Questions on hormonal communication and negative feedback

Question 1

Describe how adrenaline in the blood causes a liver cell to release glucose, naming the molecules involved in the order they act.

Accepted Answer

B1 adrenaline binds to a receptor on the outer surface of the cell surface membrane and does not enter the cell B1 binding changes the tertiary structure of the receptor, which activates adenylate cyclase on the inner face of the membrane B1 adenylate cyclase converts ATP into cyclic AMP, the second messenger B1 cyclic AMP activates a protein kinase, which phosphorylates and so activates the next enzyme in the chain A1 glycogen phosphorylase is activated and breaks glycogen down, releasing glucose, which leaves the cell

Question 2

Explain why a steroid hormone such as oestrogen can act directly on the genes of its target cell while a peptide hormone such as insulin needs a second messenger.

Accepted Answer

B1 a steroid hormone is lipid-soluble, so it diffuses straight through the phospholipid bilayer of the membrane B1 inside the cell it binds to a receptor and the hormone-receptor complex acts in the nucleus as a transcription factor, switching genes on or off B1 a peptide hormone is not lipid-soluble and cannot cross the bilayer, so it binds to a receptor on the outer surface of the membrane instead A1 the message must therefore be carried inwards by a different molecule made in the cytoplasm, and because every step of that chain is enzyme-catalysed the signal is amplified as it goes

Question 3

Explain how negative feedback holds a variable such as blood glucose close to a set point, and explain why two antagonistic hormones give tighter control than one.

Accepted Answer

B1 a receptor detects a deviation of the variable from the norm B1 a coordinator, which may be a hormone, a nerve or both, carries the information to an effector B1 the effector brings about a response that moves the variable in the direction that removes the original deviation, which is what makes the feedback negative A1 one effector can only push the variable one way and then wait for it to drift back, whereas two antagonistic hormones correct in both directions and so correct faster

Question 4

Compare nervous coordination with hormonal coordination, writing paired statements about the route the message takes, its speed, its target and how long its effect lasts.

Accepted Answer

B1 a nervous message travels along neurones to a defined destination, whereas a hormone is carried in the blood plasma and reaches every tissue B1 impulses travel at up to about 100 m per second, whereas a hormone is limited by the time the blood takes to circulate, so it is much slower B1 a neurone stimulates only the muscles or glands it connects to, whereas a hormone acts on any cell carrying a complementary receptor B1 the nervous effect is short, because the neurotransmitter is broken down at once, whereas the hormonal effect lasts until the hormone itself is broken down and often needs an antagonistic hormone to reverse it

Question 5

Describe how the hypothalamus controls the posterior pituitary and how it controls the anterior pituitary, naming one hormone released by each route.

Accepted Answer

B1 ADH and oxytocin are made in the cell bodies of neurones in the hypothalamus and travel down the axons of those neurones into the posterior pituitary, where they are stored B1 the posterior pituitary later releases them into the blood, so it stores and releases hormones rather than making its own B1 the anterior pituitary makes its own hormones, and the hypothalamus controls it chemically, by releasing factors carried in a short, dedicated set of blood vessels A1 one example is thyroid-stimulating hormone, released by the anterior pituitary in response to a releasing factor from the hypothalamus, which then acts on the thyroid gland

Question 6

Compare negative feedback with positive feedback, referring to the direction of the response, whether the mechanism is self-limiting, and one named example of each.

Accepted Answer

B1 negative feedback produces a response that reverses a deviation from the norm, whereas positive feedback produces a response that increases the original deviation B1 negative feedback is self-limiting, because removing the deviation removes the stimulus for the response, whereas positive feedback is not self-limiting and continues until something outside the loop intervenes B1 blood glucose control is an example of negative feedback: a rise in glucose triggers insulin release, which lowers glucose and removes the stimulus that caused the release B1 blood clotting is an example of positive feedback: activated platelets release chemicals that activate more platelets, so the clotting response accelerates rather than being damped down

Question 7

The hypothalamus secretes a releasing factor that stimulates the anterior pituitary to secrete thyroid-stimulating hormone (TSH), and TSH stimulates the thyroid gland to secrete thyroxine. Thyroxine in the blood inhibits secretion by both the hypothalamus and the anterior pituitary. A tumour of the anterior pituitary secretes TSH continuously, whatever else is in the blood. Suggest why the concentration of thyroxine in this person's blood is much higher than normal, and suggest what has happened to the releasing factor secreted by their hypothalamus.

