6C - Homeostasis Flashcards
Define homeostasis.
The maintenance of a stable internal environment.
What are the 3 factors you need to know about the control of in homeostasis?
1) Temperature
2) pH
3) Glucose
Why is it important to maintain the optimum body temperature?
- If body temperature is too high -> Enzymes become denatured, so metabolic reactions are less efficient
- If body temperature is too low -> Enzyme activity is reduced, so metabolic reactions are less efficient
Why are enzymes denatured above their optimum temperature?
- The enzyme’s molecules vibrate too much, which breaks the hydrogen bonds that hold them in their 3D shape
- The shape of the active site is changed so it no longer works as a catalyst
Why is it important to maintain optimum blood pH?
If the blood pH is too high or too low, enzymes become denatured, so metabolic reactions are less efficient.
Why are enzymes denatured at too high or too low pHs?
- The hydrogen bonds that hold them in their 3D shape are broken
- The shape of the active site is changed so it no longer works as a catalyst
Why is it important to maintain optimum blood glucose concentration?
- If blood glucose concentration is too high -> The water potential is reduced so water diffuses out of cells into the blood by osmosis. This can cause the cells to shrivel up and die.
- If blood glucose concentration is too low -> Cells are unable to carry out normal activities because there isn’t enough glucose for respiration to provide energy
By what mechanism do homeostatic systems respond?
Negative feedback
What are the three parts of a homeostatic system?
- Receptors
- Communication system
- Effectors
What is negative feedback?
When the change produced by the effector acts to counteract the change that caused it.
Describe briefly how a homeostatic system works.
- Receptors detect change
- Information is communicated via nervous system or hormonal system to effectors
- Effectors respond to change
What communication systems might be involved in a homeostatic system?
- Nervous system
* Hormonal system
Does negative feedback always work?
Only if the change is small, or the effectors might not be able to counteract the change.
Does homeostasis involve only one negative feedback system for each thing being controlled? Why?
- No, there are multiple negative feedback mechanisms for each thing
- This is because it gives greater control -> By being able to actively increase or decrease a level
Give an example of multiple negative feedback mechanisms for a single thing being controlled.
There is are feedback mechanism to increase your temperature and feedback mechanisms to increase it.
What would be the effect of having only one negative feedback system for a single thing being controlled?
- Slower response
* Less control
What is positive feedback?
When the change produced by the effector amplifies the change that caused it.
What is positive feedback useful for?
Rapidly activating something
Describe positive and negative feedback with platelets.
- Injury occurs
- Platelets become activated and release a chemical
- This triggers more platelets to be activated and so on
- This means a clot forms very quickly
- The process ends with negative feedback when the blood clot has been formed
When can unwanted positive feedback occur?
When a homeostatic system breaks down (e.g. if you’re cold for too long).
What type of feedback is involved in hypothermia?
Positive feedback
Describe how hypothermia occurs.
- Heat is lost from the body faster than it can be produced
- As the body temperature falls, the brain doesn’t work properly and shivering stops
- This makes the body temperature fall even more (positive feedback)
- Temperature continues to fall unless action is taken
Is positive feedback involved in homeostasis?
No, because it doesn’t keep your internal environment stable.
What is the normal concentration of glucose in the blood?
90mg per 100cm₃ of blood
What monitors glucose concentration of the blood?
Pancreas
What increases and decreases the glucose concentration of the blood?
- Increase -> Eating carbohydrates
* Decrease -> Exercise
What two hormones are involved in the control of blood glucose concentration?
- Insulin
* Glucagon
What medium do insulin and glucagon travel by?
Blood
What are clusters of cells in the pancreas called?
Islets of Langerhas
What are the two types of cell in the pancreas?
- Alpha (α)
* Beta (β)
What do alpha pancreatic cells secrete?
Glucagon
What do beta pancreatic cells secrete?
Insulin
How can you remember which cells secrete insulin and glucagon?
