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Chapter 3

Biology 1080 - Chapter 3.docx

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University of Guelph
BIOL 1080
Jim Kirkland

Biology 1080 Chapter 3a(i) Gap Junctions: Link adjacent cells and are formed by plasma membrane proteins, called connexins, that form structures called connexons. - Connexons form channels that allow ions and small molecule to pass directly from one cell to the other. - Electrically couples the cell Communication through chemical messengers - Ligands  Molecules that bind to proteins reversibly. - Happens when one cell releases a chemical into the interstitial fluid (secretion), and another cell; target cell, responds to the chemical messenger. - The target cell is the cell at which the message is aimed. - Target cell responds because of certain proteins, called receptors, which specifically recognize and bind the messenger. - The binding of messengers to receptors produces a response in the target cell through a variety of mechanisms referred to as signal transduction. Chemical Messengers 1. Paracrines  Communicate with neighboring cells. Target cell must be close enough so that the paracrine secreted can reach it by simple diffusion. Include: a. Growth factors  Proteins that stimulate proliferation and differentiation of cells. b. Clotting factors  Proteins that stimulate formation of a blood clot. c. Cytokines  Peptides, released from immune cells, that function in coordinating the body’s defense against infections. Ex. Histamine Autocrines, are a subclass of paracrines, that act on the same cell that secreted them. (Secretory cell = Target cell) 2. Neurotransmitters  Chemicals released into interstitial fluid from nervous system cells called neurons.  released by axon terminal, which is very close to the target cell. - Junction between two cells are called synapses. - Communication by neurotransmitters is called synaptic signaling. - The cell that releases the neurotransmitter is called the presynaptic neuron. - And the target cell is called the postsynaptic cell. Ex. Acetylcholine 3. Hormones  Chemicals released from endocrine glands into the interstitial fluid, where they can diffuse into the blood. - Travels through the blood to its target cell, which can be distant. Ex. Insulin 1 - Specific class  Neurohormones, released by a special class of neurons called neurosecretory cells through a mechanism similar to that of neurotransmitter release. Lipophilic (hydrophobic) molecules are lipid soluble and, therefore, readily cross the plasma membrane, but they do not dissolve in water. Hydrophilic (lipophobic) molecules are water-soluble and so not cross the plasma membrane. Hormones transported into the blood, either in dissolved form, or bound to carrier proteins. - To be dissolved, the messenger must be a hydrophilic messenger. (Peptides and amines) - Steroids are hydrophobic so they bound to carrier proteins. o Protected from degradation and have a longer life. Receptor specificity - Usually only bind one messenger or a group of messengers. - The strength is called affinity. Target’s cell’s response to chemical messenger 1. Messenger’s concentration  Response increase as the concentration increase 2. Number of receptors present  The more receptors, the more likely a. Up-regulation  Increase in number of receptors occurs when cells are exposed to low messenger concentrations for a long time. The cell adapts by producing more receptors. b. Down-regulation  Decrease in the number of receptors occurs when messenger concentrations are higher than normal for a long time. 3. Affinity of the receptor for the messenger Agonists: Ligands that bind to receptors and produce a biological response, Antagonist: Ligands that bind to receptors but do not produce a response. Signal Amplification The ability of small changes in the concentration of a chemical messenger to elicit marked responses in target cells. (Feature of second messenger system) 2 Long Distance Communication Via - The nervous system  consists of neurons and supporting cells called glial cells. o Neurons can communicate in long distances, by transmitting electrical signals, and transmitting chemical signals through the release of a neurotransmitter from the axon terminal.  The neurotransmitter then diffuses over a short distance. o Wired system, because of the cell-to-cell interaction by synapses o Short and quick, in charge of movements - The endocrine system  lacks any direct anatomical link between secretory cell and target cell. o Communicates through hormones, which travel via the bloodstream. o Slower o Coordinating metabolic activities among organ systems. Action of lipophilic hormones - Located in the cytosol or nucleus of target cell. - Easily permeate the plasma membrane o If receptor is in the nucleus  hormone diffuses into the nucleus and binds to it  forming hormone-receptor complex. o If receptor is in cytosol  hormones binds to it there  forming hormone-receptor complex  THAT THEN enters the nucleus.  Inside the nucleus  complex binds to region of DNA, called the hormone response element (HRE)  which is located at the beginning of a specific gene.  Binding of the complex to the HRE activates or deactivates the gene  affects transcription of mRNA and increases or decreases synthesis of the protein coded by the gene. (Ex. mRNA is increased and gene is activated)  The mRNA moves into the cytosol.  The mRNA is translated by ribosomes to yield proteins. cAMP Second Messenger System 1. The first messenger binds to the receptor, activating a Gsprotein 2. The G protein releases alpha subunit  binds to and activates the enzyme adenylate cyclase. 3. Adenylate cyclase catalyzes the conversion of ATP to cAMP 4. cAMP activates protein kinase A,  cAMP-dependent protein kinase. 5. The protein kinase catalyzes the transfer of a phosphate group from ATP to a protein  altering the protein’s activity. 6.  Causes a response in the cell 3 Chapter 3a(ii) Neurons 1. Sensory neurons  conduct information toward the brain and spinal chord. Generally extend from sensory receptors  specialized structures in gathering information about the conditions within and around the body. 2. Motor neurons  carry information away from the brain and spinal chord to an effector – muscle
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