B16 Homeostasis Flashcards
What is homeostasis?
Internal environment is maintained within set limits around an optimum.
Why is it important that core temperature remains stable?
Maintain stable rate of enzyme-controlled reactions & prevent damage to membranes.
Temperature too low = enzyme & substrate molecules have insufficient kinetic energy
Temperature too high = enzymes denature
Why is it important that blood pH remains stable?
Maintain stable rate of enzyme-controlled reactions (& optimum conditions for other proteins)
Acidic pH = H+ ions interact with H-bonds & ionic bonds in tertiary structure of enzymes —> shape of active site changes so no ES complexes form.
Why is it important that blood glucose concentration remains stable?
Maintain constant blood water potential: prevent osmotic lysis / crenation of cells
Maintain constant concentration of respiratory substrate: organism maintains constant level of activity regardless of environmental conditions.
Define negative feedback
Self-regulatory mechanisms return internal environment to optimum when there is a fluctuation.
Define positive feedback
A fluctuation triggers changes that result in an even greater deviation from the normal level.
Outline the general stages involved in negative feedback
Receptors detect deviation —> coordinator —> correct mechanism by effector —> receptors detect that conditions have returned to normal.
Suggest why separate negative feedback mechanisms control fluctuations in different directions
Provides more control, especially in case of ‘overcorrection’, which would lead to a deviation in the opposite direction from the original one.
Suggest why coordinators analyse inputs from several receptors before sending an impulse to effectors.
Receptors may send conflicting information
Optimum response may require multiple types of effector
Why is there a time lag between hormone production and response by an effector?
It takes time to:
> produce hormone
transport hormone in the blood
cause required change to the target protein
Name the factors that affect blood glucose concentration
> amount of carbohydrate digested from diet
> rate of glycogenolysis
> rate of gluconeogenesis
Define glycogenesis
Liver converts glucose into the storage polymer glycogen
Define glycogenolysis
Liver hydrolyses glycogen into glucose which can diffuse into blood
Define gluconeogenesis
Liver converts glycerol and amino acids into glucose
Outline the role of glucagon when blood glucose concentration decreases.
Alpha cells in islets of langerhans in pancreas detect decrease & secrete glucagon into bloodstream.
Glucagon binds to surface receptors on liver cells & activates enzymes for glycogenolysis & gluconeogenesis.
Glucose diffuses from liver into bloodstream.
Outline the role of adrenaline when blood glucose concentration decreases.
Adrenal glands produce adrenaline. It binds to surface receptors on liver cells & activates enzymes for glycogenolysis.
Glucose diffuses from liver into bloodstream
Outline what happens when blood glucose concentration increases.
Beta cells in islets of langerhans in pancreas detect increase and secrete insulin into bloodstream.
Insulin binds to surface receptors on target cells to:
> increase cellular glucose uptake
> activate enzymes for glycogenesis (liver & muscles)
> stimulate adipose tissue to synthesise fat
Describe how insulin leads to a decrease in blood glucose concentration.
> increases permeability of cells to glucose
> increases glucose concentration gradient
> triggers inhibition of enzymes for glycogenolysis
How does insulin increase permeability of cells to glucose?
Increases number of glucose carrier proteins.
Triggers conformational change which opens glucose carrier proteins.
How does insulin increase the glucose concentration gradient?
Activates enzymes for glycogenesis in liver & muscles
Stimulates fat synthesis in adipose tissue
Use the secondary messenger model to explain how glucagon and adrenaline work.
Hormone-receptor complex forms
Conformational change to receptor activates G-protein
Activates adenylate cyclase, which converts ATP to cyclic AMP (cAMP)
cAMP activates protein kinase A pathway
Results in glycogenolysis
Explain the causes of Type 1 diabetes
Body cannot produce insulin e.g. due to autoimmune response which attacks beta cells of islets of langerhans.
How can type 1 diabetes be controlled?
