IBDP Biology 2025 SL&HL: C2.1 Chemical signaling Study Notes

Additional Higher Level

C2.1.1 – Receptors as Proteins with Binding Sites for Specific Signalling Chemicals

🔑 What Are Receptors?

Receptors are proteins found on:

The cell surface membrane
The cytoplasm (in some cases)

They have a specific shape that allows them to bind only to one type of signalling molecule.

💬 What Are Ligands?

A ligand is a signalling chemical that binds to a receptor.

This binding is usually highly specific – like a lock and key.

🧠 Examples of ligands:

Hormones (e.g. insulin)
Neurotransmitters (e.g. dopamine)
Cytokines or growth factors

🎯 How It Works

A ligand travels through blood or extracellular fluid.
It binds to a receptor on/in a target cell.
This causes a change inside the cell, called a signal transduction.
It might activate an enzyme, open a channel, or switch on a gene.

🔬 Real-World Example

Ligand	Receptor Location	Response
Insulin	Cell membrane of liver cells	Increases glucose uptake
Adrenaline	Muscle cell membrane	Triggers fight-or-flight response
Dopamine	Neurons	Affects mood and movement

🔑 Key Points:
– Receptors are proteins that bind specific ligands.
– Ligands are the signalling molecules (like hormones).
– Binding triggers a cellular response – like turning on a gene or changing enzyme activity.
– The interaction is specific, like a lock and key.

C2.1.2 – Cell Signalling by Bacteria in Quorum Sensing

🧬 What Is Quorum Sensing?

Quorum sensing is how bacteria communicate with each other using chemical signals.
It helps bacteria detect their population density.
When enough bacteria are present, they change behavior as a group – like a coordinated team!

💬 How It Works:

Each bacterium releases a signalling molecule called an autoinducer.
As the bacterial population grows, the concentration of autoinducers increases.

When the concentration crosses a threshold:

The autoinducers bind to receptors on/in the bacteria.
This triggers a change in gene expression.

📌 Result? All the bacteria start doing the same thing at the same time!

🌟 Real Example: Bioluminescence in Vibrio fischeri

Feature	Description
Bacterium	Vibrio fischeri
Environment	Lives in the light organ of certain marine animals like the Hawaiian bobtail squid
Signal	Autoinducer molecule
Response	Turns on genes that produce light (bioluminescence)
Purpose	Helps the squid hide its shadow at night – camouflage!

📊 Why Quorum Sensing Matters

Coordinates virulence (infection-causing ability) in pathogens
Controls biofilm formation (slimy protective layers)
Regulates antibiotic resistance
Useful in synthetic biology and biotechnology

🔍 Key Points:
– Quorum sensing = bacterial communication based on population density
– Uses autoinducers to send and detect signals
– Triggers a group response when threshold levels are reached
– Example: Vibrio fischeri produces light only when enough bacteria are present

C2.1.3 – Functional Categories of Signalling Chemicals in Animals

🧪 What Are Signalling Chemicals?

Signalling chemicals carry messages from one cell to another.
They bind to receptors on target cells to trigger a response.
The type, speed, and target of the response depends on the category of the chemical.

The 4 Key Categories:

Category	Function	Speed	Target Range	Example
Hormones	Long-distance messengers released into the bloodstream	Slower (minutes to hours)	Whole body	Insulin, adrenaline
Neurotransmitters	Chemical signals between neurons or to muscles	Very fast (milliseconds)	Specific, local synapses	Acetylcholine, dopamine
Cytokines	Immune system messengers that regulate inflammation and cell communication	Moderate speed	Local or whole-body	Interleukins, interferons
Calcium ions (Ca²⁺)	Second messengers inside cells for quick signalling	Very fast	Intracellular (within cells)	Triggers muscle contraction, enzyme activation

🧠 Comparison: Who Does What?

