Title: InkBiology / Hormonal communication, plant responses and homeostasis
On every note of this deck, unless a note says otherwise:
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Front: DEFINE
Define: Negative feedback.
Back: A control mechanism in which a deviation from the norm triggers a response that reverses the deviation.

Front: SPOT THE ERROR
Say what is wrong with this claim:
The loop of Henle reabsorbs water.
Back: The loop reabsorbs very little water. What it does is build a concentration gradient in the medulla, salty at the tip, by actively transporting sodium and chloride out of its ascending limb. The water is reabsorbed later, from the collecting duct running back down through that gradient.

Front: DEFINE
Define: Endocrine gland.
Back: A ductless gland that secretes hormones directly into the blood.

Front: DEFINE
Define: Anterior pituitary.
Back: The lobe that makes its own hormones, under the control of releasing factors from the hypothalamus.

Front: DEFINE
Define: Posterior pituitary.
Back: The lobe that stores and releases ADH and oxytocin, both made in the hypothalamus.

Front: CHECK YOURSELF
A person has a fasting blood glucose concentration of 8.2 mmol per dm3 and a two-hour value of 13.5 in a glucose tolerance test. Their blood contains a higher concentration of insulin than a person without diabetes. Which type of diabetes do they have? Justify your answer from both pieces of evidence, and explain why their pancreas is secreting more insulin than normal.
Back: Both test values are diagnostic. A fasting concentration of 8.2 is above the 7.0 threshold, and a two-hour value of 13.5 is above 11.1, so the person has diabetes.
The high insulin concentration identifies it as type 2. In type 1 the beta cells have been destroyed by the immune system, so there would be little or no insulin in the blood. Insulin is present here in more than the usual amount, so the beta cells are working.
The cause is therefore a poor response to insulin rather than a shortage of it. The target cells have fewer receptors, or receptors that respond weakly, so fewer glucose transporters are moved into their membranes and less glucose is taken up and stored, and the concentration in the blood stays high.
That persistently high concentration is what drives the extra secretion. Beta cells respond to the glucose around them: more glucose entering means more ATP, the potassium channels stay shut, the cell stays depolarised and more insulin is secreted. The negative feedback loop is intact at the pancreas and broken at the target cell, which is why the correction never arrives.

Front: DEFINE
Define: Glycogenolysis.
Back: The hydrolysis of glycogen to glucose. Promoted by glucagon and by adrenaline.

Front: CHECK YOURSELF
A drug blocks adenylate cyclase in liver cells but leaves the adrenaline receptors, the protein kinase and glycogen phosphorylase intact. Predict the effect of the drug on the response of the liver to adrenaline, explain your prediction, and suggest why a drug blocking the receptor instead would have a similar outcome by a different route.
Back: Adrenaline would still bind normally, because the receptors are unaffected, and the receptor would still change shape. The cascade would stop at the next step: with adenylate cyclase blocked, ATP is not converted into cyclic AMP.
With no cyclic AMP the protein kinase is never activated, so phosphorylase is not activated either, although both enzymes are present and working. No glycogen is hydrolysed, so blood glucose would not rise in response to adrenaline.
Blocking the receptor gives the same end result by cutting the chain one step earlier: adrenaline could not bind, so the receptor would not change shape and adenylate cyclase would never be activated.
Together the two cases make the point of a cascade. A pathway built from separate proteins in sequence fails wherever it is interrupted, which is a nuisance in disease and useful in pharmacology, since it gives a drug several places to act.

Front: CHECK YOURSELF
A coleoptile is illuminated from the left. A student proposes two hypotheses: that light destroys auxin on the lit side, and that auxin moves across to the shaded side. Design an experiment using agar blocks that would distinguish between them, and state the result each hypothesis predicts.
Back: Cut the tips from a number of coleoptiles and stand each tip on a pair of agar blocks separated by a thin impermeable barrier running down the middle of the tip, so that auxin diffusing from the lit half and from the shaded half is collected separately. Illuminate from one side, and keep a second set of tips in complete darkness as a control.
Measure the auxin in each block by the standard bioassay: place the block on one side of a decapitated stump in the dark and measure the angle of curvature, which is proportional to the quantity of auxin present.
If light destroys auxin, the total collected from the two blocks of an illuminated tip will be less than the total from a tip kept in darkness, and the shortfall will be on the lit side. If auxin moves across instead, the total will be the same as in darkness, but more will be in the shaded block and less in the lit one.
The experiment as actually done gave the second result, so redistribution is supported and destruction is not. Adding the barrier makes the design decisive in another way: with the two halves separated, no sideways movement is possible, and the asymmetry between the blocks disappears, which is difficult to explain if light were simply destroying auxin where it fell.

