Homeostasis and Kidney Function Notes PDF

14. Homeostasis Homeostasis – The maintenance of a relatively constant internal environment for the cells within the body Homeostasis is important because it ensures the maintenance of optimal conditions for enzyme action and cell function 4 Features of tissue fluid that affects cell activity : -T...

14. Homeostasis Homeostasis – The maintenance of a relatively constant internal environment for the cells within the body Homeostasis is important because it ensures the maintenance of optimal conditions for enzyme action and cell function 4 Features of tissue fluid that affects cell activity : -Temperature, Water potential, Concentration of Glucose, pH The Negative Feedback Mechanism 1. Change occurs in a physiological factor and is no longer within the set point is a stimuli 2. This stimuli can be internal and external which can be detected by receptors in the body 3. The receptors then send out impulse/information of the change through the nervous system to the central control. This sensory information is known as input 4. The central nervous system instructs the effector to carry out an action which is known as the output (ex. Secretion of insulin in high glucose concentration, Contraction of muscle ) 5. The output reaction is also known as the corrective action- as their effect is to correct/maintain the change detected and brings the physiological factor back to the set-point Excretion Excretion – The removal of waste products of metabolism from the body. Mostly Carbon Dioxide and Urea Carbon Dioxide CO2 – Produced in aerobic respiration Urea - Produced in the liver from excess amino acids, removed as Urine. Deamination - The breakdown of excess amino acids in the liver  - NH2 group is removed with an extra Hydrogen H -The removal of NH3 produces a Keto acid which is respired or converted to glucose or fat - The ammonia NH3 with carbon dioxide CO2 to produce Urea Structure of the Kidney - Fibrous Capsule - Cortex - Medulla - Renal Pelvic - Renal Artery & Renal Vein - Ureter - Glomerulus - Bowman’s Capsule - Proximal Convoluted Tubule ( PCT ) - Loop of Henle - Distal Convoluted Tubule ( DCT ) - Collecting Duct Formation of Urine in the Kidney in 2 steps : Ultrafiltration & Selective Reabsorption Ultrafiltration Afferent Arterioles – Carry Blood to the Bowman’s Capsule Efferent Arterioles – Carry Blood away from the Bowman’s Capsule - The renal artery ( afferent and efferent arterioles ) forms a knot known as the Glomerulus, which is held in the cup-shape of the Bowman’s Capsule - The diameter of the renal artery narrows as it flows through the glomerulus, this increases the pressure of blood flowing through them - The hydrostatic pressure in the capillary pushes smaller molecule ( 60-80nm ) out of the capillary endothelium and into the Bowman’s Capsule where it is now known as filtrate - The filtrate then goes through the basement membrane located between the capillary endothelium and the epithelium of the Bowman’s capsule, the basement membrane is made up of a net work of collagen and glycoprotein, it stops large protein molecules ( 69000 Mr ) from getting through, acting like a filter for RBC, WBC and Platelets. - Lastly the filtrate reaches the epithelium of the Bowman’s Capsule, consisting of podocyte cell that have finger-like projections that have gaps in between each podocyte cell allows filtrate to move into the lumen of the Bowman’s Capsule, filtrate in the Bowman’s Capsule is known as the glomerular filtrate -Glomerular Filtrate consist of amino acids, water, glucose, urea and inorganic ions (mainly Na+, K+ and Cl-) Factor affecting glomerular filtration rate Glomerular Filtration Rate – 125cm3 min-1 Ultrafiltration occurs because of the differences in water potential between the plasma in the glomerular capillaries and the filtrate in the Bowman’s capsule, as water molecules move down it water potential gradient. Pressure – Increase in pressure increase the water potential of the blood plasma in the glomerular capillaries above the water potential of the filtrate in the Bowman’s capsule, resulting in the movement of water of blood plasma in the glomerular capillaries into the Bowman’s capsule. Solute Concentration – Some protein that are unable to flow through the basement membrane get trapped in the blood capillary, this increases solute concentration of the capillary, resulting of the movement of water from the Bowman’s capsule into the glomerular capillaries.  