Chemical Signalling: Interaction and Interdependence

C2.1 Chemical Signalling: First Exams 2025

Theme: Interaction and Interdependence
Level of Organisation: Cells

IB Guiding Questions

  • How do cells distinguish between the many different signals that they receive?

  • What interactions occur inside animal cells in response to chemical signals?

Receptors as proteins with binding sites for specific signalling chemicals (C2.1.1)

  • Definition: Receptors are specialized proteins that bind specific signalling chemicals, also known as ligands, at their binding sites.

  • Specificity: The binding is highly specific, meaning each receptor typically binds only to particular ligands, much like a lock and key.

  • Induced Fit: Upon ligand binding, the receptor often undergoes a conformational change (shape alteration) that initiates the cellular response.

Cell signalling by bacteria in quorum sensing (C2.1.2)

  • Definition: Quorum sensing is a communication mechanism in bacteria that allows them to regulate behaviors based on population density.

  • Mechanism: Bacteria release small signaling molecules called autoinducers. As the population density increases, the concentration of these autoinducers in the environment also increases.

  • Response: Once a threshold concentration is reached, bacteria detect these autoinducers, leading to coordinated gene expression across the bacterial population.

  • Regulated Behaviors: These coordinated behaviors include bioluminescence, biofilm formation, virulence factor production, and antibiotic resistance.

Hormones, neurotransmitters, cytokines, and calcium ions as functional categories of signalling chemicals in animals (C2.1.3)

  • Examples: Hormones (e.g., epinephrine), neurotransmitters (e.g., acetylcholine), cytokines, calcium ions.

  • Importance: Understanding these categories aids in comprehending cell signaling.

Chemical diversity of hormones and neurotransmitters (C2.1.4)

Hormones

  • Types: Amine hormones, peptide hormones, steroid hormones.

Neurotransmitters

  • Types: Amino acids, peptides, amines, nitrous oxide.

Localized and distant effects of signalling molecules (C2.1.5)

  • Hormones: Released into the bloodstream and act on distant targets.

  • Neurotransmitters: Act locally at synapses between neurons.

Differences between transmembrane receptors and intracellular receptors (C2.1.6)

  • Transmembrane Receptors: Embedded in plasma membranes; bind hydrophilic ligands.

  • Intracellular Receptors: Located within the cytoplasm or nucleus; bind hydrophobic ligands.

Initiation of signal transduction pathways by receptors (C2.1.7)

  • Definition: Signal transduction pathways commence when a ligand binds receptor proteins, triggering a cascade of cellular responses.

Transmembrane receptors for neurotransmitters and changes to membrane potential (C2.1.8)

  • Example: Acetylcholine receptors.

  • Mechanism: The binding of acetylcholine to its receptor, which is often a ligand-gated ion channel, causes a conformational change that opens the channel. This allows specific ions (e.g., Na^+) to flow across the membrane, changing the membrane potential. This change can lead to depolarization and potentially trigger an action potential in the postsynaptic neuron or muscle cell.

Transmembrane receptors that activate G proteins (C2.1.9)

  • G Proteins: Composed of three subunits: alpha (α), beta (β), and gamma (γ). In an inactive state, the α subunit is bound to GDP.

  • Activation: Occurs when a ligand binds to a G protein-coupled receptor (GPCR), causing the GPCR to activate the G protein. The α subunit then releases GDP and binds GTP, leading to its dissociation from the βγ subunits.

  • Signaling: Both the GTP-bound α subunit and the βγ complex can then interact with and activate various effector proteins (e.g., enzymes, ion channels) within the cell, initiating a signal transduction cascade.

  • Inactivation: The α subunit possesses intrinsic GTPase activity, hydrolyzing GTP back to GDP, which causes its reassociation with the βγ subunits, inactivating the G protein.

Mechanism of action of epinephrine (adrenaline) receptors (C2.1.10)

  • Process: Epinephrine binds to specific adrenergic GPCRs on the cell surface. This binding activates a G protein, specifically the Gs protein.

  • Effector Activation: The activated Gs-α subunit then stimulates the enzyme adenylyl cyclase, which converts ATP into cyclic AMP (cAMP), a crucial secondary messenger.

  • Downstream Effects: Elevated cAMP levels activate protein kinase A (PKA). PKA then phosphorylates various target proteins within the cell, leading to rapid metabolic changes such as glycogenolysis (breakdown of glycogen) and lipolysis (breakdown of fats), mobilizing energy stores for a "fight or flight" response.

Transmembrane receptors with tyrosine kinase activity (C2.1.11)

  • Example: Insulin receptors, typically receptor tyrosine kinases (RTKs).

  • Process: When insulin binds to its receptor, it causes the receptor molecules to dimerize (two receptors come together). This dimerization activates the intrinsic tyrosine kinase activity of the receptors, leading to autophosphorylation of tyrosine residues on the cytoplasmic tails of the receptors.

  • Signal Relay: These phosphorylated tyrosines then serve as binding sites for various intracellular signaling proteins, initiating a complex signaling cascade that includes the recruitment of adapter proteins and the activation of further kinases, ultimately leading to cellular responses such as glucose uptake and protein synthesis.

Intracellular receptors that affect gene expression (C2.1.12)

  • Example: Steroid hormones like testosterone, oestradiol, and progesterone.

  • Mechanism: Being hydrophobic, these hormones can readily diffuse across the plasma membrane into the cytoplasm. There, they bind to specific intracellular receptor proteins.

  • Gene Regulation: The hormone-receptor complex then translocates into the nucleus, where it binds to specific DNA sequences (hormone response elements) upstream of target genes. This binding acts as a transcription factor, either activating or repressing gene expression, thereby influencing protein synthesis and long-term cellular changes.

Effects of hormones oestradiol and progesterone on target cells (C2.1.13)

Oestradiol

  • Function: Stimulates the secretion of gonadotropin-releasing hormones (GnRH); promotes follicle development.

Progesterone

  • Function: Stimulates thickening of the endometrium in preparation for embryo implantation.

Regulation of cell signalling pathways by positive and negative feedback (C2.1.14)

Positive Feedback

  • Definition: Amplifies changes; e.g., oxytocin during childbirth enhances uterine contractions which further increase oxytocin levels.

Negative Feedback

  • Definition: Counteracts changes to maintain homeostasis; e.g., temperature regulation involves initiating processes to return body temperature to norm.

Key Terms

  • Receptor Proteins: Proteins that bind signalling chemicals (ligands).

  • Signalling Chemicals: Molecules involved in cell communication, such as hormones, neurotransmitters.

  • Ligand: A signalling chemical that binds to receptors.

  • Quorum Sensing: Bacterial communication mechanism regulating group behaviors.

  • Bioluminescence: Light emission by living organisms, often regulated by quorum sensing.

  • Autoinducers: Molecules that facilitate quorum sensing.

  • Hormones, Neurotransmitters, Cytokines, Calcium Ions: Categories of signalling chemicals.

  • Secondary Messengers: Intracellular signaling molecules released by the cell to trigger physiological changes.

  • Amine, Peptide, Steroid Hormones: Classes of hormones classified by their chemical structure.

  • Natural Selection: The evolutionary process where traits are passed based on survival benefits.

  • Synapse: The junction between neurons; site of neurotransmitter action.

  • G Protein, cAMP: Important components in cell signal transduction pathways.

  • Phosphorylation: The addition of a phosphate group to a molecule, often regulating activity.

Linking the IB Questions

  • What patterns exist in communication in biological systems?

  • In what ways is negative feedback evident at all levels of biological organization?