Introduction to Chemical Signalling
Chemical signalling is the fundamental mechanism by which cells communicate with each other and coordinate biological responses across an organism. From bacteria coordinating group behaviour to hormones travelling through the bloodstream to distant target tissues, chemical signalling underpins virtually every physiological process.
Receptor: A receptor is a protein that detects and responds to specific signals. Receptors bind to signalling molecules (ligands) to initiate a cellular response.
Ligand: A ligand is a molecule that binds to a specific site on a receptor protein, triggering a conformational change and initiating a cellular response.
Key types of signalling chemicals include:
- Neurotransmitters (e.g., acetylcholine, glutamate)
- Hormones (e.g., insulin, oestradiol, adrenaline)
- Cytokines (immune signalling proteins)
- Calcium ions (Ca²⁺, intracellular second messengers)
- Nitric oxide (NO, a gaseous signalling molecule)
Imagine you're in a crowded room and someone calls your name , you immediately turn to see who it is. Cells "listen" for specific signals in exactly the same way: receptors are tuned to detect only their matching ligand, ignoring everything else around them.
The binding of a ligand to its receptor is highly specific , like a lock and key , ensuring that only the correct signal triggers the appropriate response. This specificity is critical for the accuracy of biological communication.
An important dimension of specificity is tissue-specific response: different cell types express different sets of receptors, so the same signalling molecule can produce different effects in different tissues. For example, epinephrine causes heart muscle to beat faster while simultaneously causing blood vessels in the gut to constrict , because the two cell types express different receptor subtypes that activate different downstream pathways.
Receptor Location: Transmembrane vs. Intracellular
Receptors are found in two main locations within (or on) cells, and this location is directly determined by the chemical nature of the ligand.
1. Transmembrane Receptors (Cell Surface Receptors)
- Span the entire plasma membrane, with extracellular and intracellular domains
- Contain hydrophobic amino acids in the membrane-spanning regions
- Bind hydrophilic signalling molecules (e.g., proteins, peptides, charged molecules) that cannot cross the phospholipid bilayer
- Examples: acetylcholine receptors, insulin receptors, GPCRs
2. Intracellular Receptors (Cytoplasmic/Nuclear Receptors)
- Located in the cytoplasm or nucleus
- Composed mostly of hydrophilic amino acids
- Bind hydrophobic signalling molecules (e.g., steroid hormones) that can diffuse through the phospholipid bilayer
- Examples: oestrogen receptor, progesterone receptor, androgen receptor
3. Special Case: Nitric Oxide (NO)
- NO is a small, non-polar gaseous molecule that diffuses freely through membranes
- Rather than binding a classical receptor protein, NO directly activates the intracellular enzyme guanylyl cyclase, which produces the second messenger cyclic GMP (cGMP)
- This triggers smooth muscle relaxation, including vasodilation , the mechanism exploited by drugs like Viagra
The hydrophobic core of the phospholipid bilayer acts as a selective barrier. Small, non-polar molecules like steroid hormones and nitric oxide pass through freely, while large or charged molecules cannot , they must bind to a receptor on the cell surface instead.
A quick rule: hydrophilic ligand → surface receptor; hydrophobic ligand → intracellular receptor. This is a common exam distinction.
The steps of receptor-ligand interaction follow a consistent pattern regardless of receptor type:
- Binding: Ligand binds to the receptor's specific binding site
- Conformational change: The receptor changes shape upon binding
- Signal transmission: The activated receptor initiates downstream cellular responses
