Cell Signalling in Biology

Cell Signalling (HL)

• Definition: Cell signalling is the process by which cells communicate with each other to coordinate activities and respond to their environment.
• Importance: Essential for multicellular organisms, enabling response to stimuli and coordination of cellular activities across different parts of the organism.

Basic Stages of Cell Signalling

• Reception: A stimulus or signal is received by a receptor.
• Transduction: The signal is converted into a form that can be transmitted within the cell.
• Transmission: The signal is sent to a target cell (effector).
• Response: The target cell responds appropriately to the signal.

Signalling Molecules

• Ligands: Molecules that act as signalling molecules, which can include:
  • Proteins and amino acids
  • Nucleotides
  • Steroids
  • Amines
• Mechanism:
  1. Ligands are secreted from a cell into the extracellular space.
  2. Ligands travel to target cells and bind to specific surface receptors (e.g., glycoproteins).
  3. The bound ligand initiates a response inside the cell via a chain of chemical messengers.

Quorum Sensing in Bacteria

• Definition: A communication process where bacteria monitor their population size and adjust gene expression accordingly.
• Mechanism:
  1. Bacteria release ligands that bind to receptors on other bacteria.
  2. As the population increases, more ligands are released.
  3. Once a threshold is met, it triggers changes in gene expression to signal when a quorum has been reached.
• Example: Vibrio fischeri, which lives in association with certain squid, enabling bioluminescence through quorum sensing.

Categories of Signalling Molecules

• Hormones: Chemical messengers produced by glands that regulate physiology (e.g., insulin, glucagon).
  • Hormones are transported in the bloodstream and affect only target cells with specific receptors.
• Neurotransmitters: Chemicals that transmit signals between nerve cells across synaptic gaps (e.g., acetycholine, dopamine).
  • Bind to receptors on postsynaptic neurons, opening ion channels and generating nerve impulses.
• Cytokines: Signalling proteins involved in immune responses and cell growth (e.g., interleukin, interferon).
• Calcium Ions (Ca²⁺): Act as second messengers in signal transduction pathways involved in muscle contraction and neurotransmitter release.

Chemical Diversity: Hormones & Neurotransmitters

• Types of Hormones:
  • Amines: Derived from amino acids (e.g., epinephrine, thyroxine).
  • Peptides: Short chains of amino acids (e.g., insulin).
  • Steroids: Lipid-soluble hormones (e.g., testosterone).
• Function: Hormones bind to specific receptors; the binding mechanism is crucial for cell signaling efficacy.

Signal Reception

Transmembrane Receptors

• Characteristics: Extend across the cell membrane, have extracellular binding sites, and involve hydrophilic and hydrophobic regions.

Intracellular Receptors

• Ligands such as steroid hormones can diffuse through the membrane and bind to receptors inside the cell, leading to gene expression changes.

Signal Transduction Pathways

1. Receptor binding: Ligand binds to receptor, causing a conformational change.
2. Internal signal: This triggers a cascade of intracellular events leading to cellular responses, such as metabolic changes or gene expression regulation.

Types of Receptors

• G-Protein Coupled Receptors (GPCRs):
  • Largest family of receptors; when activated, they use G-proteins to propagate an intracellular signal.
• Receptor Tyrosine Kinases (RTKs):
  • Phosphorylate tyrosine residues in response to ligand binding, initiating multiple signalling pathways.

Example of Signal Transduction: Epinephrine

• Epinephrine binds to liver cell receptors, activating adenylyl cyclase and converting ATP to cAMP, which activates protein kinases leading to glycogenolysis (glucose release).

Gene Expression Regulation by Hormones

• Steroid Hormones: Can directly result in gene expression regulation by forming ligand-receptor complexes.
• Example: Oestradiol affects numerous genes and is pivotal in sexual function regulation.

Regulation of Cell Signalling

• Negative Feedback Mechanisms: Maintain homeostasis by reversing changes in physiological factors (e.g., blood glucose levels).
  • Involve receptors detecting stimuli and effectors (muscles or glands) responding accordingly.
• Positive Feedback: Amplifies the original stimulus until a specific process is completed (e.g., during childbirth).

Summary

• Cell signalling is vital for biological functions, enabling organisms to communicate and respond to environmental changes. The diversity of signalling molecules and their mechanisms plays a significant role in physiological regulation across different systems in the body.