(A)lpha -> Gluc(A)gon
What is the name for the production of glycogen from glucose?
Glycogenesis
What is the name for the production of glucose from glycogen?
Glycogenolysis
What is the name for the production of glycogen from glycerol and amino acids?
Gluconeogenesis
Remember to practise drawing out the glucose-glycogen conversion flowchart on pg 158 of revision guide.
Do it.
What are the different processes involved in the control of blood glucose concentration?
- Glycogenesis
- Glycogenolysis
- Gluconeogenesis
What processes does insulin activate?
Glycogenesis
What processes down glucagon activate?
- Glycogenolysis
* Gluconeogenesis
Describe how insulin lowers blood glucose concentration.
1) Insulin binds to specific receptors on the cell membranes of liver cells and muscle cells.
2) It increases the permeability of muscle cell membranes to glucose, so the cells take up more glucose. This involves increasing the number of channel proteins in the cell membrane.
3) Insulin also activates enzymes in the liver and muscle cells that convert glucose into glycogen, which is stored in the cytoplasm.
4) Insulin also increases the rate of respiration of glucose.
Summarise simply the 3 things insulin does.
1) Increases permeability of muscle cells to glucose
2) Stimulates glycogenesis
3) Increases the rate of respiration of glucose
What is another name for liver cells?
Hepatocytes
Describe how glucagon raises blood glucose concentration.
1) Glucagon binds to receptors on the cell membrane of liver cells.
2) It activates enzymes in the liver cells to break down glycogen into glucose.
3) Glucagon also activates enzymes that are involved in the formation of glucose from glycerol and amino acids.
4) Glucagon also decreases the rate of respiration of glucose in cells.
Summarise simply the 3 things glucagon does.
1) Stimulates glycogenolysis
2) Stimulates gluconeogenesis
3) Decreases rate of respiration of glucose
Compare hormonal and nervous responses.
Hormonal: • Slow • Long-lasting Nervous: • Quick • Short-lasting
Remember to practise drawing out the negative feedback mechanism flowchart for the control of blood glucose concentration.
See diagram pg 158 of revision guide.
What is GLUT4?
- Channel protein -> Glucose transporter
* In skeletal and cardiac muscle cells
Describe how insulin makes the cell membrane of cells more permeable to glucose.
- GLUT4 is a glucose transporter
- When insulin levels are low, GLUT4 is stored in vesicles in the cytoplasm
- When insulin binds to receptors on the cell-surface membrane, it triggers the movement of GLUT4 to the membrane
- Glucose can be transported into the cell via facilitated diffusion
Via what process can glucose be transported into cells?
Facilitated diffusion (through GLUT4 channel proteins)
Where is adrenaline secreted from?
Adrenal glands (just above the kidneys)
What does adrenaline do?
Increases blood glucose concentration (amongst other things)
When is adrenaline secreted?
Where there is:
• Low concentration of glucose in the blood
• Stress
• Exercise
How does adrenaline increase blood glucose concentration?
Binds to receptors in the cell membrane of liver cells and: • Activates glycogenolysis • Inhibits glycogenesis • Activates glucagon secretion • Inhibits insulin secretion
By what process do adrenaline and glucagon have an effect within cells?
Using a second messenger
Describe how adrenaline and glucagon work via a second messenger.
- Adrenaline and glucagon bind to their respective receptors.
- This activates an enzyme called adenylate cyclase.
- The adenylate cyclase converts ATP into a “second messenger” called cyclic AMP (cAMP).
- cAMP activates an enzyme called protein kinase A, which in turn activated a cascade of reactions that result in glycogenolysis.
What are the chemicals involved in a second messenger response?
- Adrenaline/Glucagon
- Adenylate cyclase
- ATP
- Cyclic AMP (cAMP)
- Protein kinase A
What is the second messenger in adrenaline and glucagon responses?
cAMP (cyclic AMP)
Remember to practise drawing out the way in which adrenaline and glucagon work through a second messenger.
