IB Biology HLChemical SignallingPaper 1 & 2~11 min read
Cell Signalling
A cell in your toe has no idea what your brain is doing. It does not need to. It only needs to recognise one specific molecule when that molecule arrives. That is the whole trick behind how a body made of trillions of separate cells behaves like one organism.
📚 What you need to know
Cell signalling is the process by which messages are sent to cells.
It allows multicellular organisms to control and coordinate their bodies and to respond to their environments.
Signalling pathways coordinate cells even when they are far apart in the organism.
The basic stages: a stimulus is received by a receptor, the signal is converted (transduction), the signal is transmitted to a target (effector), and a response is made.
Signalling molecules are called ligands. Examples include proteins and amino acids, nucleotides, steroids and amines.
Ligands are secreted into the extracellular space, transported to a target cell, and bind to receptors specific to that ligand.
Inside the cell the message is relayed by a chain of chemical messengers, triggering a response.
Quorum sensing lets bacteria respond to population size by altering gene expression, as in Vibrio fischeri and the bobtail squid.
Why cells need to talk
A single-celled organism has a simple life. Everything it senses, it deals with itself. A multicellular organism has a problem: the cell that detects something is almost never the cell that needs to do something about it.
Cell signalling solves that. It is the process by which messages are sent to cells, and it allows an organism to control and coordinate its body and respond to its environment. Signalling pathways can coordinate the activities of cells that sit at opposite ends of the body.
The four stages of a signalling pathway
Every pathway you meet in this topic is a version of this diagram. Only the molecules change.
If a question says “outline the stages of a cell signalling pathway”, write all four and name transduction explicitly. Students routinely jump from receptor straight to response and lose a mark for the missing middle.
Ligands: the messages themselves
A signalling molecule is called a ligand. Ligands come in several chemical families:
Proteins and amino acids
Nucleotides
Steroids
Amines
Whatever the chemistry, the journey is the same:
🧩 The life of a ligand
The ligand is secreted from a cell (the sending cell) into the extracellular space.
It is transported through that space to a target cell.
It binds to a receptor specific to that ligand. These receptors are often proteins with binding sites, such as a glycoprotein.
The message is relayed through a chain of chemical messengers inside the cell.
A response is triggered.
Why only some cells respond
Here is the part worth understanding properly. A hormone released into the blood washes past almost every cell in the body. Yet only a handful react.
That is because a ligand can only bind to a receptor with a complementary shape. A cell without that receptor simply has nothing for the ligand to attach to, so the ligand drifts past and nothing happens. Specificity comes from the receptor, not from the ligand.
The pink chain inside the target cell is the relay of chemical messengers that carries the message from the membrane to wherever the response happens.
Quorum sensing in bacteria
Bacteria signal too, and they use it to answer a surprising question: how many of us are there?
A quorum is the minimum number of individuals that must be present for something to go ahead. Bacteria estimate their own population size using ligands, and when the number is high enough they change their gene expression, which changes what the colony does.
🧩 How quorum sensing works
Bacteria release ligands that bind to receptors on other bacteria.
The more bacteria there are, the more ligand is released, so the concentration climbs.
When a threshold number of receptors is occupied, a change in gene expression is triggered.
That change in gene expression produces a change in activity, which signals that a quorum has been met.
Vibrio fischeri and the bobtail squid
Vibrio fischeri is a marine bacterium that forms a mutualistic association with some squid, including the bobtail squid. Both sides gain:
Partner
What it gains
The squid
Increased camouflage. The bacteria emit light by bioluminescence, lighting the underside of the squid so it is less visible against the bright sky from below.
The bacteria
Amino acids and sugar from the squid’s metabolic processes.
The light is switched on by quorum sensing:
Vibrio fischeri colonise a structure in the squid called the light organ and release a ligand called an autoinducer into the extracellular environment.
The more bacteria present, the more autoinducer is released.
The autoinducer enters other bacterial cells and binds to a receptor called LuxR in the cytoplasm.
When enough autoinducer–LuxR complexes have formed, a threshold is reached, causing transcription of DNA that leads to synthesis of the enzyme luciferase.
Luciferase catalyses an oxidation reaction that releases energy as bioluminescence.
This is why bioluminescence only appears when the colony is large enough to switch on luciferase synthesis. One cell alone would produce light nobody could see.
Notice what is being controlled. Quorum sensing does not switch a protein on and off — it switches a gene on and off. The response is a change in gene expression, which is slower than a nerve impulse but much longer lasting.
Worked examples
WE 1
Outline a cell signalling pathway
Outline the basic stages of a cell signalling pathway. (4 marks)
Stage 1: detection
A stimulus or signal is received by a receptor.
Stage 2: transduction
The signal is converted into a signal that can be passed on; this conversion is called transduction.
Stage 3: transmission
The signal is transmitted to a target, known as the effector.
Stage 4: response
The effector carries out an appropriate response.
Receptor → transduction → transmission → responsefour marks means four stages — do not merge transduction and transmission into one
WE 2
Explain why only some cells respond
A ligand is released into the blood and reaches every cell in the body, but only certain cells respond. Explain why. (3 marks)
Point 1: binding needs a match
A ligand can only bind to a receptor with a complementary shape, so binding is specific to that ligand.
Point 2: target cells
Only target cells carry that receptor, so only they can bind the ligand and start a chain of chemical messengers inside the cell.
Point 3: non-target cells
Cells without the receptor have nothing for the ligand to bind to, so no transduction occurs and no response is made.
The specificity is in the receptor, not in the ligandsay “complementary” — it is the word the mark scheme uses
WE 3
Explain bioluminescence in Vibrio fischeri
Explain how quorum sensing allows Vibrio fischeri to produce light only when its population is large. (4 marks)
Point 1: the ligand
Each bacterium releases a ligand called an autoinducer, so more bacteria means a higher autoinducer concentration.
Point 2: the receptor
Autoinducer enters other bacterial cells and binds to the receptor LuxR in the cytoplasm.
Point 3: the threshold
Once enough autoinducer–LuxR complexes form, a threshold is reached and transcription of DNA leads to synthesis of the enzyme luciferase.
Point 4: the light
Luciferase catalyses an oxidation reaction that releases energy as bioluminescence.
Concentration reports population size, and only a high enough concentration switches the gene onname LuxR and luciferase — a generic “a receptor and an enzyme” answer will not get full marks
💡 Exam tips
Learn all four stages and name transduction.
Use the word ligand for a signalling molecule, and complementary for the receptor fit.
Say responses are caused by a chain of chemical messengers inside the cell, not by the ligand itself.
For quorum sensing, always mention threshold and gene expression.
For Vibrio fischeri, learn the three names: autoinducer, LuxR, luciferase.
Describe the squid relationship as mutualistic and give the benefit to both partners.
⚠ Common mistakes
Skipping transduction. Receptor straight to response loses a mark.
Saying the ligand enters the cell and causes the response. Most ligands never get in; they bind on the surface and the message is relayed.
Saying the ligand is specific. The same ligand meets every cell. It is the receptor that makes the cell a target.
Describing quorum sensing as bacteria “counting” each other. They detect a concentration, nothing more.
Saying the squid makes the light. The bacteria make it; the squid provides the light organ and nutrients.
Calling the relationship parasitic. Both organisms benefit, so it is mutualism.
Up next: Signalling Molecules. We have called them all “ligands”, but a hormone travelling in your blood for hours behaves nothing like a neurotransmitter crossing a 20 nm gap in a millisecond. Next we sort them into types.
Want this explained one-to-one?
Book a free session with an experienced IB Biology tutor and get your trickiest topics made simple.
