IB DP Biology 2025: Interaction and Interdependence
C2.1 — Chemical Signaling
Theme C: Interaction and Interdependence · HL Only
C2.1.1 / C2.1.3 HL Only
Receptors and Functional Diversity
Key UnderstandingReceptors are proteins with specific binding sites for signaling chemicals (ligands). Functional categories of signaling chemicals in animals include hormones, neurotransmitters, cytokines, and calcium ions.
Chemical signaling relies on the interaction between a signaling molecule (ligand) and its specific receptor. Like an enzyme-substrate complex, this interaction is highly specific. When a ligand binds to a receptor, it causes a conformational change that triggers a cellular response.
Ligand
A signaling molecule that binds specifically to a receptor site on another molecule, initiating a response.
Receptor
A specialized protein on the cell surface or inside the cell that detects a specific ligand and undergoes a conformational change.
| Category | Source | Function |
|---|
| Hormones |
Endocrine glands |
Travel via blood to distant target cells to regulate prolonged physiological processes (e.g., growth, metabolism). |
| Neurotransmitters |
Neurons |
Cross synaptic clefts to act on adjacent neurons or muscle cells for rapid, localized communication. |
| Cytokines |
Immune cells |
Mediate and regulate immunity, inflammation, and hematopoiesis. They coordinate the immune response. |
| Calcium Ions (Ca²⁺) |
Stored internally (e.g., ER) |
Act as ubiquitous intracellular second messengers involved in muscle contraction, neurotransmitter release, and gene expression. |
Application
Drugs often act by mimicking natural ligands (agonists) or blocking receptors (antagonists) due to their structural similarity to endogenous signaling molecules. For example, beta-blockers block adrenaline receptors to slow heart rate.
C2.1.2 HL Only
Bacterial Quorum Sensing
Key UnderstandingCell signaling occurs in bacteria, exemplified by quorum sensing (e.g., Vibrio fischeri and bioluminescence).
Bacteria are not merely solitary organisms; they can communicate and coordinate behavior based on population density. This density-dependent communication is called quorum sensing. It allows a population of bacteria to act collectively, akin to a multicellular organism.
Autoinducer
Signaling molecules produced by bacteria that increase in concentration as cell density grows, triggering group behaviors.
Low Density: Autoinducers diffuse away, concentration remains low
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High Density (Quorum): Autoinducer concentration increases rapidly
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Autoinducers bind to intracellular receptors, altering gene expression
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Result: Synchronized population response (e.g., Bioluminescence, Biofilms)
A classic example is Vibrio fischeri, a marine bacterium that colonizes the light organ of the bobtail squid. Only when a quorum is reached do the bacteria switch on genes for bioluminescence. Quorum sensing is also critical for pathogenic bacteria to coordinate the release of toxins or form resistant biofilms.
Skills
Be able to describe how individual bacterial cells behave differently depending on whether they are solitary or in a dense population.
C2.1.6 / C2.1.7 HL Only
Transmembrane vs. Intracellular Receptors
Key UnderstandingReceptors may be transmembrane (in plasma membranes) or intracellular (in cytoplasm/nucleus). Binding initiates signal transduction pathways.
The chemical nature of a ligand determines the location of its receptor. The plasma membrane's hydrophobic core acts as a barrier to polar molecules but allows non-polar molecules to pass through easily.
Transmembrane Receptors
Bind to hydrophilic, polar, or large ligands (e.g., protein/peptide hormones, most neurotransmitters) that cannot cross the hydrophobic core of the plasma membrane.
- Receptor spans the plasma membrane (integral protein).
- Binding causes a conformational change on the intracellular side.
- Initiates a Signal Transduction Pathway inside the cell using second messengers.
- Response is typically rapid and reversible.
Intracellular Receptors
Bind to hydrophobic, non-polar, or small ligands (e.g., steroid hormones like testosterone, dissolved gases like NO) that easily diffuse across the plasma membrane.
- Found in the cytoplasm or the nucleus.
- Ligand-receptor complex enters the nucleus.
- Often acts directly as a transcription factor to alter gene expression.
