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a. The three-step process by which an external signal is converted into a cellular response is reception, transduction, and response. Reception is when the signal molecule (ligand) binds to a receptor protein. Receptor proteins are highly specific and the ligand binding generally causes a shape change in the receptor. Next is transduction, which involves a multistep pathway that helps produce a large or amplified cellular response. The multistep process provides more opportunities for coordination and regulation of the cellular response than simpler systems do. The process acts like falling down dominoes. The receptor activates another protein, which activates another, and so on, until the protein producing the response is activated. At each step, the signal is transduced into a different form, usually a shape change in a protein. Finally the response occurs in either the cytoplasm or in the nucleus. The signal transduction pathways usually leads to the regulation of one or more cellular activities, often regulating the synthesis of enzymes or other proteins by turning genes on or off in the nucleus. Add: During reception, a chemical signal is detected when the signal molecule binds to the receptor protein located on the cells surface or inside the cell. b. The three main types of membrane receptors are G protein-coupled receptors (GPCRs), ligand-gated ion channels, and intracellular receptors. GPCRs are plasma membrane receptors that work with the help of a G protein, which act as an on-off switch depending on whether GTP (an energy carrying molecule) is attached. A ligandgated ion channel receptor acts as a “gate” for ions when the receptor changes shape. When a signal molecule binds as a ligand to the receptor, the gate allows specific ions through a channel in the receptor. Intracellular receptor proteins are found in the cytosol or nucleus of target cells. The receptors are activated when small or hydrophobic chemical messengers readily cross the membrane. Add: tyrosine kinases, and ion channels. Delete: Intracellular receptors. Don’t need to explain** just list! c. The IP3 pathway is a tyrosine kinase specific pathway. Signals are first received by tyrosine kinase and a dimer forms and is phosphorylated by ATP. This phosphorylation activates an enzyme, phospholipase C, which targets a phospholipid in the membrane and cleaves off a molecule of IP3. IP3 then quickly diffuses to a ligand gated ion channel on the smooth ER where it docks. This ion channel then opens allowing the flood of a second messenger, Ca2 to be released from the ER. These calcium ions then in turn activate relay proteins, which eventually lead to a cellular response. Add: calcium ions also act as second messengers in signal transduction pathways, two monomers of tyrosine kinase fuse together to form the dimer, dimer is phosphorylated by six molecules of ATP, cleaves off a molecule of IP3 from PIP2, PIP2 splits into two molecules, IP3, and DAG. d. Saccharomyces cerevisiae yeast identifies their mates by chemical signaling. Type a mating type is when the yeast secrete a signaling molecule called “a” factor that binds to receptor proteins on nearby α cells. In type α the yeast secrete a signaling factor called α factor that binds receptors on nearby “a” cells. The two mating factors, a and α, cause the types of cells to grow toward each other and fuse together. The new cell contains genes from both a and α, which provides advantages to descendants of these cells that arise by subsequent cellular division. Thus, this mating in yeast involves a change (transduction) in the signaling molecule into some form that brings about the cellular response of mating. Add: the two mating factors cause the cells to grow toward each other and bring about other cellular changes, the molecular details of signal transduction in yeast and mammals are strikingly similar even though the last common ancestor of these two groups of organisms lived over a billion years ago, early versions of the cell-signaling mechanisms used today evolved well before the first multicellular creatures appeared on Earth, signaling mechanisms evolved first in ancient prokaryotes and single-celled eukaryotes and were then adopted for new uses by their multicellular descendants.