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What do antibodies, plant growth, and plaque have in common? They all have to do with the communication of cells! Cell communication, which is typically synonymous with cell signaling, is extremely important to not only multicellular organisms, but also unicellular organisms (such as bacteria and some fungi). Communication helps cells coordinate activities like synchronized swimmers. It is crucial for homeostasis and can protect cells from harmful situations. Cells have multiple ways to communicate, just as we do. They tend to use signaling, in which a chemical message is sent to a cell in order to produce a change in that cell. There are non-chemical methods of communication, using sensory stimuli and electromagnetic radiation, but not very much is known about how they work. Different types of signaling are used for different distances. Some messages are sent through direct contact, while some travel short or long distances outside of the cell. Here are 5 types of signaling: Juxtacrine signaling is another name for direct-contact signaling. Cells using this method of signaling must be directly touching in order to transmit a message. They do this by transferring the signal between cell junctions or touching cell membrane proteins for “cell-cell recognition”. Think of this contact as the cell whispering a message to another cell. Their signals are directly transmitted, rather than going elsewhere first. Paracrine signaling is a type of local signaling. A cell will secrete signaling molecules to change nearby target cells. This is similar to calling someone or even a group of friends when you want to make plans. You aren’t talking face to face but you are still able to transfer your message to them from a relatively close position. Synaptic signaling is usually classified as a type of local signaling. This type of signaling occurs with the stimulation of your nerve cells. Chemical synapses convert the electrical stimulation of the nerve cell to chemical messages, known as neurotransmitters, and diffuse *them* to another nerve cell so that they can react accordingly. Autocrine signaling is a bit like creating a reminder for yourself on a calendar. After all, the prefix “auto-“ means “self”. In this form of local signaling, a cell will release chemical messengers that it can then respond to itself. Unfortunately, this type of messaging is taken advantage of in some cells to repeatedly undergo mitosis, ultimately creating a tumor. Endocrine signaling uses hormones to send long-distance messages. In animals, these hormones are not exclusively secreted by the endocrine system and can come from specialized glands in other parts of the body. Hormones usually travel through the bloodstream until they can diffuse and bind to a target cell. In plants, hormones aren’t specialized to certain glands and they travel the course of the plant through cells, vascular tissues, or the surrounding air. Endocrine signaling is similar to sending your friends a letter in the mail. It takes time to be delivered, and when it is, your letter will pass by many people, but because the message isn’t meant for them, none of them will act on it. When your friends receive the message, it will have a long-lasting impact on them, just like endocrine signaling with an organism. A good way to memorize the names of these five different types of signaling is to remember that *Cells love to work in their PEA-Js because it helps get work done* At this point, you’re probably wondering how all these signals actually work. Cell signaling follows three steps: reception, transduction, and response. In reception, a ligand (the signal molecule) binds to a receptor protein, which may be in the cell’s cytoplasm, on the nucleus, or in the plasma membrane. This receptor is specific to the ligand that it joins with. Receptors in the cell are called intracellular receptors and they typically bind with small, hydrophobic molecules that can pass through the plasma membrane easily. Some common types of receptors embedded in the plasma membrane are G protein-coupled receptors, ion channel receptors, and receptor tyrosine kinases. Each of these have different functions and therefore work in different ways. Transduction is usually a multi-step process for signals using a membrane receptor; intracellular receptors do most of the transduction for the message in the reception stage. In transduction, the receptor protein changes and initiates other changes within the cell. This signal transduction pathway will interact with several relay molecules, which are typically protein kinases. These proteins use a phosphate from ATP to activate another molecule—in this case several more protein kinases—through phosphorylation. This method of relaying the signal through a chain of energy is called a phosphorylation cascade. The phosphates are removed from the protein kinase by protein phosphatases in order to turn off the signal when it no longer needs to be used. Pathways must only last a short time so that molecules will be available for use in other signals. G protein-coupled receptors and receptor tyrosine kinases both use calcium ions (ca2+) as a second messenger to relay the signal, rather than using just protein kinases. G protein-coupled receptors also use cyclic AMP as a second messenger. Second messengers are small, water-soluble molecules or ions that not only relay a signal, but help to amplify a signal. The signal transduced by the cell will result in some sort of response in the nucleus or cytoplasm. Typically the signal will modify the activity of proteins within the target cell but sometimes cell division or growth will occur. Occasionally transcription factors will be used to turn specific genes on and off. So why is cell communication important? For one, it adds evidence to the idea that life came from a common ancestor. This complex system of events occurs in a wide variety of both eukaryotes and prokaryotes. In addition to that, understanding how our cells are communicating can help establish new cures to prevent the onset of diseases. Even though cell signaling is very controlled and tightly regulated, mistakes can still occur. When errors (like improper responses or over-signaling) occur in cell communication, severe illnesses can develop. This includes cholera, tuberculosis, Parkinson’s disease, and some cancers. Other conditions caused by modifications to cell communication can make it difficult for organisms to react to changes in the body (pH, excess water, etc.) or sensory stimuli: temperature, pressure, light, smell, sound, and taste. This can make daily functions more difficult or even dangerous. In the case of Type II diabetes, cells cannot respond to the hormone insulin in order to break down glucose in food, resulting in high blood sugar levels. Complications with diabetes can lead to high blood pressure and nerve damage, among other things. Knowing how cells communicate has helped establish new medicines to engage or inhibit signals and receptors so that the body can maintain homeostasis. With more knowledge about cell communication, our understanding of life can only grow.