Overall, the effect of the pressure gradient outweighs the effect of solute gradient, resulting in movement of water down the water potential gradient from the blood into the Bowman’s capsule Selective Reabsorption Substances in the glomerular filtrate that need to be kept are reabsorbed into the blood as the filtrate passes along the nephron, Glucose reabsorption occurs in the proximal convoluted tubule (PMT), Water and salts are reabsorbed via the Loop of Henle and collecting duct - The lining of the Proximal Convoluted Tubules is lined with a layer of epithelial cells, which are adapted to carry out reabsorption in several ways: o Microvilli – Increase SA o Co-transporter proteins – Transportation of solutes o A high number of mitochondria – Provide energy ATP for protein pumps o Tightly packed cells – No filtration fluid through cells - The blood capillaries are located very close to the outer surface of the proximal convoluted tubule - The basal membranes of the proximal convoluted tubule epithelial cells are the sections of the cell membrane that are closest to the blood capillaries - Sodium/Potassium Pumps in the basal membrane use ATP from mitochondria to actively pumps sodium Na+ ion out of the epithelial cell and into the blood - The lower concentration of Na+ ions in the epithelial cell causes Na+ ions in the filtrate flowing down the Proximal Convoluted Tubules (PMT) to diffuse down their concentration gradient through the luminal membranes of the epithelial cells - However, the Na+ ion do not diffuse freely, they diffuse via a co-transporter protein in the luminal membrane, each co-transporter protein transport a Na+ and another solute from the solute ( eg. aa/glucose ) - Once the solute is inside the epithelial cells they diffuse down their concentration gradients, passing through transport proteins in the basal membranes (of the epithelial cells) into the blood - The movement of all these solutes from the proximal convoluted tubule into the capillaries increases the water potential of the filtrate and decreases the water potential of the blood in the capillaries, creating a steep water potential gradient and causes water to move into the blood by osmosis. Osmoregulation Osmoregulation – The control of the water potential of blood and tissue fluid by controlling the water content and or/ the concentration of ions, particularly Na+ - Osmoreceptors in the hypothalamus detect a decrease in water potential of the blood below the set-point, nerve impulses are sent along these sensory neurones to the posterior pituitary gland, stimulating the release of ADH - ADH enters the blood and travel throughout the body - ADH is a hormone responsible for the reabsorption of more water in the kidney by reducing the loss of water in the urine Effect of ADH in the Kidney Water is reabsorbed by osmosis from the filtrate in the nephron This reabsorption occurs at the collecting ducts ADH causes the luminal membranes of the collecting duct cells to become more permeable to water ADH does this by causing an increase in the number of aquaporins in the luminal membranes of the collecting duct cells. This occurs in the following way: - Collecting duct cells contain vesicles containing aquaporins - ADH binds to receptor proteins which in cell membrane of the cells ling the collecting duct - The binding of the causes the production of cyclic AMP (cAMP), which is a secondary messenger, cAMP activates a signalling cascade leading to the phosphorylation of aquaporin molecules - Activation of aquaporin molecule causes the vesicle to move towards the luminal membrane and fuse with it - Fusion released aquaporin forming water permeable channel in the cells lining the collecting duct allowing water from the filtrate to move down its water potential gradient and into the in the blood - The reabsorption of water from the collecting duct prevent the excess loss of water from urine by producing low volume high salt concentration urine In the case where the body has more that enough water in the body : - osmoreceptors in the hypothalamus detects the increase in water potential in blood, neurone in the posterior pituitary glands stop secreting ADH - Without stimulus from ADH the aquaporin are moved out of the cell membrane of the cels lining the collecting duct, making the collecting duct cells to be impermeable to water - Producing large volume and low salt concentration urine The control of blood glucose concentration Blood glucose concentration is controlled by two hormones secreted by endocrine tissue in the pancreas This tissue is made up of groups of cells known as the islets of Langerhans The islets of Langerhans contain two cell types: - α cells that secrete the hormone glucagon - β cells that secrete the hormone insulin α and β cells act as the receptors and initiate the response for controlling blood glucose concentration by glucagon which can be used to demonstrate the principles of cell signalling Increase in Blood Glucose Concentration - α and β cells detect the increase in glucose concentration. α cells respond by stopping the secretion of glucagon, whereas the β cells respond by secreting insulin into the blood plasma Insulin causes muscle and liver cells to: increase the rate of glucose absorption - increase glucose respiration rate - increase conversion from glucose to glycogen Insulin carries out a mechanism to increase the membrane permeability of glucose. - Insulin is a protein and cannot directly pass through membrane, insulin