All the cells in our bodies communicate with each other all the time. Every cell in the multicellular organism’s body sends and receives millions of messages through chemical signalling molecules. A process by which cells in multicellular organisms send and receive messages from each other or with the external environment is called cell signalling. It is found in all multicellular organisms and is absent in unicellular. Because of cell signalling, our body acts as one unit rather than as a collection of individual cells acting independently.

Ligands & Receptors
The cell that wants to communicate with other cells sends a signalling molecule called ligands. This molecule initiates signalling and triggers a specific response in the target cell. Ligands can be proteins, lipids, amino acids, metabolites, gas, or any other molecule. It is carried by blood or tissue fluid towards target cells. Signalling molecules or ligands pass through thousands of cells; however, they bind only with a specific protein called a receptor (located in/on the target cell). Every ligand requires specific receptors.

Events of cell signalling at the  target site;
Events of cell signalling at the target site are divided into three stages: reception, transduction, and response.
Reception: Reception is the target cell’s detection of a signalling molecule coming from outside the cell. A chemical signal is “detected” when the signalling molecule binds to a receptor protein located at the cell’s surface (or inside the cell).
Transduction: The binding of the signalling molecule changes the receptor protein in some way, initiating the process of transduction. The transduction stage converts the signal to a form that can bring about a specific cellular response.
Response: In the third stage of cell signalling, the transduced signal finally triggers a specific cellular response. Different ligands trigger different responses, such as catalysis by an enzyme, rearrangement of the cytoskeleton, or activation of specific genes in the nucleus.

Pathway of a signal from outside to inside;
Transduction sometimes occurs in a single step but more often requires a sequence of changes in a series of different molecules called the signal transduction pathways. The molecules in the pathway are called relay molecules.
Protein Signaling
Protein and peptide hormones are water-soluble, so they cannot pass through the plasma membrane of the cell. These hormones or environmental stimuli are the first messenger. They bind with their receptors on the plasma membrane of the target cell, starting a series of events in the cell that generate a second messenger, e.g., cAMP (cyclic adenosine monophosphate). The second messenger then triggers various changes in the cell, including activation of enzymes, gene activation, etc.

Federal Board (FBISE) Islamabad | National Books Foundation Biology Notes