Biology › Hormonal communication, plant responses and homeostasis › Hormones: a message every cell receives and only some can read
Hormones: a message every cell receives and only some can read
A hormone is poured into the blood and reaches your toes, your liver and the back of your eye within a minute. Only the cells carrying a receptor complementary to it do anything about it, and that is the whole of hormonal specificity.
Before this Membrane-bound receptor proteins and complementary shape · Nervous coordination and why a reflex is fast
Before you start
Adrenaline gets into the liver cell and switches on the enzyme that breaks glycogen down. Almost everyone starts with a picture like that, because it is what a messenger ought to do — arrive, go in, give the message. Adrenaline never enters the cell at all. It binds to a receptor on the outside of the membrane and stops there. What carries the instruction inwards is a different molecule, made in the cytoplasm as a result. The arrangement looks wasteful until you notice what it buys: every step between the receptor and the glycogen is an enzyme, so one hormone molecule at the surface releases thousands of glucose molecules.
What you should be able to do
- Distinguish an exocrine gland from an endocrine gland by where its secretion goes.
- Describe the relationship between the hypothalamus and the two lobes of the pituitary.
- Explain why a peptide hormone needs a second messenger and a steroid hormone does not.
- Describe the adrenaline cascade in order, from binding to glucose leaving the liver cell.
- Explain negative feedback using receptor, coordinator and effector, and say what positive feedback does instead.
- Compare nervous and hormonal coordination on speed, route, duration and target.
Two kinds of gland, told apart by one thing
Glands come in two kinds, and the difference between them is not what they secrete or what the secretion does. It is where the secretion is put.
An exocrine gland releases its product into a duct that carries it to a surface — the gut lumen, the skin, the mouth. Salivary glands, sweat glands and the enzyme-secreting cells of the pancreas are exocrine, and their product arrives at one place, in bulk.
An endocrine gland has no duct at all. It releases its product — a hormone — straight into the capillaries running through it, and the blood takes it everywhere. The thyroid, the adrenals, the pituitary and the islets of Langerhans are endocrine.
The pancreas is both. Most of it is exocrine tissue making pancreatic juice, which leaves down the pancreatic duct into the duodenum; scattered through it are about a million patches of endocrine cells, the islets of Langerhans, which secrete insulin and glucagon into the blood and have no connection to the ducts at all. The next lesson depends on keeping the two apart.
Delivering a hormone everywhere raises an obvious problem. If thyroxine reaches every cell, why does it not affect every cell? Only a cell carrying a receptor with a binding site complementary to the hormone can respond; the rest are washed in it and do nothing. Specificity lives in the receptor rather than in the delivery.
- Exocrine gland
- A gland that secretes its product into a duct, which carries it to a surface.
- Endocrine gland
- A ductless gland that secretes hormones directly into the blood.
- Hormone
- A chemical messenger, secreted by an endocrine gland into the blood, that acts on target cells carrying a complementary receptor.
- Target cell
- A cell with receptors complementary to a particular hormone, and therefore the only kind of cell that responds to it.
The gland that runs the other glands
Under the brain, on a stalk, sits a structure the size of a pea that controls most of the endocrine system: the pituitary gland. Wired into it from above is the hypothalamus, the region of the brain that monitors the blood passing through it — its temperature, its water potential, the hormones already circulating in it. The two lobes of the pituitary are connected to the hypothalamus in quite different ways, and questions like to test exactly that.
The posterior pituitary is not really a gland. Its hormones, 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 lobe, where they are stored and later released into the blood. So the posterior pituitary stores and releases what the hypothalamus makes.
The anterior pituitary makes its own hormones, and the hypothalamus controls it chemically: releasing factors travel the short distance in a private set of blood vessels and tell the anterior lobe to secrete. Most of what it secretes is aimed at other glands — thyroid-stimulating hormone at the thyroid, ACTH at the adrenal cortex, FSH and LH at the ovaries and testes. Thyroxine in the blood then inhibits both the hypothalamus and the anterior pituitary, so a rise in thyroxine reduces the signal that produced it, which is the last section's feedback loop drawn in three glands.
The adrenal glands, one on top of each kidney, are the other pair to know by name. The outer cortex secretes steroid hormones under the control of ACTH, including aldosterone, which acts on the kidney tubule. The inner medulla secretes adrenaline under direct nervous control rather than hormonal control, which is why the response to a shock is so fast. OCR A examines the adrenal glands and that cortex and medulla split explicitly; AQA meets adrenaline inside the glucose topic, and CAIE frames the whole business as cell signalling.
- Hypothalamus
- The region of the brain that monitors the blood and controls the pituitary gland.
- Posterior pituitary
- The lobe that stores and releases ADH and oxytocin, both made in the hypothalamus.
- Anterior pituitary
- The lobe that makes its own hormones, under the control of releasing factors from the hypothalamus.