molecule binds to the specific receptor on a cell - In the cell cytoplasm, vesicles containing GLUT proteins which are transporter protein that facilitate the movement of glucose into the cell - The binding of the insulin molecule, causes the vesicles containing GLUT to move towards the cell membrane and fuse with it, the GLUT protein help facilitate glucose into the cell where it can be respired and converted GLUT proteins are specific to type of cell, of which glucose in trying to enter: Brain cells -> GLUT 1 Liver cells -> GLUT 2 Muscle cells -> GLUT 4 Insulin stimulates the activation of the enzyme glucokinase, which is responsible for the phosphorylation of glucose Glucokinase – Enzyme that phosphorylate glucose This traps glucose inside cells, because phosphorylates glucose cannot pass out of cell by GLUT transporter proteins Insulin also stimulates the activation of 2 enzymes - Phosphofructokinase - Glycogen Synthase The 2 work together to catalyse the reaction where glucose molecule form 1,4 glycosidic bonds with each other to produce a polysaccharide glycogen molecule – a process known as Glycogenesis The glycogen produced is a short-term energy storage unit found in liver and muscle cells that can quickly be converted back to glucose when energy is needed. Therefore, when insulin is secreted, there is an increase in the size of the glycogen granules in the liver and muscle cells. Decrease in Blood Glucose concentration α and β cells detect the decrease in glucose concentration. α cells respond by secreting glucagon, whereas the β cells respond by stopping the secreting insulin into the blood plasma The rate at which glucose is absorbed is reduced Glucagon binds to different specific receptor molecule in the cell membrane of liver cells ONLY as there are no glucagon receptors on the muscle cells Glucagon mode of action mechanism: - Glucagon binds to the receptor protein on the cell membrane of liver cells - The binding causes a confirmational change in the receptor protein that activates a G- Protein - The G-Protein then turn activates the enzyme adenylyl cyclase, which is a part of the cell surface membrane - Adenylyl cyclase catalyses the conversion of ATP to cyclic AMP (cAMP), which then acts as a second messenger - Molecules of cyclic AMP binds to protein kinase A enzyme in the cytoplasm and activates them - The active protein kinase enzyme then activates phosphorylase kinase enzyme by adding a phosphate group to them - The active phosphorylase kinase enzymes activate glycogen phosphorylase enzymes by adding a phosphate group to them - When activated, glycogen phosphorylase catalyses the break down of glycogen to glucose – a process known as glycogenolysis → This is an enzyme cascade that amplifies the original signal from glucagon The concentration of glucose inside the cell increases and the molecules of glucose diffuse out through GLUT 2 transporter protein and into the blood Glucagon also stimulate the formation of glucose from amino acids, fatty acids, glycerol, pyruvate and lactate in a process known as Gluconeogenesis, meaning ‘new’ glucose Negative Feedback Control of Blood Glucose Blood glucose concentration is regulated by negative feedback control mechanisms In negative feedback systems: o Receptors detect whether a specific level is too low or too high o This information is communicated through the hormonal or nervous system to effectors o Effectors react to counteract the change by bringing the level back to normal In the control of blood glucose concentration: o α and β cells in the pancreas act as the receptors o They release the hormones glucagon (secreted by α cells) and insulin (secreted by β cells) o Liver cells act as the effectors in response to glucagon and liver, muscle and fat cells act as the effectors in response to insulin  Test Strip & Biosensors Measuring Urine glucose concentration People with diabetes cannot control their blood glucose concentration so that it remains within normal, safe limits The presence of glucose in urine is an indicator that a person may have diabetes - Test strips can be used to test urine for the presence and concentration of glucose - Two enzymes are immobilised on a small pad at one end of the test strip. These are: o glucose oxidase o peroxidase enzyme The pad is immersed in the urine sample, and if glucose in present: o Glucose oxidase catalyses the oxidation of glucose to form gluconic acid and hydrogen peroxide o Peroxidase then catalyses a reaction between the hydrogen peroxide and a colourless chemical in the pad to form a brown compound and water - The colour of the pad is compared to a colour chart – different colours represent different concentrations of glucose (the higher the concentration of glucose present, the darker the colour) Cons of using a test trip - Urine tests only show whether or not the blood glucose concentration was above the renal threshold whilst urine was collecting in the bladder – they do not indicate the current blood