IB Biology HLChemical SignallingPaper 1 & 2~13 min read
Signalling Molecules
Adrenaline and acetylcholine are both signalling molecules, yet one can flood your whole body for minutes and the other crosses a gap thinner than a virus and is destroyed almost immediately. The difference is not the message. It is how far it travels and how long it lasts.
📚 What you need to know
Hormones are made by endocrine glands, secreted into capillaries, carried in the blood, and alter the activity of target cells with complementary receptors.
Neurotransmitters cross the synaptic cleft, bind to receptors on the postsynaptic membrane, open sodium ion channels and can generate a nerve impulse.
Neurotransmitter signals are short-lived and localised; hormone signals can be distant and longer lasting.
Cytokines are proteins released by nearly all cells; they bind to cell surface receptors and trigger a cascade affecting gene expression.
Calcium ions act in muscle contraction and at synapses, and often act as second messengers.
Hormones fall into three groups: amines, proteins (peptides) and steroids.
Hydrophilic molecules bind to receptors on the outside of the membrane; hydrophobic steroids cross the membrane and bind to receptors inside cells.
Signalling chemicals must be able to bind to receptors and be small and soluble enough to move around the body.
Hormones
A hormone is a chemical substance produced by a gland and carried by the blood, which alters the activity of one or more specific target organs. In other words, hormones transmit information from one part of the organism to another and bring about a change.
Endocrine glands produce hormones and secrete them into capillaries in the gland tissue. Together these glands form the endocrine system.
Hormones are transported in the blood to target cells.
They only affect cells that have target receptors the hormone can bind to. These receptors are found either on the cell surface membrane or inside the cell.
Receptors must be complementary to the hormone for binding to occur, so they are specific to a particular hormone.
Twenty nanometres versus the length of your body. That single difference explains almost every contrast between the two systems.
Feature
Neurotransmitter
Hormone
Distance travelled
very short — about 20 nm across the cleft
much longer, carried in the blood anywhere in the body
Where it acts
localised, at one synapse
any cell with the correct receptor, near the gland or far from it
Duration
short-lived, broken down quickly
longer lasting
If you are asked to compare the two, do not just say “one is fast and one is slow”. Explain why: diffusion across 20 nm takes almost no time, while travelling in the blood takes seconds to minutes. The distance causes the speed difference.
Cytokines
Cytokines are proteins released by nearly all cells in the human body.
There are several types, and each one plays a role in determining the activity of another cell.
They bind to receptors on the cell surface membrane — they cannot enter the cytoplasm.
Binding leads to a cascade of events inside the cell that affects gene expression, and therefore cell activity.
They are involved in signalling between white blood cells during an immune response, and in regulating the cell cycle for cell growth and proliferation during embryonic development.
Not every signal is a big organic molecule. Calcium ions (Ca2+) are involved in many signalling pathways:
Where
What Ca2+ does
Why the response is brief
Muscle contraction
An influx of Ca2+ changes the shape of specific proteins, allowing fibres in muscle tissue to contract
Calcium ions are pumped back out of the muscle fibres, so the response is rapid and short-lived
At a synapse
A nerve impulse causes Ca2+ to move into the presynaptic knob, triggering vesicles to release neurotransmitter
Calcium ions are pumped back into the synaptic cleft, so the response is rapid and short-lived
Calcium ions also act as second messengers: they form part of the cascade of reactions that happens inside a cell after another signalling molecule binds to an external membrane receptor.
Why pumping matters. A signal is only useful if it can be switched off. Calcium responses are short because the cell actively pumps the ions back where they came from. The same logic runs through this whole topic — every signal has an off switch.
Chemical diversity
Within the categories of hormones and neurotransmitters there are several different chemical configurations, and each one is fundamental to how well the molecule works as a signal.
A signalling chemical must be:
Able to bind to receptors, which depends on the shape and chemical properties of the molecule
Small and soluble, so it can move around the body
Hormone categories
Amines
Proteins (peptides)
Steroids
Melatonin
Insulin
Oestradiol
Thyroxin
Glucagon
Progesterone
Epinephrine
ADH
Testosterone
The category matters because it decides where the receptor is:
Amines and proteins are hydrophilic, which makes it difficult for them to cross the phospholipid bilayer. They function by binding to external membrane receptors.
