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Signaling into cells Communication between cells
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Cell Recognition and Adhesion
Cells are able to arrange themselves into groups through Cell recognition: one cell specifically binds to another cell of a certain type. Cell adhesion, the relationship between the two cells is “cemented”. Tissue-specific aggregation occur because of plasma membrane recognition proteins.
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Cell Recognition and Adhesion
Cell-Cell and Cell-Matrix Physical interaction Extra Cellular Matrix (ECM)
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Cell Recognition and Adhesion
Specialized cell junctions form between cells in a tissue. Animals have three types of cell junctions: tight junctions, desmosomes, and gap junctions.
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Figure 5.6 Junctions Link Animal Cells Together (Part 1)
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Cell Recognition and Adhesion
Desmosomes act like spot welds on adjacent cells, holding them together. Desmosomes have dense plaques that are attached both to cytoplasmic fibers and to membrane cell adhesion proteins. The membrane cell adhesion proteins bind to the proteins of an adjacent cell. (Cadherins) The cytoplasmic fibers are intermediate filaments of the cytoskeleton.
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Figure 5.6 Junctions Link Animal Cells Together (Part 3)
Hemidesmosome
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Adhesion to surface during cell movement
Adhesion and cell movement Adhesion to surface during cell movement
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Cell Recognition and Adhesion
Gap junctions are connections that facilitate communication between cells. Gap junctions are made up of specialized protein channels called connexons. Connexons span the plasma membranes of two adjacent cells and protrude from them slightly. Connexons are made of proteins called connexins, which snap together to generate a pore.
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Figure 5.6 Junctions Link Animal Cells Together (Part 4)
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Direct Intercellular Communication
Some cells send signals directly from their interior to the interior of adjacent cells. This transfer occurs by way of specialized structures called gap junctions in animal cells, and plasmodesmata in plant cells.
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Cell Recognition and Adhesion
Plasmodesmata in plant cells allow connections between ER compartment of adjacent cells
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Cell Recognition and Adhesion
Tight junctions are specialized structures at the plasma membrane that link adjacent epithelial cells. They have two primary functions: To restrict the migration of membrane proteins and phospholipids from one region of the cell to another To prevent substances from moving through the intercellular space
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Figure 5.6 Junctions Link Animal Cells Together (Part 2)
Freeze Fracture
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Figure 5.6 Junctions Link Animal Cells Together (Part 2)
Tight junctions restrict the migration of membrane proteins and phospholipids from one region of the cell to another Cell Polarity
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Cell Signaling and Communication
Signals Receptors Signal Transduction Effectors Effects on Cell Function
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Signals Both prokaryotic and eukaryotic cells must process information from their environment and respond appropriately. Signals may be chemical molecules or physical stimuli such as light. Cells must be set up to interpret signals—not all cells can interpret all signals. To interpret a signal, a cell must have the appropriate receptor protein. Membrane
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Signal transduction The entire signaling process, from signal detection to final response, is called a signal transduction pathway. Membrane
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Signal transduction A signal transduction pathway involves a signal, a receptor, transducers, effectors and effects. Signal Receptor Transducers Effector Effect
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Signals Multicellular organisms’ internal cells are exposed to extracellular fluids and other cells, from which they receive information. A few of the many types of signals in animal cells are hormones, neurotransmitters, chemical messages from the immune system, CO2, and H+.
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Signals In large animals, local signals reach targets via diffusion (autocrine or paracrine signals )when the target is close. Paracrine signals diffuse to and affect nearby cells.
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Signals Cells can sense signals only if they express the right receptor Autocrine signals are signals generated by the same cells upon which they act. Autocrine Paracrine
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Signals When the target is distant, signals travel by circulation in the blood.(hormones, Pheromones)
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Signals A review of the steps in this signal transduction pathway (Cascade): one signal can induce several coordinated effects Receptor Target/Effector
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Receptors The type of receptors each cell makes is genetically determined. A cell responds to only a few of the many signals it receives. Receptors have specific binding sites for their signals.
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Figure 15.3 A Signal Bound to Its Receptor
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Ligands A ligand is the signaling molecule that binds the receptor. Binding of the ligand causes the receptor to change shape. The ligand has no further involvement in the pathway. Ligand binding is reversible. Inhibitors can bind (reversibly or irreversibly) to the ligand binding sites on receptor molecules and block it. Natural and artificial inhibitors are important in medicine.
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Ligands There are two classes of signaling molecules: Ligands with plasma membrane receptors: large and/or polar molecules that can not cross, such as insulin. Receptors are usually transmembrane proteins. Ligands with cytoplasmic receptors: small and/or nonpolar molecules that can cross the plasma membrane, such as steroids.
