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Note On A Level- Homeostasis

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Published in: Biology
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Biology A level- Homeostasis

Areesha A / Dubai

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  1. Homeostasis Maintaining a constant internal environment despite external changes Examples Body temperature Blood glucose concentrations Blood salt concentration Water potential of blood Blood pressure Carbon dioxide concentration
  2. The importance of homeostasis To maintain a constant internal environment of blood and tissue fluids within narrow limits / set point, Effects: Low temperature, consequence: slowed metabolism / enzymes less active High temperature, consequence: enzymes denatured Low water potential, consequence: water leaving cells / cells shrink High water potential, consequence: water enters cells / cells burst Low blood glucose, consequence: effect on respiration High blood glucose, consequence: water leaving cells / cells shrink Control of pH, consequence: enzymes become less active
  3. Reversal of any change in internal environment to return to an optimum steady state. Structures required for pathway to work Sensory receptors Communication system Effector cells
  4. Control mechanisms use a negative feedback Receptor (sensor) detects changes in both internal and external stimuli (any change in a physiological factor being regulated) away from the set-point. Nerve impulse sent to a central control or hormone released, which then reaches the effectors (muscles and glands) / target organs. Effector performs corrective action, hence factor returns to set-point Negative feedback: the mechanism to keep changes in the factor within narrow limits, by increasing or decreasing accordingly during a change in the factor Two coordination systems in mammals: Nervous system, by electrical impulses transmitted along neurones Endocrine system, by hormones (chemical messengers) travel in the blood
  5. Thermoregulation It is the control of body temperature involving both coordination systems, controlled by the hypothalamus . The hypothalamus receives constant input of sensory information about temperature of the blood (by the themorecepter cells monitoring the core temperature) and the surroundings (skin receptors)
  6. Decrease in temperature Hypothalamus sends impulses that activate several physiological responses which decreases the loss of heat from the body and increases heat production: Vasoconstriction — contraction of the muscles in the walls of the arterioles in skin surface, narrowing the lumens, reducing the supply of blood, hence less heat lost from the blood Shivering — involuntary contraction of the skeletal muscles generate heat, absorbed by the blood Raising body hairs - contraction of the muscles attached to the hairs, increasing the depth of fur and the layer of insulation, trapping air close to the skin Decrease in sweat production — reduces heat loss by evaporation from skin surface Increase secretion of adrenaline — increases the rate of heat production in the liver A decrease in temperature gradually (e.g. winter), the hypothalamus releases a hormone which activates the anterior pituitary gland to release thyroid stimulating hormone (TSH) TSH stimulates the thyroid gland to secrete thyroxine hormone into the blood, increases the metabolic rate, increases the heat production . When temperature starts to increase again, the hypothalamus responds by reducing the release of TSH by the anterior pituitary gland, hence less thyroxine released from the thyroid gland
  7. Increase in temperature Hypothalamus increases the loss of heat from the body and reduces heat production: Vasodilation — relaxation of the arterioles in skin, hence it widens, more blood flows to the capillaries, heat energy lost Increasing sweat production — sweat glands increase production of sweat which evaporates on the surface of the skin, removing heat from the body Lowering body hairs — relaxation of the muscles attached to the hairs, hence they lie flat, reducing the depth of fur and layer of insulation
  8. Excretion: the removal of unwanted products (e.g. ammonia — toxic) of metabolism Urea is produced in the liver from excess amno acids. Deamination / removal of amine group and ammonia (NH3) formed, which is then combined with carbon dioxide forming the urea cycle Ammonia is a soluble and toxic compound, hence needed to be converted into urea which less soluble and less toxic It is transported to the kidney in solution in the blood plasma through diffusion from liver cells, which will then be removed from the blood, dissolved in water and excreted as urine 2NH, CO. keto acid (respired C — COOH comerted to gl«OSe Or fat) NH, Figure 14.4 a Deamination and b urea formation.