Pg 159 of revision guide
What is diabetes mellitus?
A condition where blood glucose concentration can’t be controlled properly.
How is Type 1 diabetes treated?
- Insulin therapy -> Regular insulin injections or an insulin pump
- Eating regularly and controlling simple carb intake helps avoid a sudden rise in glucose
Why must insulin therapy be carefully controlled?
Too much insulin can produce a dangerous drop in blood glucose levels (hypoglycaemia).
Describe what causes Type 2 diabetes.
• β cells don’t produce enough insulin
OR
• Insulin receptors on body cells do not respond properly to insulin
What are some risk factors of Type 2 diabetes?
- Obesity
- Genetics
- Lack of exercise
- Age
- Poor diet
How is Type 2 diabetes treated?
- Eating a healthy, balanced diet
- Losing weight
- Regular exercise
- Glucose-lowering medication
- Insulin injections
Describe what causes Type 1 diabetes.
- The immune system attacks the β cells in the islets of Langerhans, so they can’t produce insulin.
- This can be due to genetics or a viral infection.
What is low blood glucose concentration called?
Hypoglycaemia
What is high blood glucose concentration called?
Hyperglycaemia
What are some additional health problems associated with Type 2 diabetes?
- Visual impairment
* Kidney failure
Remember to revise how Type 2 diabetes is being tackled on a national scale.
Pg 160 of revision guide
What is the normal concentration of urine in the blood?
0 to 0.8mM
What happens if diabetes is left untreated?
- The blood glucose rises and stays high (hyperglycaemia).
* This can result in death if left untreated.
How can diabetes easily be detected and why?
Glucose in the urine, because the kidneys can’t reabsorb all the glucose.
Describe how you can work out the concentration of glucose in a urine sample.
1) Make a serial dilution with a dilution factor of 2 and an initial glucose concentration of 4mM.
2) Use the same volume of quantitive Benedict’s reagent with each solution. The blue colour is lost, but a brick-red precipitate is not produced.
3) Use a colorimeter with a red filter to measure the absorbance of each solution.
4) Plot a calibration curve of absorbance against glucose concentration.
5) Do the same with the urine sample and use the calibration curve to find the concentration of glucose.
What is the difference between Benedict’s solution and quantitive Benedict’s solution?
Quantitive does not produce a brick-red precipitate.
What are the functions of the kidney?
- Excrete waste products
* Regulate water potential of the blood
Describe the structure of a kidney.
- Inner medulla
- Outer cortex
- Very inner pelvis
- Connected to renal artery and renal vein
- Connected to ureter, which connects to bladder and then urethra
(See diagram pg 162 of revision guide)
Remember to practise drawing out the structure of the kidney.
See diagram pg 162 of revision guide
What are the three stages of kidney blood filtration?
1) Ultrafiltration
2) Selective reabsorption
3) Concentrating sodium ions in the medulla
4) Water reabsorption
Which part of the kidney does ultrafiltration happen in?
Cortex
What is the different between the urethra and ureter?
- Ureter -> From kidney to bladder
* Urethra -> Away from bladder
Describe the structure of a nephron.
IN CORTEX:
• Afferent arteriole brings blood in
• Glomerulus (bundle of blood capillaries) is surrounded by the Bowman’s capsule
• Efferent arteriole takes blood away from glomerulus
• Proximal convoluted tubule (PCT) is connected to Bowman’s capsule
IN MEDULLA:
• Loop of Henle is connected to PCT
IN CORTEX AGAIN:
• Distal convoluted tubule (DCT) is connected to the Loop of Henle
• This feeds into the collecting duct, which goes from the cortex to the medulla
(See diagram pg 162 of revision guide)
Remember to practise drawing out the structure of a nephron.
Pg 162 of revision guide
What is a nephron?
A long tubule along with a bundle of capillaries where blood is filtered in the kidneys.
