Topic 6.4 - Homeostasis

Question 1

Describe homeostasis in mammals (2)

Answer 1

1) Maintenance of a stable internal environment within restricted limits

2) By physiological control systems (normally involve negative feedback)

Question 2

Explain the importance of maintaining a stable core temperature (2)

Answer 2

1) If temperature is too high:
- Hydrogen bonds in tertiary structure of enzyme break
- Enzymes denature, active sites change shape and substrates can’t bind
- So fewer enzyme substrate complexes

2) If temperature is too low:
- Not enough kinetic energy so fewer enzyme substrate complexes

Question 3

Explain the importance of maintaining a stable blood pH (3)

Answer 3

1) Above or below optimal pH, ionic / hydrogen bonds in tertiary structure break

2) Enzymes denature - active sites change shape and substrates can’t bind

3) So fewer enzyme substrate complexes

Question 4

Explain the importance of maintaining stable blood glucose concentration (3)

Answer 4

Too low (hypoglycaemia):

1) Not enough glucose (respiratory substrate) for respiration

2) So less ATP produced

3) Active transport can’t happen -> cell death

Too high (hyperglycaemia):

1) Water potential of blood decreases

2) Water lost from tissue to blood via osmosis

3) Kidneys can’t absorb all glucose -> more water lost in urine causing dehydration

Question 5

Describe the role of negative feedback in homeostasis (3)

Answer 5

1) Receptors detect change from optimum

2) Effectors respond to counteract change

3) Returning levels to optimum / normal

Question 6

Give examples of negative feedback in homeostasis (4)

Answer 6

1) Control of blood glucose concentration

2) Blood pH

3) Core temperature

4) Blood water potential

Question 7

Explain the importance of conditions being controlled by separate mechanisms involving negative feedback (2)

Answer 7

1) Departures in different directions from the original state can all be controlled / reversed

2) Giving a greater degree of control (over changes in the internal environment)

Question 8

Describe positive feedback (3)

Answer 8

1) Receptors detect change from normal

2) Effectors respond to amplify change

3) Producing a greater deviation from the normal

Question 9

Give examples of positive feedback (2)

Answer 9

NOT involved in homeostasis

1) Onset of contractions in childbirth

2) Blood clotting

Question 10

Describe the factors that influence blood glucose concentration (2)

Answer 10

1) Consumption of carbohydrates -> glucose absorbed into blood

2) Rate of respiration of glucose e.g. increases during exercise due to muscle contraction

Question 11

Describe the role of glycogenesis

Answer 11

Converts glucose -> glycogen

Question 12

Describe the role of glycogenolysis

Answer 12

Converts glycogen -> glucose

Question 13

Describe the role of gluconeogenesis

Answer 13

Converts amino acids and/or glycerol -> glucose

Question 14

Why is insulin secreted?

Answer 14

Beta cells in islets of Langerhans in pancreas detect blood glucose concentration is too high -> secrete insulin

Question 15

Explain the action of insulin in decreasing blood glucose concentration (3)

Answer 15

1) Attaches to specific on cell surface membranes of target cells e.g. liver / muscles

2) This causes more glucose channel proteins to join cell surface membrane
- Increasing permeability to glucose
- So more glucose can enter cell by facilitated diffusion

3) This also activates enzymes involved in conversion of glucose to glycogen (glycogenesis)
- Lowering glucose concentration in cells, creating a concentration gradient
- So glucose enters cell by facilitated diffusion

Question 16

Why is glucagon secreted?

Answer 16

Alpha cells in islets of Langerhans in pancreas detect blood glucose concentration is too low -> secrete glucagon

Question 17

Explain the action of glucagon in increasing blood glucose concentration (40

Answer 17

1) Attaches to specific receptors on cell surface membranes of target cells e.g. liver

2) Activates enzymes involved in hydrolysis of glycogen to glucose (glycogenolysis)

3) Activates enzymes involved in conversion of glycerol / amino acids to glucose (gluconeogenesis)

4) This establishes a concentration gradient -> glucose enters blood by facilitated diffusion

Question 18

When / where is adrenaline secreted?