  9. The maintenance of a stable body temperature is an important aspect of horneostasis in endotherms. This is known as thermoregulation. (a) (i) State where the core body temperature is monitored. (ii) Name the type of sensory cell in the skin that detects changes in environmental temperature. (iii) Name the corrective homeostatic mechanism that works to restore any changes in body temperature to the normal range. [11 1)Thermoregulatory centre in the hypothalamus 2) Thermoreceptors Negative feedback machanism
  10. KIDNEY Structure of kidney: Each kidney receives blood from a renal artery; return blood via a renal vein Narrow tube — ureter — carries urine from kidney to bladder Urinary bladder- Temporary stores the unne. Urethra — single tube — carries urine to the outside of the body renal artery renal vein
  11. L.S of Kidney A longitudinal section through a kidney shows its main areas: Capsule covering the whole kidney. Cortex lying beneath the capsule Medulla — central area of kidney Pelvis — where ureter joins capsule medulla branch of renal vein branch Of renal artery
  12. Nephron- Basic unit of kidney Bowman's capsule proximal convoluted tubule distal convoluted tubule loop of Hen le collecting duct medulla pelvis afferent arteriole from renal artery proximal convoluted Bowman 's tubule capsule glomerul descending limb Of ascending limb Of loop Of Henle distal co n voluted medulla collecting pelvis
  13. Parts of Nephron Each glomerulus is supplied with blood from a branch of renal artery called an afferent arteriole The capillaries of the glomerulus rejoins to form an efferent arteriole, which leads off to form a network of capillaries running closely alongside the rest of the nephron, where it then flows into a branch of the renal vein The kidney makes urine in a two-stage process: Ultrafiltration — filtering of small molecules including urea into the Bownman's capsule from the blood Selective reabsorption — taking back useful molecules from the fluid in the nephron as it flows along
  14. Ultrafiltration The blood in a glomerulus is separated from the space inside the renal capsule by: • the capillary wall (endotheüum) which is one cell thick and has pores in it: the basement membrane of the wall of the renal capsule: the layer of cells making up the wall of the renal capsule, called podocytes: these cells have slits between them. The blood in a glomerulus is at a relatively high pressure, because the efferent arteriole is narrower than the afferent arteriole. This forces molecules from the blood through these three structures, into the renal capsule. The pores in the endothelium and the slits beb.veen the podocytes will let all molecules through, but the basement membrane acts as a filter and will only let small molecules pass through. Substances that can pass through include water, glucose, inorganic ions such as Na+, K+ and Cl- and urea. Substances that cannot pass through include red and white blood cells and plasma proteins (such as albumen and fibrinogen). The liquid that seeps through into the renal capsule is called glomerular filtrate.
  15. Reabsorption in the proximal convoluted tubule Lining of the proximal convoluted tubule is made of a sinole layer of cuboidal epithelial cells which are adapted to fheir function of reabsorption by having: Microvilli to increase the surface area of the inner surface facing the lumen to increase absorption of Na+ / glucose / amino acids. Tight junctions to hold adjacent cells together so that fluid cannot pass between the cells (all reabsorbed substances must go through the cells). Many mitochondria to provide ATP for sodium-potassium (Na+ -K + ) pump proteins in the outer membranes of the cells. Many co-transporter proteins in the membrane facing the lumen . Folded basal membrane to increase surface area to increase sodiumpotassium pumps to move Na+ into the blood . More ER for increase in protein synthesis mal ccmvduted of opillu-y —and
  16. Reabsorption in the proximal convoluted tubule plasma endothelium of capillary — passive I in the basal membrane Of proximal convoluted tubule cells use ATP made by the mitochondria. These pumps decrease the concentration of sodium ions in the cytoplasm. The basal membrane is to give a large surface area for many Of these carrier basement membrance ADP* prox ima I convoluted tubule cen glucose —and amino acids proxi mal tubule lumen glucose amino Very nearby, the blood 3 helping plasma rapidly removes absorbed uptake of solutes. Na- moves passively Na•, CI-, glucose and amino into the cell down its concentration acids. This helps further uptake gradient. It moves in using protein from the lumen Of the t co- transporter in the bring in and acids at the same time.
  17. Reabsorption in the proximal convoluted tUbUIe Active transport is used to move Na+ out of the outer surface of a cell in the wall of the proximal convoluted tubule, into the blood. This lowers the concentration of Na+ inside the cell, so that Na+ ions diffuse into the cell from the fluid inside the tubule. The Na+ ions diffuse through protein transporters in the cell surface membrane of the cell As the Na+ions diffuse through these transporter proteins, they carry glucose molecules with them. This is called co-transport. The glucose molecules move through the cell and diffuse into the blood. The movement of Na+ and glucose into the blood decreases the water potential in the blood. Water therefore moves by osmosis from the fluid inside the tubule, down a water potential gradient through the cells making up the wall of the tubule and into the blood. Aa a result, the fluid inside the nephron now has: no glucose a lower concentration of Na+than the filtrate originally had less water than the filtrate originally had About 50% of the urea is also reabsorbed in the proximal convoluted tubule.
