14. Homeostasis Notes Flashcards
Homeostasis & Negative feedback 02.03.2021 Thermoregulation Ultrafiltration Selective reabsorption 03.03/05.03.2021 Osmoregulation Blood glucose concentration Urine analysis Homeostasis in plants 17.03/26.03.2021
Negative feedback
How negative feedback is involved in homeostatic mechanisms
A change sets off events that counteract the change
- Change in parameter from set point
- Detected by RECEPTOR
- Hormone released / Nerve impulse sent
- Hormone/Impulse reaches EFFECTOR
- Effector performs corrective action
- Parameter returns to set point
Roles of the nervous system and endocrine system in co-ordinating THERMOREGULATION
- Core temperature 37°C
- Hypothalamus is the central control, has thermoreceptors
Temperature drops,
1. Vasoconstriction
- Arterioles in skin get narrower
(Muscles in walls of arterioles in skin contract)
- Less blood flow through skin surface CAPILLARIES
- Less heat loss from blood at skin surface to surroundings
2. Shivering
- Skeletal muscles contract involuntarily, releasing heat energy
3. Hair erector muscles contract to raise body hair & trap air for insulation
4. Increase secretion of ADRENALINE by adrenal gland
- to break down glycogen –> glucose in liver in order to increase RATE of respiration, more heat energy released
*Anterior pituitary gland releases TSH (thyroid stimulating hormone)
- stimulates thyroid gland to release thyroxine hormone, which increases metabolic rate, more heat released
(vice versa for increase in temp.)
Temperature rises, 1. Vasodilation - Arterioles in skin widen (Muscles in walls of arterioles relax) - More blood flow to capillaries - Heat loss from blood to surroundings 2. Sweat production - More sweat produced (by sweat glands); evaporation of sweat removes heat ENERGY from body 3. Hair erector muscles relax to make hair lie flat & reduce insulation
✧ Excretion
2 major excretory products in mammals are CO2 and urea.
Removal from organisms of the waste products of metabolism (chemical reactions in cells including respiration), toxic materials, and substances in excess in requirements
Deamination of amino acids + Formation of urea in the urea cycle
Deamination: The removal of AMINE GROUP (the nitrogen-containing part of amino acids) to form urea
- excess Amino acids cannot be stored
- In the LIVER, deaminated/Hydrolysed –> kept acid + AMMONIA (toxic, highly soluble)
- Ammonia is converted to UREA (less toxic, less soluble), by combining with CO2, in the UREA CYCLE
- urea is excreted by kidneys
- keto acid is respired or converted to glycogen/fat for storage
Gross structure of the kidney and the detailed structure of the nephron with its associated blood vessels
In cortex {outer}:
- Bowman's capsule & Glomerulus
- Proximal & distal convoluted tubules
In medulla {inner}:
- Loop of Henle
- Proximal convoluted tubule has microvilli, seen as hairs in micrographs, but not Distal convoluted tubule
Formation of urine in the nephron involves:
I. Ultrafiltration
II. Selective reabsorption
- Creatine is converted to creatinine for excretion
- Most water reabsorbed into blood at PCT
Ultrafiltration (involves 3 layers)
- Afferent arteriole has a larger diameter than efferent arteriole, causing high blood pressure in GLOMERULUS
- So fluid forced out of glomerulus into Bowman’s capsule
- Fluid passes through gaps in ENDOTHELIUM of capillaries…
- …through BASEMENT MEMBRANE, which is selectively permeable; prevents large molecules (RMM>68000) from passing through - plasma proteins, RBC, WBC
* 100% glucose, 100% amino acids, water, mineral ions, urea, uric acid + creatinine filtered through - …then through gaps in PODOCYTES - epithelial cells making up the wall of Bowman’s capsule
- Filtrate enters Bowman’s capsule
Selective reabsorption
❖mostly in Proximal convoluted tubule,
1. BASAL membrane {nearest capillaries} of PCT cells ACTIVEly transport Na+ out of cells into tissue fluid, by sodium-potassium pumps), & into blood
2. Conc. of Na+ inside cell decreases
3. Na+ in filtrate (lumen) enter tubule cells by facilitated DIFFUSION through CO-TRANSPORTER proteins in APICAL membrane (facing lumen)
4. Na+ cotransport glucose/amino acids into cells (secondary active transport)
- Facilitated diffusion of glucose out into blood, via GLUT proteins
In loop of Henle,
❖in ascending limb - thick wall, impermeable to water
5. Cells in wall ACTIVELY TRANSPORT Na+ and Cl- out of tube into tissue fluid (in medulla)
6. Conc. of these ions increase in MEDULLA, decrease in ascending limb
❖in descending limb - thin wall, permeable to water + Na+, Cl- ions
7. Water leaves descending limb by OSMOSIS into tissue fluid {and carried away by capilaries}
8. Na+ and Cl- ions diffuse into tubule down [ ] gradient
- fluid most concentrated at apex of loop b/c less water and more Na+/Cl-