What is the name for the blood vessel that takes blood to the glomerulus?
Afferent arteriole
What is the name for the blood vessel that takes blood away from the glomerulus?
Efferent arteriole
Describe how the process of ultrafiltration works in a nephron.
1) Blood from the renal artery enters arterioles (called afferent arterioles) in the cortex.
2) Each arteriole splits into a glomerulus (bundle of capillaries), which is surrounded by Bowman’s capsule.
3) Blood is taken away by the efferent arteriole. This has a smaller diameter than than the afferent arteriole, so the pressure in the glomerulus is very high. Also, the capillaries are twisted into a knot to increase pressure.
4) The high pressure forces liquid and small molecules into the Bowman’s capsule through 3 layers.
5) Larger molecules, like proteins and blood cells stay in the blood.
How is the pressure in the glomerulus made high enough for ultrafiltration?
- Efferent arteriole is narrower than afferent arteriole
* Capillaries are twisted into a knot
What is filtered out and what remains in the blood in ultrafiltration?
Filtered out: • Water • Amino acids • Glucose • Urea • Inorganic ions Remains in blood: • Blood cells • Proteins
What 3 layers do liquids and small molecules pass through between the glomerulus and the Bowman’s capsule?
1) Capillary wall
2) Basement membrane
3) Epithelium of Bowman’s capsule
What is the term for the liquids and small molecules that enter the Bowman’s capsule?
Glomerular filtrate
What is the difference between epithelial and endothelial cells?
- Epithelial -> Any cells that line a cavity or surface
* Endothelial -> Specifically the cells that line the inside of blood vessels
Describe briefly how the process of selective reabsorption occurs in a nephron.
• Reabsorption of the glomerular filtrate occurs along the proximal convoluted tubule, loop of Henle, distal convoluted tubule and collecting duct
IN THE PCT:
• The PCT epithelium cells have microvilli to provide a large surface area for absorption from the PCT
• Sodium ions are moved by active transport in a Na⁺-K⁺ pump from the PCT epithelium cells into the capillaries
• Now, useful substances, like glucose, are absorbed into the PCT epithelium cells by co-transport with sodium.
• They then diffuse into the capillaries.
IN THE PCT, LOOP OF HENLE, DCT, AND COLLECTING DUCT:
• The water potential of the blood is lower than the water potential of the filtrate
• This means water is reabsorbed by osmosis
Where are useful solutes reabsorbed in the nephron?
Proximal convoluted tubule
Where is water reabsorbed in the nephron?
- Proximal convoluted tubule
- Loop of Henle
- Distal convoluted tubule
- Collecting duct
What does urine usually contain?
- Water
- Some dissolved salts
- Urea
- Hormones and excess vitamins
What doesn’t urine usually contain?
- Protein
- Blood cells
- Glucose
- Amino acids
Describe the way in which selective reabsorption occurs in the proximal convoluted tubule.
- Na⁺-K⁺ pump actively transport sodium ions from the cells lining the PCT into the capillaries
- Na⁺ ions now diffuse (facilitated diffusion) into the PCT epithelium cell from the fluid in the PCT
- This facilitated diffusion involves co-transport, so each Na⁺ carries glucose/chloride/vitamins with it
- The carries molecule diffuses into the capillaries
- Overall, this increases the water potential of the fluid in the PCT, so it moves into the epithelial cells and then the capillaries by osmosis
By what process is water reabsorbed in the nephron?
Osmosis
What is the name for the maintenance of the right amount of water in the blood by the kidney’s?
Osmoregulation
What happens in terms of osmoregulation when the water potential of the blood is too low?
- More water is reabsorbed into the blood by osmosis
* So the urine is more concentrated
What happens in terms of osmoregulation when the water potential of the blood is too high?
- Less water is reabsorbed into the blood by osmosis
* So the urine is less concentrated
Which parts of the nephron regulate the water potential of the of blood?