  18. Reabsorption in the loop of Henle and collecting duct The descending limb of the loop of Henle is permeable to water, but relatively impermeable to NO+CI-. The ascending limb of the loop of Henle is permeable to salts, but impermeable to water As fluid fows down the descending limb of the loop of Henle, water moves out of it by osmosis. By the time the fluid reaches the bottom of the loop, it has a much lower water potential than at the top of the loop. As it flows up the ascending limb, Na+ and Cl- move out of the fluid into the surrounding tissues, first by cfffusion and then by active transport. The function of the loop of Henle is to build up a high concentration of Na+ and Cl- in the tissues of the medulla. This allows highly concentrated urine to be produced. o limb 2 This Of Na• CV in the tissue nuid. from the 4 Water • in limb. concentrated in the wall to N'
  19. Reabsorption in the distal convoluted tubule and collecting duct In the distal convoluted tubule, Na+ is actively transported out of the fluid. K + •ons are actively transported into the tubule, where the rate of transfer of the two ions are variable, helps regulate the concentration of these ions in the blood. As the fluid continues to flow through the collecting duct, water moves down the water potential gradient from the collecting duct and into the tissues of the medulla. This further increases the concentration of urea in the tubule. The fluid that finally leaves the collecting duct and flows into the ureter is urine. 2 o The tissue in the deeper layers Of the medulla contains a very concentrated solution of Na-, Cl- and urea. As urine passes down the collecting duct. water can pass out of it by osmosis. The reabsorbed water is carried away by the blood in the capillaries.
  20. Osmoregulation: fluid from distal convoluted tu bule lumen Of collecting duct urine I ADH binds to s in the cell surface membrane Of the cells lining the collecting duct. 2 This activates a series of enzyme-controlled reactions. ending with the production of an active phosphorylase enzyme. 3 The phosphorylase causes vesicles, surrounded by membrane containing water-permeable channels (aquaporins). to move to the cell surface membrane. with tissue fluid 4 The vesicles fuse with the cell surface membrane. Water can now move freely through the membrane, down its Water potential gradient. into the concentrated tissue fluid and plasma in the medulla of the kidney.
  21. Osmoregulation: Roles of hypothalamus, posterior pituitary, collecting ducts and ADH •n osmoregulation: Hypothalamus detects changes in water potential of the blood, as osmoreceptors (in the hypothalamus) shrink when there is a low water potential (ADH produced in hypothalamus), and released into the blood via the posterior pituitary g and. Nerve impulses are sent from the hypothalamus to posterior pituitary gland ADH bind to receptor proteins on the collecting duct cell surface membranes and affects the collecting duct by activating series of enzyme controlled reactions, activating ves•cles containing aquaporins in their membranes to move to cell surface membrane on lumen side; fuses with the cell surface membrane. This increases water permeability of collecting duct cells, causing more water reabsorption / more concentrated urine, as water moves through the aquaporins, out of the tubule into the tissue fluid down water potential gradient.
  22. The Control of Blood Glucose: In a healthy human, each 1000cm•3 of blood contains between 80-120 mg of glucose. The homeostatic control of blood glucose concentration is carried out by two hormones secreted by endocrine tissues in pancreas. The tissue consists of a group of cells, known as the Islets of Langerhans, which are scattered throughout the pancreas. The islets contain two types of cells: a cells (secrete glucagon) cells (secrete insulin) The a and ß cells act as the receptors and the central control of the homeostatic mechanism with the hormones coordinating the actions of the effectors. Cee Ceil
  23. Regulating Blood Glucose Concentrations Exercise Adipose cells take up glucose 000 Decreasing blood sugar Pancreatic alpha cells release glucagon Pancreatic beta cells release insulin blood sugar Liver breaks down glycogen to glucose Eating
  24. Increase in Blood Glucose Level: As the blood containing high g UCOSe concentration flows through the pancreas, the a and ß cells detect this increase. The a cells respond by stopping the secretion of glucagon and the 13 cells respond by secreting insulin into the blood plasma.
  25. Insulin: Insulin is a signaling molecule. It's a protein and cannot pass directly through the cell surface membranes. It binds to a receptor in the cell membrane and affects the cell indirectly through the mediations of intracellular messengers. Insulin stimulates the cells, containing its specific receptors, to increase the rate at which they absorb glucose from the blood and convert it into glycogen and use it for respiration.
  26. 3 Glucose Ca n now diffuse into the cell down its concentration gradient. receptor cell surface membrane cytoplasm I Insulin binds to a receptor in the cell surface membrane. insulin glucose 2 The receptor signals to the cell and makes vesicles glucose transporter proteins merge with the cell surface membrane. glucose transporter GLUT4
  27. Role of Insulin Glucose can only enter cells via the GLUT transporter proteins. There are several different types of GLUT (Glucose Transporter)proteins: GLUT-I (for brain) GLUT-2 (for liver) GLUT-4 (for muscles) When insulin molecules bind to the receptors on the muscle cells, the vesicles with GLUT-4 proteins are moved to the cell surface membrane and fused with it. Hence, they facilitate the movement of glucose into the cell. GLUT 1 and GLUT 2 proteins are always in the cell membrane and their distribution is not altered by insulin.
