How Cells Convert External Signals Into Internal Responses
Multicellular organisms rely on precise communication between cells to coordinate growth, metabolism, immunity, and homeostasis. This communication often begins with a ligand–receptor interaction at the cell surface or inside the cell. When a ligand binds to its receptor, it triggers a series of biochemical events known as an intracellular signalling cascade. Understanding how these cascades work is essential for IB Biology students studying cell communication and regulation.
A ligand is any molecule that binds to a receptor. It can be a hormone, neurotransmitter, cytokine, or other signalling molecule. Receptors, usually proteins, are highly specific and only bind ligands with matching shape and chemistry. This specificity ensures that each signal produces the correct response.
When a ligand binds, it causes a conformational change in the receptor. This change activates the receptor, allowing it to interact with other molecules inside the cell. The activated receptor often initiates a cascade, where one activated molecule activates another, amplifying the signal at each step.
Different types of receptors trigger signalling in different ways:
1. G Protein–Coupled Receptors (GPCRs)
When activated, GPCRs stimulate G proteins, which in turn activate enzymes such as adenylyl cyclase. This leads to the production of second messengers like cyclic AMP (cAMP), which activate protein kinases that regulate metabolism, gene expression, or ion channels.
2. Receptor Tyrosine Kinases (RTKs)
These receptors dimerize and autophosphorylate when ligands bind. The phosphorylated regions serve as docking sites for intracellular proteins, initiating pathways such as the MAP kinase cascade, which often regulates cell growth and differentiation.
3. Ligand-Gated Ion Channels
Binding opens ion channels, allowing ions to flow into or out of the cell. Changes in ion concentration can trigger electrical signals or downstream biochemical responses.