glucose concentration Measuring Blood Glucose Concentration A biosensor can be used by people with diabetes to show their current blood glucose concentration Biosensors uses glucose oxidase ONLY which is immobilised on a recognition layer Covering the recognition layer is a partially permeable membrane that only allows small molecules from the blood to reach the immobilised enzymes Glucose oxidase catalyses a reaction in which any glucose in the blood sample is oxidised to form gluconic acid and hydrogen peroxide The hydrogen peroxide produced is oxidised at an electrode that detects electron transfers The electron flow is proportional to the glucose concentration of the blood sample The biosensor amplifies the current, which is then read by a processor to produce a digital reading for blood glucose concentration This process is complete within a matter of seconds Homeostasis in Plants Plants carry out homeostasis –to maintain a constant internal environment Stomata (specifically the guard cells) control the diffusion of gases in and out of leaves This means stomata control the entry of carbon dioxide into leaves stomata respond to changes in environmental conditions by opening and closing and that regulation of stomatal aperture balances the need for carbon dioxide uptake by diffusion with the need to minimise water loss by transpiration Environmental stimuli causing stomata to open - Increase in light intensity - Low CO2 concentration between air space in the leaf Environmental stimuli causing stomata to close - High CO2 concentration between air space in the leaf - Low humidity - High temperature - Water stress Stomata open during the day Adv- Increase the gain of CO2 for photosynthesis Dis- Water can be lost through transpiration Stomata close during the day Adv- Water is retained inside the leaf keeping moisture Dis- Supply of CO2 decrease so the rate of photosynthesis decreases Opening and Closing of the Stomata Stomata open and close in a daily rhythm The stomata are open during the day and close in the night Guard Cells Each stomata have 2 guard cells surrounding it Guard cells have features such as: - Thick cell wall facing the air outside the leaf - Cellulose microfibrils arranged in bands around the cell - No plasmodesmata in cell wall - Folded cell membrane and contains many channel and carrier proteins - Cytoplasm have a high density of chloroplasts and mitochondria Mechanism for Opening Stomata Stomata water potential decreases In response to light the ATP Powered-proton pumps in the cell membrane active transport pumps H+ ions out of the guard cell The decrease of H+ ions inside the cell causes channel protein to open allowing K+ ion to enter the cell down their electrical gradient towards negative regions This done to correct the electrical imbalance caused by the removal of H+ ions The K+ ions also go down its concentration gradient Electrical Gradient + Concentration Gradient = Electrochemical Gradient The K+ ions increase the solute the concentration of the stomata, decrease the water potential Creating a water potential gradient between inside and outside the cell The gradient allows water to move into the cell by osmosis through aquaporins in the membrane and most enter the vacuole The addition of water increases the size of the cell, and the turgor pressure of the guard cells increases, the bands of cellulose microfibrils only allow the guard cells to increase in length This causes the guard cells to become curved, opening the stoma Mechanism for Closing the Stomata - When certain environmental stimuli are detected (that lead to the closing of the stomata), the proton pumps in the guard cell surface membranes stop actively transporting hydrogen (H+) ions out of the guard cell - The potassium (K+) ions leave the guard cells - The water potential gradient is now reversed and water leaves the guard cells by osmosis - This causes the guard cells to become flaccid, closing the stoma Abscisic Acid & Stomatal Closure During times of water stress, the hormone abscisic acid (ABA) is produced by plants to stimulate the closing of their stomata - Guard cells have ABA receptors on their cell surface membranes - ABA binds with these receptors, inhibiting the proton pumps and therefore stopping the active transport of hydrogen (H+) ions out of the guard cells - ABA also causes calcium (Ca2+) ions to move into the cytoplasm of the guard cells through the cell membranes - The calcium ions act as second messengers: o They cause channel proteins to open that allow negatively charged ions to leave the guard cells o This stimulates the opening of further channel proteins that allow potassium (K+) ions to leave the guard cells o The calcium ions also stimulate the closing of channel proteins that allow potassium (K+) ions to enter the guard cells - This loss of ions increases the water potential of the guard cells - Water leaves the guard cells by osmosis - The guard cells become flaccid, causing the stomata to close