Cell Communication

Cell-to-cell communication is critical for the survival of cells. These are responsible for the growth and development of organisms. Cells communicate through 3 ways: direct contact, local signaling, and long-distance signaling. Direct contact is communication through cell junctions. Signaling substances and other materials dissolved in the cytoplasm can pass freely between adjacent cells. Animal cells use gap junctions and plant cells use plasmodesmata. For example, Immune cells like antigen-presenting cells communicate with T cells through direct contact. Local signaling is when a secreting cell releases chemical messages (ligands) that travel a short distance. The chemical messages will cause a response in a target cell. For example, Paracrine signaling and synaptic signaling. Paracrine signaling is when secretory cells release local regulators via exocytosis to an adjacent cell. Synaptic signaling occurs in animal nervous systems and is when neurons secrete neurotransmitters. These diffuse across the synaptic cleft between nerve cells and target cells. Long-distance signaling is when plants and animals use hormones to communicate. Plants release hormones that travel into the vascular tissue or through the air to reach the target tissue. Animals use endocrine signaling, which is specialized cells that release hormones into the circulatory system, where they reach a target system; such as insulin, which is released by the pancreas into the bloodstream and circulates through the body and binds to target cells. Cell signaling overview: there are 3 stages: reception, transduction, and response. Reception is the detection and reception of a ligand receptor in a target cell. The receptor is a macromolecule that binds to a ligand. All receptors have an area that interacts with the ligand and an area that transmits a signal to another protein. The binding between ligand and receptor is highly specific. When a ligand binds to the receptor, the receptor is activated and allows the receptor to interact with other cellular molecules. This initiates a transduction signal. Receptors can be in the plasma membrane or intracellular. Plasma membrane receptors are the most common type of receptor. These bind to ligands that are polar, water-soluble, large, GPCRs and ligand-gated ion channels. Intracellular receptors are found in the cytoplasm or nucleus of target cells. They bind to ligands that pass through the plasma membrane, hydrophobic molecules, gases like nitric oxide. Stage 2 Transduction: conversion of an extracellular signal to an intracellular signal that will bring about a cellular response. This requires a sequence of changes in a series of molecules known as a signal transduction pathway. A signal transduction pathway regulates protein activity through the protein kinase, which relays the signal, and the protein phosphatases, which shut off pathways. During transcription, the signal is amplified. The second messengers are small non-protein molecules that help ions relay the message and amplify the response. Cyclic AMP is a common second messenger. Stage 3 Response: The final molecule in the signaling pathway converts the signal to a response that will alter a cellular process. Examples: Proteins that can alter membrane permeability, enzymes that will change a metabolic process, proteins that turn genes on or off. Signal Transduction Pathways can influence how a cell responds to its environment. They can result in changes in gene expression and cell function ( can alter phenotypes or result in cellular death). A mutation would result in a change to the transduction of the signal. In eukaryotic organisms, there are 2 main categories of cell membrane receptors: GPCRs and Ion channels. GPCRs or G protein-coupled receptors are the largest category of cell surface receptors. They are important in animal sensory systems and bind to a G protein that can bind to GTP, which is an energy molecule similar to ATP. The GPCR, enzyme, and G protein are inactive until ligand binding to the GPCR on the extracellular side. Ligand binding causes the cytoplasmic side to change shape and allows for the G protein to bind to the GPCR and activate the GPCR and G protein. Part of the activated G protein can then bind to the enzyme, which activates the enzyme and amplifies the signal and leads to a cellular response. Ion channels: Ligand-gated ion channels are located in the plasma membrane and are important in the nervous system. Receptors act as a gate for ions. When a ligand binds to a receptor, the gate opens or closes, allowing diffusion of specific ions and initiating a series of events that lead to a cellular response.