Types of Cell Signalling

Cells communication

Cell signalling is the way in which a cell communicates with itself or with another cell through signals emitted by them. Particularly, cells in multicellular organisms (like algae*, fungi*, bryophytes*, pteridophytes*, plants* and animals* etc) communicate with each other through signals that they emit to each other. These different types of signals stimulate the target cell to develop a certain function, therefore, they are essential for multicellular organisms. How cell signalling occurs? Signalling cells require some components which make it possible for the cell to initiate cell signalling pathway within itself or communicate with other cells. In the absence of these components, cells are unable to communicate and initiate biological response. These components are 1). the signalling cell, 2). the signalling molecule, also known as “ligand”, 3). and the target cell. Basic requirements of cells communication require only few components which enable cells to communicate with each others. However, cell signalling is divided into many types. In this post we will focus on different types of cells signalling.

For basic introduction of cell signalling please read the following post, “Introduction to Cell Signalling“

General Classification of Living Organisms

S.#	Types of Living Organism	Description of Organisms
1	*Fungai	are kingdom of heterotrophs (i.e., cannot synthesized their food) organisms. Mostly Fungai species are saprotrophic (i.e., an organism that feeds on or derives nourishment from decaying organic matter (Oxford Dictionary of English)
2	*Algae	are aquatic group of organisms. Unlike fungai kingdom, they synthesize their own food. They can not be differentiated like plants into ture roots, shots, leaves etc.
3	*Plants	are, (classified as; shrubs, herbs, grasses, mosses, ferns and, trees), habitually static living organisms. They are autotrophs (i.e., using photosynthesis mechanism to synthesize their nutrition in leaves from water, carbon dioxide in the presence of sunlight)
4	*Bryophytes	are non-vascular plants (i.e., absence of Xylem and Phloem tissue)
5	*Pteridophytes	are categorizes as vascular plants.
6	*Animals	are basically breathing organisms. The word “Animal” derived from Greek word “animalis” which stands for “having breath, having soul”.

Who synthesise Signalling Molecules?

Functionally, the signalling cell is responsible for the production of the signalling molecule, which, in turn, will be responsible for carrying information between the cells. Each cell is programmed to respond to certain signals. For this reason, they are equipped with receptors that recognize signalling molecules. These signalling molecules can be proteins, amino acids, hormones, and many other substances. The target cell will receive the signalling molecule, which will bind to specific receptors. These receptors can be on the membrane, known as extracellular receptor or inside the cell, known as intracellular receptor.

We have now solid but basic knowledge of two types of cells; the first type of cell which is responsible for initiating of signalling process, known as “signalling cells”and second type of cells which initiates biological response according to the received signalling molecule, known as “target cells”

Basic steps in cell signalling

The whole cell signalling process accomplish in the following common steps.

1. Synthesis of signalling molecules. The signalling cells synthesize and release the signalling molecule.
2. Transportation of signalling molecules. The signalling molecule moves towards the target cell, which may or may not be located close to the signalling cell.
3. Reception of signalling molecules. The signalling molecule binds to a specific receptor located on the target cell.
4. Biological response of target cell. A signal is issued; Modifications in cell metabolism ensure a cellular response.

Types of cell signalling

Signalling between signalling cells and targets cells are mainly differentiated by the route established by the signalling molecule until it reaches the target cell. Therefore, we have five major different types of cell signalling.

1. Autocrine signalling. The signalling molecule is produced by a signalling cell, which in turn received by the producing cells and initiate some biological activity. In this signalling cell is also the target cell. 
   • Autocrine signalling = (Signalling cells = Target cells)
2. Paracrine signalling. In this type of signalling, the signalling molecule is released and acts on cells that are in closed proximity of the signalling cell. In paracrine signalling process, the signalling molecule finds the target cell through a process of diffusion*.
   • *Diffusion: The process of passive movement of molecules (from higher concentration towards lower concentration) between two adjacent regions.In paracrine signalling, movement of signalling molecules from signalling cells toward the target cells. Concentration of signalling molecules remains higher in signalling cells as compared to target cells.
3. Endocrine signalling. In endocrine signalling, the signalling molecules, such as proteins, hormones etc, are synthesized and released by signalling cells into the circulatory system (i.e., bloodstream) to reach and bind to specific receptors on distant target cells and trigger biological response accordingly.
4. Neuroendocrine signalling: this type of signalling occurs in specialized neurons, which release neurohormones, which will be released into the bloodstream, triggering a response in distant target cells
5. Synaptic signalling. Synaptic signalling is special types of signalling. In these types of signalling, it is observed that signalling molecules, such as neurotransmitters, are launched at specialized junctions between neurons and target cells, called synapses*.
   • *Synapses are contact points of sharing information between two neurons. Both neurons connect with each other via axons and dendrites. Synapses are small gaps between two adjacent neurons.