Accepted Answer

B1 the continuous supply of TSH makes the thyroid secrete thyroxine continuously B1 the high concentration of thyroxine inhibits the hypothalamus, so it secretes very little releasing factor B1 the tumour does not respond to that inhibition, so negative feedback cannot lower the TSH, and thyroxine stays high even though the releasing factor has fallen

Question 8

Explain why adrenaline can be released from the adrenal medulla within seconds of a stressful stimulus, while cortisol from the adrenal cortex takes several minutes to rise.

Accepted Answer

B1 the adrenal medulla is stimulated directly by sympathetic neurones, so adrenaline is released as soon as the nerve impulse arrives at the gland B1 the adrenal cortex has no equivalent direct nerve supply; it is controlled instead by ACTH, a hormone released from the anterior pituitary A1 a hormonal signal has to be secreted, enter the blood and be carried by the circulation to its target before the target can respond, which takes far longer than a nerve impulse reaching a gland directly

Question 9

Give one example of a process in the body controlled by positive feedback rather than negative feedback, and give two ways in which its control differs from negative feedback.

Accepted Answer

B1 an example is the release of oxytocin during childbirth: pressure of the baby's head on the cervix causes contractions that press the head harder still B1 in positive feedback the response increases the original deviation rather than reversing it, unlike negative feedback, which brings the variable back towards a set point B1 positive feedback is not stopped by a return to normal; it continues until an event outside the loop ends it, such as the birth itself

Question 10

One activated adrenaline receptor produces 50 molecules of cyclic AMP, each cyclic AMP activates one protein kinase, each kinase activates 60 molecules of the next enzyme, and each of those releases 500 molecules of glucose from glycogen. Calculate the number of glucose molecules released from one molecule of adrenaline.

Accepted Answer

M1 multiply along the chain: the number of cyclic AMP molecules by the number of next-enzyme molecules each kinase activates, 50 × 60 = 3000 molecules of active phosphorylase M1 multiply that by the glucose released per phosphorylase molecule: 3000 × 500 A1 1 500 000 glucose molecules from one molecule of adrenaline

Question 11

A drug used to treat high blood pressure blocks the receptors for adrenaline on the heart, but has no effect on receptors elsewhere in the body. Suggest why a person taking this drug feels less shaky and their heart beats more slowly during a stressful event, and suggest why they may still sweat heavily.

Accepted Answer

B1 with the adrenaline receptors on the heart blocked, adrenaline cannot bind there, so its usual effect of increasing heart rate is prevented, even though the hormone is still released and still circulating B1 the racing heartbeat that contributes to the shaky feeling no longer occurs, so the person feels calmer than they otherwise would A1 sweating is controlled directly by the nervous system rather than by adrenaline binding to a receptor on the heart, so a drug blocking only heart receptors leaves the sweat glands responding normally to the event

Question 12

Name the hormone secreted by the adrenal medulla, and name the second messenger produced inside a liver cell when that hormone binds to its receptor.

Accepted Answer

B1 the hormone is adrenaline B1 the second messenger is cyclic AMP

Question 13

State the structural difference between an exocrine gland and an endocrine gland, and state which type of gland the islets of Langerhans in the pancreas are.

Accepted Answer

B1 an exocrine gland secretes into a duct that carries the product to a surface, whereas an endocrine gland has no duct and secretes its product straight into the blood B1 the islets of Langerhans are endocrine gland tissue

Question 14

Outline what is meant by a hormone's target cell, and outline why a hormone reaches cells that it does not affect.

Accepted Answer

B1 a target cell is a cell carrying receptors complementary in shape to a particular hormone, which is the only kind of cell able to respond to it B1 a hormone travels in the blood to every tissue the circulation reaches, but only cells with the complementary receptor respond, so specificity depends on the receptor rather than on where the blood goes

Question 15

State what is meant by homeostasis, and state one variable in the body that is maintained by it.

Accepted Answer

B1 homeostasis is the maintenance of a stable internal environment within narrow limits, despite changes in the external environment B1 acceptable variables include blood glucose concentration, core body temperature or blood water potential