Treat by injecting insulin
Explain the cause of type 2 diabetes.
Glycoprotein receptors are damaged or become less responsive to insulin.
Strong positive correlation with poor diet/obesity.
How can type 2 diabetes be controlled?
Treat by controlling diet and exercise regime.
Name some signs and symptoms of diabetes.
> high blood glucose concentration
glucose in urine
polyuria
polyphagia
polydipsia
blurred vision
sudden weight loss
Suggest how a student could produce a desired concentration of glucose solution from a stock solution.
Volume of stock solution = required concentration x final volume needed / concentration of stock solution.
Volume of distilled water = final volume needed - volume of stock solution
Outline how colorimetry could be used to identify the glucose concentration in a sample.
- Benedict’s test on solutions of known glucose concentration. Use colorimeter to record absorbance.
- Plot calibration curve: absorbance ( y-axis), glucose concentration (x-axis).
- Benedict’s test o unknown sample. Use calibration curve to read glucose concentration as its absorbance value.
Define osmoregulation
Control of blood water potential via homeostatic mechanisms.
Name the gross structure of a mammalian kidney.
Fibrous capsule
Cortex
Medulla
Renal pelvis
Ureter
Renal artery
Renal vein
What is the purpose of the fibrous capsule in the mammalian kidney.
Fibrous capsule : Protects the kidney
What is the cortex in the kidney?
Cortex : outer region consists of Bowman’s capsules, convoluted tubules, blood vessels.
What is the medulla in the mammalian kidney?
Medulla : inner region consists of collecting ducts, loops of Henle, blood vessels.
What is the renal pelvis in the mammalian kidney?
Renal pelvis : cavity collects urine into ureter.
What is the ureter in the mammalian kidneys?
Ureter : tube that carries urine to the bladder
What is the renal artery in the mammalian kidneys?
Renal artery : Supplies kidney with oxygenated blood
What is the renal vein in the mammalian kidney?
Renal vein : returns deoxygenated blood to the heart
State the structure of a nephron.
Bowman’s capsule
Proximal convoluted tubule (PCT)
Loop of Henle
Distal convoluted tubule (DCT)
Collecting duct
Describe the structure of the bowman’s capsule in the nephron.
Is at the start of nephron
Is cup-shaped, surrounds glomerulus, inner layer of podocytes.
Describe the structure of the proximal convoluted tubule ( PCT ) in the nephron.
Series of loops surrounded by capillaries, walls made of epithelial cells with microvilli
Describe the structure of the Loop of Henle in the nephron.
Hairpin loop extends from cortex into medulla
Describe the structure of the distal convoluted tubule (DCT) in the nephron.
Similar to PCT but fewer capillaries
Describe the structure of the collecting duct in the nephron.
DCT from several nephrons empty into collecting duct, which leads into pelvis of kidney.
Name the blood vessels associated with a nephron.
Wide afferent arteriole
Efferent arteriole
Describe the wide afferent arteriole
Wide afferent arteriole from renal artery enters renal capsule & forms glomerulus: branched knot of capillaries which combine to form narrow efferent arteriole.
Describe the efferent arteriole
Efferent arteriole branches to form capillary network that surrounds tubules.
Why might desert animals have long loops of Henle. But why?
Animals in dry environments would have longer loops of Henle to give a longer counter current multiplier + so more absorption of water by the collecting duct.
Explain how glomeluar filtrate is formed.
Ultrafiltration in Bowman’s capsule.
High hydrostatic pressure in glomerulus forces small molecules (urea, water, glucose, mineral ions) out of capillary fenestrations AGAINST osmotic gradient.
Basement membrane acts as filter. Blood cells and large molecules e.g. proteins remain in capillary.
How are cells of the Bowman’s capsule adapted for ultrafiltration?
Fenestrations between epithelial cells of capillaries.
Fluid can pass between and under folded membrane of podocytes.
State what happens during selective reabsorption and where it occurs.
