Year 13 - Homeostasis

Question 1

What is meant by homeostasis?

Answer 1

Maintaining a constant internal environment within restricted limits by a physiological control system

Question 2

Using an example, explain why homeostasis is important.

Answer 2

Temperature/ pH needs to be kept at optimum ​
Blood glucose concentration as need glucose to be available to cells as a respiratory substrate.​
Blood glucose concentration/Water potential kept constant as osmotic imbalance can cause to much water to leave cell (crenation) or enter cell (cell lysis).

Question 3

What is meant by negative feedback and why is it important?

Answer 3

A change to the normal level initiates a response which reduces the effect of a change (this means it acts against the change/reverses) ​
So restores systems to their original level preventing named risk/damage ​
or ​
So prevents continuation of behaviour preventing named risk/damage

Question 4

What is meant by positive feedback and explain why info/figure/table is example of positive feedback?

Answer 4

-A change to the normal level initiates a response which causes additional change in the same direction ​
-1st named thing causes release/formation/production of 2nd named thing ​
- 2nd named thing causes more release/formation/production of 1st named thing. ​
-1st named thing causes more release/formation/production of 2nd named thing

Question 5

Describe the response to higher than normal blood glucose concentration (negative feedback)

Answer 5

Increase in glucose detected by hypothalamus / beta cells in pancreas ​
Beta cells secrete insulin by exocytosis ​
Insulin binds to complementary receptor on cell surface membrane of target liver/muscle cells
Activates enzymes involved in glycogenesis so maintains concentration gradient of glucose for continues diffusion from the blood
Causes more carrier and channel proteins for glucose to move to the cell surface membrane of target cells, increasing permeability to glucose so more glucose enters by facilitated diffusion, lowering blood plasma glucose concentration.

Question 6

Describe the response to lower than normal blood glucose concentrations (negative feedback)

Answer 6

Decrease in glucose detected by hypothalamus / alpha cells in pancreas ​
Alpha cells secrete glucagon by exocytosis ​
Glucagon binds to complementary receptor on cell surface membrane of target liver/muscle cells
Acitivates enzymes involved in glycogenolysis which is the hydrolysis of glycogen to release glucose
Activates enzymes involved in gluconeogenesis which is the conversion of fatty acids, glycerol and amino acids to glucose
Glucose leaves cells into blood by facilitated diffusion so increases blood plasma glucose concentration.

Question 7

Describe the second messenger model of adrenaline and glucagon action

Answer 7

1. Adrenaline binds to complementary receptors on cell surface membrane​
2. Activates adenylate cyclase which produces the second messenger cAMP from ATP. ​
3. cAMP activates protein kinase ​
4. Which activates enzymes involved in glycogenolysis and gluconeogensis
5. Glucose produced so leaves cells into blood by facilitated diffusion increasing blood glucose concentration.

Question 7

Describe the causes of type 1 and why they may have a higher blood glucose concentration than a non diabetic person

Answer 7

Pancreatic beta cells destroyed/don’t work​
Doesn’t produce enough insulin but remains sensitive to insulin. ​
Less insulin binds to complementary receptors so doesn’t effect target cells
Less carrier and channel proteins complementary to glucose to move to cell surface membrane of target cells. ​
Reduced uptake of glucose into cells by facilitated diffusion ​
Reduced activation of enzymes involved in glycogenesis which is the conversion of glucose to glycogen ​
Blood glucose concentration not lowered when higher than normal as remains in blood

Question 8

Describe the causes of type 2 and why they may have a higher blood glucose concentration than a non diabetic person

Answer 8

Pancreatic beta cells not destroyed​
Still produces and releases insulin but reduced sensitivity to insulin so insulin has less of an effect on target cells
Less carrier and channel proteins complementary to glucose to move to cell surface membrane of target cells. ​
Reduced uptake of glucose into cells by facilitated diffusion ​
Reduced activation of enzymes involved in glycogenesis which is the conversion of glucose to glycogen ​
Blood glucose concentration not lowered when higher than normal as remains in blood

Question 9

Describe and explain the treatments for type 1 diabetes

Answer 9

Must be given insulin by injections (or way to make it eg stem cells/transplant) ​
As target cells still sensitive to insulin so increase in insulin so see insulin function fc​
Controlled Diet that lowers sugar intake and regular exercise so don’t have spike of higher-than-normal blood glucose concentration.

Question 10

Describe and explain the treatments for type 2 diabetes

Answer 10

Diet low in carbohydrates so less glucose enters the blood so over time cells will regain sensitivity to insulin

Exercise so causes blood glucose to be taken up and used in glycolysis in respiration​

Drugs so lower blood glucose by altering energy metabolism/ inhibiting gluconeogenesis/action of glucagon/adenylate cylase ect. So over time cells regain sensitivity to insulin

Drugs so increase sensitivity of pancreas cells to glucose so increases secretion of insulin

Question 11

Describe symptoms of diabetes

Answer 11

Higher than normal blood glucose concentration ​

Excessive urination/drinking/diluted urine/ thirst

Question 12

Why is glucose found in the urine of a person with untreated diabetes?

