6.4 - Homeostasis is the maintenance of stable environment

Question 1

What is homeostasis?

Answer 1

Internal environments is maintained within set limits around an optimum

Question 2

Why is it important that core temperature remains stable?

Answer 2

• Maintains stable rate of enzyme-controlled reactions & prevents damage to membranes
• Temperature too loww = enzyme & substrate molecules have insufficient kinetic energy 7
• Temp to high = enzymes denature

Question 3

Why is it important that blood pH remains stable?

Answer 3

Maintains stable rate of enzyme-controlled reactions
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

• Maintains constant blood water potential: prevent osmotic lysis/crenation of cells.
• Maintains 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 trigger 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 corrdinator -> corrective mechanism by effector -> receptors detect that conditions have returned to notmal.

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 orginal one.

Question 8

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

Answer 8

• Receptors may senf 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 taked time to:
- produce hormones
- transport hormones 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. a 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 the liver into the 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. B cells in Islets of Langerhans in pancreas detect increase & secrete insulin into bloodstream.
2. 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

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 glucogon and adrenaline work.

Answer 18

1. Hormone-receptor complex forms.
2. Conformational change to receptoractivate 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 B 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 damages 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 22

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

Answer 22

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 23

Describe the gross structure of a mammalian kidney.

Answer 23

• 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 23

Define osmoregulation.

Answer 23

Control of blood water potential via homeostatic mechanisms.

Question 23

Describe the structure of a nephron.

Answer 23

• Bowman’s capsule at the start of nephron: cup-shaped, surrounds glomerulus, inner layer of podocytes.
• Proximal convulated 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 convulated tubule: similar to PCT but fewer capillaries.
• Collecting Duct: DCT from several nephrons empty into collecting duct, which leads into pelvis of kidney.

Question 24

Describe the blood vessels associated with a nephron.

Answer 24

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

Question 25

Explain how glomerular filtrate is formed.

Answer 25

• Ultrafiltration in Bowman’s capsule.
• High hydrostatic pressure in glomerulus forces sall 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 26

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

Answer 26

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

Question 27

State what happens during selective reabsorption and where it occurs.

Answer 27

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

Question 28

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

Answer 28

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

Question 29

What happens in the loop of Henle?

Answer 29

1. Active transport of Na+ & Cl- out of ascending limb.
2. WP of interstitial fluid decreases.
3. Osmosis of water out of descending limb (ascending limb is impermeable to water).
4. WP of filtrate decreases going down descending limb: lowest in medullary region, highest at top of ascending limb.