Useful molecules from glomerular filtrate e.g. glucose are reabsorbed into the blood.
Occurs in proximal convoluted tubule.
How are cells in the proximal convoluted tubule adapted for selective reabsorption?
Microvilli - large SA for co-transporter proteins.
Many mitochondria - ATP for active transport of glucose into intercellular spaces.
Folded basal membrane - Large SA
What happens in the loop of Henle?
1 - active transport of Na+ & Cl- out of ascending limb.
2 - water potential of interstitial fluid decreases.
3 - osmosis of water out of descending limb (ascending limb is impermeable to water).
4 - water potential of filtrate decreases going down descending limb: lowest in medullary region, highest at top of ascending limb.
Explain the role of the distal convoluted tubule.
Reabsorption:
a) of water via osmosis
b) of ions via active transport
Permeability of walls is determined by action of hormones.
Explain the role of the collecting duct.
Reabsorption of water from filtrate into interstitial fluid via osmosis through aquaporins.
Explain why it’s important to maintain a Na+ gradient.
Countercurrent multiplier : filtrate in collecting ducts is always beside an area of interstitial fluid that has a lower water potential.
Maintains a water potential gradient for maximum reabsorption of water.
What might cause blood water potential to change?
> level of water intake
> level of ion intake in diet
> level of ions used in metabolic processes or excreted
> sweating
Explain the role of the hypothalamus in osmoregulation.
- Osmosis of water out of osmoreceptors in hypothalamus causes them to shrink.
- This triggers hypothalamus to produce more antidiuretic hormone (ADH).
Explain the role of the posterior pituitary gland in osmoregulation.
Stores and secreted the ADH produced by the hypothalamus.
Explain the role of ADH in osmoregulation.
- Makes cells lining collecting duct more permeable to water:
> binds to receptor —>activates phosphorylase —> vesicles with aquaporins on membrane fuse with cell-surface membrane. - Makes cells lining collecting duct more permeable to urea:
> water potential in interstitial fluid decreases
> more water reabsorbed = more concentrated urine
How can Benedict’s solution be used to measure the concentration of glucose in a solution?
Use a colorimeter to measure the absorbance of a series of solutions of known concentrations to Create a calibration curve. Compare the absorbance of an unknown sample to the calibration curve.
What is a serial dilution?
A dilution where successive concentrations increase/decrease in a logarithmic fashion.
Outline the procedure of this practical.
- Make a serial dilution of glucose, ranging from 0 to 10 mmol dm^-3
- Place 2cm3 of each of the unknown samples in separate boiling tubes.
- Add 2cm3 of Benedict’s solution to all boiling tubes.
- Place boiling tubes in a water bath at 90*C for 4 minutes.
- Zero the colorimeter using a cuvette with distilled water and set to red filter.
- Place known samples into cuvette and measure the absorbance of each using the colorimeter.
- Make a calibration curve.
- Measure the absorbance of the unknown samples using the colorimeter. Use the calibration curve to determine glucose concentrations.
When does blood glucose fall/increase
Blood glucose increases following ingestion of food or drink containing carbohydrates and will fall following exercise or if you have not eaten.
What does the pancreas
The pancreas detects changes in the blood glucose levels. It contains endocrine cells in the Islets of Langerhans which release the hormones insulin and glucagon to bring blood glucose levels back to normal.
What does adrenaline do
Adrenaline is released by adrenal glands when your body anticipates danger and this results in more glucose being released from stores of glycogen in the liver.
What happens when blood glucose increases
Blood glucose levels increases
Detected by the beta cells in the islets of Langerhans (pancreas)
Beta cells release insulin
Liver cells become more permeable to glucose and enzymes are activated to convert glucose to glycogen
Glucose is removed from the blood and stored as glycogen in cells
What happens when blood glucose decreases
Blood glucose levels decrease
Detected by the alpha cells in the islets of Langerhans (pancreas)
Alpha cells release glucagon
Adrenal gland release adrenaline
Second messenger model occurs to activate enzymes to hydrolyse glycogen
Glycogen is hydrolysed to glucose and more glucose is release back into the blood.
