  3. Hydrogen peroxide oxidised by peroxidase enzyme
  4. Generates a current - detected by electrodes 
  5. Gives a digital reading/numerical value
• Outline how a biotest strip works.

  1. Pad immerse in urine sample
  2. If glucose present: glucose oxidase catalyse the conversion of glucose to gluconic acid and hydrogen peroxide
  3. Hydrogen peroxide + colourless chemical → brown compound + water (peroxidase enzyme)
  4. Use colour chart
• Mechanism by guard cells to opening of stomata

  1. Respond to light, ATP-powered proton pump in CSM of guard cell actively transport H+ out
  2. H+ ions leave the cell
  3. Using energy/ATP
  4. Low H+ = negative charge - create electrochemical gradient
  5. K+ channel proteins open
  6. K+ move into cell by facilitated diffusion (influx of K+ ions)
  7. WP decreases, water moves in by osmosis
  8. Cell volume increases
  9. Cell become turgid - turgor pressure increases
  10. Thick inner cell wall of guard cells
• Mechanism by guard cells to stomata closure

  1. Abscisic acid (ABA) binds to receptor on CSM of guard cell
  2. Proton pumps are inhibited
  3. Stimulates Ca2+ uptake // opens Ca2+ channels
  4. Ca2+ acts as second messenger
  5. K+ diffuses out of the guard cell
  6. WP of guard cell increases
  7. Water leaves guard cell by osmosis - down WP gradient
  8. Guard cell becomes flaccid - cell volume decreases
• Describe structure and function of guard cell

  • Thick cell walls facing stoma
  • Thin cell wall facing adjacent epidermal cell
  • Cellulose microfibril
  • Cell surface membrane folded → many channel and carrier proteins 
  • Cytoplasm
  • Mitochondria - many cristae