What is homestasis
- Changes in external environment can affect your internal environment. Homeostasis is the maintenance of a stable internal environment. It involves control systems that keep your internal environment roughly constant. This means our internal environment is kept in a state of dynamic equilibrium. Keeping your internal environment stable is vital for cells to function normally and stop them being damaged.
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Importance of homeostasis: temperature
- It is important to maintain the right body temperature. Temperature affects enzyme activity, and enzymes control metabolic reactions.
- The rate of metabolic reactions increase when temperature's increased. More energy= more kinetic energy. This makes the substrate molecules more likely to collide with the enzymes active sites. But, if temperature gets too high for example over 40oC in the human body, the reaction essentially stops. The rise in temperature causes the enzymes molecules to vibrate more. If the vibrations reach a certain level then they make break some of the hydrogen bonds that hold the enzyme in its 3D shape. This means that the active site changes shape and the enzyme and substrate will not longer fit together, this means that the enzymes is denatured and no longer functions as a catalyst
- If body temp is too low then enzyme activity is reduced, slowing the rate of metabolic reactions.
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Importance of homeostasis: pH
- If blood pH is too high or low enzymes become denatured. The ionic bonds that hold them in their 3D shape are broken so the shape of the active site is changed.
- The highest rate of enzyme activity happens at their optimum pH, so this is when metabolic reactions are fastest. Optimum pH is around pH 7 but some enzymes work best at other pHs for example enzymes in the stomach work best at low pH(acidic)
- pH is calculated based on the concentration of hydrogen ions in the environment. The greater the concentration of hydrogen, the lower the pH will be and vice versa. You can work out the pH of a solution by pH= -log^10 (hydrogen conc). A logarithmic scale is a scale that uses the logarithm of a number instead of the number itself. Each value on a logarithmic scale using log^10 is X10 larger than the value before. so a solution of pH3 will contain X10 more hydrogen ions that pH4
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Importance of homeostasis: blood glucose
- If blood glucose concentration is too high, the water potential of blood is reduced to a point where water molecules diffuse out of cells into the blood by osmosis ( the diffusion of water molecules from an area of higher water potential to an area of lower water potential, across a partially permeable membrane). This can cause the cells to shrivel up and die
- If blood glucose concentration is too low, then cells cannot carry out there functions due to a lack of glucose which is required for respiration
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- Homestatic systems involve receptors. They detect when a level is too high or too low, and the information is communicated via the nervous system or the hormonal system. The effectors respond to counteract the change, brining the level back to normal
- Negative feedback only works within certain limits though, if the change is too big then the effectors may not be able to counteract it.
- Homeostasis involves multiple negative feedback mechanisms for each thing that is controlled. Having multiple negative feedback mechanisms means you can actively increase or decrease a level so it returns to normal. If you only had 1 negative feedback mechanisms, all you would be abe to do is turn it off and on. You'd only be able to actively change a level in 1 direction so it returns to normal. Only having 1 negative feedback mechanisms, slows response and control
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- Some changes trigger a positive feedback mechanism, which amplfies the change. The effectors respond to further increase the level away from normal level. The mechanism that amplifies a change away from the normal level is called a positive feedback mechanism.
- Positive feedback isnt involved in homeostasis because it doesnt keep your internal environment stable. Positive feedback is useful to rapidly active processes in the body.
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Positive and negative feedback
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Glucose concentration in the blood
- All cells need constant energy to work, so blood glucose must be carefully controlled. The concentration of glucose in blodd is normally 90 mg per 100 cm^3. It is monitored by cells in the pancreas. Blood glucose concentration rises after eating food containing carbohydrate and it falls after exercise, as more blood glucose is used in respiration to release energy required in exercise
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Hormone control of blood glucose
- The hormonal system controls blood glucose concentration using 2 hormones called insulin and glucagon. They are chemical messengers that travel in the blood to their target cells. They are both secreted by clusters of cells in the pancreas called islet of Langerhans. The islets contain beta and alpha cells. Beta cells secrete insulin into the blood and alpha cells secrete glucagon into the blood.
- Insuilin- lowers blood glucose when its too high. It binds to specific receptors on the cell membranes of muscle cells and liver cells (hepatocytes). It increases the permeability of of muscle cell membranes, so they take up more glucose. This involves increasing the number of channels proteins in the cell membranes. Insulin also activates enzymes in muscle and liver cells that convert glucose into glycogen. The process of forming glycogen from glucose is called glycogenesis.