What is glycogenesis
The process of excess glucose being converted to glycogen when blood glucose is higher than normal. This occurs mainly in the liver.
(genesis means to make)
What is Glycogenolysis
Glycogenolysis (lysis means to breakdown)
The hydrolysis of glycogen back into glucose in the liver. This occurs when blood glucose levels are lower than normal.
What is Gluconeogenesis
Gluconeogenesis (Amino acids to glucose)
The process of creating gluçose from non-carbohydrate stores in the liver. This occurs if all glycogen has been hydrolysed into glucose and your body still needs more glucose.
What do beta cells do
Beta cells in the islets of Langerhans detect when blood glucose levels are too high and secrete insulin in response to this.
What are the different ways that insulin decreases blood glucose
Attaching to receptors on the surfaces of target cells. This changes the tertiary structure of the channel proteins resulting in more glucose being abs›rbed by facilitated diffusion.
2 More protein channels are incorporated into cell membranes so that more glucose is absorbed from the blood into cells.
3. Activating enzymes involved in the conversion of glucose to glycogen. This results in glycogenesis in the liver.
What do alpha cells do
Alpha cells in the islets of Langerhans detect when blood glucose is too low and will secrete glucagon in response to this.
What are the different ways that glucagon increases blood glucose
Attaching to receptors on the surfaces of target cells (liver cells).
When glucagon binds it causes a protein to be activated into adenylate cyclase and to convert ATP in a molecule called cyclic AMP (cAMP). CAMP activates an enzyme, protein kinase, that can hydrolyse glycogen into glucose.
Activating enzymes involved in the conversion of glycerol and amino acids into glucose.
What are the different ways that adrenaline increases blood glucose
- Adrenaline attaches to receptors on the surfaces of target cells. This causes a protein (G protein) to be activated and to convert ATP into cAMP.
- CAMP activates an enzyme that can hydrolyse glycogen into glucose.
- This is known as the second messenger model of adrenaline and glucagon action, because the process results in the formation of cAMP, which acts as a second messenger.
What is diabetes
This is when blood glucose cannot be controlled.
What is type 1 diabetes
Type I diabetes is due to the body being unable to produce insulin, it starts in childhood and could be the result of an autoimmune disease where the beta cells were attacked. Treatment involves injections of insulin.
What is type 2 diabetes
Type Il diabetes is due to receptors on the target cells losing their responsiveness to insulin, it usually develops in adults because of obesity and poor diet. It is controlled by regulating intake of carbohydrates, increasing exercise and sometimes insulin injections.
What happens in second messenger model
Glucagon binds to glucagon receptors
Once bound, it causes a change in shape to the enzyme adenyl cyclase, which activates it
Activated adenyl cyclase enzymes converts ATP into cyclic AMP (cAMP)
cAMP is the second messenger
Where does osmoregulation occur
Within the nephron
Where are nephrons found
In the kidneys
What are nephrons
Nephrons are long tubules surrounded by capillaries
There are approximately I million nephrons each kidney
What’s the function of the nephron
To Filter the blood to remove waste and selectively reabsorb useful substance back into the blood.
What does urine contain
Water
Dissolved salts
Urea
Other small substances e.g. hormones and excess vitamins
What does urine not contain
Proteins & Blood cells
Glucose
Why does urine not contain proteins and blood cells
Proteins are too large to be filtered out of the blood
Why does urine not contain glucose
All glucose is absorbed at the selective reabsorption stage in the PCT
How filtering and reabsorption occurs
Stage I: Ultrafiltration occurs due to high hydrostatic pressure. Water and small molecules are forced out of the glomerulus capillaries into the renal capsule.
