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Cells are the building blocks of life and they can detect whats going on around them. They are the living part of our body and they can respond to the activities going in the environment. Every moment the cells in your body are sending and receiving millions of messages. These messages are in the form of chemical signaling molecules. There are some basic principles with the help of which the cells communicate with one another.
Overview of cell signaling
Cells typically communicate via chemical signals, which are the proteins. Some molecules are produced by a sending cell and released into the extracellular space. They can float over to neighboring cells.
The Sending cell secretes a ligand and the Target cell has a receptor that can bind to the ligand which was produced. The ligand binds to the receptor and triggers a signaling expression inside the cell which causes the response. The Non-target cell does not have a receptor for the ligand but they may have other kinds of receptors. Therefore, the cell does not perceive the ligand which was produced and thus does not respond.
All the cells in the body cannot “hear” the particular chemical message is produced. To detect a signal by the target cell, a neighbor cell must have the right receptor for the signal produced. During the binding with the signal molecule to its receptor, the shape or activity of the receptor is altered, and there is a change inside the cell. Signaling molecules are called ligands, a general term for molecules that bind specifically to the receptors.
Ligand carries a message that often relayed through a chain of chemical messengers of the cell. The result is change in the cell, such as alteration in the activity of a gene. In some cases there is an induction of a whole process, like cell division.
Principles of cell signaling
The principle of cell signaling is based on the fact that cellular communication involves converting signals. The signals involved are the ones that carry information from one place to another. The signaling cell releases a particular signaling molecule during the cell communication that is detected by the target cell.
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Animal cells send and receive signals therefore; they act as both the signaling and the target cells. Animal cells can mainly communicate through the direct contact. They can also secrete local regulators, neurotransmitters.
There are three stages of cell signaling:
- Reception– During this stage the target cell detects a signaling molecule which is present in the exogenous environment.
- Transduction– In this stage the conversion of the signal to a form takes place that can bring a specific cellular response.
- Response– During this stage the specific cellular effect occurs by the signaling molecule.
Reception: In this stage the ligand binds to a receptor protein and thus causes it to change the shape. The ligand and receptor interaction is highly specific. Receptors can be found in two places;
- Intracellular proteins- These are found inside the plasma membrane either in the cytoplasm or the nucleus. The molecule that is used for signaling must cross the plasma membrane and should be hydrophobic like testosterone and Nitric Oxide.
- Cell-surface proteins- These are embedded in the plasma membrane and they bind to water-soluble ligands.
G-Protein is known as guanine nucleotide-binding protein. These are the proteins that act as molecular switches. They are present inside the cell and are involved in transmitting signals.
Transduction: In this stage the molecular interactions help in transmitting signals from the receptors to the target molecule in the cell. Enzymes called protein kinases phosphorylate activate many proteins at every step up to the next level. This process enhances the signal and hence, allows a large cellular response. Signal transduction pathways are proteins but some can be small non-protein water-soluble ions and they are termed as secondary messengers.
Response: In this stage the cell signaling causes the regulation of transcription activities. The signaling pathways affect protein synthesis and helps in turning specific genes on or off. Through the signaling pathway the final activated molecule functions as the transcription factor. Signaling pathways often regulate the activity of proteins in the cytoplasm rather than their synthesis.
Forms of signaling
Cell-cell signaling is a process of transmission of a signal from a sending cell to a receiving cell. All sending and receiving cells are next-door neighbors and all the exchange signals do not occur in the same way. There are four methods of chemical signaling found in multicellular organisms-
- Paracrine signaling
- Autocrine signaling
- Endocrine signaling
- Signaling by direct contact
Difference between the different signaling methods is the distance that the signal travels from the organism to reach the target cell.
Cells that are near one another communicate by the paracrine signaling through the release of chemical messengers. They contain ligands that can diffuse through the space between the cells. This is type of signaling that occurs over short distances.
Paracrine signaling allows cells coordinate activities with other neighboring cells. This type of signaling is also used in many different tissues and contexts. Paracrine signals are evident during the development as they allow one group of cells to tell which identity to take on to the other neighboring group of cells.
Paracrine signaling has a good example and that is known as synaptic signaling. In this type of signaling the nerve cells transmit the signals. This process is named after the synapse. Synapse is the junction between two nerve cells which are helpful in signal transmission.
An electrical impulse moves rapidly through the cell when a sending impulse is generated which travels down the long, fiber-like extension which is known as the axon. The impulse reaches the synapse and triggers the release of ligands which are known as neurotransmitters. They quickly cross the small gap which is present between the two nerve cells. Neurotransmitters arrive at the receiving cell and thus they bind to the receptors. This binding causes a chemical change inside of the cell like opening of the ion channels or changing of the electrical potential across the membrane.
The neurotransmitters that are released are quickly degraded and are taken up by the sending cell. This leads to the resetting of the system so the synapse can be prepared to respond quickly to the next signal which will be produced.
Synaptic signaling- Neurotransmitters are released from vesicles at the end of the axon of the sending cell and they diffuses across the small gap between sending and target neurons and also binds to the receptors on the target neuron.
Autocrine signaling is a method in which the cell signals to itself. The cell releases a ligand which binds to receptors that is present on its own surface. Autocrine signaling is very important for many processes thought this process may seem a little weird as both the ligand and the receptor are present on the same cell.
Autocrine signaling is very important during development and also helps the cells to reinforce their correct identities. It is also important in cancer as it plays a key role in metastasis, which is spread of cancer from its original site to other parts of the body. Signal can be both autocrine and paracrine and thus can bind to the sending cell as well as other cells in the area.
The endocrine signaling is helpful when the signals need to be sent over a long distance. They use the circulatory system as a distribution network for the messages.
In long-distance endocrine signaling, specialized cells lead to the production of the signal and thus they are released into the bloodstream. Circulation system carries the signal to the target cells which is present in the distant parts of the body. Therefore, in this signaling method the signals that are produced in one part of the body and the circulation system help them to travel to the far-away targets which are known as hormones.
Endocrine glands that release hormones are the thyroid, the hypothalamus, the pancreas, the pituitary, and the testes and ovaries. Each endocrine gland releases different hormones which lead to the regulation of many important steps that causes developmental growth of the body.
The pituitary releases growth hormone (GH), which promotes growth of the body mainly the skeleton and cartilage. GH affects many different types of cells that increase the size of the body.
Signaling through cell-cell contact
Gap junctions are tiny channels that directly connect the neighboring cells. These gap junction allows the small signaling molecules, called intracellular mediators, to diffuse through the two cells. Large molecules like proteins and DNA cannot fit through the channels and therefore, they need special assistance.
The transfer of signaling molecules helps in the transmission of the current state of one cell to its neighbor. They allow in coordinating their response to a signal that only one of them may have received. Plants have plasmodesmata between the cells which help in making the entire plant one giant network.
Direct signaling is a method in which two cells may bind to one another. They carry complementary proteins on their surfaces which help in binding. After the binding the interaction changes the shape of one or both proteins and thus helps in transmitting a signal. This kind of signaling is evident for the immune system.