Feature	Hormones	Neurotransmitters	Cytokines	Calcium Ions
Released by	Endocrine glands	Neurons	Immune cells	Stored in organelles (e.g., ER)
Transport method	Bloodstream	Synaptic cleft	Blood or local tissue	Cytoplasm
Target distance	Long	Very short	Short or long	Within same cell
Response speed	Slow	Very fast	Moderate	Instant
Example function	Regulate blood sugar	Move a muscle	Trigger fever	Trigger muscle contraction

🔍 Key Points:
– Hormones: Long-distance, slow-acting signals in the bloodstream
– Neurotransmitters: Fast, local messengers at nerve synapses
– Cytokines: Immune signalling molecules for inflammation and immunity
– Calcium ions: Act inside cells as second messengers for quick responses

C2.1.4 – Chemical Diversity of Hormones and Neurotransmitters

🔍 Why So Many Different Chemicals?

Cells use a wide variety of signalling molecules because:

Different chemicals can have different properties (e.g. solubility, size, speed of action).
Each chemical type can target different receptors and produce specific responses.
Variety allows specialization for different tissues, organs, or functions.

🧪 Types of Hormones

Hormones belong to three main chemical groups:

Hormone Type	Chemical Nature	Solubility	Example
Amines	Modified amino acids	Water-soluble	Adrenaline, thyroxine
Proteins/Peptides	Chains of amino acids	Water-soluble	Insulin, ADH
Steroids	Lipid-based (from cholesterol)	Fat-soluble	Testosterone, cortisol

Note: Solubility affects how the hormone is transported in the blood and whether it can pass through cell membranes.

⚡ Types of Neurotransmitters

Neurotransmitters are even more chemically diverse. They include:

Type	Chemical Group	Examples
Amino acids	Simple building blocks of proteins	Glutamate, GABA
Peptides	Small chains of amino acids	Substance P, endorphins
Amines	Modified amino acids	Dopamine, serotonin, histamine
Gases	Very small, diffusible molecules	Nitric oxide (NO)

💬 Some neurotransmitters act quickly (e.g., glutamate), while others modulate slow, long-term responses (e.g., neuropeptides).

🧠 Why Use Different Chemicals?

Specificity: Each chemical activates specific receptors.
Timing: Some work in milliseconds, others take minutes or longer.
Function: Some stimulate, others inhibit.
Solubility: Determines how and where the chemical can act (e.g., in blood vs inside cells).

🔍 Key Points:
– Hormones can be amines, proteins/peptides, or steroids, with different solubilities and functions.
– Neurotransmitters include amino acids, peptides, amines, and even gases.
– The diversity allows precise control over cell signalling across body systems.

C2.1.5 – Localized and Distant Effects of Signalling Molecules

🧬 What Are Signalling Molecules?

Signalling molecules like hormones and neurotransmitters help cells communicate — but how far their effects reach can vary:

Some act right next to the target cell.
Others travel to distant parts of the body.

🔄 Local vs Distant Effects: Key Differences

Feature	Neurotransmitters	Hormones
Distance of effect	Local (across a synaptic gap)	Distant (travel via bloodstream)
Speed of action	Very fast (milliseconds)	Slower (seconds to minutes or longer)
Duration of effect	Short-lived	Longer-lasting
Mode of transport	Diffusion across synapse	Carried by the circulatory system
Examples	Dopamine, serotonin, acetylcholine	Insulin, adrenaline, thyroxine

📍 Local Effects: Neurotransmitters

Released by neurons at synapses.
Act only on nearby target cells (e.g., muscle cells, another neuron).
Highly specific – acts on a single synapse.

🧠 E.g., Acetylcholine triggers muscle contraction right where the nerve meets the muscle.

🚀 Distant Effects: Hormones

Secreted by endocrine glands (e.g., pancreas, adrenal gland).
Travel through bloodstream to reach target cells far away.
Can affect many cells in different parts of the body.

💉 E.g., Insulin lowers blood sugar throughout the body.

🧠 Key Points:
– Neurotransmitters act locally, are fast, and have short-lived effects.
– Hormones act distantly, are slower, but have long-lasting effects.
– This contrast shows how the body uses different signalling strategies for different needs.

C2.1.6 – Differences Between Transmembrane and Intracellular Receptors

📡 What Are Cell Receptors?

Receptors are proteins that bind signaling molecules (like hormones or neurotransmitters).

Their main job is to receive messages and trigger responses inside the cell.

Receptors can be located:

On the plasma membrane (outer cell surface)
Inside the cytoplasm or nucleus

🧪 Transmembrane Receptors (Plasma Membrane)

Found spanning the cell membrane (outside to inside).
Bind hydrophilic (water-soluble) molecules that can’t cross the cell membrane.

