Difficulty: Introductory | Prerequisites: Part 1 (Signal Reception notes), basic enzyme function, gene expression
Once a signal molecule binds its receptor (covered in Part 1), the cell needs to relay that message inward and produce a response. This is where transduction and response come in. Transduction converts the extracellular signal into a cascade of intracellular events, often amplifying it along the way, so that a small number of signal molecules can produce a large cellular effect. The response can range from changes in gene expression to altered enzyme activity to programmed cell death (apoptosis). This material builds directly on receptor types and signal reception.
After a signal binds a receptor, the message is passed along through phosphorylation cascades and second messengers (cAMP, IP₃, Ca²⁺). Each step can amplify the signal, so one signal molecule can trigger a massive response. The end result is a change in cell behaviour: altered gene transcription, enzyme activation, or even cell death via apoptosis.
Second messenger
A small, non-protein molecule that relays and amplifies a signal inside the cell after the first messenger (the signal molecule) has bound the receptor. Examples include cyclic AMP, IP₃ and Ca²⁺. Think of it as the internal courier that runs the message through the building after the letter arrives at the front desk.
Cyclic AMP (cAMP)
A second messenger produced from ATP by the enzyme adenylyl cyclase. cAMP activates protein kinase A, which then phosphorylates target proteins. In simple terms, cAMP is the cell's internal alarm bell.
IP₃ (inositol trisphosphate)
A second messenger produced when phospholipase C cleaves PIP₂. IP₃ binds to ligand-gated ion channels on the endoplasmic reticulum, releasing stored Ca²⁺ into the cytoplasm.
Calcium ions (Ca²⁺)
Act as second messengers. Stored in the endoplasmic reticulum and released into the cytoplasm when IP₃ opens ER ion channels. Ca²⁺ then activates proteins that produce the cellular response.
Phosphorylation cascade
A chain of reactions in which one protein kinase phosphorylates and activates the next, and so on. Each step amplifies the signal. In simple terms, it is a molecular domino chain, but each domino is bigger than the last.
Protein kinase
An enzyme that transfers a phosphate group from ATP to a target protein, changing its activity. Protein kinases are the workhorses of signal transduction.
Phosphatase
An enzyme that removes phosphate groups from proteins, reversing the action of kinases. Phosphatases turn off the signal transduction pathway. Think of them as the off switch.
Amplification
The process by which a single signal molecule produces an increasingly large response at each step of the transduction pathway. A protein kinase activating many other protein kinases is a textbook example.
Transcription factor
A protein that binds to DNA and triggers the transcription of a specific gene. Many signal transduction pathways end by activating a transcription factor.
Apoptosis
Programmed cell death. The cell shrinks, its DNA is fragmented and it is packaged into small bodies that are engulfed by neighbouring cells. Apoptosis does not involve lysis (bursting) of the cell.
Adenylyl cyclase
The enzyme that converts ATP to cyclic AMP. It is activated by G proteins in the GPCR pathway.
Phospholipase C
An enzyme activated by G proteins that cleaves PIP₂ into DAG and IP₃. This is the starting point of the IP₃/Ca²⁺ second-messenger pathway.
A signal transduction pathway is initiated when a signal molecule binds to a receptor. The purpose of transduction is to convert that extracellular event into a chain of intracellular events. This typically involves phosphorylation cascades and second messengers.
A protein kinase adds a phosphate group to a target protein, changing its shape and activity.
That activated protein can itself be a kinase, which phosphorylates the next protein in the chain, and so on.
This relay is a phosphorylation cascade.
Phosphatases reverse this process: they remove phosphate groups from proteins, turning off the signal.
A protein kinase activating many other protein kinases is an example of amplification.
Second messengers are small molecules that spread the signal rapidly through the cytoplasm. The main ones tested in this course:
Cyclic AMP (cAMP) pathway
Signal molecule binds a GPCR.
The activated G protein switches on adenylyl cyclase.
Adenylyl cyclase converts ATP to cyclic AMP.
cAMP activates protein kinase A.
Protein kinase A activation is one possible result of signal molecules binding to G protein-coupled receptors.
IP₃ / Ca²⁺ pathway
Signal molecule binds a GPCR.
The activated G protein switches on phospholipase C.
Phospholipase C cleaves PIP₂, forming DAG and IP₃.
IP₃ binds to a ligand-gated ion channel in the ER membrane.
Calcium ions flow through the channel into the cytoplasm.
Calcium ion concentration increases in the cytosol.
Calcium ions activate proteins, leading to a cellular response.
Summary: phospholipase C catalyses the production of IP₃, which then opens an ion channel that releases Ca²⁺ into the cell's cytoplasm.
Histamine is released during inflammatory and allergic responses. The H₁ receptor is a GPCR that activates phospholipase C.
Histamine binds extracellularly to the H₁ receptor (it does not diffuse across the membrane, because it is hydrophilic).
When histamine binds, the receptor undergoes a conformation change and binds the inactive G protein.
Once the G protein is active, it binds the enzyme phospholipase C, activating it.
Histamine is likely hydrophilic (which is why it binds on the outside, not inside).
What determines the specific cellular response to histamine depends on: the type of histamine receptor, the type of cell, the enzyme activated by the G protein, the types of second messengers involved, and the proteins activated by those second messengers.