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Cytoplasmic Receptors
Cytoplasmic receptors which are located inside the cell bind with ligands that can cross the plasma membrane. The receptor changes shape and can then enter the nucleus where it acts as a transcription factor. Steroid hormones are an example of such signal molecules. Bind to promoter and regulate transcription
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Transmembrane Receptors
Three well-studied types of transmembrane receptors in complex eukaryotes: Ion channel receptors Protein kinases G protein-linked receptors
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Receptors Some ion channel proteins, acting as “gates,” are signal receptors. Channel proteins can open to let certain ions in or out, or close to restrict them. The signal to open or close the channel can be chemical, light, sound, pressure, or voltage. An example of a gated ion channel is the acetylcholine receptor of muscle cells.
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Nicotine = Agonist = increased blood pressure
Figure A Gated Ion Channel Nicotine = Agonist = increased blood pressure Curare = Antagonist = Paralysis
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Receptors Some eukaryotic receptor proteins become kinases when activated. A phosphate is transferred from ATP to a protein, the target protein, changing its shape or activity. Sometimes the protein kinase phosphorylates itself. This is called autophosphorylation. Insulin receptors are examples of Tyrosine kinase receptors.
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Figure 15.6 A Protein Kinase Receptor
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Figure 15.9 A Protein Kinase Cascade
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Signal Transduction There are at least three advantages to having many kinase steps in signal transduction: A signal at the cell membrane is transferred to the nucleus. Each activated protein kinase can phosphorylate many target proteins, so amplification of the signal occurs at each step. Having many steps affecting different target proteins allows for a variety of responses by different cells to the same signal.
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Receptors The G protein-linked receptors are proteins with seven regions that pass through the lipid bilayer. A ligand binds to the extracellular side and changes the shape of the protein on the cytoplasmic side. This exposes a binding site for the G protein. G protein also has a binding site for GTP. The GTP-bound subunit separates and moves along the membrane until it finds an effector protein. The effector protein may catalyze many reactions, amplifying the signal.
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Figure 15.7 A G Protein-Linked Receptor (Part 1)
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Figure 15.7 A G Protein-Linked Receptor (Part 2)
Ga,Gi
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Signal transduction pathway
Adrenaline Transcription Factor
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Signal Transduction Transducers convert signals from one form to another. Direct transduction results from the action of the receptor itself on effector proteins. Direct transduction occurs at the plasma membrane. Indirect transduction uses a second messenger to mediate the interaction between receptor binding and cellular reaction. In both direct and indirect transduction the signal initiates a series of events that eventually lead to a final response.
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Signal Transduction Indirect transduction is more common than direct transduction. Scientists investigating the effects of epinephrine on the liver enzyme phosphorylase discovered cyclic AMP (cAMP) as a second messenger. The second messenger carries the signal from the membrane receptor to the cytoplasm (does not act by itself). Second messengers affect many cell processes, amplifying the signal.
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Signal Transduction The cAMP molecule is a small cyclic nucleotide generated from ATP. The enzyme adenylyl cyclase produces cAMP using ATP as a substrate. Adenylyl cyclase is activated by an activated G protein subunit. Like other second messengers, cAMP is not an enzyme. Second messengers act as cofactors or allosteric regulators of target proteins. cAMP has two major kinds of targets: ion channels and protein kinases.
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Figure 15.10 The Formation of Cyclic AMP
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Signal Effects: Changes in Cell Function
Sensory nerve cells of the sense organs are stimulated through the opening of ion channels. Each of the thousands of nerve cells in the nose expresses just one of these receptors. When an odorant molecule binds to its receptor, a G protein becomes activated, which leads to formation of the second messenger, cAMP. The cAMP binds to ion channels, causing them to let in Na+. The change in Na+ ion concentration stimulates the neuron to send a signal to the brain.
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Figure 15.14 A Signal Transduction Pathway Leads to the Opening of Ion Channels (Part 1)
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Figure 15.14 A Signal Transduction Pathway Leads to the Opening of Ion Channels (Part 2)
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Membrane potential Animal cells use ATP to pump Na+ and K+ against their concentration. Na+-K+ ATPase
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Membrane potential Animal cells use ATP to pump Na+ and K+ against their concentration. Na+-K+ ATPase
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Signal Effects: Changes in Cell Function
Neuronal cell communication.
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Signal Effects: Changes in Cell Function
Voltage gated Na+ Channels have three states
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Signal Effects: Changes in Cell Function
Voltage gated Na+ Channels have three states
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Signal Effects: Changes in Cell Function
Electric signal in cells is propagated by transient opening of Na+ channels.
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Signal Effects: Changes in Cell Function
Electric signal in cells is propagated by transient opening of Na+ channels.
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Signal Effects: Changes in Cell Function
Electric signal is translated into secreted chemical signal in the synapse
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