Structure:

Outside part: Recognizes and binds the signaling molecule.
Inside part: Passes the signal into the cell, often triggering a cascade.

Amino Acid Distribution:

Hydrophilic amino acids on the extracellular and cytoplasmic ends (face watery environments).
Hydrophobic amino acids in the middle, anchoring it in the membrane.

Examples:

Insulin receptor
Neurotransmitter receptors

🧬 Intracellular Receptors (Cytoplasm or Nucleus)

Located inside the cell – in the cytoplasm or nucleus.
Bind hydrophobic signaling molecules (e.g. steroid hormones) that can pass through the membrane.

Structure:

Fully inside the cell, no membrane-spanning region.
Activate gene expression or other intracellular processes.

Amino Acid Distribution:

Mostly hydrophilic amino acids (soluble inside the cell).
Have specific binding sites for DNA or enzymes.

Examples:

Estrogen receptor
Thyroid hormone receptor

📊 Comparison Table

Feature	Transmembrane Receptors	Intracellular Receptors
Location	In plasma membrane	In cytoplasm or nucleus
Ligand Type	Hydrophilic (can’t cross membrane)	Hydrophobic (can cross membrane)
Signal Entry	Signal stays outside	Signal enters the cell
Amino Acid Features	Hydrophobic in membrane, hydrophilic outside	Mostly hydrophilic
Examples	Insulin, adrenaline receptors	Steroid hormone receptors

🔍 Key Takeaways:
– Transmembrane receptors detect external, hydrophilic signals and transmit them into the cell.
– Intracellular receptors detect internal, hydrophobic signals that diffuse into the cell.
– Amino acid distribution reflects the receptor’s location and function.

C2.1.7 – Initiation of Signal Transduction Pathways by Receptors 🧬

🧬 What is Signal Transduction?

Signal transduction is the process by which a cell responds to signals from its environment.

It begins when a signalling molecule (ligand) binds to a specific receptor protein on or inside the cell.

This binding triggers a chain of events inside the cell → like falling dominoes → leading to a cellular response.

🧪 Step-by-Step: How Signal Transduction Begins

Signal Binding
A signalling molecule (like a hormone or neurotransmitter) binds to its receptor. This binding is highly specific — like a key fitting into a lock.
Receptor Activation
Binding changes the shape or activity of the receptor (a process called conformational change). This activates the receptor so it can now pass on the message.
Signal Transduction Cascade Begins
The activated receptor triggers a chain reaction of molecular events inside the cell. These reactions involve relay proteins, enzymes, or second messengers (like cAMP or calcium ions).
Cellular Response
The cell finally responds in some way, such as:
- Activating a gene
- Changing its metabolism
- Releasing another chemical
- Moving or dividing

🧠 Example: Hormone Signal Cascade

Step	What Happens
1	Adrenaline binds to a receptor on a liver cell
2	The receptor activates an enzyme inside the cell
3	A second messenger (like cAMP) is produced
4	Enzymes are activated to break down glycogen
5	Glucose is released into the bloodstream

🧩 Types of Signalling Molecules

Hydrophilic (like adrenaline) → bind to membrane receptors
Hydrophobic (like steroid hormones) → pass through membrane → bind to intracellular receptors

📌 Key Takeaways:
– Signal transduction begins when a ligand binds to a receptor.
– This activates a cascade of molecular events inside the cell.
– The result is a specific cellular response, allowing the cell to react to changes in its environment.

C2.1.8 – Transmembrane Receptors for Neurotransmitters and Changes to Membrane Potential

🧬 Neurotransmitters and Transmembrane Receptors

Neurotransmitters are chemical messengers released by neurons to communicate with other cells.

They bind to specific receptors on the postsynaptic cell membrane, triggering a response.

Some receptors are transmembrane proteins that also act as ion channels.

🎯 Key Example: The Acetylcholine Receptor

Acetylcholine (ACh) is a common neurotransmitter found at neuromuscular junctions and in the brain.

The acetylcholine receptor is a ligand-gated ion channel:

It spans the membrane (transmembrane receptor).
When ACh binds, the channel opens.

