Chapters
In this article, we will discuss cell signalling in detail. We will specifically focus on the main stages involved in the process of cell signalling leading to specific responses. So, let us get started.
Cell Signalling
The process by which messages are sent to the cells is referred to as cell signaling.
Cells should interface not only with their environment but also with other cells around them. Unicellular organisms (singular cellular organisms) should identify nutrients in their environment. On the other hand, multicellular organisms participate in a complicated communication system.
It is an extremely critical process because it enables multicellular organisms in controlling their bodies and be responsive to their environments. The activities of the cells are coordinated by the cell signalling pathways, even if they are far away within the organism.
Cells identify signals with cell receptors on their plasma membrane which are typically glycolipids and glycoproteins. Due to the complementary shape of the receptor, the signalling molecule binds to the receptor. This then triggers a chain of reactions within the cell, leading to a response.
In the next section, we will discuss the main stages of the cell signalling pathway.
Main Stages of Cell Signalling Pathway
The primary stages of the cell signalling pathway are:
- The receptor receives a stimulus or signal
- The signal is transformed into a message that can be passed on. This process is referred to as transduction
- The message is passed on to a target, also known as an effector
- A proper response is made
Messages that are transmitted in cell signalling pathways need barrier crossing, such as cell surface membranes. Hence, cell surface membranes play a crucial role in signalling pathways because the membrane controls the movement of molecules between the internal and external environments of the cell.
Usually, the signalling molecules are extremely small for easy transmission across the cell membranes. Generally, in signalling pathways, signalling molecules are required to cross or interact with cell membranes.
The categories of the cell signalling pathways are based on the distance over which the signalling occurs.
- Endocrine signalling refers to the signalling over large distances, generally where the signalling molecule is moved in a circulatory system
- Paracrine signalling takes place between the cells that are close to each other. The cells can be close together sometimes directly or sometimes through extracellular fluid
- Autocrine signalling occurs when the cell releases signals for its receptors to stimulate a response within itself
In multicellular organisms, hormones are often employed as signalling molecules. They are produced in a cell, sometimes in response to changes in the environment. The hormones initiate the response by binding to receptor sites on a target cell when they are released. The working of some medicinal drugs is attributed to the fact that they are complementary to some cell receptor sites.
Viruses attack cells by binding to cell receptor sites that are generally employed in cell signalling. Although they are not a cell, however, they have receptor sites. Some poisons also bind to receptor sites, thus stopping the targeted cells from working properly.
What are Ligands?
Signalling molecules are often referred to as ligands. Ligands participate in the following stages of a cell signalling pathway:
- The sending cell secretes them into the extracellular space
- They are then transmitted through the extracellular space to the target cell
- They bind to surface receptors that are meant for that ligand on the target cell. Remember that these receptors are created from glycolipids and glycoproteins
- A chain of chemical messengers inside the cell relays the message carried by the ligand and thus triggers the response.
Thus we can say that the primary stages of the cell signalling that leads to certain responses include secretion of certain chemicals (ligands) from cells, transportation of ligands to the cells, and binding of ligands to cell surface receptors on target cells.
In the next section of the article, we will discuss the nature of the cell signals.
Nature of Cell Signals
The majority of the cell signals are chemical. For instance, the sensors in prokaryotic organisms help to identify nutrients and assist them to steer towards sources of food. Examples of signals in multicellular organisms include hormones, growth factors, extracellular matrix components, and neurotransmitters.
Either these substances can exert their effects locally or they may move over long distances. For example, neurotransmitters refer to the class of short-range signalling molecules that move across small regions between adjacent neurons or between neurons and muscle cells. Other signalling molecules must travel further to reach their destinations. An example of such a signalling molecule is the follicle-stimulating hormone which moves from the mammalian brain to the ovary where it causes the release of an egg.
Some cells are also responsive to mechanical stimuli. For instance, the skin has sensory cells which respond to the pressure of touch. On the other hand, the same cells in the ear react to the movement of sound waves. Additionally, specialized cells in the human vascular system identify changes in blood pressure which is the information used by the body to maintain a constant cardiac load.
The Role of Receptors in Cell Signaling
The proteins known as receptors in the cells bind to signalling molecules and start a physiological response. Various receptors are specific to different molecules. For instance, insulin receptors bind insulin, dopamine receptors bind dopamine, etc. Receptors are generally transmembrane proteins that bind to signalling molecules outside the cell. They transfer signals through a sequence of molecular switches to internal signalling pathways.
Membrane Receptors
There are three main categories of membrane receptors:
- G-protein coupled receptors
- Ion channel receptors
- Enzyme-linked receptors
The names of these receptor classes mean the mechanism by which the receptors convert the external signal into internal ones through protein action, ion channel opening, or activation of the enzyme. As membrane receptors interface with both extracellular molecules and signals within the cell, they allow signalling molecules to influence the function of the cell without entering the cell. This is crucial because the majority of the signalling molecules are too large or too charged to cross the plasma membrane of the cell.
Did you like this article? Rate it!
Loading...
Keep on teaching us,you are excellent teachers
This is great
Thanks a lot for this book,it really helped me a lot
It’s useful to me
Thanks a lot for your Better book!
It’s a perfect article, go ahead