IB Biology HLChemical SignallingPaper 1 & 2~12 min read
Epinephrine Receptors
A few molecules of adrenaline land on a liver cell and, seconds later, that cell dumps a large amount of glucose into your blood. The hormone never gets inside. Everything that happens is done by a relay team, and each member of that team is louder than the last.
📚 What you need to know
Epinephrine (also called adrenaline) increases blood glucose concentration in response to biological stress.
It binds to receptors on the outside of the cell and works through the second messenger model.
Second messengers are molecules or ions inside cells that relay signals received by cell-surface receptors.
Binding activates adenylyl cyclase, which converts ATP into cyclic AMP (cAMP).
cAMP activates protein kinase A, which activates phosphorylase kinase by adding phosphate groups.
Phosphorylase kinase activates glycogen phosphorylase, which breaks glycogen down into glucose — a process called glycogenolysis.
The cascade amplifies the signal: one hormone molecule leads to many cAMP molecules, which activate many enzymes.
NOS: “adrenaline” and “epinephrine” are both in common use, an unusual example of naming conventions and international cooperation in science.
The second messenger model
Epinephrine is an amine, so it is hydrophilic and cannot cross the membrane. It has to work from outside. The solution is a second messenger: a small molecule made inside the cell that carries on the message the hormone started outside.
The idea
first messenger (epinephrine, outside) → second messenger (cAMP, inside)
🧩 From adrenaline to glucose
Epinephrine binds to specific receptors on the membrane of a liver cell.
This causes the enzyme adenylyl cyclase to change shape and become activated.
Active adenylyl cyclase catalyses the conversion of ATP into the second messenger, cyclic AMP (cAMP).
cAMP binds to protein kinase A enzymes, activating them.
Active protein kinase A activates phosphorylase kinase by adding phosphate groups to it.
Active phosphorylase kinase activates glycogen phosphorylase.
Active glycogen phosphorylase catalyses the breakdown of glycogen to glucose (glycogenolysis).
Glucose is released from the liver cell, raising blood glucose concentration.
Read the chain downwards. Notice that after the first step, every single stage is one enzyme switching on the next — that is what makes it a cascade rather than a queue.
Students often try to memorise this as five random enzyme names. Do not. Learn it as a sentence: cAMP wakes up protein kinase A, which wakes up phosphorylase kinase, which wakes up glycogen phosphorylase, which chops up glycogen. The names then hang off a story instead of floating loose.
Why a cascade at all?
It would seem simpler for the hormone to switch on glycogen phosphorylase directly. There are two reasons it does not.
First, epinephrine cannot get into the cell. Second — and more importantly — a chain of enzymes gives you amplification. Each enzyme molecule is a catalyst, so it can act on many molecules of the next substance before it is switched off. The effect of adrenaline is amplified so that each molecule can stimulate many molecules of cAMP, which in turn activate many enzymes.
The diagram understates it badly. In a real cell each tier is thousands of times larger than the one above, not five times.
Where this fits with the last page. The receptor epinephrine binds is a GPCR, and the G-protein it activates is what switches on adenylyl cyclase. Everything you learned about GTP replacing GDP sits between step 1 and step 2 of this cascade.
NOS: two names for one molecule
The syllabus asks you to see naming conventions as an example of international cooperation in science for mutual benefit, and adrenaline is the case study.
Science does not happen inside one laboratory or one country, so scientists must be able to communicate clearly about their work.
For collaboration to work, scientific language must let everyone be sure they are talking about the same thing.
So when new names are coined, there are mechanisms for constructing a logical and universal term.
Both names for this hormone were built from the same idea: it is made in the adrenal glands, which sit just on top of the kidneys.
Name
Language
Literal meaning
Adrenaline
Latin: ad + ren
“at kidney”
Epinephrine
Greek: epi + nephros
“above kidney”
Usually one name wins and the other disappears. Here, unusually, both names remain in common use in different parts of the world — which is exactly why you need to recognise both in an exam.
🧠
Same hormone, same logic
Ad-ren and epi-nephros are Latin and Greek for the same address. If a question uses one name and your notes use the other, nothing has changed — answer as normal.
Worked examples
WE 1
Describe the second messenger model
Describe how epinephrine causes glucose to be released from a liver cell. (5 marks)
Step 1: binding
Epinephrine binds to a specific receptor on the outside of the liver cell membrane.
Step 2: the second messenger
This activates adenylyl cyclase, which converts ATP into cyclic AMP, the second messenger.
Step 3: the kinases
cAMP activates protein kinase A, which activates phosphorylase kinase by adding phosphate groups.
Step 4: the final enzyme
Phosphorylase kinase activates glycogen phosphorylase.
Step 5: the response
Glycogen phosphorylase catalyses the breakdown of glycogen to glucose, which is released to raise blood glucose concentration.
Receptor → cAMP → kinases → glycogen phosphorylase → glucosefive marks, five named stages — the enzyme names are the marking points
WE 2
Explain the advantage of a cascade
Explain the advantage to the body of epinephrine acting through an enzyme cascade rather than directly. (3 marks)
Point 1: it has to
Epinephrine is hydrophilic, so it cannot cross the membrane and must act through a second messenger.
Point 2: amplification
Each enzyme is a catalyst, so one molecule activates many molecules of the next stage; one epinephrine molecule leads to many cAMP molecules, which activate many enzymes.
Point 3: the benefit
A very small amount of hormone therefore produces a large, rapid release of glucose, which is useful in a stress response.
A small signal becomes a very large responsethe word amplified is what the mark scheme wants here
WE 3
Explain why two names exist
Explain what the existence of both “adrenaline” and “epinephrine” shows about naming in science. (2 marks)
Point 1: why names are agreed
Scientific research is international, so clear shared terminology is needed to be sure scientists are describing the same thing.
Point 2: this case
Both terms were coined logically from the adrenal glands above the kidneys, one from Latin and one from Greek, and unusually both remain in common use.
Naming conventions exist for cooperation; here two logical names survivedanswer the NOS point, not the biology — the marks are about communication between scientists
💡 Exam tips
Define a second messenger as a molecule or ion inside the cell that relays a signal received by a surface receptor.
Learn the enzyme order and say activates at every step.
Name glycogenolysis for the final breakdown.
Say protein kinase A activates phosphorylase kinase by adding phosphate groups.
Use the word amplified whenever a question asks about the size of the response.
Recognise adrenaline and epinephrine as the same hormone.
⚠ Common mistakes
Saying epinephrine enters the liver cell. It binds on the outside; cAMP does the inside work.
Calling cAMP the first messenger. The hormone is first, cAMP is second.
Saying cAMP breaks down glycogen. It only activates protein kinase A.
Confusing glycogen with glucagon. Glycogen is the storage polysaccharide; glucagon is a different hormone.
Saying the cascade makes more glucose. The glucose already existed as glycogen; it is released, not created.
Writing that adrenaline and epinephrine are different hormones. They are the same molecule.
Up next: Intracellular Receptors. We have followed a hormone that has to shout through the wall. Next we follow one that walks straight in, all the way to the DNA, and changes which genes a cell is using.
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