Cell Communication1. Cell signaling evolved early in the history of life.- Saccharomyces cerevisiae, the yeast of bread, wine, and beer, identifies potential mates by chemical signaling.- There are two sexes, a and , each of which secretes a specific signaling molecule, a factor and  factor, respectively.- These factors each bind to receptor proteins on the other mating type.- Once the mating factors have bound to the receptors, the two cells grow toward each other and undergo other cellular changes.- The two cells fuse, or mate, to form an a/ cell containing the genes of both cells.- The process by which a signal on a cell’s surface is converted into a specific cellular response is a series of steps called a signal-transduction pathway.- The molecular details of these pathways are strikingly similar in yeast and animal cells, even though their last common ancestor lived more than a billion years ago.- Signaling systems of bacteria and plants also share similarities.- These similarities suggest that ancestral signaling molecules evolved long ago in prokaryotes and have since been adopted for new uses by single-celled eukaryotes and multicellular descendents.2. Communicating cells may be close together or far apart.- Multicellular organisms release signaling molecules that target other cells.- Cells may communicate by direct contact.- Both animals and plants have cell junctions that connect to the cytoplasm of adjacent cells.- Signaling substances dissolved in the cytosol can pass freely between adjacent cells.- Animal cells can communicate by direct contact between membrane-bound cell surface molecules.- Such cell-cell recognition is important to such processes as embryonic development and the immune response.- In other cases, messenger molecules are secreted by the signaling cell.- Some transmitting cells release local regulators that influence cells in the local vicinity.- One class of local regulators in animals, growth factors, includes compounds that stimulate nearby target cells to grow and multiply.- This is an example of paracrine signaling, which occurs when numerous cells simultaneously receive and respond to growth factors produced by a single cell in their vicinity.- In synaptic signaling, a nerve cell produces a neurotransmitter that diffuses across a synapse to a single cell that is almost touching the sender.- The neurotransmitter stimulates the target cell.- The transmission of a signal through the nervous system can also be considered an example of long-distance signaling.- Local signaling in plants is not well understood. Because of their cell walls, plants must have different mechanisms from animals.- Plants and animals use hormones for long-distance signaling.- In animals, specialized endocrine cells release hormones into the circulatory system, by which they travel to target cells in other parts of the body.1- Plant hormones, called growth regulators, may travel in vessels but more often travel from cell to cell or move through air by diffusion.- Hormones and local regulators range widely in size and type.- The plant hormone ethylene (C2H4), which promotes fruit ripening and regulates growth, is a hydrocarbon of only six atoms, capable of passing through cell walls.- Insulin, which regulates blood sugar levels in mammals, is a protein with thousands of atoms.- What happens when a cell encounters a signal?- The signal must be recognized by a specific receptor molecule, and the information it carries must be changed into another form, or transduced, inside the cell before the cell can respond.3. The three stages of cell signaling are reception, transduction, and response.- E. W. Sutherland and his colleagues pioneered our understanding of cell signaling.- Their work investigated how the animal hormone epinephrine stimulates breakdown of the storage polysaccharide glycogen in liver and skeletal muscle.- Breakdown of glycogen releases glucose derivatives that can be used for fuel in glycolysis or released as glucose in the blood for fuel elsewhere.- Thus one effect of epinephrine, which is released from the adrenal gland during times of physical or mental stress, is mobilization of fuel reserves.- Sutherland’s research team discovered that epinephrine activated a cytosolic enzyme, glycogen phosphorylase.- However, epinephrine did not activate the phosphorylase directly in vitro but could only act via intact cells.- Therefore, there must be an intermediate step or steps occurring inside the cell.- The plasma membrane must be involved in transmitting the epinephrine signal.- The process involves three stages: reception, transduction, and response.- In reception, a chemical signal binds to a cellular protein, typically at the cell’s surface or inside the cell.- In transduction, binding leads to a change in the receptor that triggers a series of changes in a series of different molecules along a signal-transduction pathway. The molecules in the pathway are called relay molecules.- In response, the transduced signal triggers a specific cellular activity.B. Signal Reception and the Initiation of Transduction1. A signal molecule binds to a receptor protein, causing the protein to change shape.- The cell targeted by a particular chemical signal has a receptor protein on or in the target cell that recognizes the signal molecule.- Recognition occurs when the signal binds to a specific site on the receptor that is complementary in shape to the signal.- The signal molecule behaves as a ligand, a small molecule that binds with specificity to a larger molecule.- Ligand binding causes the receptor protein to undergo a change in shape.- This may activate the receptor so that it can interact with other molecules.- For other receptors, this causes aggregation of receptor molecules, leading to further molecular events inside the cell.2- Most signal receptors are plasma membrane proteins, whose ligands are large water-soluble molecules that are too large to cross the plasma membrane.2. Some receptor proteins are intracellular.- Some signal receptors are dissolved in the cytosol or nucleus of target cells.- To reach these receptors, the signals pass through the target cell’s plasma membrane.- Such chemical messengers are either hydrophobic enough or small enough to cross the phospholipid interior of the plasma membrane.- Hydrophobic messengers include the steroid and thyroid hormones of animals.- Nitric oxide (NO) is a gas whose small size allows it to pass between