6.4 Homeostatis is the maintenance of a stable internal environment

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 is 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 no 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 production and response by an effector?

Answer 9

It 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 glycogenolysis.
● Rate of gluconeogenesis.

Question 11

Define glycogenesis, glycogenolysis and gluconeogenesis.

Answer 11

Glycogenesis: liver converts glucose into the storage polymer 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. 𝞪 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 glycogenolysis & gluconeogenesis.
3. Glucose diffuses from liver into bloodstream.

Question 13

Outline the role of 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. 𝝱 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. ● Increases 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 the 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 𝝱 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

Define osmoregulation.

Answer 21

Control of blood water potential via homeostatic mechanisms.

Question 22

Name some signs and symptoms of diabetes.

Answer 22

● High blood glucose concentration
● Glucose in urine
● Polyuria
● Polyphagia
● Polydipsia
● Blurred vision
● Sudden weight loss
● Blurred vision

Question 23

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

Answer 23

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 24

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

Answer 24

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 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 structure of a nephron.

Answer 26

Bowman’s capsule at start of nephron: cup-shaped, surrounds glomerulus, inner layer of podocytes.
Proximal convoluted tubule (PCT): series of loops surrounded by capillaries, walls made of epithelial cells with microvilli.
Loop of Henle: hairpin loop extends from cortex into medulla. Distal convoluted tubule : similar to PCT but fewer capillaries. Collecting duct: DCT from several nephrons empty into collecting duct, which leads into pelvis of kidney.

Question 27

Describe the blood vessels associated with a nephron.

Answer 27

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 28

Explain how glomerular filtrate is formed.

Answer 28

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 & large molecules e.g. proteins remain in capillary.

Question 29

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

Answer 29

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

Question 30

State what happens during selective reabsorption and where it occurs.

Answer 30

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

Question 31

Outline the transport processes involved in selective reabsorption.

Answer 31

check

Question 32

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

Answer 32

● 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 33

What happens in the loop of Henle?

Answer 33

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 34

Explain the role of the distal convoluted tubule.

Answer 34

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

Question 35

Explain the role of the collecting duct.

Answer 35

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

Question 36

Explain why it is important to maintain an Na+ gradient.

Answer 36

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 37

What might cause blood water potential to change?

Answer 37

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

Question 38

Explain the role of the hypothalamus in osmoregulation.

Answer 38

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

Question 39

Explain the role of the posterior pituitary gland in osmoregulation.

Answer 39

Stores and secretes the ADH produced by the hypothalamus.

Question 40

Explain the role of ADH in osmoregulation.

Answer 40

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.