*counter-current multiplier mechanism: builds up high conc. of ions in tissue fluid, so more water can pass out of collecting duct by osmosis
*thus loop of Henle allows production of concentrated urine
In Distal convoluted tubule & Collecting duct,
9. Na+ pumped out, K+ actively transported into tubule
10. Tissue fluid in medulla has higher solute concentration & lower water potential, so water moves out of collecting duct by OSMOSIS
^controlled by ADH
- the longer the loop of Henle, the greater conc. that can build up in medulla, the greater conc. of urine that can be produced
eg. in camels - live in dry environments, need to conserve water
5 adaptations of proximal convoluted tubule cells for selective reabsorption
- Many mitochondria
- produce ATP for active transport of Na+ out of cells by sodium-potassium pumps - Microvilli on apical membrane
- large surface area for reabsorption of Na+ ions, glucose, amino acids {and water} - Co-transporter proteins on apical membrane for Na+, glu, amino acids
- Tight junctions between cells that hold adjacent cells together
- fluid cannot pass BETWEEN cells; all substances that are reabsorbed must pass through cells - Folded basal membrane
- large surface area for many pumps & transport proteins
Roles of the hypothalamus, posterior pituitary gland, ADH and collecting ducts in OSMOREGULATION
/ Effect of ADH on kidneys
ADH = antidiuretic hormone
*ADH also binds to receptors on CSM of distal convoluted tubule
- When Osmoreceptors {cells} in hypothalamus detect decrease in water potential in blood,
+ Neurosecretory cells stimulated to produce ADH
-nerve impulses are sent to posterior pituitary gland - POSTERIOR pituitary gland secretes ADH into blood
- ADH reaches cells of COLLECTING DUCT & binds to receptors on CSM
- This activates an enzyme cascade…
- …ending with the production of ACTIVE PHOSPHORYLASE enzyme, which causes vesicles containing AQUAPORINS to move to and fuse with CSM
- This increases membrane permeability to water
- More water moves out of collecting duct, by osmosis down Ψ gradient, then into BLOOD
- Urine more concentrated & in low volume
- Ψ of blood increases back to set point
When ADH conc. in blood reduces,
- Aquaporins move out of CSM and back into cytoplasm
- Collecting duct less permeable to water
- Time needed for ADH in blood to break down, so collecting ducts do not respond immediately to reduction in ADH secretion
2 types of glands
What type of gland is the pancreas?
- Endocrine glands
- Ductless glands that secrete hormones - Exocrine glands
- Secrete substances that are not hormones (eg. enzymes) into ducts
Pancreas acts as both.
- In islets of Langerhans (scattered throughout pancreas), α cells & β cells secrete hormones that are carried away by blood
- Pancreas secretes pancreatic juice (contains digestive enzymes) to duodenum via pancreatic duct
How blood glucose concentration is regulated by negative feedback control mechanisms, with reference to insulin and glucagon
- α cells & β cells detect changes in blood glucose conc. (acts as receptors)
- effectors: liver & muscle (insulin only) + α cells & β cells
Increase in blood glucose conc.,
- β cells secrete insulin, insulin [ ] increases
- α cells do not secrete glucagon, glucagon [ ] decreases
1. Insulin binds to receptors on CSM of liver cells and muscle cells
2. & stimulates Increase in uptake of glucose from blood
3. stimulates Increased use of glucose in Respiration
3. More glucose transporter molecules, GLUT4, fuse with CSM of muscle cells (NOT liver cells)
4. Insulin activates enzymes to increase conversion of glucose –> glycogen
5. Brings about a decrease in blood glucose conc. {to set point} - insulin stimulates activation of:
- glucokinase - phosphorylate glucose (glycolysis)
- phosphofructokinase - phosphorylate fructose 6-phosphate (glycolysis)
- glycogen synthase - add glucose to glycogen inside cell
Decrease in blood glucose conc.,
- α cells secrete Glucagon, glucagon [ ] increases, binds to receptors in CSM of liver cells (NOT muscle cells)
- β cells do not secrete Insulin –> insulin [ ] decreases - Stimulates breakdown of glycogen to glucose
- Liver releases glucose into blood
- Blood glucose conc. increases (back to set point)
Role of cyclic AMP as a second messenger with reference to the stimulation of liver cells by adrenaline and glucagon
+
3 main stages of cell signalling in the control of blood glucose by adrenaline
• hormone-receptor interaction at the cell surface
• formation of cyclic AMP which binds to kinase proteins
• an enzyme cascade involving activation of enzymes by phosphorylation to amplify the signal
*Adrenal gland secretes adrenaline when blood glucose conc. decreases OR due to fear/shock/excitement/stress
- Glucagon acts as cell signalling molecule