Front: SPOT THE ERROR
Say what is wrong with this claim:
Insulin converts glucose into glycogen.
Back: Insulin never touches a glucose molecule. It binds receptors on liver, muscle and fat cells and changes what they do: more glucose transporters in the membrane, the glycogen-building enzymes activated, respiration raised. Enzymes do the conversion; insulin is the signal.

Front: DEFINE
Define: Positive feedback.
Back: A mechanism in which a deviation from the norm triggers a response that increases the deviation.

Front: DEFINE
Define: Homeostasis.
Back: The maintenance of a stable internal environment within narrow limits, despite changes in the external environment.

Front: DEFINE
Define: Nephron.
Back: The functional unit of the kidney: a renal capsule and its tubule, running from the cortex into the medulla and back.

Front: SPOT THE ERROR
Say what is wrong with this claim:
Adrenaline gets into the liver cell and switches on the enzyme that breaks glycogen down.
Back: Adrenaline never enters the cell. It binds a receptor on the outside of the membrane, and a second messenger made in the cytoplasm carries the instruction inwards. Every step after that is an enzyme, so one hormone molecule releases thousands of glucose molecules.

Front: DEFINE
Define: Hormone.
Back: A chemical messenger, secreted by an endocrine gland into the blood, that acts on target cells carrying a complementary receptor.

Front: DEFINE
Define: Hypothalamus.
Back: The region of the brain that monitors the blood and controls the pituitary gland.

Front: DEFINE
Define: Glycogenesis.
Back: The synthesis of glycogen from glucose, in liver and muscle cells. Promoted by insulin.

Front: DEFINE
Define: Exocrine gland.
Back: A gland that secretes its product into a duct, which carries it to a surface.

Front: DEFINE
Define: Antagonistic.
Back: Describes two effectors or hormones whose effects on a variable are opposite.

Front: DEFINE
Define: Osmoregulation.
Back: The control of the water potential of the blood and body fluids.

Front: DEFINE
Define: Ultrafiltration.
Back: Filtration under pressure through a partially permeable barrier, separating small molecules from large ones.

Front: DEFINE
Define: Target cell.
Back: A cell with receptors complementary to a particular hormone, and therefore the only kind of cell that responds to it.

Front: CHECK YOURSELF
A person drinks two litres of water quickly. Describe and explain what happens to the volume and concentration of urine they produce over the following hour, naming the receptors, the hormone and the part of the nephron involved, and state what kind of feedback this is.
Back: The water is absorbed from the gut into the blood, so the water potential of the blood rises, becoming less negative. Osmoreceptors in the hypothalamus detect the change: water enters them by osmosis and they swell, and they send fewer impulses to the posterior pituitary.
Less ADH is therefore released into the blood. With less ADH binding to receptors on the collecting duct cells, fewer aquaporins are held in the membrane facing the filtrate, because the vesicles carrying them are withdrawn, so the duct wall becomes much less permeable to water.
Less water can therefore leave the duct down the gradient the loop of Henle maintains in the medulla, and a large volume of dilute urine is produced.
This is negative feedback. The response, losing the excess water in the urine, lowers the blood's water potential back towards the norm, which removes the stimulus that caused the response. Notice that the medulla's gradient is unchanged throughout: what the hormone altered was the permeability of the duct, not the driving force acting on the water.

Front: DEFINE
Define: Gluconeogenesis.
Back: The production of glucose from non-carbohydrate sources such as amino acids and glycerol, in the liver. Promoted by glucagon.

Front: SPOT THE ERROR
Say what is wrong with this claim:
Light destroys the auxin on the lit side of a shoot, so there is less of it there and that side grows less.
Back: The evidence supports movement, not destruction. Auxin collected in agar from the lit and shaded halves of an illuminated coleoptile totals the same as in the dark; only its distribution has changed. Insert a barrier down the middle and the asymmetry disappears.

Front: DEFINE
Define: Selective reabsorption.
Back: The return of useful substances from the filtrate to the blood, by transport proteins in the tubule wall.