Stage 2: Selective reabsorption occurs in the proximal convoluted tubule.
Stage 3 + 4: The loop of Henle maintains a sodium ion gradient so water can be reabsorbed by the blood.
Stage 5+6: Water moves out of the distal convoluted tubule and collecting duct to return back to the blood.
The collecting duct then carries the remaining liquid (urine) to the ureter.
Simple explanation of how filtering and reabsorption occurs and where they occur
- Glomerulus: filters small solutes from the blood
- Proximal convoluted tubule: reabsorbs ions, water, and nutrients; removes toxins and adjusts filtrate pH
- Descending loop of Henle: aquaporins
allow water to pass from the filtrate into the interstitial fluid - Ascending loop - of Henle: reabsorbs Na+ and Cl-from the filtrate into the interstitial fluid
- Distal tubule: selectively secretes and absorbs different ions to maintain blood pH and electrolyte balance
- Collecting duct: reabsorbs solutes and water from the filtrate
What happens in ultrafiltration
Blood enters through the afferent arteriole, and this splits it lots of smaller capillaries which make up the glomerulus. This causes a high hydrostatic pressure of the blood.
Water and small molecules, such as glucose and mineral ions are forced out of the capillaries and for the glomerulus filtrate.
Large proteins and blood cells are too big to fit through the gaps in the capillary endothelium, so remain in the blood. This blood leaves via the efferent arteriole
Where does selective reabsorption occur
Occurs in the proximal convoluted tubule. Here 85% of the glomerulus filtrate is reabsorbed back into the blood, leaving urea and excess mineral ions and urea behind.
Adaptations for selective reabsorption
Microvilli provide a large surface area for reabsorption.
Lots of mitochondria to provide energy for active transport
Process of selective reabsorption
- The concentration of sodium ions in the PCT cell in decreased as the sodium ions are actively transport out of the PCT cells into the blood in the capillaries.
- Due to the concentration gradient, sodium ions diffuse down the gradient from the lumen of the PCT into the cells lining the PCT. The is an example of co-transport, as the proteins which transport the sodium ions in carry glucose with it.
- The glucose can then diffuse from the PCT
epithelial cell into the blood stream. - This is how all the glucose is reabsorbed.
Maintaining a sodium ion gradient by the loop of henle
A sodium ion gradient, to enable reabsorption of water, is maintained in the medulla by the loop of Henle
The loop of Henle has an ascending and descending limb
ascending limb - wall are impermeable to water. It has much thicker walls. Sodium ions are actively transported out
Descending limb limb - wall are permeable to water. The wall are much thinner
Why is the filtrate that reaches the top of the PCT very dilute
Due to all the sodium ions being actively transported out of the PCT, when the filtrate reaches the top of the PCT it is very dilute.
Reabsorption of water at the DCT and the collecting duct
Due to all the sodium ions being actively transported out of the PCT, when the filtrate reaches the top of the PCT it is very dilute.
This filtrate moves into the distal convoluted tubules and collecting duct. This section of the medulla surround these two parts of the nephron are very concentrated.
Therefore, even more water diffuses out of the DCT and collecting ducts.
What remains is transported to and forms urine
SUGGEST HOW THE LENGTH OF THE LOOP OF HENLE WILL DIFFER FOR A DESERT ANIMAL COMPARED TO A HUMAN
EXPLAIN WHY?
Desert animals will have a longer loop of Henle.
The longer the loop of Henle, the more sodium ions that are actively transported out, and therefore an even more negative water potential is created.
This results in more water being rebsorbed into the blood and very concentrated urine
Where is glomerular filtrate produced
In the renal capsule
What are reabsorbed into the blood by the PCT
Glucose and water
Where and how is a sodium ion gradient maintained
A sodium ion gradient, to enable reabsorption of water, is maintained in the medulla by the loop of Henle
Where does reabsorption of water into the blood occur
Reabsorption of water into the blood occurs in the DCT and collecting ducts.
