3.6.4 Homeostasis

Question 1

What is homeostasis?

Answer 1

Internal environment is maintained within set limits around an optimum

Question 2

Why is it important that core temperature remains stable?

Answer 2

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

Question 3

Why is it important that blood pH remains stable?

Answer 3

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 ES complexes form

Question 4

Why is it important that blood glucose concentration remains stable?

Answer 4

• 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.

Question 5

Define negative and positive feedback

Answer 5

negative feedback: self-regulatory mechanisms return internal environment to optimum when there is a fluctuation
positive feedback; a fluctuation triggers changes that result in an even greater deviation from the normal level

Question 6

outline the general stages involved in negative feedback.

Answer 6

Receptors detect deviation –> coordinator –> corrective mechanism by effector –> receptors detect that conditions have returned to normal

Question 7

Suggest why separate negative feedback mechanisms control fluctuations in different directions.

Answer 7

Provides more control, especially in case of ‘overcorrection’, which would lead to a deviation in the opposite direction from the original one.

Question 8

Suggest why coordinators analyse inputs from several receptors before sending an impulse to effectors.

Answer 8

• Receptors may send conflicting information
• Optimum response may require multiple types of effector

Question 9

Why is there a time lag between hormone productions and response by effector?

Answer 9

takes time to
- produce hormone
- transport hormone in the blood
- cause required change to the target protein

Question 10

Name the factors that affect blood glucose concentration.

Answer 10

• Amount of carbohydrate digested from diet
• Rate of glycogenesis
• Rate of gluconeogenesis

Question 11

Define glycogenesis, glycogenolysis and gluconeogenesis

Answer 11

glycogenesis: liver converts glucose into glycogen
glycogenolysis: liver hydrolyses glycogen into glucose which can diffuse into blood
gluconeogenesis: liver converts glycerol & amino acids into glucose.

Question 12

Outline the role of glucagon when blood glucose concentration decreases.

Answer 12

1. alpha cells in Islets of Langerhans in pancreas detect decrease & secrete glucagon into bloodstream
2. Glucagon binds to surface receptors on liver cells & activates enzymes for glycogenesis & gluconeogenesis
3. Glucose diffuses from liver into bloodstream

Question 13

Outline the role if adrenaline when blood glucose concentration decreases.

Answer 13

1. Adrenal glands produce adrenaline. It binds to surface receptors on liver cells & activates enzymes for glycogenolysis
2. Glucose diffuses from liver into bloodstream

Question 14

Outline what happens when blood glucose concentration increases.

Answer 14

1. beta cells in Islets of Langerhans in pancreas detect increase & secrete insulin into bloodstream.
2. insulin binds to surface receptors on target cells to:
   a) increase cellular glucose uptake
   b) activate enzymes for glycogenesis (liver & muscles)
   c) stimulate adipose tissue to synthesise fat

Question 15

Describe how insulin leads to a decrease in blood glucose concentration.

Answer 15

• Increases permeability of cells to glucose
• Increase glucose concentration gradient
• Triggers inhibition of enzymes for glycogenolysis

Question 16

How does insulin increase permeability of cells to glucose?

Answer 16

• Increases number of glucose carrier proteins
• Triggers conformational change which opens glucose carrier proteins

Question 17

How does insulin increase glucose concentration gradient?

Answer 17

• Activates enzymes for glycogenesis in liver & muscles
• Stimulates fat synthesis in adipose tissue.

Question 18

Use the secondary messenger model to explain how glucagon and adrenaline work.

Answer 18

1. Hormone-receptor complex forms.
2. Conformational change to receptor activates G-protein
3. Activates adenylate cyclase, which converts ATP to cyclic AMP (cAMP)
4. cAMP activates protein kinase A pathway
5. Results in glycogenolysis

Question 19

Explain the causes of Type 1 diabetes and how it can be controlled

Answer 19

Body cannot produce insulin e.g. due to autoimmune response which attacks beta cells of Islets of Langerhans
Treat by injecting insulin

Question 20

Explain the causes of Type 2 diabetes and how it can be controlled.