- Loop of Henle
- DCT
- Collecting duct
(Reabsorption happens all along the nephron, but regulation only happens in these parts)
What is the role of the Loop of Henle?
Maintains a sodium ion gradient between the medulla and nephron tubules, so water can move out of the tubules and be reabsorbed into the blood.
What are the two parts of the loop of Henle?
- Ascending limb
* Descending limb
Describe how the Loop of Henle works.
1) Na⁺ ions are actively transported out of the top of ascending limb. Water stays in because the ascending limb is impermeable to water.
2) This increases the Na⁺ ion concentration in the medulla.
3) So water leaves the descending limb by osmosis and is reabsorbed by the capillaries. Na⁺ ions stay in because the descending limb is impermeable to water, so the fluid becomes more concentrated.
4) At the bottom of the ascending limb, Na⁺ ions diffuse out into the medulla.
5) Overall, the medulla ends up very salty, with a low water potential.
6) This means water moves out of the DCT and collecting duct by osmosis. It is reabsorbed into the blood.
7) The result is more concentrated urine.
What is the effect of a longer Loop of Henle? Why? (4)
More concentrated urine because:
• Longer ascending limb -> More water can be reabsorbed into the blood
• Longer descending limb -> More ions are actively transported into the medulla, so there is an even lower water potential in the medulla, so more water moves out of the collecting duct and into the capillaries
Describe how the Loop of Henle acts as a counter current multiplier.
(NOTE: The interstitial space is the region between the ascending and descending limbs of the LOH)
• The concentration of the interstitial space increases as you go down the ascending limb of the LOH
• The concentration of the fluid in the collecting duct increases as you go down it
• So the reverse direction of flow means that a concentration gradient is maintained along the entire length
Describe briefly the role of each limb of the Loop of Henle.
- Descending -> Water moves out of it and is reabsorbed into the blood.
- Ascending -> Sodium ions are moved out of it, increasing the Na+ concentration in the medulla, so that water moves out of the descending limb, DCT and collecting duct.
Describe the permeability of each limb of the Loop of Henle.
- Descending limb -> Impermeable to Na⁺ ions
* Ascending limb -> Impermeable to water
What controls the volume of water reabsorbed from the DCT and collecting duct?
Hormones (ADH)
What monitors the water potential of the blood?
Osmoreceptors in the hypothalamus
Which hormone is involved in the control of water potential of the blood?
ADH (Antidiuretic hormone)
What produces ADH?
Posterior pituitary gland
What is the effect of ADH?
It makes the walls of the DCT and collecting duct more permeable to water, so more water is reabsorbed from them and the urine is more concentrated.
Describe how ADH works.
- ADH binds to receptors on the cell membranes of DCT and collecting duct walls
- Phosphorylase is activated
- This causes vesicles containing aquaporins to be inserted into the cell surface membrane
- This makes the walls more permeable to water, so more is reabsorbed and the urine is more concentrated
Describe briefly how the body responds when you are dehydrated.
1) Osmoreceptors in the hypothalamus detect the drop in water potential.
2) Posterior pituitary gland is stimulated to release more ADH into the blood.
3) More ADH means that the DCT and collecting duct become more permeable, so more water is reabsorbed into the blood by osmosis.
4) A small amount of highly concentrated urine is produced.
Describe briefly how the body responds when you are well-hydrated.
1) Osmoreceptors in the hypothalamus detect the rise in water potential.
2) Posterior pituitary gland releases less ADH into the blood.
3) More ADH means that the DCT and collecting duct become less permeable, so less water is reabsorbed into the blood by osmosis.
4) A large amount of dilute urine is produced.
Describe how osmoreceptors detect and respond to a drop in the water potential of the blood.
- When the water potential of the blood drops, water will move out of the osmoreceptor cells by osmosis
- This causes the cells to decrease in volume, which sends a signal to other cells in the hypothalamus
- These cells then send a signal to the posterior pituitary gland
- The pituitary gland secretes ADH
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