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.
4. Intracellular Receptors
Some ligands, such as steroid hormones, pass through the membrane and bind receptors in the cytoplasm or nucleus. These receptor–ligand complexes act as transcription factors to regulate gene expression directly.
A key feature of signalling cascades is amplification. A single ligand-binding event can activate many downstream molecules, allowing cells to respond powerfully even when ligand concentration is low.
Signalling cascades also allow fine control of responses. Molecules can be activated, inhibited, or modified through phosphorylation, dephosphorylation, or feedback loops. This flexibility allows cells to respond dynamically to internal and external conditions.
Finally, cascades often converge on the nucleus, where they alter gene expression. This leads to long-term changes in cell behavior, such as growth, division, or differentiation. Alternatively, cascades may affect immediate responses such as enzyme activity or ion movement.
Overall, ligand–receptor interactions convert external information into coordinated internal responses, enabling the complex regulation required for multicellular life.
FAQs
Why are signalling cascades so complex?
Complexity allows amplification, regulation, and integration of multiple signals. These cascades enable precise control over cell responses.
What is a second messenger?
A second messenger is a small molecule, like cAMP or calcium ions, that transmits signals inside the cell after ligand–receptor binding.
How do cells ensure signalling specificity?
Receptors only bind specific ligands, and each cascade involves unique proteins that ensure the correct cellular response is triggered.
Study Cell Communication with RevisionDojo
RevisionDojo provides clear, exam-focused explanations that help IB Biology students master signalling pathways and cell communication. With structured notes and student-friendly support, you can revise effectively and build confidence. Strengthen your biology study with RevisionDojo today.