⚙️ How the Process Works

Neurotransmitter Release: A neuron releases acetylcholine into the synaptic cleft.
Binding to Receptor: ACh binds to its receptor on the postsynaptic membrane.
Ion Channel Opens: The receptor changes shape and opens a channel for ions to flow through.
Ions Diffuse In: Na⁺ (sodium ions) rush into the cell. Sometimes K⁺ may flow out too, but sodium influx dominates.
Change in Membrane Potential: The inside of the cell becomes less negative (this is called depolarization).
- May trigger an action potential (if threshold is reached)
- Activate other proteins or channels
- Cause a muscle contraction (if it’s a muscle cell)

🧪 Acetylcholine Receptor at a Glance

Feature	Description
Type of receptor	Ligand-gated ion channel
Neurotransmitter	Acetylcholine (ACh)
Ions involved	Mainly Na⁺, sometimes K⁺
Result of activation	Depolarization (change in membrane potential)
Possible outcome	Action potential, muscle contraction, signaling

💡 Did You Know?

The nicotinic acetylcholine receptor is also activated by nicotine, which mimics ACh.

Blocking this receptor (e.g., with curare) can cause paralysis!

📌 Key Takeaways:
– Acetylcholine binds to a transmembrane receptor that acts as an ion channel.
– This allows positively charged ions (mainly Na⁺) to enter the cell.
– The result is a change in membrane potential, which may trigger further cellular responses.

C2.1.9 – Transmembrane Receptors That Activate G Proteins

🧠 What Are G Protein-Coupled Receptors (GPCRs)?

GPCRs are a large family of transmembrane proteins.

Found across all tissues in the human body.

Their job: Detect external signals (like hormones or neurotransmitters) and start a response inside the cell.

🔗 Structure of GPCRs

GPCRs span the plasma membrane 7 times (they are also called 7-transmembrane receptors).
Outside: Binding site for the signaling molecule (ligand).
Inside: Interacts with a G protein (made of α, β, and γ subunits).

🔄 How GPCRs Work — Step-by-Step

Signal Arrival: A ligand (e.g. adrenaline, dopamine) binds to the extracellular part of the GPCR.
Receptor Activation: The GPCR changes shape and activates the G protein on the inner membrane surface.
G Protein Activation: The α-subunit exchanges GDP for GTP, becoming active. It separates from the βγ subunits.
Message Relay: The active α-subunit or βγ complex interacts with target proteins like:
- Enzymes (e.g. adenylate cyclase → makes cAMP)
- Ion channels
- Second messengers
Signal Termination: GTP is hydrolyzed back to GDP, and the subunits rejoin, resetting the system.

🔍 Why Are GPCRs Important?

Involved in vision, smell, taste, mood, heart rate, and more.
~800 GPCRs in humans!
Targeted by many medicines (e.g., antihistamines, beta blockers).

🧪 GPCR Signaling Pathway Overview

Step	Event	Location
1	Ligand binds to GPCR	Outside cell
2	GPCR activates G protein	Inner membrane
3	G protein subunits split	Cytoplasm
4	Target protein is activated	Cytoplasm or membrane
5	Cellular response triggered	Inside cell

💡 Real Example: Adrenaline and Heart Rate

Adrenaline binds to β-adrenergic receptors (a type of GPCR).

Activates G protein → adenylate cyclase → cAMP → faster heart rate.

📌 Key Takeaways:
– GPCRs detect external signals and activate G proteins inside the cell.
– The G protein then triggers internal responses, often through second messengers.
– GPCRs are involved in many body systems and are major drug targets.

C2.1.10 – Mechanism of Action of Epinephrine (Adrenaline) Receptors

🧬 What Is Epinephrine / Adrenaline?

A hormone and neurotransmitter.

Released by the adrenal glands (above the kidneys).

Prepares the body for “fight or flight” — increases heart rate, dilates pupils, boosts glucose.

📌 Naming Note (NOS):

Adrenaline = Latin: ad (at) + ren (kidney)
Epinephrine = Greek: epi (above) + nephros (kidney)
Both mean the same thing – naming reflects international cooperation in science!
“Adrenaline” is common in the UK.
“Epinephrine” is more common in the US.

How Epinephrine Triggers a Response in Cells

This is an example of G protein-coupled receptor (GPCR) signaling.

