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Unformatted text preview: LECTURES 26, 27 & 28 28, 30 Oct; 2 Nov 2009 (P. J. Hollenbeck) BIOL 231 Reading: for lect 26, pp. 531-40 (& DVD 16.1,3,4); for lects 27/28, pp. 540-67 (& DVD 16.2, 6) Probs: 71-75; Exam II '04, #6; Exam II05, #5; Exam III06, #3 CELL SIGNALING I. Varieties of signaling between cells (A) Origins & diversity (1) Intercellular communication was essential for the evolution of complex multicellular organisms. However, the origins of signaling systems are clear in unicellular organisms, esp. those that interact extensively such as yeast or Dictyostelium . And even the simplest unicellular organism must be able to locate food and suitable environments, and avoid danger. All of these require that cells receive and interpret molecular signals every cell is a target cell for signals. (2) A target cell responds to a signaling molecule via a receptor . Many signaling molecules act at very low concentration and their receptors bind them with high affinity and specificity . But there are significant variations on this theme. The target cell then transfers the signal inside the cell, amplifies and diversifies the signal, responds to it through the activation of effector molecules, and finally inactivates the signal, returning to the resting state ( figs 16-6, 16-7 ). (3) Hundreds of different signaling molecules are employed in animals - they can be proteins, small peptides, amino acids, nucleosides, steroids, fatty acid derivatives or other lipids, or even dissolved gases (e.g., NO or CO). See table 16-1 to get an idea. Any individual target cell responds to a subset of these signaling molecules, those for which it has receptors. But each signaling molecule can produce a complex array of effects within a target cell. (B) Paracrine, endocrine, synaptic, and contact-dependent signaling <Most of what we will talk about is focused on the cell surface and what happens within the cell. But it is worth knowing a bit about how signaling is organized in the body on a larger scale.> Endocrine : Global - Specialized secreting cells release the signaling molecule (usually a hormone) into the bloodstream; the specificity of a hormones effects depends upon target cells having the right specific receptors. Hormones are diluted to nM or lower concentration by the bloodstream, so they must act at extremely [low]. Thus, the receptors have extremely high affinities for hormone. Paracrine : More local - the extracellular signaling molecule diffuses through the extracellular fluid to reach its nearby target cells. The signal doesnt get far from the signaling cell because it is taken up by target cells, bound to the extracellular matrix, or destroyed by extracellular enzymes close to where it was secreted....
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- Fall '08
- Cell Signaling