Cells need to interact with their environment and other cells around them. This is called Cell Signalling. Single cellular organisms need to detect nutrients in their environment, and cells in multicellular organisms are involved in a complex system of communication with each other.
Cells detect signals with Cell Receptors on their plasma membrane, which are usually Glycoproteins or Glycolipids. The signalling molecule binds to the Repeptor because its shape is complementary. This then instigates a chain of reaction withing the cell, leading to a response.
Cell Signalling Pathways can be categorised based the distance over which the signalling occurs.
Endocrine Signalling involves signalling over large distances, often where the signalling molecule is transported in the circulatory system
Paracrine Signalling occurs between cells which are close together, sometime directly, sometimes via extracellular fluid
Autocrine Signalling is where the cell stimulates a response within itself by releasing signals for its own Receptors
Hormones are often used as cell signalling molecules in multicellular organisms. Hormones are produced in a cell, sometimes in response to environmental changes. The Hormones are are released and bind to Receptor Sites on a Target Cell, which starts a response.
An example of a hormone mediated cell signalling pathway is in the use of Insulin to lower blood glucose levels. In response to high glucose levels, Beta-Cells in the pancreas release the hormone Insulin in to the blood, which binds to cells such as muscle and liver cells. This causes them to take up more glucose.
Some Medicinal Drugs work because they are complementary to certain Cell Receptor Sites. Some drugs block these Receptors so that they natural signalling molecules cannot instigate a response. Others are designed to mimic natural signalling molecules that the body cannot produce, such as drugs to treat some mental conditions.
Viruses invade cells by binding to Cell Receptor Sites that are normally used in cell signalling. They themselves have Receptor Sites, despite not being cells. Some poisons also bind to Cell Receptors, preventing the targeted cells from working properly.