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Cell Communication
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The “Cellular Internet”
Biologists have discovered some universal mechanisms of cellular regulation that involve cell-to-cell communication. External signals are converted into responses within the cell
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Methods used by Cells to Communicate
Cell-Cell communication Cell Signaling using chemical messengers Local signaling over short distances Cell-Cell Recognition Local regulators Paracrine (growth factors) Synaptic (neurotransmitters) Long distance signaling Hormones
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Cell-Cell Communication
Animal and plant cells Have cell junctions that directly connect the cytoplasm of adjacent cells Plasma membranes Plasmodesmata between plant cells Gap junctions between animal cells Figure 11.3 (a) Cell junctions. Both animals and plants have cell junctions that allow molecules to pass readily between adjacent cells without crossing plasma membranes.
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Cell-Cell Communication
Animal cells use gap junctions to send signals Cells must be in direct contact Protein channels connecting two adjoining cells Gap junctions between animal cells
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Cell-Cell Communication
Plant cells use plasmodesmata to send signals Cells must be in direct contact Gaps in the cell wall connecting the two adjoining cells together Plasmodesmata between plant cells
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Local Signaling: Cell-Cell Recognition
In local signaling, animal cells may communicate via direct contact Membrane bound cell surface molecules Glycoproteins Glyolipids Figure 11.3 (b) Cell-cell recognition. Two cells in an animal may communicate by interaction between molecules protruding from their surfaces.
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Local Signaling: Local Regulators
In other cases, animal cells Communicate using local regulators Only work over a short distance (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: Hormones
In long-distance signaling Both plants and animals use hormones 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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Long-Distance Signaling
Nervous System in Animals Electrical signals through neurons Endocrine System in Animals Uses hormones to transmit messages over long distances Plants also use hormones Some transported through vascular system Others are released into the air
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The Three Stages of Cell Signaling
Earl W. Sutherland (1971) Discovered how the hormone epinephrine acts on cells Sutherland suggested that cells receiving signals went through three processes Reception Transduction Response Called 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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Overview of cell signaling
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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Three Stages of Cell Signaling
EXTRACELLULAR FLUID CYTOPLASM Plasma membrane 1 1 Reception The receptor and signaling molecules fit together (lock and key model, induced fit model, just like enzymes!) Receptor Signaling molecule Signaling molecule binds to the receptor protein
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Three Stages of Cell Signaling
EXTRACELLULAR FLUID CYTOPLASM Plasma membrane 1 1 Reception 2 Transduction Receptor 2nd Messenger! Relay molecules in a signal transduction pathway Signaling molecule The signal is converted into a form that can produce a cellular response
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Three Stages of Cell Signaling
EXTRACELLULAR FLUID CYTOPLASM Plasma membrane 1 Reception 2 Transduction 3 Response Receptor Activation of cellular response Relay molecules in a signal transduction pathway Can be catalysis, activation of a gene, triggering apoptosis, almost anything! Signaling molecule The transduced signal triggers a cellular response
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Signal Transduction Animation
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Calcium ions and Inositol Triphosphate (IP3)
Calcium, when released into the cytosol of a cell acts as a second messenger in many different pathways Calcium is an important second messenger because cells are able to regulate its concentration in the cytosol EXTRACELLULAR FLUID Plasma membrane ATP CYTOSOL Ca2+ pump Endoplasmic reticulum (ER) Nucleus Mitochondrion Key High [Ca2+] Low [Ca2+] Other second messengers such as inositol triphosphate and diacylglycerol can trigger an increase in calcium in the cytosol
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3. Response Many possible outcomes
Growth factor 3. Response Receptor Reception Many possible outcomes This example shows a transcription response Phosphorylation cascade Transduction CYTOPLASM Inactive transcription factor Active transcription factor Response P DNA Gene NUCLEUS mRNA
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Specificity of the signal
Signaling molecule Specificity of the signal The same signal molecule can trigger different responses Many responses can come from one signal! Receptor Relay molecules Response 1 Response 2 Response 3 Cell A. Pathway leads to a single response. Cell B. Pathway branches, leading to two responses.
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The signal can also trigger an activator or inhibitor
The signal can also trigger multiple receptors and different responses Activation or inhibition Response 4 Response 5 Cell C. Cross-talk occurs between two pathways. Cell D. Different receptor leads to a different response.
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REVIEW: Signal-transduction pathway
Definition: Signal on a cell’s surface is converted into a specific cellular response Local signaling (short distance): √ Paracrine (growth factors) √ Synaptic (neurotransmitters) Long distance: hormones
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Stages of cell signaling
3 steps: •Reception: target cell detection •Transduction: single-step or series of changes •Response: triggering of a specific cellular response
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The three stage of cellular signaling: Reception, Transduction, and Response.
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Tight junction
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Desmosome junction
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