Answer 18

Fear / stress / exercise -> adrenal glands secrete adrenaline

Question 19

Explain the role of adrenaline in increasing blood glucose concentration (3)

Answer 19

1) Attaches to specific receptors on cell surface membranes of target cells e.g. liver

2) Activates enzymes involved in hydrolysis of glycogen to glucose (glycogenolysis)

3) This establishes a concentration gradient -> glucose enters blood by facilitated diffusion

Question 20

Describe the second messenger model of adrenaline in increasing blood glucose concentration (4)

Answer 20

Adrenaline / glucagon (first messenger) attach to specific receptors on cell membrane which:

1) Activates enzyme adenylate cyclase (changes shape)

2) Which converts many ATP to many cyclic AMP (cAMP)

3) cAMP acts as the second messenger -> activates protein kinase enzymes

4) Protein kinases activate enzymes to break down glycogen to glucose

Question 21

Suggest an advantage of the second messenger model (3)

Answer 21

1) Amplifies signal from hormone

2) As each hormone can stimulate production of many molecules of second messenger (cAMP)

3) Which can in turn activate many enzymes for rapid increase in glucose

Question 22

Compare the causes of types I and II diabetes (2/3)

Answer 22

Both -> higher and uncontrolled blood glucose concentration; higher peaks after meals and remain high

Type I:

• Key point = B cells in islets of Langerhans in pancreas produce insufficient insulin
• Normally develops in childhood due to an autoimmune response destroying B cells of islets of Langerhans

Type II:

• Key point = receptor (faulty) loses responsiveness / sensitivity to insulin (but insulin still produced)
• So fewer glucose transport proteins -> less uptake of glucose -> less conversion of glucose to glycogen
• Risk factor = obesity

Question 23

Describe how type I diabetes can be controlled (3)

Answer 23

1) Injections of insulin (as pancreas doesn’t produce enough)

2) Blood glucose concentration monitored with biosensors; dose of insulin matched to glucose intake

3) Eat regularly and control carbohydrate intake e.g. those that are broken down / absorbed slower
- To avoid sudden rise in glucose

Question 24

Suggest why insulin can’t be taken as a tablet by mouth (2)

Answer 24

1) Insulin is a protein

2) Would be hydrolysed by Endopeptidases / Exopeptidases

Question 25

Describe how type II diabetes can be controlled 5)

Answer 25

1) Not normally trade with insulin injections (as pancreas still produces it) but may use drugs which target insulin receptors to increase their sensitivity - To increase glucose uptake by cells / tissues 2) Reduce sugar intake (carbohydrates) / low glycemic index -> less absorbed 3) Reduce fat intake -> less glycerol converted to glucose 4) More (regular) exercise -> uses glucose / fats by increasing respiration 5) Lose weight -> increased sensitivity of receptors to insulin

Question 26

Describe how you can evaluate the positions of health advisors and the food industry in relation to the increased incidence of type II diabetes (2)

Answer 26

1) Health advisors aim -> reduce risk of type II diabetes due to health problems caused (e.g. kidney failure) - So need to reduce obesity as it is a risk factor 2) Food industry aim -> maximise profit

Question 27

Describe the structure of a nephron (2)

Answer 27

1) Nephron - basic structural and functional unit of the kidney (millions in the kidney) 2) Associated with each nephron are a network of blood vessels

Question 28

What is the role of the Bowman’s / renal capsule?

Answer 28

Formation of glomerular filtrate (ultrafiltration)

Question 29

What is the role of the proximal convoluted tubule?

Answer 29

Reabsorption of water and glucose (selective reabsorption)

Question 30

What is the role of the Loop of Henle?

Answer 30

Maintenance of a gradient of sodium ions in the medulla

Question 31

What is the role of the distal convoluted tubule?

Answer 31

Reabsorption of water (permeability controlled by ADH)

Question 32

What is the role of the collecting duct?