Answer 20

Glycoprotein receptors are damaged or become less responsive to insulin.
Strong positive correlation with poor diet / obesity.
Treat by controlling diet and exercise regime.

Question 21

Name some signs and symptoms of diabetes

Answer 21

• high blood glucose concentration
• glucose in urine
• polyuria
• polyphagia
• polydipsia
• blurred vision
• sudden weight loss

Question 22

Suggest how a student could produce a desired concentration of glucose solution from a stock solution.

Answer 22

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

Question 23

outline how colorimetry could be used to identify the glucose concentration in a sample.

Answer 23

1. benedict’s test on solutions of known glucose concentration. use colorimeter to record absorbance.
2. Plot calibration curve: absorbance (y-axis), glucose concentration (x-axis)
3. Benedict’s test on unknown sample. Use calibration curve to read glucose concentration at its absorbance value

Question 24

define osmoregulation

Answer 24

Control of blood water potential via homeostatic mechanisms

Question 25

describe the gross structure of a mammalian kidney

Answer 25

Fibrous capsule: protects kidney
Cortex: outer region consists of Bowman’s capsules, convoluted tubules, blood vessels
Medulla: inner region consists of collecting ducts, loops of Henle, blood vessels
Renal pelvis: cavity collects urine into ureter
Ureter: tube carries urine to bladder
Renal artery: supplies kidney with oxygenated blood
Renal vein: returns deoxygenated blood from kidney to heart

Question 26

Describe the blood vessels associated with a nephron

Answer 26

wide afferent arteriole from renal artery enters renal capsule & forms glomerulus: branched knot of capillaries which combine to form narrow efferent arteriole.
efferent arteriole branches to form capillary network that surrounds tubules

Question 27

Explain how glomerular filtrate is formed.

Answer 27

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 a filter. Blood cells & large molecules e.g. proteins remain in capillary

Question 28

How are cells of the Bowman’s capsule adapted for ultrafiltration?

Answer 28

• Fenestrations between epithelial cells of capillaries
• Fluid can pass between & under folded membrane of podocytes

Question 29

State what happens during selective reabsorption and where it occurs.

Answer 29

Useful molecules from globular filtrate e.g. glucose are reabsorbed into the blood.
Occurs in proximal convoluted tubule.

Question 30

How are cells in the proximal convoluted tubule adapted for selective reabsorption?

Answer 30

• Microvilli: large surface area for co-transporter proteins
• Many mitochondria: ATP for active transport of glucose into intercellular spaces
• folded basal membrane: large surface area

Question 31

What happens in the loop of Henle?

Answer 31

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

Question 32

Explain the role of the distal convoluted tubule.

Answer 32

Reabsorption:
a) of water via osmosis
b) of ions via active transport
permeability of walls is determined by action of hormones.

Question 33

Explain the role of the collecting duct.

Answer 33

Reabsorption of water from filtrate into interstitial fluid via osmosis through aquaporins

Question 34

Explain why it is important to maintain an Na+ gradient

Answer 34

Countercurrent multiplier: filtrate in collecting ducts is always beside an area of interstitial fluid that has a lower water potential.
Maintains water potential gradient for maximum reabsorption of water.

Question 35

What might cause blood water potential to change?

Answer 35

• level of water intake
• level of ion intake in diet
• level of ions used in metabolic processes or excreted
• sweating

Question 36

Explain the role of the hypothalamus in osmoregulation

Answer 36

1. Osmosis of water out of osmoreceptors in hypothalamus causes them to shrink.
2. This triggers hypothalamus to produce more antidiuretic hormone (ADH)

Question 37

Explain the role of the posterior pituitary gland in osmoregulation

Answer 37

Stores and secretes the ADH produced by the hypothalamus.

Question 38

Explain the role of ADH in osmoregulation.

Answer 38

1. Makes cells lining collecting duct more permeable to water:
   - binds to receptor –> activates phosphorylase –> vesicles with aquaporins on membrane fuse with cell-surface membrane.
2. Makes cells lining collecting duct more permeable to urea:
   - water potential in interstitial fluid decreases
   - more water reabsorbed = more concentrated urine