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Cell Communication
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Communication Methods
Cell-to-cell contact Local signaling Long distance signaling
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Cell-to-Cell Communications
Cell junctions directly connect the cytoplasm of adjacent cells Ex: cardiac cells for rhythmicity Surface receptors can give/send information Ex: specific immune response Plasma membranes Plasmodesmata between plant cells Gap junctions between animal cells
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diffuses across synapse
Local Signaling Adjacent cells are signaled. Chemical messengers released Ex: Neurotransmitters via neurons (a) Paracrine signaling. A secreting cell acts on nearby target cells by discharging molecules of a local regulator (a growth factor, for example) into the extracellular fluid. (b) Synaptic signaling. A nerve cell releases neurotransmitter molecules into a synapse, stimulating the target cell. Local regulator diffuses through extracellular fluid Target cell Secretory vesicle Electrical signal along nerve cell triggers release of neurotransmitter Neurotransmitter diffuses across synapse is stimulated Local signaling
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Long Distance Signaling
Use of hormones Both plants and animals use hormones (e.g. Insulin) Can affect many cells in Other parts of the body Protein or Steroid types Hormone travels in bloodstream to target cells (c) Hormonal signaling. Specialized endocrine cells secrete hormones into body fluids, often the blood Hormones may reach virtually all body cells. Long-distance signaling Blood vessel Target cell Endocrine cell Figure 11.4 C
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How Do Cells Communicate?
Signal Transduction Pathways Convert signals on a cell’s surface into cellular responses Are similar in microbes and mammals, suggesting an early origin
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Relay molecules in a signal transduction pathway
3 Phases of Signal Transduction EXTRACELLULAR FLUID Receptor Signal molecule Relay molecules in a signal transduction pathway Plasma membrane CYTOPLASM Activation of cellular response Figure 11.5 Reception 1 Transduction 2 Response 3
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Step One - Reception Reception occurs when a signal molecule (ligand) binds to a receptor protein. Receptor protein is on the cell surface Ligand and receptor have a unique bonding
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Step Two - Transduction
Signal initiated by conformational change of receptor protein Signal is turned into a cellular response. Signaling cascades relay signals to target Multistep pathways can amplify a signal Second messengers involved
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A phosphorylation cascade
Signal molecule Active protein kinase 1 2 3 Inactive protein kinase Cellular response Receptor P ATP ADP PP Activated relay molecule i Phosphorylation cascade P A relay molecule activates protein kinase 1. 1 2 Active protein kinase 1 transfers a phosphate from ATP to an inactive molecule of protein kinase 2, thus activating this second kinase. Active protein kinase 2 then catalyzes the phos- phorylation (and activation) of protein kinase 3. 3 Enzymes called protein phosphatases (PP) catalyze the removal of the phosphate groups from the proteins, making them inactive and available for reuse. 5 Finally, active protein kinase 3 phosphorylates a protein (pink) that brings about the cell’s response to the signal. 4 Figure 11.8
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Cyclic AMP example… First messenger (signal molecule
ATP GTP cAMP Protein kinase A Cellular responses G-protein-linked receptor Adenylyl cyclase G protein First messenger (signal molecule such as epinephrine)
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Ex: Inositol P3 and calcium
2 1 IP3 quickly diffuses through the cytosol and binds to an IP3– gated calcium channel in the ER membrane, causing it to open. 4 The calcium ions activate the next protein in one or more signaling pathways. 6 Calcium ions flow out of the ER (down their con- centration gradient), raising the Ca2+ level in the cytosol. 5 DAG functions as a second messenger in other pathways. Phospholipase C cleaves a plasma membrane phospholipid called PIP2 into DAG and IP3. A signal molecule binds to a receptor, leading to activation of phospholipase C. EXTRA- CELLULAR FLUID Signal molecule (first messenger) G protein G-protein-linked receptor Various proteins activated Endoplasmic reticulum (ER) Phospholipase C PIP2 IP3 (second messenger) DAG Cellular response GTP Ca2+ (second messenger) IP3-gated calcium channel Ex: Inositol P3 and calcium
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Step Three - Response Cell signaling leads to regulation of cytoplasmic activities or transcription Signaling pathways regulate a variety of cellular activities
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Pathways can also regulate genes by activating transcription factors that turn genes on or off
Reception Transduction Response mRNA NUCLEUS Gene P Active transcription factor Inactive DNA Phosphorylation cascade CYTOPLASM Receptor Growth factor Figure 11.14
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Types of Receptors There are three main types of plasma membrane receptors: G-protein-linked Tyrosine kinases Ion channel
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G-protein-linked receptors
Very common, diverse functions Only results in single pathway response G-protein-linked Receptor Plasma Membrane Enzyme G-protein (inactive) CYTOPLASM Cellular response Activated enzyme Activated Receptor Signal molecule Inactivate GDP GTP P i
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Receptor tyrosine kinases
Multiple pathway response Regulates/coordinates many cell functions Signal molecule Signal-binding site CYTOPLASM Tyrosines Signal molecule Helix in the Membrane Tyr Dimer Receptor tyrosine kinase proteins (inactive monomers) P Cellular response 1 Inactive relay proteins Activated relay proteins Cellular response 2 Activated tyrosine- kinase regions (unphosphorylated dimer) Fully activated receptor tyrosine-kinase (phosphorylated 6 ATP ADP Figure 11.7
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Ion channel receptors When ligand binds, channel can open or close.
Cellular response Gate open Gate close Ligand-gated ion channel receptor Plasma Membrane Signal molecule (ligand) Figure 11.7 Gate closed Ions When ligand binds, channel can open or close. Ex: neurotransmitters bind as ligands for ion channels
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*Intracellular Receptors
Target protein is INSIDE the cell Must be hydrophobic molecule Hormone (testosterone) EXTRACELLULAR FLUID Receptor protein DNA mRNA NUCLEUS CYTOPLASM Plasma membrane Hormone- receptor complex New protein Figure 11.6 1 The steroid hormone testosterone passes through the plasma membrane. Testosterone binds to a receptor protein in the cytoplasm, activating it. 2 The hormone- receptor complex enters the nucleus and binds to specific genes. 3 The bound protein stimulates the transcription of the gene into mRNA. 4 The mRNA is translated into a specific protein. 5
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Evolutionary Significance
Unicellular and multicellular cell communication have similarities Yeast cells signal for sexual reproduction through signal transduction process. Bacteria secrete molecules to sense density of own population. Quorum Sensing (survival purpose)
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Yeast Sexual Reproduction
factor Receptor Exchange of mating factors. Each cell type secretes a mating factor that binds to receptors on the other cell type. 1 Mating. Binding of the factors to receptors induces changes in the cells that lead to their fusion. New a/ cell. The nucleus of the fused cell includes all the genes from the a and a cells. 2 3 Yeast cell, mating type a mating type a/ a Yeast cells identify their mates by cell signaling. Suggests early evidence of cell signaling.
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