🔄 Step-by-Step Mechanism:

Epinephrine Binds to Receptor: The hormone binds to β-adrenergic receptor (a GPCR) on the plasma membrane.
Activation of G Protein: Receptor changes shape → activates a G protein inside the cell.
GDP is replaced with GTP on the G protein’s α-subunit.
Activation of Adenylate Cyclase: The GTP-bound α-subunit activates adenylate cyclase (an enzyme in the membrane).
Production of Second Messenger: cAMP: Adenylate cyclase converts ATP → cyclic AMP (cAMP).
cAMP is a second messenger — it carries the signal deeper into the cell.
Activation of Protein Kinase A (PKA): cAMP activates PKA, which then phosphorylates (adds phosphate groups to) target proteins.
Cellular Responses:
- Increased heart rate (cardiac muscle)
- Breakdown of glycogen → glucose (liver)
- Relaxation of airways (smooth muscle in lungs)

📊 Signal Amplification

One epinephrine molecule can activate many G proteins, leading to thousands of cAMP molecules.

This causes a huge cellular response from a tiny hormone signal!

🧪 Epinephrine Signaling Pathway Overview

Step	Event	Molecules Involved
1	Epinephrine binds to GPCR	Epinephrine, β-adrenergic receptor
2	G protein is activated	GTP replaces GDP
3	Adenylate cyclase is activated	Enzyme at membrane
4	cAMP is produced	Second messenger from ATP
5	Protein kinase A is activated	Phosphorylation of target proteins
6	Cell responds	e.g., energy release, heart rate

📌 Key Takeaways:
– Epinephrine binds to a GPCR, starting a signal transduction cascade.
– G proteins activate adenylate cyclase, which creates cAMP from ATP.
– cAMP activates protein kinases, causing changes like increased heart rate or glucose release.
– This is a great example of signal amplification and international naming conventions in biology.

C2.1.11 – Transmembrane Receptors with Tyrosine Kinase Activity

🧪 What Is a Tyrosine Kinase Receptor?

A type of transmembrane receptor with built-in enzyme activity.
Specifically: it phosphorylates tyrosine residues on proteins inside the cell.
These receptors help transmit signals from protein hormones like insulin.

Insulin: A Classic Example

Insulin is a protein hormone secreted by the pancreas when blood glucose levels are high.
Its main job is to promote glucose uptake into cells, especially muscle and fat cells.

🔗 How Insulin Triggers a Cellular Response

🧬 Step-by-Step Mechanism:

Insulin Binds to Receptor: Insulin binds to the insulin receptor on the plasma membrane.
This receptor is a tyrosine kinase receptor.
Receptor Dimerization & Autophosphorylation: Binding causes the receptor to dimerize (two parts join).
The receptor autophosphorylates (adds phosphate groups to its own tyrosine residues inside the cell).
Signal Cascade Begins: These phosphorylated tyrosines act as docking sites for intracellular proteins.
A signal transduction cascade begins — a sequence of reactions inside the cell.
Vesicle Movement: The cascade causes vesicles containing GLUT4 (glucose transporters) to move to the cell membrane.
Glucose Uptake: GLUT4 transporters are inserted into the membrane.
Glucose enters the cell via facilitated diffusion → blood sugar levels drop.

📦 Summary Table: Insulin Signaling Pathway

Step	Event	Key Molecules
1	Insulin binds receptor	Insulin, insulin receptor
2	Receptor phosphorylates tyrosines	ATP, tyrosine residues
3	Signal transduction cascade starts	Intracellular proteins
4	Vesicles with GLUT4 move to membrane	GLUT4-containing vesicles
5	Glucose transporters allow glucose uptake	Glucose enters cell

📌 Key Takeaways:
– The insulin receptor is a tyrosine kinase receptor found in the plasma membrane.
– Binding of insulin triggers phosphorylation of tyrosine residues inside the cell.
– This activates a cascade that leads to GLUT4 transporters moving to the membrane.
– Result: Glucose enters cells, helping to regulate blood sugar levels.

C2.1.12 – Intracellular Receptors That Affect Gene Expression

🧪 What Are Intracellular Receptors?

These are receptor proteins inside the cell, usually in the cytoplasm or nucleus.
They’re activated by lipid-soluble signalling chemicals (like steroid hormones) that can pass through the cell membrane.

