packet #17 chapter #11 introduction to cell communication
TRANSCRIPT
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PACKET #17CHAPTER #11
Introduction to Cell Communication
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Why Communication Between Cells is Important
A typical free-living cell must be able to Sniff out nutrients Sense the difference between light and dark Detect and avoid poisons and predators
Cells must also be able to communicate to provide long range integration of metabolism.
Therefore, cells must be able to communicate with each other.
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Players of Cell Signaling
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The Players
The Signal Signaling Cell
Cell sending the message Target Cell
Cell receiving the message Ligand
Molecule such as a hormone or a neurotransmitter that binds to a specific site on a protein (receptor protein)
Receptor Protein* Recognizes and responds specifically to the signal molecule Performs the first step in a series of transduction processes
at the receiving end
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Molecules Used As Signals
ProteinsPeptidesAmino AcidsNucleotidesSteroidsFatty Acid DerivativesDissolved Gases
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Target Cell
Contain receptor proteins in the cell membrane Recognizes and responds specifically to the signal
molecule Performs the first step in a series of transduction
processes at the receiving end
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General Steps of Cell Signaling
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Three Steps
Three Steps Signal reception Transduction Response
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Signal Reception & Types of Receptors
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Reception
Cells are stimulated by an extracellular signalSignal binds to, and activates, a receptor
proteinEach receptor protein recognizes a specific
signal molecule Once signal is recognized, a new intracellular signal is
generated. This is the first step of transduction
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Receptors I
There are two basic mechanisms by which chemical signals cause a biologic effect within the cell Hence there are two
types of receptors Intracellular receptors
Inside the cell Cell surface receptors
Found on the surface of the cell
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Classes of Cell-Surface Receptors
Three classes Ion-channel linked receptor G-protein linked receptor Enzyme-linked receptor
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Intracellular Receptors
Used for molecules that are sufficiently small and hydrophobic to diffuse across the membrane Best known hydrophobic signal molecules
Steroid hormones Cortisol Estradiol Testosterone
Thyroid hormones Thyroxine
Vitamin D Retinoic Acid
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Intracellular Receptors
Receptors lie in the interior of the target cell Cytosol or nucleus When steroid hormone binds, receptor undergoes
large conformational change that enables it to bond to its corresponding sequence in the DNA Promotes or inhibits transcription of a set of genes
Generally regulatory proteins or enzymesMore details to come later.
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Cell Surface Receptors
Largest class of receptorsUsed for signals that are too large or too
hydrophillic to cross the plasma membraneLie in the plasma membrane of the cellRelays message across the membrane
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Signal Transduction & Transduction Pathways
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Signal Transduction
Signal transduction is the continuation and/or conversion of signals from one form to the next. The signals, although
sometimes in different forms, represent the same information.
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Signal Transduction II
After reception, when the original signal molecule binds on the receptor protein, the message is passed through the cell membrane through a set of intracellular signaling molecules—transduction.
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Pathways
Two classifications Catabolic pathway
Degradable pathway Reactions serve to capture chemical energy {ATP} from
the degradation of energy-rich fuel molecules Reactions that break down complex molecules, such as
proteins, polysaccharides, and lipids, to a few simple molecules
Anabolic Synthetic pathway
Combine small molecules, such as amino acids to form complex molecules such as proteins
Require energy ATP is broken down to ADP + P
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Introduction of Pathways
Multistep Pathways Can amplify a signal Provide more opportunities for coordination and regulation
At each step in a pathway The signal is transduced into a different form, commonly a
conformational change in a protein Many signal pathways
Include phosphorylation cascades In this process
A series of protein kinases add a phosphate to the next one in line, activating it
Phosphatase enzymes then remove the phosphates
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Phosphorylation Cascades
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Cascade Utilizing G-proteins
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Regulation of Pathways
How does the organism regulate metabolism? Regulatory signals inform individual cells of the
metabolic state of the body as a whole Hormones Nervous system Availability of nutrients
These influence signals that are generated within the cell itself
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Signals Within the Cell
Regulatory signals Substrates Allosteric activators Allosteric inhibitors Product Cause a rapid response
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Signaling Cascades
The relay chain during transduction Physically transfer the point of reception to the end
point where the cell machinery will make the response Transform the signal into a form that stimulates the
response Amplify the signal received Distribute the signal so that many processes are
influenced Each step is open to interference from other factors so
that the signal of the cell can me modulated according to conditions prevailing inside or outside the cell
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Questions
How do cells receive the signal?Where do the cells receive the signal?What do cells do once they have received the
signal?How do cells respond to the signals received?Among all the hundreds of signals being sent,
and received, how do cells determine which signal was sent to them?
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“Types” of Communication
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Private vs. Public
Private Messages Only a select few cells receive the message
Public Messages Message sent throughout entire body
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Long Distance vs. Short Distances
Long Distance Messages Are messages that are sent
over a long path from the point of origin location up until the signal's final spot.**
Short Distance Messages Are messages that are sent
over a short distance from the point of origin up until the signal’s final stop.** Sometimes within the
immediate area.
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ExamplePublic & Long Distance
Hormonal Signaling Hormones may be sent throughout the bodies of
plants and animals In animals, hormonal signals are secreted into the
bloodstream In plants, hormonal signals are secreted into plant sap.
Hormones, produced in animals, are produced in endocrine cells and help make up the Endocrine System.
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ExamplePublic and Short Distance
Paracrine Signaling Signal molecules diffuse locally through the
extracellular medium Signal remains in the neighborhood of the cell that
secretes them Signal acts as local mediators
Regulate inflammation at sites of infection Regulate cell proliferation in wound healing
Growth factorsStimulate target cells to grow and multiply
• Part of the bigger Immune SystemUsed in the healing process
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Private & Short DistanceSynaptic Signaling
Signals sent via the nervous system are sent over long distances. However, the signal must travel along multiple individual nerve cells and the communication between individual nerve cells is called synaptic signaling.
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Private & Short DistanceIntimate Messaging
Most short range of all Face to face conversation
No molecules are released Different in comparison to the other three.
Cells make direct contact, through signaling molecules, in their plasma membrane Message is delivered by the binding of a signal molecule
anchored in the plasma membrane of the signaling cell to a receptor molecule embedded in the plasma membrane of the target cell Cell to cell connection
Remember from earlier packet the functions of proteins in cell membrane.
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Private & Short DistanceIntimate Messaging II
In embryonic development, this type of signaling has an important role in tissues where adjacent cells that are initially similar have to become specialized in different ways Plays role in determining what type of cell a stem cell
becomes.
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Types of Cell to Cell Connection
Gap JunctionsTight JunctionsDesomosomesPlasmodesmata*
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CELL TO CELL CONNECTIONS
Intimate Messaging
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Gap Junctions
Communicating cell to cell junction that allows ions and small molecules to pass from the cytoplasm of one cell to the cytoplasm of the next Found in animal cells
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Tight Junctions
Cell to cell junction that seals adjacent epithelial cells together, preventing dissolved molecules in the extracellular medium from passing from one side of the epithelial sheet to the other
Found in animal cells.
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Desmosomes
Specialized to cell to cell junction, usually formed between two epithelial cells, mediated by cadherin molecules and characterized by dense plaques of protein into which intermediate filaments in the two adjoining cells insert
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Plasmodesmata
Organelle found in plant cells that allow adjacent cells to communicate, relay signals and move materials between them
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Private & Short Distance
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Yeast Cells
When a yeast cell is ready to mate, it secretes a small protein called a mating factor to which other yeast cells are sensitive They detect the mating
factor They put out a
protrusion toward the source of the factor
Halt the cell cycle