In this tutorial, you will learn about what is cell signaling and its involvement in biological pathways. You will also learn about different types of cell signaling and their processes.
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Overview of Cell Signaling
Cell signaling correlates with cellular communication (the process of cells releasing and sending signals). It is the vast network of communication between and within the cells in our bodies. Cells communicate using cell signals through signal transduction pathways. The chemical signals can be proteins or other molecules produced by sending cells. The signals are secreted from the cell and then released in the extracellular space (space around cells).
The cell that receives the “message” is the target cell (or receiving cell). However, not every cell can detect that particular “message”. A neighboring cell must have a receptor for that signal, where it can bind and alter the shape or activity of the receptor. These signaling molecules are often called as ligands (term for molecules that bind to other molecules).
Signal Transduction Pathway
Signal transduction is the process by which cells respond to ligands. Cells can have intracellular (within the cell) receptors which bind to the ligands. However, in most cases the receptors are extracellular (outside the cell). These receptors are located on the cell membrane and the signal then has to be passed through other molecules within the cell.
One common signal transduction pathway found in many different organisms is G Protein Signaling.
Intracellular signal transduction pathways are the chains of molecules inside the cell that aid in relaying signals from extracellular receptors.
Process of Cell Signaling
Cell signaling varies based on the types of receptors and signaling molecules used. However, there is a common pattern that emerges.
- First, the cell releases a signal in response to certain conditions (temperature, other signaling molecules, etc.).
- Cell produces the signal. Some molecules are actively produced while others (neurotransmitters) are stored and readily released.
- These signals either stay within a cell or travel outside the cell to find their target proteins.
- The receptors bind to the signaling molecule where they transduce the signal. The protein binds to the signaling molecules, changes its shape, and stimulates a particular process within the cell. The transduction of the message within the cell often involves ATP.
- Lastly, the signaling molecule will unbind from the receptor protein, stop the transduction pathway, and allow the protein to return to its original configuration.
Types of Cell Signaling
The most common cell signaling is that between cells that are next to one another. However, not all sending and receiving cells are by one another. The four basic categories of signaling for multicellular organism are listed below.
- Paracrine signaling
- Autocrine signaling
- Endocrine signaling
- Signaling by direct contact
Paracrine signaling occurs when cells need to communicate over short distances. This happens when cells release chemical messengers (which can diffuse through the extracellular space) to other cells.
Paracrine signaling allows cells located in close proximity to coordinate their activities with neighbors. Even though these signals are important in many different tissues and processes, they are very important during development. This is because they allow for cells to tell their neighbors what identity to take.
Autocrine signaling is when a cell signals itself. The cell releases a ligand that binds to the surface of its cell.
This signaling is important in development as it helps cells take on their specific identities. It is extremely important in cancer and plays a role in metastasis (cancer spreading from one part of the body to another).
Endocrine signaling is used in long-distance signal transmission. The signals come from specialized cells that are released into the bloodstream, this carries the signal to target cells in different parts of the body. Also, these signals are hormones which are released from endocrine glands.
These endocrine glands are in the thyroid, hypothalamus, pituitary, pancreas, and the gonads (reproductive organs). As a result, these glands can release one or more types of hormones, usually which regulate development and physiology.
Cell-to-Cell Contact Signaling
There are gap junctions (animal cells) or plasmodesmata (plant cells) that are tiny channels that connect neighboring cells. Small signaling molecules and ions are able to move through these channels, while large molecules cannot.
As a result, the signaling molecules are able to transmit the status of one cell to its neighbor. This process allows for a group of cells to have coordinated responses to a signal one cell receives.
Another form of direct cell-to-cell signaling involves the binding of two cells to one another, due to complementary proteins located on their surfaces. This interaction causes the shape of one or both proteins to change to transmit a signal. This process serves important functions in the immune system.