Types of Cell Communication
Cells are constantly interacting with each other and their environment. Cells receive information from direct contact with neighboring or circulating cells through the binding of membrane glycoproteins and glycolipids. They can receive information from the immediate environment through the binding of various chemicals to proteins on the surface or inside the cell. In multicellular organisms, cells receive information from distant locations in the form of chemicals such as hormones and cytokines.
Cell communication is important to cells in a variety of ways. The recognition of similar cells plays a role in the reproduction of many organisms such as amoeba and yeast. The recognition of cells that do not belong or are damaged is key for immune cells to keep the organism free of disease. Communicating information to neighboring cells is involved in the development of multicellular organisms as tissue layers develop. It also controls the growth of cells, as contact with neighboring cells prevents uncontrolled cell growth in normal cells.Receiving information from throughout the organism is necessary for each cell to function properly within the organism. The function of the organism as a whole is dependent on cell communication to maintain balanced internal conditions, called homeostasis.
Steps of Cell Communication
Cell Signal Cascade
The process of cellular communication requires that external signals reach the inside of the cell so that the cell can respond appropriately to the information. This process is called a cell signal cascade because it progresses through multiple mediators from the outside of the cell to the inside.
Cells recognize signals by using receptors. A receptor is a protein to which a signaling molecule, or ligand, can bind in order to elicit a physiological response. There are many different receptor types found in cells. Each receptor type participates in a specific ligand binding. Some receptors are responsive to other stimuli such as light and pressure changes. An intracellular receptor is found inside the cell membrane. Signaling molecules, such as some steroids, must diffuse across the plasma membrane in order to interact with these proteins. Thus, these signaling molecules must typically be hydrophobic in nature. A cell-surface receptor is found embedded in the cell membrane. Because they interact with the extracellular space, water-soluble signaling molecules will bind to these proteins. These receptors are also involved in direct cell-to-cell communication. Cell-surface receptors are associated with peripheral proteins on the inside of the cell membrane.
The binding of the ligand to the receptor leads to a multistep process to direct the signal into the cell, where it can be acted upon. This process is known as signal transduction. Signal transduction often involves successive activation of signaling molecules by phosphorylation. The pathway is commonly a multistep event that helps to amplify the overall signal. Some small water-soluble ions called second messengers may also be involved in this process. Because they are small, second messengers can diffuse through the cytoplasm quickly, amplifying the signal and increasing the speed of the cellular response.
Ultimately, the signal transduction pathway leads to a cell response. In some cases the target of the signal is a cytoplasmic or membrane protein. The signal can change the activity of the protein, such as activating a previously inactive protein. In other cases the signal will be relayed to the nucleus. There, the signal will result in the regulation of different genetic activities, specifically those that affect protein synthesis. This is accomplished by turning specific genes on or off. For example, the binding of growth hormone to its receptor increases the production of proteins involved in tissue growth.