Stage | Components that belong here |
|---|---|
Reception | Signalling molecule, receptor tyrosine kinase, G protein-coupled receptor |
Transduction | IP₃, phosphorylation cascade, cAMP, second messenger, Ca²⁺, adenylyl cyclase |
Response | Protein synthesis |
Many signal transduction pathways end by activating a transcription factor. The transcription factor binds to DNA and triggers the transcription of a specific gene. This is how signals from outside the cell ultimately change what proteins the cell makes.
Amplification explains how a tiny number of signal molecules can produce a large effect. At each step of the cascade, one activated enzyme can activate many copies of the next enzyme. By the time the signal reaches the response, the effect has been magnified enormously.
Apoptosis is a controlled process of cell death. The cell does not burst (lyse). Instead:
The cell shrinks.
The DNA is fragmented.
The cell is broken into small membrane-bound pieces.
Neighbouring cells engulf the fragments.
Apoptosis involves all of the above. Lysis of the cell is the exception: lysis is a feature of necrosis (uncontrolled cell death), not apoptosis.
IP₃ / Ca²⁺ pathway (memorise this sequence):
Enzyme (phospholipase C) cleaves PIP₂, forming DAG and IP₃.
IP₃ binds to a ligand-gated ion channel in the ER membrane.
Calcium ions flow through the ligand-gated ion channel.
Calcium ion concentration increases in the cytosol.
Calcium ions activate a protein, leading to a cellular response.
Phosphorylation cascade (general pattern):
Signal molecule → receptor → G protein → enzyme (adenylyl cyclase or phospholipase C) → second messenger (cAMP or IP₃) → protein kinase → phosphorylation of target proteins → cellular response.
Phosphatases reverse each phosphorylation step to shut the pathway down.
Caffeine works by inhibiting a phosphodiesterase, the enzyme that breaks down cAMP. With cAMP levels staying elevated, the signalling pathway stays on longer, which is part of why caffeine keeps you alert. Many drugs target specific steps in transduction pathways: beta-blockers interfere with the cAMP pathway in heart cells, and some cancer therapies target overactive receptor tyrosine kinase cascades.
Students often think phosphatases activate the signalling pathway. They do the opposite: phosphatases remove phosphate groups and turn the pathway off.
Students confuse second messengers with signal molecules. The signal molecule (first messenger) is extracellular; second messengers (cAMP, IP₃, Ca²⁺) are intracellular.
Students sometimes think apoptosis involves the cell bursting open. It does not. Apoptosis is orderly: the cell shrinks and is packaged, with no lysis. Lysis is necrosis.
Students mix up where Ca²⁺ is stored. Calcium ions used as second messengers are stored in the endoplasmic reticulum, not the mitochondria or Golgi.
Know the complete IP₃/Ca²⁺ pathway in order, from phospholipase C cleaving PIP₂ through to Ca²⁺ activating a protein.
Be able to assign each component (cAMP, IP₃, Ca²⁺, adenylyl cyclase, protein kinase, etc.) to its correct stage: reception, transduction or response.
The role of phosphatases (turning off the signal) is a common exam question. Do not confuse them with kinases.
Amplification comes up frequently. Understand why a single signal molecule can produce a massive cellular response.
The apoptosis "all but which" format is standard. Remember that lysis is the answer that does not belong.
Fill in the blank: ____ converts ATP to cyclic AMP. (Adenylyl cyclase)
True or false: Phosphatases activate signal transduction pathways. (False)
Fill in the blank: Calcium ions used as second messengers are stored in the ____. (Endoplasmic reticulum)
True or false: Apoptosis involves lysis of the cell. (False)
Fill in the blank: A protein kinase activating many other protein kinases is an example of ____. (Amplification)
Q: What role do phosphatases play in signal transduction pathways?
A: They remove phosphate groups from proteins (inactivate protein kinases), turning off signal transduction.
Q: Which molecule acts as a second messenger?
A: Cyclic AMP (cAMP). Other examples include IP₃ and Ca²⁺.
Q: What role does a transcription factor play in a signal transduction pathway?
A: By binding to DNA, it triggers the transcription of a specific gene.
Q: Phospholipase C catalyses the production of ____, which opens an ion channel that releases ____ into the cytoplasm.
A: IP₃; Ca²⁺.
Q: Apoptosis involves all but which of the following: cell shrinkage, DNA fragmentation, packaging into membrane-bound fragments, lysis of the cell?
A: Lysis of the cell. Apoptosis is orderly and does not involve bursting.
Q: When histamine binds to the H₁ receptor, what determines the specific cellular response?
A: The type of histamine receptor, the type of cell, the enzyme activated by the G protein, the types of second messengers involved, and the proteins those second messengers activate.
This material connects to enzyme function (protein kinases and phosphatases are enzymes, so understanding enzyme specificity and regulation helps). It also links to gene regulation: the endpoint of many signalling pathways is a transcription factor switching a gene on or off. Apoptosis connects to developmental biology (sculpting fingers, removing larval tails) and to cancer biology (cancer cells often have defective apoptosis pathways). See the companion notes on Signal Reception (Part 1) for receptor types and how signals first enter the cell.
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