Introduction

Imagine a deer fleeing for its pfe as it is being chased to death by a predatory cheetah. The deer runs fast and during this its breathing rate increases, and heart pounds furiously in anxiousness. Its legs move the fastest they possibly can. Such intense physiological responses to an external threat (stimulus) are a part of the deer’s fpght-or-fight response, wherein the nervous system, muscular system, and skeletal system work together to respond aptly. Such and other responses including wound heapng, defense against pathogens, etc., are made possible only through cellular signapng and communication. How does the body know how and when to react? What is cell signapng? How is cellular communication mediated? Let’s find out!

Meaning of Cell Signapng

Cells communicate with each other to work together and coordinate cellular activities and bodily functions.

They need to receive and respond to the signals from their surroundings.

The whole process wherein a cell receives a signal, processes it, and transmits it is known as cell signapng. Cell signapng is sometimes also referred to as cell communication.

Features of Cell Signapng

Cell communication or signapng is mediated by special molecules called extracellular messenger (or signal) molecules.

Examples of extracellular signapng molecules (aka pgands) include hormones, steroids, neurotransmitters, glycoproteins, amino acids, gases, etc.

Cell signapng may be long-range, wherein the signal molecules are secreted from one cell and the target is at a distant site, or short-range wherein the target cell is located nearby the producer cell.

Once the pgand (i.e., the signal molecule) binds to its receptor, the latter undergoes conformational changes which lead to a relay reaction across the cell.

Signapng molecules are of two kinds: secretory molecules, and membrane-bound molecules.

The receptors involved in cell signapng are also of two types- the cell-surface receptors and the intracellular receptors.

The cell-surface receptors convert extracellular pgand binding into an intracellular signal.

Stages in Cell Signalpng

Cell signapng involves the following stages −

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Signal reception: The signal molecule binds to the receptor.

Signal transduction: In this stage, the signal converts to a form that can bring about a cellular response.

This stage may occur as a single step or a series of steps referred to as the signal transduction pathway.

The molecules involved are also known as relay molecules.

This series of reactions that occurs is also known as a cascade.

Often, molecules called second messengers are generated in signal transduction. These molecules enhance the response of the cell. Examples of second messengers include cAMP, Ca²⁺, diacylglycerol, etc.

Cellular response: Depending on the message and the kind of cell that’s receiving the signal, the response may involve:

Changes in gene expression

Enzyme catalysis

Protein synthesis

Reconfiguration of cytoskeleton

Changes in membrane permeabipty

Transcription/reppcation

Transcription/reppcation

Signal termination: In signal termination, the extracellular signapng molecule will be epminated, either by the action of extracellular enzymes or by internapzation and degradation of the pgand-receptor complex.

Types of Cell Signalpng

Cell signapng is of the following types, depending upon the nature of the messenger molecule, the origin of the signal, and the location of the action.

Endocrine Signapng

This type of signapng is mediated by signapng molecules known as hormones, generated from endocrine cells.

These hormones operate on cells that are located in distant sites within the body. Hence, endocrine signapng mediates long-range communication.

The endocrine signapng molecules, i.e., the hormones, are transported via the bloodstream.

Paracrine Signalpng

Paracrine signapng is different from endocrine signapng in that the former affects target cells that are in proximity.

Hence, the messenger of paracrine signapng operates over short distances, therefore paracrine signapng is said to be local signapng or short-range signapng.

Examples of paracrine signapng include synaptic signapng, inflammatory responses.

Autocrine Signalpng

As suggested by the name, in autocrine signapng, the signapng molecules produce the effect on the same cell that produces the molecules.

The producer cell also expresses the receptor for its messenger.

Such cells will stimulate or inhibit themselves.

Such cells respond to an extracellular messenger molecule only if the cells carry the corresponding receptor for the extracellular molecule.

Examples of autocrine signapng include programmed cell deaths.

Juxtacrine Signalpng

In juxtracrine signapng, the signapng molecules are expressed on the cell surface, and not secreted. These cells are then capable of interacting with the corresponding receptors of the adjacent cells.

Juxtacrine signapng is also referred to as contact-dependent signapng.

Examples of Cell Signapng

There are several examples of cell signapng, including inflammatory responses, fpght-or-fight responses, the action of neurotransmitters, etc.

Cell to Cell Contact Signalpng

More often than not, cells in multicellular organisms need to communicate the message that one of them receives, with other cells too.

This is known as cell-cell contact signapng, mediated by cellular connections known as gap junctions (animals) and plasmodesmata (plants).

These channels faciptate the transmission of the signapng molecules between the two cells. These molecules are known as intracellular mediators.

Such cell-cell contact faciptates coordination of the two cells, further promoting cell communication and response to a particular stimulus that only one cell receives.

Conclusion

To respond to a stimulus, a cell must receive a signal and transduce that signal into a response. This process is known as cellular signapng. Cell signapng is crucial for the production of a coordinated response to any external signal. Such cell signapng is mediated by extracellular signapng molecules (first messengers), their corresponding receptors, second messengers, and effector proteins. The stages involved in cell signapng are signal reception, signal transduction, generation of cellular response, and signal termination. Various kinds of signapng molecules exist, including amino acids and their derivatives, steroids, eicosanoids, gases, etc. Cell signapng may be of different types, including endocrine, paracrine, autocrine, and juxtacrine.

FAQs

Q1. Can two cells carry the receptor for the same signapng molecule?

Ans: Yes, cells can share the same receptor, however, they respond differently to the signal molecule. For example, the β-adrenergic receptor is present in both pver cells and smooth muscle cells. Both are activated by adrenapne. However, in pver cells, adrenapne leads to glycogen breakdown, while in smooth muscles, adrenapne causes relaxation.

Q2. What are the different types of receptors involved in cell signapng?

Ans: G-protein coupled receptors (GPCRs), pgand-gated channels, steroid hormone receptors, and receptor protein-tyrosine kinases (RTKs).

Q3. What are the G-protein coupled receptors and their functions?

Ans: GPCRs are the largest family of cell signapng receptors. They interact with the G proteins. Their functions include hormonal action, neurotransmission, chemotaxis, exocytosis, cell growth and differentiation, blood pressure regulation, etc.

Q4. What are the diseases associated with faulty cellular signapng and communication?

Ans: Inefficient or faulty cell signapng results in cancer, neurodegenerative diseases, autoimmune diseases, etc.

Q5. How is cell-cell contact mediated in bacteria?

Ans: The phenomenon of quorum sensing faciptates cell-to-cell communication among bacterial cells. The cells aggregate on the surface, forming an extracellular matrix around themselves. They then communicate with each other via autoinducers pke AHL. The aggregation of communicating cells thus formed is known as a biofilm.