A2 homeostasis, glucose diabetes, kidney (topic 6)

Question 1

(negative) feedback loop def

Answer 1

a deviation from the normal level that causes a return to the normal level
negative reduces magnitude of the stimulus
positive intensifies stimulus

Question 2

normal value for glucose in blood

Answer 2

90 mg per 100cm^3

Question 3

how does glucose enter the blood

Answer 3

• absorption from the gut after digestion of carbohydrates
• hydrolysis of stored glycogen to glucose (glycogenolysis)
• production of glucose from non-carb sources eg lactate, glycerol, amino acids (gluconeogenesis)

Question 4

factors effecting blood glucose concentration (3)

Answer 4

• food: amount of glucose absorbed into blood from digestion can fluctuate greatly
• activity: during exercise, muscles are respiring rapidly, quickly deplete glucose in blood
• body’s control systems: remove excess glucose from blood after a carb-rich meal and release glucose rapidly from storage during exercise

Question 5

the role of the pancreas in regulating blood glucose

Answer 5

exocrine gland - secrete pancreatic juice into duodenum via pancreatic duct

endocrine gland - secrete 2 hormones directly into the bloodstream
=> located in isolated groups of cells called islets of Langerhans

Question 6

what happens when blood glucose conc rises

Answer 6

detected by β cells in pancreas, of which cell membranes contain carrier proteins for transport of glucose
glucose absorbed into β cells by facilitated diffusion
stimulates vesicles containing insulin to move to cell surface membrane and release insulin into surrounding capillaries

Question 7

action of insulin

Answer 7

lowers blood sugar (hypoglycaemic)
- stimulates uptake of glucose by muscle cells, adipose tissue and the liver (adds more channel proteins in membrane)
- activates enzymes which convert glucose into glycogen (glycogenesis) and inhibits breakdown of glycogen stored in liver and muscle cells
- increases rate of glucose used in respiration
- favours fat formation by promoting glucose uptake and metabolism in adipose tissue

Question 8

action of insulin on liver cells

Answer 8

glucose enters liver cells where it activates an enzyme that rapidly converts glucose to glucose phosphate
- lowers glucose conc in cell, maintains steep diffusion gradient between blood and liver cells
- other enzymes catalyse reactions that make glycogen from glucose-phosphate (glycogenesis)

Question 9

what happens when glucose conc falls back to normal again

Answer 9

β cells detect level is normal, so produce less insulin (negative feedback mechanism)
- lower glucose level stimulates α cells in islets of Langerhans to secrete glucagon

Question 10

action of glucagon in liver

Answer 10

raises blood sugar levels (hyperglycaemic)
- activates enzymes that breakdown stored glycogen to glucose in the liver (glycogenolysis)
- activates enzymes that produce glucose from glycerol, amino acids (gluconeogenesis)
- stimulates mobilisation of fat stores, fatty acids used in respiration instead of glucose

Question 11

action of adrenaline

Answer 11

secreted by adrenal gland at times of stress or excitement
=> raises blood glucose conc
activates enzymes in liver cells which promote glycogenolysis, converting stored glycogen to glucose

Question 12

second messenger model of hormone action

Answer 12

glucagon and adrenaline work differently to insulin, they don’t have a direly effect on liver cells
- they use a second messenger known as cyclic adenosine monophosphate (cAMP) to bring about effects

• adrenaline/glucagon binds to a specific receptor protein in cell-surface membrane of target cell
• hormone-receptor complex causes a change in the shape of enzyme adenyl cyclase in the cell membrane
• this activates enzyme which hydrolyses ATP removing 2 phosphate groups to form cAMP
• cAMP binds to enzyme protein kinase , activating it by exposing it’s active site
• PK hydrolyses glycogen to glucose-phosphate, converted to glucose and released in blood

Question 13

kidney nephrons - ultrafiltration

Answer 13

blood enters kidney via renal artery => afferent arteriole => glomerulus, under pressure from the heart.
water and small components are forced out of the glomerulus.
high pressure is aided by the efferent arteriole (leaving glomerulus) being narrower than the afferent one (entering glomerulus)

Question 14

kidney nephrons - selective reabsorption

Answer 14

• in proximal convoluted tubule

works like NaK pump

=> Na+ active transported out of the epithelial cell into blood
=> lowers conc so Na+ facilitated diffusions from lumen of the PCT into epithelial cell.
=> co transports glucose with it, which then facilitated diffusion’s into blood

Question 15

kidney nephrons - loop of Henle

Answer 15

creates high conc of Na+ and Cl- in tissue fluid of the medulla to allow water to be reabsorbed from the nephron as they pass through collecting duct

=> Na+ actively transported out of the ascending limb using ATP

=> creates low water potential between the 2 limbs (ascending limb is impermeable to water so water only moves out of descending limb by osmosis into the area of low WP)

=> water then enters blood capillaries by osmosis

Question 16

kidney nephrons - collecting duct and distal convoluted tubule

Answer 16

tissue fluid deep in the medulla has low WP / high Na+ conc
water moves down the collecting duct and WP gradient decreases further into the medulla
water enters blood via osmosis

permeability of collecting duct altered by ADH being released. osmoreceptors in the hypothalamus detect change in blood WP. when it falls ADH released and binds to receptors on the surface of collecting duct and activates enzyme phosphorylase. causes aquaporins (protein carriers) to be embedded in the cell surface membrane. increases permeability to water as well as urea so they get reabsorbed