  28. Role of Insulin Insulin stimulates the activation of the enzyme gulcokinase, which phosphorylates glucose. This traps glucose inside the cells because phosphorylated glucose cannot pass through the transporters in the cell membrane. Insulin also stimulates the activation of two other enzymes, phosphofructokinase and glycogen synthase, wh'ch together add glucose molecules to glycogen. This increases the size of glycogen granules in the cell.
  29. liver cea glucagon I Glucagon binds to membrane receptor. receptor 2 Activation of G-protein and then enMne. 3 Active enzyme produces cyclic AMP from ATP. cyclic AMP 4 Cyclic AMP stimulates inactive active g b glycogen glucose
  30. Decrease in Blood Glucose Level: The a cells respond by secreting glucagon, while the cells respond by stopping the secretion of insulin. Glucagon binds to different receptor molecules in the cell membranes of the liver cells. This binding activates a G-protein, that in turn activates an enzyme ( adenylyl cyclase)that catalyzes the conversion of ATP to cyclic AMP, which is a secondary messenger. Cyclic AMP binds to kinase enzymes within the cytoplasm, which activates other enzymes. Kinase enzymes activate enzymes by adding phosphate group to them through a process known as phosphorylation. This enzyme cascade amplifies the original signal from glucagon. Glycogen phosphorylase is the end product of the enzyme cascade, and catalyzes the breakdown of glycogen to glucose. It does this by removing glucose units from the numerous 'ends' of glycogen. This increases the concentration of glucose inside the cell so that it diffuses out, via the GLUT-2 transporter proteins, into the blood. Muscle cells don't have receptors for glucagon and hence, don't respond to it.
  31. NORMAL PHYSIOLOGY o Insulin binds to insulin receptors and triggers the opening of glucose transporters in fat and muscle cells, allowing glucose removal from the bloodstream Key: O glucose TYPE 1 DIABETES Insulin is not produced by beta cells in the pancreas and hence glucose is not removed from the bloodstream, causing diabetes TYPE 11 DIABETES Prolonged overproduction of insulin leads to desensitization Of the insulin receptors and hence glucose is not removed from the bloodstream, causing diabetes insulin receptor (t) glucose transporter (Glut-4)
  32. Type 1 Diabetes In this type, the pancreas seems incapable of secreting insulin. This is thought to be due to a deficiency in the gene that codes for the production of insulin, or due to an attack on the ß cells by the body's own immune system. This type usually begins very early in life and hence, is sometimes referred to as juvenile-onset diabetes. Normally, there's no glucose in the urine, but if the glucose concentration in the blood becomes very high, the kidneys cannot reabsorb all the glucose, so that some passes out in the urine. Extra water and salts accompany this glucose. The person consequently feels extremely hungry and thirsty. Uptake of glucose in a diabetic person is slow, even when there's a plenty of glucose in blood. Thus, cells lack glucose and, metabolize fats and proteins as alternative energy sources. This leads to a buildup of substances in blood, called ketones (keto acids). These are produced when the body switches to metabolizing fats and they decrease the pH of blood (this can cause comma if accompanied with dehydration). Sufferers of type-I diabetes receive regular injections of insulin.
  33. Type 2 Diabetes Mellitus: In this type the pancreas does secrete insulin, but the liver and muscle cells don't respond to it properly. This type begins relatively late in life and is often associated with diet and obesity. The symptoms of this diabetes mellitus are the same as the first one. People with type-2 rarely need to have insulin injections: they can use diet and, regular and frequent exercise to keep their blood glucose concentration within normal limits.
  34. Urine Analysis: Dipsticks Can be used to test urine for a range of different factors including, pH, glucose, ketones and proteins. Dipsticks for detecting glucose contain the enzymes, glucose oxidase and peroxidase immobilized on a small pad on one end ot the stick. The pad is immersed in urine and if the urine contain glucose, glucose oxidase catalyzes a chemical reaction in which glucose is oxidized into a substance called gluconolactone. Hydrogen peroxide is also produced. Peroxidase catalyzes a reaction between hydrogen peroxide and a colorless chemical in the pad to form a brown compound. The resultina color of the pad is matched against a color chart. The chart shows the colors that indicate different concentrations of glucose. Larger the amount of glucose present, darker the color. Dipsticks indicate that whether the concentration of glucose was higher than the renal threshold in the period of time while the urine was beng collected in the bladder. The dipsticks don't indicate the current blood glucose concentration. 10.0 200
  35. Urine Analysis:Biosensors Used to measure the glucose level in the blood Biosensor is a device which makes use of a biological molecule to detect and measure a chemical compound. i.e. a pad impregnated with glucose oxidase. A small sample of blood is placed on the pad which is inserted into the machine. Glucose oxidase catalyzes the reaction to produce gluconolactone and at the same time, a tiny electric current is generated. The current is detected by an electrode, amplified, and read by the meter which produces a reading for blood glucose concentration within seconds. The more glucose that is present, the greater the current and the greater the reading from the biosensor.

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