What is the nephron made up of
The nephron is made up of the renal (Bowman’s) capsule, proximal convoluted tubule, loop of Henle, distal convoluted tubule and colleting ducts.
They are surrounded by capillaries.
What is water potential of the blood controlled by
Osmoregulation
Why is homeostasis of water potential of the blood (osmoregulation) essential? **think about osmosis
Blood with too low a water potential (hypertonic)
Too much water will leave the cells and move into the blood by osmosis. Cells will shrivel (crenation)
Blood with too high a water potential (hypotonic)
Too much water will move from the blood into the cells by osmosis. Cells will burst (lysis)
Osmoregulation - blood with too low a water potential (hypertonic)
Blood with too low a water potential (hypertonic)
• Too much sweating
• Not drinking enough water
• Lots of ions in diet (lots of salt)
Corrective mechanism:
More water is reabsorbed by osmosis into the blood from the tubules of the nephrons. This means the urine is more concentrated as less water is lost in the urine.
Osmoregulation - Blood with too high a water potential (hypotonic)
- Drinking too much water
- Not enough salt in diet
Corrective mechanism:
Less water is reabsorbed by osmosis into the blood from the tubules of the nephrons. This means the urine is more dilute and more water is lost in the urine.
What are changes in water potential in the blood detected by
Changes in the water potential of the blood are detected by osmoreceptors found in the hypothalamus.
What happens if the water potential of the blood is too low
If the water potential of the blood is too low water leaves the osmoreceptors by osmosis and they shrivel. This stimulates the hypothalamus to produce more of the hormone ADH
What happens if the water potential of the blood is too high
If the water potential of the blood is too high water enters the osmoreceptors by osmosis.
This stimulates the hypothalamus to produce less
ADH
Where is ADH produced
The hypothalamus is where ADH is produced. ADH then moves to the posterior pituitary and from here it is released into capillaries and into the blood.
ADH travels through the blood to its target organ, the kidney.
What does ADH stand for
ADH - ANTIDIURETIC HORMONE
What happens when ADH reaches the kidneys
When ADH reaches the kidney it causes an increase in the permeability of the walls of the collecting duct and distal convoluted tubule to water.
This means more water leaves the nephron and is reabsorbed into the blood, so urine is more concentrated.
What are aquaporins
There are receptors on the cell membranes of the DCT and collecting duct which ADH binds to.
What happens when aquaporins are bounds
When bound, it activates a phosphorylase enzyme in the cells.
Phosphorylase causes the vesicles containing aquaporins to fuse with the cell membrane and the aquapoins embed.
Aquaporins are protein channels for water to pass through
With more aquaporins in the cell membrane, more water leaves the DCT and collecting tubule and is reabsorbed into the blood.
What happens when water potential of blood increases
Water potential of blood increases (too much water)
Detected by osmoreceptors in hypothalamus.
Hypothalamus releases less ADH
DCT and collecting duct walls become less permeable to water
Less water is reabsorbed into the blood and more is lost in the urine (dilute urine)
What happens when water potential of blood decreases
Water potential of blood decreases ( not enough water)
Detected by osmoreceptors in hypothalamus.
Hypothalamus releases more ADH which is released into the blood by the posterior pituitary gland
DCT and collecting duct walls become more permeable to water
More water is reabsorbed into the blood and less is lost in the urine (concentrated urine)
MAINTAINING A SODIUM ION GRADIENT BY THE LOOP OF HENLE - process
I. Mitochondria in the walls of the cells provide energy to actively transport sodium ions out of the ascending limb of the loop of Henle.
- The accumulation of sodium ions in outside of the nephron in the medulla lowers the water potential.
- Therefore water diffuses out by osmosis into the interstitial space and then the blood capillaries (water is reabsorbed into the blood).
- At the base of the ascending limb some sodium ions are transported about by diffusion, as there is now a very dilute solution due to all the water that has moved out.