💡 How Do They Work?

Steroid hormone (e.g., oestradiol, progesterone, testosterone) diffuses through the cell membrane.
It binds to its specific intracellular receptor inside the cell.
This binding activates the receptor.
The hormone-receptor complex moves into the nucleus (if not already there).
It binds to specific DNA sequences (called hormone response elements).
This activates gene transcription – turning genes on or off!

🧬 Examples of Steroid Hormones

Hormone	Function
Oestradiol	Regulates development of female secondary sex characteristics and menstrual cycle.
Progesterone	Prepares the uterus for pregnancy and maintains early stages.
Testosterone	Regulates development of male secondary sex characteristics and sperm production.

🧠 Key Idea: Hormones Can Control Gene Expression

These steroid hormones act as gene switches, telling the cell to make specific proteins.

This is slower than neurotransmitter signalling but causes long-lasting changes in cell behavior.

📌 Key Points:
– Intracellular receptors are found in the cytoplasm or nucleus.
– They bind to steroid hormones like oestradiol, progesterone, and testosterone.
– Once activated, they bind DNA and affect gene expression.
– This leads to long-term changes in cell function by changing which proteins are made.

C2.1.13 – Effects of the Hormones Oestradiol and Progesterone on Target Cells

🧬 What Are Target Cells?

Target cells are cells that have specific receptors for a hormone.
When the hormone binds, it triggers a response unique to that cell type.

🌸 Oestradiol – Effects on Hypothalamic Cells

Target: Cells in the hypothalamus that secrete gonadotropin-releasing hormone (GnRH).
Effect:
- Oestradiol stimulates these cells to increase secretion of GnRH.
- GnRH then acts on the pituitary to trigger FSH and LH release – both key to the menstrual cycle and ovulation.

In short: Oestradiol → Hypothalamus → ↑ GnRH → ↑ FSH & LH → Ovulation

🌙 Progesterone – Effects on Endometrial Cells

Target: Cells in the endometrium (lining of the uterus).
Effect:
Stimulates endometrial cells to prepare for implantation by:
- Thickening the uterine lining
- Increasing blood vessel growth
- Promoting secretion of nutrients
Helps maintain the lining during early pregnancy.

In short: Progesterone → Endometrium → Thickens & nourishes uterine lining

🔁 Hormonal Coordination Summary

Hormone	Target Cell	Main Effect
Oestradiol	Hypothalamic cells (GnRH-secreting)	Increases GnRH → Stimulates ovulation
Progesterone	Endometrial cells	Prepares uterus for pregnancy by thickening the lining

📌 Key Points:
– Oestradiol acts on the hypothalamus, boosting GnRH and influencing the menstrual cycle.
– Progesterone acts on the endometrium, preparing it for possible implantation and pregnancy.
– Both hormones are essential for proper reproductive function in females.

C2.1.14 – Regulation of Cell Signalling Pathways by Positive and Negative Feedback

⚙️ What Is Feedback in Cell Signalling?

Feedback refers to how the output of a signalling pathway affects its own activity.
It helps cells regulate responses, keeping them balanced or amplifying them when needed.

➕ Positive Feedback

Definition: A process where the response enhances or amplifies the original signal.
Effect: Makes the signal stronger and usually leads to a rapid change or self-perpetuating loop.
Example: Oestrogen and LH Surge
Oestradiol increases LH secretion via positive feedback on the hypothalamus.
Result: LH surge triggers ovulation.

➖ Negative Feedback

Definition: A process where the response reduces or suppresses the original signal.
Effect: Maintains homeostasis by preventing overreaction or overstimulation.
Example: Insulin Regulation
High blood glucose → insulin released → glucose uptake by cells → blood glucose drops
Drop in glucose reduces insulin secretion (negative feedback).

🔁 Feedback Regulation Summary Table

Feedback Type	Definition	Effect	Example
Positive	Enhances the original signal	Amplifies the response	Oestradiol → LH surge
Negative	Suppresses the original signal	Maintains balance	Insulin → lowers blood glucose

📌 Key Points:
– Positive feedback amplifies signalling and promotes a rapid or irreversible response.
– Negative feedback stabilizes the system by limiting further signalling.
– These mechanisms are vital for precise control of cellular processes like reproduction and metabolism.