Covers how cells communicate through ligands and receptors, from the basic signalling idea and its evolutionary roots to direct junctions and classical signalling across membranes. It then sets out the steps after ligand binding and five receptor categories, including ion channels, catalytic, enzyme-associated, integrins, GPCRs and intracellular receptors. Closing sections compare five modes of signalling by distance, note how HIV exploits CD4, and give four mnemonics.
Topic 31 of 105 in the MU Sofia syllabus for Biology (Medicine, Year 1). The free AceMed note covers it in 18 sections, about 1,222 words (9 min of reading), with 14 flashcards.
In this note
Cell signaling. Cell receptors
What is Cell Signaling?
The Role of Receptors and Ligands
Evolutionary Origins of Cell Signaling
How Cells Communicate: Two Main Ways
Through Physical Connections (Intercellular Bridges)
Cell signaling is the communication system of life at the microscopic level. It allows cells to detect cues from their environment or other cells and respond accordingly. These cues could be nutrients, toxins, hormones, or signals from neighboring cells.
This communication is vital for survival, growth, development, immune responses, and repair. It helps maintain balance (homeostasis) in the body and is crucial in every stage of life, from a developing embryo to a healing wound.
However, when this communication goes wrong—like when cells receive the wrong signals or fail to respond—it can lead to serious diseases such as cancer, autoimmune disorders, or diabetes.
The Role of Receptors and Ligands
To understand how cells talk, imagine each cell is equipped with antennae-like structures on their surfaces called receptors—proteins specifically shaped to detect and bind to molecules called ligands.
Ligands are the signaling molecules. They can be hormones, neurotransmitters, or even environmental cues like toxins.
Receptors are selective—they recognize specific ligands, much like a lock and key.
This receptor-ligand interaction forms the first step of cell signaling, triggering internal changes in the cell (a process called signal transduction) that ultimately results in a reaction—like changing gene expression, moving the cell, or producing a protein.
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