CELL TO CELL COMMUNICATION Cells do not operate in a void and must be able to communicate with cells beyond their borders. Cells can both send out and receive different signals that get unified in order to react in a certain way. Most of the signals a cell receives are purely chemical. Prokaryotes can detect nutrients and toxins. Multicellular organisms can detect hormones, neurotransmitters, growth factors, and components of the extracellular matrix. Cells can also respond to certain mechanical stimuli, such as with sound waves and sensory receptor cells in the skin. Most cells have receptors on them, which are proteins that respond to signals. We will talk about signaling systems in a later chapter. There are different receptors for the different molecules. Cells have hundreds of receptors on their surface that respond to different things. There are three different receptor types: G-protein-coupled receptors, enzyme-linked receptors, and ion channel receptors. As you will see in later chapters, they respond in different ways. They are important because they allow larger molecules to have access to the cell without actually entering the cell. There are also some receptors within the cell itself, such as those that respond to steroid hormones. Receptors act by undergoing conformational changes that change the interior of the cell biochemically. There are signaling pathways called signal transduction cascades that amplify the chemical message, leading to secondary messengers that act within the cell. An example of this is cyclic AMP, which is involved as a secondary messenger in several situations. It acts until it gets acted on by phosphodiesterase, which degrades it. Cells respond to numerous signals all at the same time. There are multiple signal transduction pathways operating within the cell. A single secondary messenger or protein kinase can be functional in several different pathways, leading to more than one cellular activity happening at one time. As you will find out later, there are three major stages of cellular communication. The first is reception. This happens outside of the cell that binds to a ligand or signaling molecule. Many of these receptor proteins span the membrane so they can act on the
37
