IB Biology D3.3: Homeostasis worked solutions

Negative feedback reverses the change that started it, so the variable returns towards its set point (B). A and D describe responses that move the variable away from, or ignore, the set point. C is wrong because the loop also acts when the variable falls below the set point.

Beta cells secrete insulin when blood glucose is high (B). Alpha cells secrete glucagon when it is low, so A (alpha, glucagon, high) and D (alpha, insulin) have a wrong pairing, and C pairs beta cells with glucagon.

Vasodilation of skin arterioles brings more blood to the surface and sweating lets evaporation remove heat (A). Vasoconstriction, hair erection, shivering and more thyroxin all conserve or generate heat.

The hypothalamus signals the pituitary gland, which releases TSH; TSH makes the thyroid gland secrete thyroxin, which raises the metabolic rate (A). The other orders reverse the direction of control.

In type 2 diabetes target cells respond poorly to insulin, and obesity is a risk factor (B). Autoimmune destruction of beta cells describes type 1 (A). Type 2 is often first managed by diet and exercise, not insulin (C). Glucagon is not secreted in excess (D).

Protons return across the inner mitochondrial membrane through a channel that bypasses ATP synthase, so the energy of the gradient is released as heat (B). ATP synthase is bypassed, not speeded up (A).

The loop of Henle dips into the medulla (D). The glomerulus, Bowman's capsule and proximal convoluted tubule are in the cortex.

Glucose is reabsorbed by active transport, linked to the sodium gradient made by pumps that use ATP (B). The filtrate has the same glucose concentration as plasma, so diffusion and osmosis cannot return it.

It pumps sodium ions out into the medulla by active transport, and its wall is impermeable to water. This builds the hypertonic medulla (C). Filtration (A) happens in the glomerulus, and ADH is made in the hypothalamus (D).

The blood is diluted, less ADH is secreted, and aquaporins are withdrawn into vesicles, so little water is reabsorbed (B).

(a) The falls are 0.060 (60 to 90 min: 1.8/30), 0.040 (1.2/30) and 0.008\,\mathrm{mmol\,dm^{-3}\,min^{-1}} (0.5/60), so the fastest fall is from 60 to 90 min at 0.060 (1 for the interval, 1 for the rate). (b) Beta cells of the islets secrete insulin in response to the high glucose (1); insulin makes liver and muscle cells take up glucose and store it as glycogen (1). (c) If the person then fasts, low glucose makes alpha cells secrete glucagon, and the liver converts glycogen to glucose and releases it (1).

(a) The hypothalamus detects the temperature of the blood (1). (b) Vasodilation: skin arterioles widen (1), so more blood flows near the surface and loses heat by radiation (1). Sweating: sweat glands secrete sweat (1) and its evaporation takes heat from the skin (1). (c) Negative feedback: the response reverses the rise in temperature (1).

(a) The set point is 37.0\,{}^\circC, the value the temperature returns to at 0, 40 and 70 min (1). (b) Below the set point: a fall of 37.0-36.2=0.8\,{}^\circC by 20 min (1); above the set point: a rise to 37.7, which is 0.7\,{}^\circC above it, at 60 min (1). (c) It is negative feedback because each disturbance was followed by a response that reversed it, from below and from above the set point; name one response for each direction, for example vasoconstriction or shivering after the fall and sweating after the rise (1).

(a) Type 1 is an autoimmune attack that destroys the beta cells, so little or no insulin is made (1); in type 2 the target cells are less responsive to insulin (insulin resistance) (1). (b) Two from: keep a healthy body mass, exercise regularly, eat less sugar and saturated fat (1 each). (c) Without insulin the liver and muscle cells do not take up glucose and store it, so the concentration stays high after the meal (1).

(a) Excretion is the removal of waste products of metabolism from the body (1). (b) Plasma proteins are too large to pass through the pores of the capillary wall and the basement membrane of the glomerulus (1), so they stay in the blood although small solutes are forced into the capsule by high pressure (1). (c) Excretion removes metabolic waste such as urea, which is filtered and left in the urine; osmoregulation regulates the osmotic concentration of body fluids, such as ADH changing water reabsorption in the collecting duct (1 for each correct distinction with example, maximum 2).

(a) Patient P (1): plasma protein, which should stay in the blood, is in the urine at 2.4\,\mathrm{g\,dm^{-3}} against 0, so the filtration barrier of the glomerulus is damaged (1). (b) In patient Q the filtrate carries glucose at the plasma concentration (1); glucose is in the urine because the amount exceeds what the tubule transporters can reabsorb, as when blood glucose is high in untreated diabetes (1).