Getting the message through the membrane
A hormone arriving at its target cell faces a physical problem: the instruction has to end up affecting enzymes in the cytoplasm, and the membrane is in the way. There are two solutions, and a hormone's chemistry decides which one it uses.
Steroid hormones — oestrogen, testosterone, cortisol, aldosterone — are lipid-soluble. They diffuse straight through the phospholipid bilayer, bind to a receptor inside the cell, and the hormone-receptor complex acts in the nucleus as a transcription factor, switching genes on or off. Their effects are therefore slow to appear and long-lasting, because making a protein takes time and the protein persists.
Peptide and protein hormones — insulin, glucagon, ADH, FSH — and adrenaline, which is derived from an amino acid, are not lipid-soluble and cannot cross. They bind to a receptor on the outer surface of the membrane, and something else has to carry the message inwards. That something is the second messenger.
Take adrenaline at a liver cell, the case every board uses. Adrenaline is the first messenger: it binds to a receptor on the membrane, and binding changes the receptor's tertiary structure so that it activates adenylate cyclase on the inner face. That enzyme converts ATP into cyclic AMP, the second messenger, which activates a protein kinase; the kinase phosphorylates the next enzyme, and a short chain later glycogen phosphorylase is hydrolysing glycogen into glucose, which leaves the cell.
Glucagon works through exactly the same cascade in exactly the same cells, which is why the two hormones have such similar effects on the liver despite arriving for different reasons. Write the sequence out once properly and you have answered a question on either of them.
The reason for the elaborate route is amplification. Every step is catalysed, so one molecule at one step produces many at the next, and a hormone present at nanomolar concentrations can change what a whole organ is doing within seconds.
Putting a number on amplification
In a liver cell one activated receptor causes about 100 molecules of cyclic AMP to be made. Each cyclic AMP molecule activates one kinase, and each activated kinase activates about 100 molecules of the next enzyme. If that last enzyme releases 1000 glucose molecules from glycogen before it stops, how many glucose molecules does one adrenaline molecule account for?
Work along the chain, multiplying at each catalysed step. One receptor gives 100 cyclic AMP, so 100 kinases are activated.
Each kinase activates 100 of the next enzyme: 100 × 100 = 10 000 molecules of phosphorylase.
Each of those releases 1000 glucose molecules, so the total is 10 000 × 1000 = 10 million glucose molecules from one hormone molecule.
The arithmetic is idealised — real cascades leak, and enzymes are being switched off while others are switched on — but the order of magnitude is the point. An answer that says 'the second messenger amplifies the signal' scores; an answer that explains the amplification comes from each step being enzyme-catalysed scores better.
Negative feedback, and the thing it is not
Homeostasis is the maintenance of a stable internal environment, and the mechanism that maintains it is almost always negative feedback: a change in a variable triggers a response that reverses the change.
Every loop you will be asked about has the same four parts. A receptor detects a deviation from the norm; a coordinator — hormone, nerve, or both — carries the information; an effector brings about a response; and that response moves the variable in the direction that removes the original deviation, which is what makes the feedback negative. Name the four parts before you reach the biology and you will already have most of the marks.
Notice what the left-hand graph does not show: the variable is never simply held at the norm. It overshoots, comes back, overshoots less, and oscillates within a narrow band. Any system that acts only once a deviation has been detected must let the deviation happen first, so your blood glucose, core temperature and water potential all wander a little all day.
Two effectors working in opposite directions give tighter control than one. Insulin lowers blood glucose and glucagon raises it; sweating cools you and shivering warms you. One effector can push the variable one way and then wait for it to drift back, while two antagonistic effectors correct in both directions and correct faster.
Positive feedback is the opposite arrangement: the response increases the original change rather than reversing it. It is used when the body needs to commit to something quickly rather than hold it steady. In labour, the baby's head presses on the cervix, oxytocin is released, contractions strengthen and the head presses harder, and only birth ends the loop. In a neurone, the first sodium ions in open more sodium channels, which is why the rising phase of an action potential is so steep. In clotting, activated platelets activate more platelets.
Positive feedback is not a malfunction, then: negative feedback keeps you alive minute to minute and positive feedback gets a slow process over with quickly. Hypothermia is where the distinction turns dangerous — below about 33 °C the metabolic rate falls, which produces less heat, which lowers the temperature further.
- Homeostasis
- The maintenance of a stable internal environment within narrow limits, despite changes in the external environment.
- Negative feedback
- A control mechanism in which a deviation from the norm triggers a response that reverses the deviation.
- Positive feedback
- A mechanism in which a deviation from the norm triggers a response that increases the deviation.
- Antagonistic
- Describes two effectors or hormones whose effects on a variable are opposite.