Answer 12

High concentration of glucose in blood. ​

More glucose filtered out through basement membrane at bowman’s capsule than non-diabetic individual. ​

High concentration of glucose in glomerular filtrate. ​

Not all glucose is reabsorbed at the proximal convoluted tubule by facilitated diffusion/active transport ​

Due to all carrier proteins being saturated so working at maximum rate​

So some glucose lost in urine

Question 13

State at which part of the nephron ultrafiltration occurs

Answer 13

Glomerulus

Question 14

State at which part of the nephron selective reabsorption occurs

Answer 14

Proximal convoluted tubule

Question 15

State at which part of the nephron countercurrent multiplication occurs

Answer 15

Loop of Henle

Question 16

State at which part of the nephron ADH binds to receptors and causes absorption of water

Answer 16

Distal convoluted tubule and collecting duct.

Question 17

Describe ultrafiltration in the glomerulus

Answer 17

Ultrafiltration at glomeruli and bowman’s capsule ​

High hydrostatic pressure of blood (not higher) ​

Causes water, glucose, ions, amino acids and urea (and fatty acids and glycerol) to pass out of capillaries through fenestrations in endothelium ​and Through basement membrane that acts as filter ​forming glomerular filtrate in the proximal convoluted tubule​

Proteins and cells are too large so can’t pass through

Question 18

What does the proximal convoluted tubules selectively reabsorb?

Answer 18

100% of glucose/a.a should be absorbed here as done by active transport and Facilitated diffusion. Some water follows by osmosis/some ions absorbed here.

Question 19

Describe the adaptions of the cells of proximal convoluted tubule that allow for efficient selective re-absorption

Answer 19

1. Microvilli (folded membrane) provide a large surface area​
2. Many channel/carrier proteins for facilitated diffusion​
3. Many carrier proteins for active transport (main type of transport for glucose)​
4. Many mitochondria produce ATP for active transport

Question 20

How is urine made more concentrated than the blood plasma?​
Describe the countercurrent multiplication principle in the LOH in your answer

Answer 20

Countercurrent so fluid moves in opposite directions ​
Multiplication as concentrations of Na+ ions increases down medulla establishing water potential gradient

Ascending limb of the loop of henle actively pumps Na+ ions out and is impermeable to water. ​
This makes a high concentration of ions so a lower water potential in the tissue fluid in the medulla. ​
Descending limb of the loop of henle in permeable to water so water leaves by osmosis ​
Descending limb of the loop of henle is permeable to Na+ ions so some move in by facilitated diffusion. ​
Water potential of tissue fluid in medulla is lower than filtrate in tubules/collecting duct​
So water leaves collecting duct and distal convoluted tubule down its water potential gradient by osmosis.

Urine more concentrated and smaller volume

Question 21

State the location of osmoreceptors.

Answer 21

Hypothalamus

Question 22

State the part of the body which releases antidiuretic hormone (ADH) into blood

Answer 22

Posterior pituitary

Question 23

​State where ADH binds to

Answer 23

Complementary receptors in the cell surface membrane of the collecting duct and distal convoluted tubule

Question 24

State the symptoms of a decrease in ADH

Answer 24

Less water absorbed back into blood so:​ - Dehydration and thirst ​ - Frequent urination with larger volumes of urine ​ - Less concentrated urine

Question 25

Describe the role of osmoreceptors in increasing water potential

Answer 25

Blood becomes highly concentrated in solutes (eg glucose) so water potential decreases. ​ Water moves by osmosis from osmoreceptor to blood. ​ Shrinking the receptor cell causing Na+ channels to open -> increasing rate of fire of AP *see AP FC* ​ Results in ADH being released from the posterior pituitary *see ADH FC* (And behaviour changes eg drinking more water)

Question 26

Describe the response that brings about an increase in water potential of the blood

Answer 26

Osmoreceptors in the hypothalamus detect water potential of the blood is too low. ​*see osmoreceptor fc*​ Posterior pituitary secretes more antidiuretic hormone (ADH)​ ADH binds to complementary receptors in the cell surface membrane of the distal convoluted tubule and collecting duct ​ Causing vesicles containing aquaporins to move and fuse with cell surface membranes of the distal convoluted tubule and collecting duct. ​ Inserting aquaporins into the membranes increasing permeability to water ​ More water reabsorbed as it enters cells of distal convoluted tubule and collecting duct through aquaporins​ From cells to capillary via interstitial fluid ​ Water moves down water potential gradient by osmosis​ Smaller volume of urine and more concentrated urea in urine (than glomerular filtrate)