1. Glucagon binds to receptor in CSM of liver cell & activates G protein
2. G protein activates an enzyme that converts ATP –> cyclic AMP, a SECOND MESSENGER
3. cAMP binds to kinase enzymes in cytoplasm, to activate an ENZYME CASCADE + Amplifies signal
4. leads to Activation of GLYCOGEN PHOSPHORYLASE, which breaks down glycogen –> glucose
5. Glucose [ ] in cell increases & glucose diffuses out of cell into blood, through GLUT2 transporter protein
similar mechanism for Adrenaline
differences:
❖ Glucagon cannot stimulate muscle cells to release glucose, b/c muscle cells don’t have receptors for glucagon
❖ Adrenaline can stimulate MUSCLE cells (GLUT4) to release glucose
How urine analysis is used in diagnosis with reference to glucose, protein and ketones
Type 1 diabetes
- Autoimmune disease in which β cells are destroyed, causing failure of pancreas to secrete enough insulin
- Require regular insulin injections
Type 2 diabetes
- Liver/muscle cells do not respond to insulin
Renal threshold = the blood glucose conc. above which some glucose appears in the urine
- Urine is easier to collect than blood
1. Presence of glucose & ketones indicates person may have diabetes - If blood glucose conc. above renal threshold, it may indicate diabetes, as glucose is not completely reabsorbed in proximal convoluted tubule
✧ b/c carriers in PCT are saturated - In a diabetic, glucose uptake is slow, so cells lack glucose & metabolise fats instead, producing ketones
- Presence of proteins indicates kidney problems
Principles of operation of dip sticks and biosensors that can be used for QUANTITATIVE measurements of glucose in blood and urine
3 advantages of biosensors over dip sticks
- Give more precise reading / quantitative
- Give accurate reading of blood glucose conc.
- Dip sticks require judgement to match colour on stick to colour chart; esp. difficult if colour is between 2 colours on chart - Can be reused; dip sticks are discarded after use
2 advantages of dip sticks
- Easier to use & painless, as no need to prick the skin
- Cheaper
Dip sticks - for measuring glucose in urine
1. Have a small pad containing immobilised enzymes glucose oxidase & peroxidase
2. GLUCOSE OXIDASE converts glucose + oxygen –> gluconolactone + hydrogen peroxide
- gluconolactone converts spontaneously to GLUCONIC ACID
3. PEROXIDASE breaks down H2O2 –> H2O + O2
4. Oxygen oxidises a colourless CHROMOGEN {in dip stick pad} & the chromogen becomes coloured
5. Compare result with colour chart: more glucose present, more O2 produced, darker colour
*However, dip sticks do not give current blood glucose conc.
❖This method is only ‘semi-quantitative’ b/c
- only a small range of colours
- no numerical values measured
Biosensors - for measuring blood glucose conc.
- Pad has immobilised GLUCOSE OXIDASE enzyme
- Glucose oxidase binds to glucose in blood & forms GLUCONIC ACID + H2O2
- Gluconic acid releases H+ ions,
- Current is generated and detected by an electrode
- More glucose present, larger current, larger numerical value of blood glucose conc. from biosensor
Importance of homeostasis in mammals
- Maintenance of a constant internal environment,
- around a Set point within narrow limits
- Blood or tissue fluid as example of internal environment
✦ Low temp + consequence
- slowed metabolism, enzymes less active - High temp
- enzymes denatured
✦ Low water potential
- water leaves cells, cells shrink - High water potential
- water enters cell, cells burst
✦ Low blood glucose
- less respiration b/c less respiratory substrate - High blood glucose
- water leaves cells, cells shrink
Stomata have ___ ___ of opening and closing
+ Respond to changes in environmental conditions to allow diffusion of carbon dioxide and regulate water loss by transpiration
Plants in constant light
- Rhythm continues, but stomatal opening is smaller each time
- High photosynthesis increases demand for gas exchange
- Too much water vapour lost by transpiration
- Also, higher rate of photolysis (more water molecules used) - Thus, smaller opening to prevent leaves losing too much water; to conserve water
Plants in constant dark
1. Part of rhythm to open, but low rate of photosynthesis, so Stomata open slightly to prevent water loss
❖Stomata have DAILY RHYTHMS of opening and closing
❖Stomata open in response to:
- Increase in light intensity
- Gains CO2 for photosynthesis
- Allows O2 out
- Allows transpiration to occur…
- …which brings water & mineral ions in
- For photosynthesis
❖Stomata close in response to:
- Darkness / Decrease in light intensity
- CO2 not required as no photosynthesis - Low humidity, High temperature, High wind speed, Water stress - when water supply from roots is limited and/or high rates of transpiration
- To prevent water loss by transpiration
- to prevent wilting