(a) The solute concentration of the blood rises (1); osmoreceptors in the hypothalamus detect it and the pituitary gland secretes more ADH (1). (b) The collecting ducts become more permeable to water, so little, concentrated urine is made (1). (c) Once the blood concentration is back to normal, less ADH is secreted: negative feedback (1).

(a) \dfrac{0.55-1.10}{1.10}\times100=-50\% (1). (b) Wider arterioles bring more blood, so more oxygen and glucose reach the muscle for respiration (1), and carbon dioxide and heat are carried away (1). (c) The brain flow is unchanged at 0.75\,\mathrm{L\,min^{-1}} (1).

(a) \dfrac{18-6}{6}\times100=+200\% (1). (b) Group B has the higher value at 120 min, 8.1 against 5.2\,\mathrm{mmol\,dm^{-3}}, a difference of 2.9 (1); the standard deviation ranges do not overlap (4.6 to 5.8 and 7.0 to 9.2), so the difference is likely to be real (1). (c) Group B secretes three times as much insulin (1) but cannot lower its glucose as well (1), so its target cells respond poorly to insulin: insulin resistance, the pattern of type 2 diabetes (1). (d) The data show an association between obesity and a weaker response, not proof of cause (1); each group has only 20 people and age, diet and activity were not controlled, so a longer study with measured changes in body mass is needed (1).

(a) The air temperature rose by 40-10=30\,{}^\circC (1) but the core temperature rose by only 37.3-36.9=0.4\,{}^\circC, so the core is held close to 37\,{}^\circC by negative feedback (1). (b) At 10\,{}^\circC oxygen use is 6.8, about twice the 3.2 at 30\,{}^\circC (1): shivering, more thyroxin and brown fat raise the rate of aerobic respiration, and the heat released replaces the heat lost (1). (c) At 40\,{}^\circC the skin (36\,{}^\circC) is only 4\,{}^\circC cooler than the air, so little heat leaves by radiation, conduction or convection (1); evaporation of sweat, which rises to 380\,\mathrm{g\,h^{-1}}, removes the heat (1).

One mark for each point, maximum 8, in this order: the solute concentration of the blood rises (1); osmoreceptors in the hypothalamus detect the change (1); the pituitary gland releases more ADH (1); ADH binds to receptors on the collecting duct cells and aquaporins move from vesicles into the plasma membrane (1), making the duct more permeable to water (1); the ascending limb of the loop of Henle pumps sodium ions into the medulla by active transport and makes it hypertonic (1); water leaves the collecting duct by osmosis and returns to the blood (1); a small volume of concentrated urine is produced (1); as the blood concentration returns to the set point, ADH secretion falls: negative feedback (1). Credit also: ultrafiltration of plasma in the glomerulus and reabsorption of glucose and some water in the proximal tubule as the first stages (1), and the distinction that this is osmoregulation while urea removal is excretion (1).

(a) Convert first: urine =1.20 and 5.40\,\mathrm{osmol\,L^{-1}} (1). Ratios: human \dfrac{1.20}{0.30}=4.0, rodent \dfrac{5.40}{0.32}=16.9 (1). (b) The ascending limb of the loop pumps sodium ions into the medulla by active transport (1) and is impermeable to water, so the medulla becomes hypertonic (1); with ADH the collecting duct is permeable, so water leaves by osmosis into the medulla and the urine left behind is more concentrated than plasma (1). (c) The rodent probably has longer loops of Henle (1), which build a steeper gradient in the medulla and allow more water to be reabsorbed, an advantage where water is scarce (1).

(a) Water is polar, so it crosses the hydrophobic fatty acid core of the phospholipid bilayer only slowly (1); without ADH there are few aquaporins in the membrane (1). (b) ADH binds to its receptor and vesicles carrying aquaporins fuse with the plasma membrane (1), adding channel proteins through which water passes rapidly (1). (c) Net movement is from the duct to the medulla (1) by osmosis, from the lower to the higher solute concentration (1).

(a) Insulin is a large, polar polypeptide, so it cannot pass through the hydrophobic core of the plasma membrane (1) and acts by binding to a receptor on the outer surface (1). (b) With fewer working receptors, the signal is weaker (1), so less glucose is taken up and stored as glycogen and the blood glucose stays high after a meal (1). (c) A polypeptide taken by mouth is digested by proteases in the gut (1) to amino acids, so no active hormone would reach the blood (1).

(a) Water molecules are held by hydrogen bonds (1); evaporation needs energy to break them, taken from the skin, so the skin cools (1). (b) Less ATP is made (1) because protons return through the uncoupling channel, and more of the energy is released as heat (1). (c) The proton gradient is dissipated, so the electron transport chain runs faster to rebuild it (1), and this uses oxygen as the final electron acceptor, so oxygen consumption rises (1).