Steroid hormones are hydrophobic, so they can cross cell membranes and bind to receptors inside cells.
Steroids are small and oily, so the membrane is no barrier to them. Amines and proteins are water-loving and have to shout through the wall instead.
Neurotransmitter categories
Amines
Gases
Amino acids
Peptides
Dopamine
Nitrous oxide
Glutamate
Endorphins
Epinephrine
—
Glycine
—
Most neurotransmitters are hydrophilic, so they bind to receptors on cell surface membranes.
Some neurotransmitters also act as hormones, for example epinephrine, also known as adrenaline.
Some neurones produce only one neurotransmitter; others produce multiple which can be released simultaneously, stimulating several different outcomes at the same time.
🧠
One molecule, two jobs
Epinephrine appears in the amine column of both tables. Released at a synapse it is a neurotransmitter; released from the adrenal gland into the blood it is a hormone. What a molecule is called depends on how it is delivered, not on what it is made of.
Worked examples
WE 1
Compare hormones and neurotransmitters
Compare the effects of hormones and neurotransmitters as signalling molecules. (4 marks)
Point 1: distance
Neurotransmitters diffuse a very short distance, about 20 nm across the synaptic cleft, whereas hormones travel much longer distances in the blood.
Point 2: how widespread
Neurotransmitter effects are localised to one synapse; hormones can reach any cell in the body that has the correct receptor.
Point 3: duration
Neurotransmitter signals are short-lived because the molecules are broken down; hormone effects last longer.
Point 4: what is the same
Both are ligands that bind to complementary receptors and trigger a response inside the target cell.
Short and local versus far and lasting — but the same binding mechanisma “compare” question wants at least one similarity, not only differences
WE 2
Explain the position of a receptor
Explain why insulin binds to a receptor on the cell surface membrane, while testosterone binds to a receptor inside the cell. (3 marks)
Point 1: insulin
Insulin is a protein hormone and is hydrophilic, so it cannot pass through the hydrophobic centre of the phospholipid bilayer.
Point 2: the consequence
It must therefore bind to an external membrane receptor, and the message is relayed into the cell by other molecules.
Point 3: testosterone
Testosterone is a steroid and is hydrophobic, so it diffuses through the membrane and binds to a receptor in the cytoplasm or nucleus.
Solubility decides the receptor’s locationname the property (hydrophilic or hydrophobic) — “insulin is big” is not the mark scheme answer
WE 3
Describe the role of calcium ions
Describe two roles of calcium ions in cell signalling. (3 marks)
Role 1: at a synapse
A nerve impulse causes Ca2+ to move into the presynaptic knob, which triggers vesicles to release neurotransmitter into the cleft.
Role 2: in muscle
An influx of Ca2+ changes the shape of specific proteins, allowing muscle fibres to contract.
Why it stops
In both cases the ions are pumped back out, so the response is rapid and short-lived. Calcium ions can also act as second messengers within a cascade.
Trigger vesicle release, trigger contraction, and relay signals as a second messengermentioning the pumping is what earns the “short-lived” mark
💡 Exam tips
Define a hormone fully: made by a gland, carried in the blood, alters activity of specific target organs.
Learn the three hormone categories and one example of each.
Link hydrophilic to surface receptor and hydrophobic to internal receptor every time.
Quote the 20 nm figure for the synaptic cleft if you are comparing distances.
For cytokines, say they bind to surface receptors and cannot enter the cytoplasm.
For calcium ions, always add that they are pumped back, which is why the response is short.
⚠ Common mistakes
Saying hormones travel in the nervous system. They travel in the blood.
Saying cytokines enter the cell. They bind on the surface only.
Treating epinephrine as only a hormone. It is both a hormone and a neurotransmitter.
Saying steroid hormones bind to surface receptors. They cross the membrane and bind inside.
Forgetting that neurotransmitters are broken down. Without that, stimulation would never stop.
Writing “calcium” when you mean calcium ions. Use Ca2+.
Up next: Receiving the Signal. We have sorted the messages. Next we look properly at the two kinds of receptor that pick them up, and at what “transduction” actually involves inside the cell.
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