Nerves and hormones, compared properly
The nervous unit taught the other coordination system, and questions here like to set the two against each other. Done badly the comparison collapses into 'nerves are fast and hormones are slow', which is true and worth about one mark. Done properly, every row of the table below follows from two facts: the message travels along a fixed private cable in one system and in the bloodstream in the other, and the messenger is cleared in milliseconds in one and in minutes in the other.
| Nervous | Hormonal | |
|---|---|---|
| Signal | Electrical impulse, then a neurotransmitter at synapses | A chemical, the hormone itself |
| Route | Along neurones, to a defined destination | In the blood plasma, everywhere |
| Speed of transmission | Very fast, up to about 100 m per second | Slower: limited by circulation time |
| Target | Specific muscles or glands the neurone connects to | Any cell with a complementary receptor |
| Duration of effect | Short: the transmitter is broken down at once | Longer: the hormone persists until it is broken down |
| Nature of response | Usually rapid and localised | Often widespread, and sometimes permanent |
| Reversal | Rapid, when impulses stop | Slow, and often needs an antagonistic hormone |
The last row does more work than it looks. When a motor neurone stops firing, acetylcholinesterase clears the cleft within milliseconds; when a gland stops secreting, the hormone already in the blood keeps working until the liver removes it. That is why a hormonal system needs an antagonistic partner and a nervous one does not.
Some responses use both. A shock sends impulses along sympathetic neurones to the adrenal medulla in a fraction of a second — that is why you jump before you have thought — and the adrenaline released keeps heart rate and blood glucose raised for minutes afterwards. Fast system for the jump, slow system for the aftermath.
TRY IT — Choosing the right system for a job
A person steps on a drawing pin and pulls their foot away, then stays shaky and alert for the next ten minutes. Explain why the withdrawal is coordinated by the nervous system and the shakiness by a hormone, referring to speed, route and duration.
Check your answer
The withdrawal has to happen in a fraction of a second to limit tissue damage. Impulses travel along a fixed pathway of neurones at up to 100 m per second, through the spinal cord and across only two synapses, so the response arrives almost immediately; a hormone would have to be secreted, enter the blood and be carried to the leg, which takes many seconds.
The response also needs to be localised. Only the muscles of that leg should contract, and a neurone reaches exactly the effector it connects to, while a hormone reaches every tissue.
The shakiness is adrenaline from the adrenal medulla, and here the properties that made a hormone useless for the reflex make it the right choice. The effect has to be widespread — heart, liver, bronchioles and skeletal muscle at once — and it has to last, which it does because the adrenaline stays in the blood until it is broken down. Speed and precision go to the nervous system, breadth and duration to a hormone, and the adrenal medulla is where the two systems are joined.
In the exam
- 'Ductless' is the mark for an endocrine gland. Answers that describe what the hormone does without saying where it is secreted rarely score.
- Specificity is about receptors. If a question asks why a hormone affects only some cells, the answer names the complementary receptor, not the blood supply.
- For the second messenger, name the molecules in order: adrenaline, receptor, adenylate cyclase, ATP to cyclic AMP, protein kinase, phosphorylase, glycogen to glucose. Never write that adrenaline enters the cell, and never that cyclic AMP arrives from outside it.
- In a negative feedback answer, use the words receptor, coordinator and effector, and finish by saying that the response reverses the change. An answer that stops at 'the level returns to normal' has described the outcome rather than the mechanism.
- When comparing nervous and hormonal control, write paired statements. 'Nervous is fast' earns nothing; 'impulses travel along neurones, whereas hormones are carried in the blood and are therefore slower' earns the mark.
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.
Answer
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.
Questions
Question 15 marks
Describe how adrenaline in the blood causes a liver cell to release glucose, naming the molecules involved in the order they act.
Mark scheme
- 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 hydrolyses glycogen into glucose, which leaves the cell
Question 24 marks
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.
Mark scheme
- 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 34 marks
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.
Mark scheme
- 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 44 marks
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.
Mark scheme
- 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 53 marks
A tumour of the anterior pituitary secretes thyroid-stimulating hormone 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.
Mark scheme
- B1 thyroid-stimulating hormone acts on the thyroid gland, so a continuous supply of it makes the thyroid secrete thyroxine continuously
- B1 thyroxine in the blood normally inhibits both the hypothalamus and the anterior pituitary, so the hypothalamus will be secreting very little releasing factor
- A1 the tumour is not under that control, so the negative feedback loop is broken: the signal that normally falls when thyroxine rises no longer does, and the thyroxine concentration stays high
Question 62 marks
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.
Mark scheme
- B1 the hormone is adrenaline
- B1 the second messenger is cyclic AMP
Worth remembering
- Exocrine into a duct, endocrine into the blood. That is the whole distinction.
- A hormone reaches every cell; only cells with a complementary receptor respond.
- Steroid hormones cross the membrane and act on genes; peptide hormones cannot cross and use a second messenger.
- Adrenaline, receptor, adenylate cyclase, cyclic AMP, kinase, phosphorylase, glucose — in that order.
- Negative feedback reverses a change; positive feedback amplifies one, and something outside the loop has to stop it.