- Glucagon- raises blood glucose concentration when its too low. It binds to specific receptors on the cell membranes of liver cells and activates enzymes that break down glycogen into glucose, this is called glycogenolysis. It also activates enzymes that are involved in the formation of glucose from glycerol and amino acids, this is called gluconeogenesis.
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Negative feedback mechanisms and glucose concentra
- Rise in blood glucose- when the pancreas detects blood glucose concentration is too high, the beta cells secrete insulin and the alpha cells stop secreting glucagon. Insulin then binds to receptors on liver and muscle cells. The liver and muscle cells respond to decrease the blood glucose concentration. Glycogenesis is activated and cells take up more glucose and respire more glucose.
- Fall in blood glucose- when the pancreas detects blood glucose is too low, the alpha cells secrete glucagon and the beta cells stop secreting insulin. Glucagon binds to receptors on liver cells. The liver cells increase the blood glucose concentration. Glycogenolysis and gluconeogenesis is activated and cells respire less glucose
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- The transporters are channel proteins which allow glucose to be transported across a cell membrane. Skeletal and cardiac muscle cells contain a glucose transporter called GLUT4. When insulin is low the GLUT4 is stored in vesicles in the cytoplasm of cells, but when insulin binds to receptors on the cell-surface membrane, it triggers the movement of GLUT4 to the membrane. Glucose can then be transported into the cell through the GLUT4 protein by facilitated diffusion.
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Adrenaline and second messengers
- Adrenaline is a hormone secreted from the adrenal glands and is secreted when there's a low concentration of glucose in the blood especially when you are stressed and are exercising. Adrenaline binds to receptors in the cell membrane of liver cells and does these things to increase blood glucose concentration. It activates glycogenoloysis and inhibits glycogenesis. It also activates glucagon secretion and inhibits insulin secretion. It allows the body to be ready by making glucose available.
- Both adrenaline and glucagon can activate glycogenoloysis inside a cell even though they bind to receptors on the outside of the cell. They do this by the second messenger model- the binding of the hormone to cell receptors activates an enzyme on the inside of the cell membrane, which then produces a chemical which activates other enzymes in the cell to give a response. The receptors for adrenaline and glucagon have tertiary structures that make them complementary to their hormone.
- To activate glycogenoloysis, adrenaline and glucagon bind to receptors and activate an enzyme called adenylate cyclase, this enzyme converts ATP in cyclin AMP (cAMP). cAMP activates a enzyme called protein kinase A which activates a chain of reactions which breaks glycogen into glucose (glycogenolysis)
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Diabetes: type 1 and 2
- Type 1- the immune system attack beta cells in the islet of langerhans so they can't make any insulin. After eating, the blood glucose level rises and stays high, this is called hyperglycaemia and can cause death. The kidneys can't reabsorb all the glucose so some of it is excreted in the urine.
- Type 2- it usually appears later in life and is often linked with obestiy and is more likely in people with a family history of the condition. Type 2 occurs when the beta cells dont produce enough insulin or the body does not respond to the insulin. Cells dont work properly because their receptors dont work properly so no glucose is taken up.
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Excretion of waste products
- Blood enters the kidneys through the renal artery and then passes through capillaries in the cortex of the kidneys. As blood passes through the capillaries in the cortex, substances are filitred out of the blood and into long tubules that surround capillaries and this is called ultrafiltration. Useful substances, such as glucose and the right amount of water, are then reabsorbed back into the blood. This process is called selective reabsorption.
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The nephrons- ultrafiltration
- Blood from the renal artery enters smaller arterioles in the cortex of the kidney. Each arteriole splits into a structure called a glomerulus- a bundle of capillaries looped inside a hollw ball called a bowmans capsule.
- The arteriole that takes blood into each glomerulues is called the afferent arteriole, and that arteriole that takes the filtered blood away from the glomerulus is called the efferent arteriole. The efferent arteriole is smaller in diameter than the afferent, so that blood in the glomerulus is under high pressure. The high pressure forces liquid and small molecules in the blood out of the capillary and into the bowmans capsule. The liquid and small molecules pass through 3 layers into the bowmans capsule and enter the nephron tubules- the capillary endothelium, a membran and the epithelium of the bowmans capsule.
