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AQA A LEVEL BIOLOGY Paper 2
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AQA A LEVEL BIOLOGY Paper 23.6.4 Homeostasis - ANS
What is negative feedback? - ANS-A response that returns a change in levels, back to
normal.
What is positive feedback? - ANS-A response that amplifies a change from the normal
level.
Which factors influence blood glucose concentration? - ANS-Eating food containing
carbohydrates, glucose absorbed from intestine to blood.
Exercising, more glucose is used in respiration to release energy.
What is the effect of insulin? - ANS-Decreases blood glucose concentration.
How does insulin work? - ANS-Secreted by beta cells in islets of Langerhans in
pancreas, when blood glucose too high.
Insulin binds to specific receptors on cell surface membranes of target cells, increases
permeability of cell membrane to glucose (increases number of channel proteins in cell
membrane), cells take up more glucose by facilitated diffusion, enzymes convert
glucose to glycogen and so stored in cytoplasm. Rate of respiration increases.
What is the effect of glucagon? - ANS-Increases blood glucose concentration.
How does glucagon work? - ANS-Secreted by alpha cells in the islets of Langerhans in
pancreas.
Binds to specific receptors on cell surface membranes of target cells, activates enzymes
to convert glycogen to glucose. Rate of respiration decreases.
What is the role of adrenaline? - ANS-Increases blood glucose concentration by
activating secretion of glucagon.
How does adrenaline work? - ANS-Binds to specific receptors on cell surface
membranes of target cells, activates enzymes to convert glycogen to glucose.
What is the secondary messenger model? - ANS-Demonstrated by adrenaline and
glucagon as they cause glycogenolysis to occur inside cell even though they bind to
, receptors on the outside of the cell.
Adrenaline or glucagon bind to specific complimentary receptors on the cell membrane.
They activate adenylate cyclase. Convert ATP to cyclic AMP. cAMP activates protein
kinase A. Protein kinase A activates a cascade to break down glycogen to glucose.
What is Type I diabetes? - ANS-Gene mutation.
Autoimmune response on beta cells of islets of Langerhans. Body can't produce insulin.
What is Type II diabetes? - ANS-Poor diet, lack of exercise, obesity. Glycoprotein
loses responsiveness to insulin. Cells don't take up enough glucose.
What is osmoregulation? - ANS-The control of water and salt levels in the body, by
ADH.
How does the body respond to a decrease in water potential? - ANS-Detected by
osmoreceptors in hypothalamus. Hypothalamus produces more ADH, posterior pituitary
gland secretes more ADH into blood. ADH travels in blood to kidney and attaches to
receptors on collecting duct. ADH increases permeability of cell membranes to water so
more water is absorbed by osmosis. Less water lost in urine so smaller volume of urine,
more concentrated.
How does the body respond to an increase in water potential? - ANS-Detected by
osmoreceptors in hypothalamus. Hypothalamus produces less ADH, posterior pituitary
gland secretes less ADH into blood.
Less ADH travels in blood to kidney and attaches to receptors of collecting duct. ADH
decreases permeability of cell walls to water so less water absorbed by osmosis. More
water lost in urine so large volume of urine, less concentrated.
What role does the nephron have in osmoregulation? - ANS-Formation of glomerular
filtrate, readsorption of glucose and water by the PCT, maintenance of a gradient of Na+
in the medulla by the loop of Henle, reabsorption of water by the DCT and collecting
duct.
How is glomerular filtrate formed? - ANS-Diameter of efferent arteriole smaller than
afferent arteriole. Buildup of hydrostatic pressure in glomerulus. Water, glucose, mineral
ions squeezed out of capillaries into Bowmans capsule to form glomerular filtrate,
through pores in capillary endothelium, basement membrane, podocytes. Large
proteins aren't pushed out, too big.
How are glucose and water reabsorbed by PCT? - ANS-Na+ actively transported out of
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