Answer 32

Reabsorption of water (permeability controlled by ADH)

Question 33

Describe the formation of glomerular filtrate (3)

Answer 33

1) High hydrostatic pressure in glomerulus - As diameter of different arteriole (in) is wider than efferent arteriole (out) 2) Small substances e.g. water, glucose, ions, urea forced into glomerular filtrate, filtered by: - Pores / fenestrations between capillary endothelial cells - Capillary basement membrane - Podocytes 3) Large proteins / blood cells remain in blood

Question 34

Describe the reabsorption of glucose by the proximal convoluted tubule (3)

Answer 34

1) Na+ actively transported out of epithelial cells to capillary 2) Na+ moves by facilitated diffusion into epithelial cells down a concentration gradient, bringing glucose against its concentration gradient 3) Glucose moves into capillary by facilitated diffusion down its concentration gradient

Question 35

Describe the reabsorption of water by the proximal convoluted tubule (2)

Answer 35

1) Glucose etc… in capillaries lower water potential 2) Water moves by osmosis down a water potential gradient

Question 36

Describe and explain how features of the cells in the proximal convoluted tubule allow the rapid reabsorption of glucose into the blood (5)

Answer 36

1) Microvilli / folded cell-surface membrane -> provides a large surface area 2) Many channel / carrier proteins -> for facilitated diffusion / co-transport 3) Many carrier proteins -> for active transport 4) Many mitochondria -> produce ATP for active transport 5) Many ribosomes -> produce carrier / channel proteins

Question 37

Suggest why glucose is found in the urine of an untreated diabetic person (2)

Answer 37

1) Blood glucose concentration is too high so not all glucose is re absorbed at the proximal convoluted tubule 2) As glucose carrier / co-transporter proteins are saturated / working at maximum rate

Question 38

Explain the importance of maintaining a gradient of sodium ions in the medulla (concentration increases further down)

Answer 38

1) So water potential decreases down the medulla (compared to filtrate in collecting duct) 2) So a water potential gradient is maintained between the collecting duct and medulla 3) To maximise reabsorption of water by osmosis from filtrate

Question 39

Describe the role of the loop of Henle in maintaining a gradient of sodium ions in the medulla

Answer 39

1) In the ascending limb: - Na+ is actively transported out (so filtrate concentration decreases) - Water remains as ascending limb is impermeable to water - This increases concentration of Na+ in the medulla, lowering water potential 2) In the descending limb: - Water moves out by osmosis then re absorbed by capillaries (so filtrate concentration increases) - Na+ ‘recycled’ -> diffuses back in

Question 40

Suggest why animals needing to conserve water have long loops of Henle (3)

Answer 40

1) More Na+ moved out -> Na+ gradient is maintained for longer in medulla / higher Na+ concentration 2) So water potential gradient is maintained for longer 3) So more water can be reabsorbed from collecting duct by osmosis

Question 41

Describe the reabsorption of water by the distal convoluted tubule and collecting ducts (2)

Answer 41

1) Water moves out of distal convoluted tubule and collecting duct by osmosis down a water potential gradient 2) Controlled by ADH which increases their permeability

Question 42

What is osmoregulation?

Answer 42

Control of water potential of the blood (by negative feedback)

Question 43

Describe the role of the hypothalamus in osmoregulation (2)

Answer 43

1) Contains osmoreceptors which detect increase or decrease in blood water potential 2) Produces more ADH when water potential is low or less ADH when water potential is high

Question 44

Describe the role of the posterior pituitary gland in osmoregulation

Answer 44

Secretes (more/less) ADH into the blood due to signals from the hypothalamus

Question 45

Describe the role of antidiuretic hormone (ADH) in osmoregulation (5)

Answer 45

1) Attaches to receptors on collecting duct (and distal convoluted tubule) 2) Stimulating addition of channel proteins (aquaporins) into cell-surface membranes 3) So increases permeability of cells by collecting duct and DCT to water 4) So increases water reabsorption from collecting duct / DCT (back into blood) by osmosis 5) So decreases volume and increases concentration of urine produced

Question 46

Give 2 key features of the ascending limb in the loop of Henle

Answer 46

1) Impermeable to water 2) Na+ moves out (mostly active transport)

Question 47

Give 2 key features of the descending limb in the loop of Henle

Answer 47

1) Water leaves by osmosis 2) Na+ moves in (recycled)