- Larger molecules like proteins and blood cells cant pass through so stay in the blood. The substances that enter the bowmans capsule are known as glomerular filtrate. The glomerular filtrate passes along the rest of the nepron and substances are reabsorbed. Finally the filtrate flows through the collecting duct and passes out of the kidney along the ureter
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The nephrons- selective reabsorption
- Selective reabsorption of useful substances takes place as the glomerular filtrate flows along the proximal convoluted tubule (PCT), through the loop of henle, and along the distal convoluted tubule (DCT). Useful substances leave the tubules of the neprons and enter the capillary network around them
- The epithelium of the wall of the PCT has microvilli to provide a large surface area for reabsorption of useful materials. Useful solutes like glucose are reabsorbed along the PCT by active transport and facilitated diffusion.
- Water enters the blood by osmosis because the water potential of the blood is than that of the filtrate. Water is reabsorbed from the PCT, loop of henle, DCT and the collecting duct. The filtrate that remains is urine which goes form the ureter to the bladder
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The nephrons- urine
- Urine is made up of water and dissolved salts, ureas and other substances such as hormones and excess vitamins. Urine doesnt usually contain proteins or blood because they are too big to be filtered out. Glucose if actively reabsorbed back into the blood.
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Regulation of water content
- The amount of water kept in the blood needs to be kept constant. Water is lost during excretion and sweat. The kidneys regulate water potential of the blood so that the body has the right amount of water this is called osmoregulation.
- If the water potential of the blood is too low, more water is reabsorbed by osmosis into the blood from the tubules of the nephrons. This means the urine is more dilute, so more water is lost during excretion.
- If the water potential of the blood is too high, less water is reabsorbed by osmosis into the blood from the tubules of the nephrons. This means the urine is more dilute, so more water is lost during excretion.
- Water is reabsorbed into the blood along almost all of the nephron, but water regulation of water potential mainly takes place in the loop of henle, DCT and collecting duct. The volume of water reabsorbed by the DCT and collecting duct is controlled by hormones
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The loop of henle
- It is located in the medulla of the kidneys and is made up of the ascending and descending limb. They control the movement of sodium ions.
- Near the top of the ascending limb, sodium ions are pumped into the medulla. The ascending limb is impermeable to water, so the water stays inside the tubule. This creates a low water potential in the medulla because there's a high concentration of ions.
- Because there's a lower water potential in the medulla than in the descending limb, water moves out of the descending limb into the medulla by osmosis. This makes the glomerular filtratre more concentrated so ions cant diffuse out. The water is reabsorbed into the blood
- Near the bottom of the ascending limb sodium ions diffuse out into the medulla, further lowering the water potential in the medulla.
- Water moves out of the DCT by osmosis and reabsorbed intot the blood.
- The first 3 stages increase the ion concentration in the medulla, which lowers water potential. This causes water to move out of the collecting duct by osmosis. As before, the water in the medulla is reabsorbed into the blood through the capillary network
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Antidiuretic hormone (ADH)
- The water potential of the blood is monitored by cells called osmoreceptors in the hypothalamus. When the water potential of the blood decreases, water will move out of the osmoreceptor cells by osmosis. This causes the cells to decrease in volume. This sends a signal to other cells in the hypothalamus which sends a signal to the posterior pituitary gland. This causes the posterior pituitary to release a hormone called ADH into the blood
- ADH molecules bind to receptors on the plasma membranes of the cells in the DCT and the collecting duct. When this happens, protein channels called aquaporins are inserted into the plasma membrane. These channels allow water to pass through via osmosis, making the walls of the DCT and collecting duct more permeable to water. This means water is reabsorbed from these tubules into the medulla and into the blood by osmosis.
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- Dehydration is what happens when you lose water
- The water content of the blood drops, so its water potential drops
- This is detected by osmoreceptors in the hypothalamus.
- The posterior pituitary gland is stimulated to release more ADH into the blood.
- More ADH means that the DCT and collecting duct are more permeable so more water is reabsorbed into the blood by osmosis
- A small amount of highly concentrated urine is produced and less water is lost
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- If you are hydrated, you have taken in lots of water, so the water content of the blood needs to be reduced
- The water content of the blood rises, so its water potential rises
- This is detected by the osmoreceptors in the hypothalamus
- The posterior pituitary gland releases less ADH into the blood
- Less ADH means that the DCT and collecting duct are less permeable, so less water is reabsorbed into the blood by osmosis
- A large amount of dilute urine is produced and more water is lost
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