- Response is typically slower but long-lasting.
C2.1.10 HL Only
GPCRs and Epinephrine Action
Key UnderstandingThe mechanism of action of epinephrine receptors involves G proteins and cyclic AMP (cAMP) as a second messenger.
G-Protein Coupled Receptors (GPCRs) are a massive and diverse family of transmembrane receptors characterized by seven trans-membrane alpha-helices. Epinephrine (adrenaline) uses this mechanism to rapidly trigger the "fight or flight" physiological response without entering the cell.
Second Messenger
Small intracellular molecules (like cAMP or Ca2+) that relay and amplify signals received at receptors on the cell surface.
1. Epinephrine binds to GPCR (causing a conformational change)
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2. GPCR activates a G-protein (GDP is exchanged for GTP)
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3. Activated G-protein subunit separates and activates adenylyl cyclase
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4. Adenylyl cyclase converts ATP into cAMP (Second Messenger)
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5. cAMP activates Protein Kinase A (PKA), triggering a phosphorylation cascade
Signal Amplification: A single epinephrine molecule can result in the activation of millions of enzymes because each step in the cascade multiplies the number of activated molecules.
C2.1.12 / C2.1.13 HL Only
Intracellular Receptors & Gene Expression
Key UnderstandingIntracellular receptors affect gene expression. Oestradiol and progesterone exert their effects on target cells this way.
Steroid hormones are lipids synthesized from cholesterol. Because they are hydrophobic and non-polar, they can easily diffuse through the phospholipid bilayer of cell membranes. They bypass transmembrane receptors and enter the cell directly to influence cellular activity at the genetic level.
Mechanism of Steroid Hormones:
- Oestradiol and Progesterone (female sex hormones) diffuse across the plasma membrane of target cells (e.g., in the uterus or breasts).
- They bind to specific intracellular receptors located in the cytoplasm or nucleus.
- The hormone-receptor binding causes the receptor to change shape, forming an active transcription factor complex.
- This complex binds to specific DNA control sequences (promoters or enhancers).
- It directly regulates (promotes or inhibits) the transcription of mRNA, thereby altering the synthesis of specific proteins.
Note: Because steroid hormones require the transcription of mRNA and the subsequent translation of new proteins, their physiological effects take much longer to manifest (hours to days) compared to the rapid second-messenger cascades of peptide hormones (seconds to minutes).
Self-Assessment
Check Your Understanding
Test your knowledge of the C2.1 Chemical Signaling topics covered in this module.
1. Why do peptide hormones require transmembrane receptors while steroid hormones do not?
Peptide hormones are hydrophilic (polar) and cannot cross the hydrophobic core of the phospholipid bilayer, so they must bind to receptors on the cell surface. Steroid hormones are hydrophobic (non-polar) lipids that can freely diffuse across the membrane and bind to intracellular receptors.
2. Describe the role of a second messenger in a signal transduction pathway.
A second messenger (like cAMP or Ca2+) is a small intracellular molecule generated inside the cell in response to the binding of a first messenger (ligand) to a surface receptor. It relays and heavily amplifies the signal throughout the cytoplasm, activating kinases and triggering a widespread cellular response.
3. Explain how quorum sensing allows bacteria to coordinate their behavior.
Bacteria release signaling molecules called autoinducers. At low population densities, these diffuse away. At high densities, the local concentration of autoinducers increases sharply. Once a threshold (quorum) is reached, they bind to receptors inside the bacteria, simultaneously altering gene expression across the population to trigger group behaviors like bioluminescence or biofilm formation.
4. Outline the steps of the epinephrine signaling pathway via GPCR.
1. Epinephrine binds to a GPCR. 2. The GPCR activates a G-protein (exchanging GDP for GTP). 3. The G-protein activates adenylyl cyclase. 4. Adenylyl cyclase converts ATP into cAMP. 5. cAMP activates Protein Kinase A (PKA), which triggers a phosphorylation cascade leading to the cellular response.
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C2.1 Chemical Signaling Complete!
You have successfully reviewed all HL understandings for Biology Theme C2.1.
