Cell Communication Flashcards
Significance of Communication
Allows for multicellular organisms to coordinate activities for their trillions of cells Responsible for: – Recognition of pathogens – Creating responses to the environment – Growth & division – Movement – Metabolic activities
Local & Long Distance Signaling
• Cells use chemical signals for means of communication
– Chemical messengers may travel a very short distance or throughout an entire organism to reach their target cell
Local Regulators
For communication with cells that are touching (juxtacrine) or within a very short distance (paracrine)
• These messages can reach several local cells quickly to elicit a response
– The chemical messenger will be short lived & degraded easily
• Examples:
– Growth factors released to induce cell growth & division
– Neurotransmitters crossing the synapse
Long Distance Signaling
Hormones are used to send messages throughout animals to target specific cells (endocrine signaling)
– These travel great distances & create responses in multiple cells simultaneously
– Moved by the circulatory system
– Do not degrade quickly
Three Stages of Cell Signaling
Cells will have three stages of response to a signal
– Reception
– Transduction
– Response
Reception
The target cell detects a signaling molecule from outside the cell
• The molecule will bind with receptors embedded in the plasma membrane or receptors located within the cell
Transduction
Conversion of the signal into a form that creates a cellular response
• Typically, this is a multistep process that involves numerous relay molecules
– AKA: the signal transduction pathway
– Different pathways exist for different cells, even with the same signaling molecule
Response
The last transduced molecule will trigger a specific cellular response
– This can be almost any cell activity: activation of an enzyme, rearrangement of the cytoskeleton, activation or deactivation of a gene
• This process ensures that only the correct cells are completing the proper activities at a specific time
Intracellular Responses
• Receptor proteins are located within the cytoplasm or nucleus of a cell
• Chemical messengers must pass through the plasma membrane to stimulate the target cell
– Signal molecule must be hydrophobic
• Example: testosterone
– Secreted from the testes this hormone crosses the plasma membrane & attaches to its receptor, where together they travel to the nucleus & turn on genes that control male sex characteristics
Receptors on the Plasma Membrane
• Most molecules bind to specific proteins that span the cell membrane
– The binding of the ligand will cause a change to the shape of receptor protein starting thetransduction pathway within the cytoplasm of the cell
• Key examples:
– G-protein-linked receptors (aka: G protein-coupled receptors)
– Receptor tyrosine kinases
– Ion channel receptors (ligand-gated ion channels)
G-Protein-Linked Receptors
A receptor protein that works with the help of a G protein
– G proteins bind the energy-rich molecule GTP
• A signaling molecule bonds to the extracellular side of the receptor which will become activated & change shape
• The cytoplasmic side of the receptor binds & activates a G protein which carries GTP
• The activated G protein leaves the receptor & diffuses along the membrane until it binds to an enzyme
• The activated enzyme can then trigger the next step in the transduction pathway
• All these processes are reversible & all molecules will inactivate for reuse
Receptor Tyrosine Kinases
Receptors that can trigger more than one signal transduction pathway at once by phosphorylating multiple relay molecules
– 10 or more pathways can be activated simultaneously
• A ligand will bond to the active site on both individual proteins which causes them to form a dimer (work together)
• The dimerization activates the tyrosine-kinase region adding a phosphate from ATP to each tyrosine
• The fully activated protein is recognized by relay proteins
which bind to the
phosphorylated tyrosine becoming active & triggering a transduction pathway
Ion Channel Receptors
A membrane receptor that has a gate that blocks the passage of ions until a ligand bonds to the receptor to open/close the gate
– The gate will return to its original position once the ligand dissociates
– Crucial in the nervous system
Signal Transduction Pathway
Multistep pathway that turns the signal into a cellular response
• Typical pathway include the activation of one protein by the addition/removal of a phosphate groups or the release of small molecules or ions that act as second messengers
• Benefits:
– Ability to amplify the signal
– Opportunity for more coordination & regulation
Phosphorylation Cascades
Phosphorylation (add a phosphate group) & dephosphorylation are mechanisms for regulating protein activity
– Protein kinase is an enzyme that transfers phosphate groups from ATP to proteins
– When multiple protein kinases act upon each other a phosphorylation cascade can be created
Steps of a Phosphorylation Cascade
A relay molecule is activated by a ligand bonding to the receptor which will activate protein kinase 1
• Protein kinase 1 will activate protein kinase 2 by adding a phosphate group from ATP
– This process continues until the protein that regulates the cellular response is activated
• Protein phosphatases removes the phosphates from the activated proteins to return them to an original state
Second Messengers
Many signal transduction pathways utilize small, water soluble molecule or ions to relay messages within the cytoplasm
• Two most common:
– Cyclic AMP
– Calcium (Ca2+)
Cyclic AMP
A ligand (epinephrine) will bond to a G-protein-linked receptor which activates adenylyl cyclase that will convert ATP into cyclic AMP
• Large quantities of cAMP are quickly produced to broadcasts the message
through the cell
• cAMP is short lived and converted to AMP
• The typical cAMP pathway will stimulate a protein kinase which phosphorylates another molecule
Calcium
The goal is to increase calcium concentration in the cytoplasm
– This creates a cellular response such as muscle contraction, secretion of neurotransmitters, & the trigger of transduction pathways
• Calcium is released from the endoplasmic reticulum in response to a transduction
pathway that open a calcium gated ion channel
Cellular Response
The ultimate goal is the cellular response
• In the cytoplasm:
– Cellular response may regulate the activity of a protein
• In the nucleus:
– Many pathways regulate gene expression
– Activated molecules may act as a transcription factor (which can regulated numerous genes)
