Ch. 6: Cell Communication Flashcards
How do cells become different?
gene expression varies among cells; in some cells genes are turned off, while in others those genes are turned on, influenced by the environment
How do cells receive signals from other cells?
direct contact (gap junctions, plasmodesmata), synaptic signaling, paracrine signaling, endocrine signaling
Direct contact signaling
allows proteins, carbs, and lipids of plasma membrane to transmit info., common among cells of early development
2 ways: gap junctions, plasmodesmata
Gap junctions
in animal cells allow for chemical and electrical signaling between cells
ions and small molecules can pass but larger molecules like proteins/ nucleic acids cannot
Plasmodesmata
in plant cells tunnels of cytoplasm between cells that provide passageways across plasma membranes and cell walls for movement of ions, amino acids, sugars, small proteins and miRNA
Synaptic signaling
occurs between junctions of nerve cells or between nerve and muscle cells
neurotransmitters (short lived chemical signals) cross synapse to stimulate/ inhibit nerve impulse/ muscle contraction
Paracrine signaling
mechanism for local communication
cells secrete substances, like growth factors, that will affect only nearby cells who will readily absorb the hormones
Endocrine signaling
provides mechanism for long range communication throughout multicellular organism
ex. hormones made in one part of the body target cells in another part of body
Signal transduction pathway
sequence of molecular interactions that transforms an extracellular signal into a specific cellular response
signal (1st messenger) –> receptor –> proteins/ other 2nd messengers –> cellular response
Signaling molecules (ligands) 2 types
first messengers
small molecules that bind to larger receptor proteins of specific target cells to induce change in 3D structure of receptor protein that initiates activity
2 types:
Hydrophilic –> cannot cross bilayer so bind to membrane receptors
Hydrophobic –> able to cross membrane unaided so bind to intracellular receptors in cytoplasm/ nucleus
Receptor proteins
2 types
molecules that have binding sites for ligands, when activates they initiate a series of reactions to activate a cellular process
2 types:
Membrane receptors: transmembrane proteins w/ binding site on outside and cytoplasm side initiating chem. rxn
Intracellular receptors: proteins in cytoplasm/ nucleus
Second messengers
small, nonprotein, hydrophilic/ hydrophobic/ gaseous molecules that relay signal from inside face of receptor protein to other molecules that may initiate a cell response
ex. Ca2+, IP3, cAMP, DAG
Signaling cascade
series of enzymatic reactions, first enzyme activates next, and that one the next and so on… a chain reaction
enzymes can be used repeatedly so products of each reaction magnify as sequence progresses
signal that could have begun w/ one molecule can be amplified to produce a huge # of molecules for a strong cellular response
Kinase cascade (phosphorylation cascade)
signaling cascade wherein each kinase phosphorylates, thus activates, the next kinase in the sequence, ultimately phosphorylating and activating a protein that initiates cell response
amplifies signaling response
Scaffold proteins
improve efficiency of signaling cascade by holding all participating enzymes close to each other
also keep different signaling cascades apart
Protein phosphatase
enzyme that dephosphorylates its substrate, so dephosphorylate kinases in kinase cascade to stop signaling response
Advantages of complex STP (3)
- Amplification: amplify effect of signaling molecule
- Control: cell has more accuracy of signal pathway; bc all components of pathway must be functioning properly, there is small chance that error will occur
- Multiplicity: single signaling molecule can activate multiple cytoplasmic proteins, each generating a dif. response so many processes can be coordinated to produce one response
Gated ion receptor
transmembrane protein containing gated channel that opens in response to ligand (ligand-gated ion receptor) or voltage (voltage-gated ion receptor)
Ligand-gated- ion receptor (steps)
- Ligand-gated ion receptor receives signal: messenger binds
- Receptor channel opens and ions pass through: 3D shape changes
- Ions initiate chemical response: in cytoplasm
- Ligand-gated ion receptor deactivates when ligand detaches from receptor: broken, blocked allosterically, obstructed by blocker
Ligand-gated ion example (nerve impulse)
neuron transmitting signal releases acetylcholine into synapse, when acetylcholine binds to ligand-gated receptor, receptor molecules open the channel and allow sodium (Na+) to enter the cell. as Na+ enters cell it becomes (+). this change in membrane voltage causes action potential and initiates nerve impulse
neurons in muscles contract in similar process
G protein-coupled receptor (GPCR)
transmembrane protein that activates a G protein which ten activates another membrane protein which then triggers cell response/ activates 2nd messenger
has GTP attached to it
largest fam. of signal receptors: vision, tase, odor, hormones, immune system
GPCR steps
- GPCR receives signal: messenger ligand binds
- GPCR activates G protein by making GDP –> GTP
- G protein binds to effector protein
- Effector protein initiates response: enzymatic activity like activating enzyme, produce 2nd messenger cAMP which activates cytoplasmic response like protein kinase, produce 2nd messengers IP3 and DAG or Ca+
- GPCR signaling is deactivated when GTP is hydrolyzed: GTP –> GDP + Pi
cAMP signaling pathway example (Glycogen breakdown)
in muscle and liver cells
G protein activated by epinephrine (signaling molecule) to change GTP for GDP on effector protein adenylyl cyclase to convert ATP to cAMP. cAMP phosphorylates (activates) protein kinase PKA which leads to activation of enzyme that removes glucose from glycogen
Protein kinase receptor
transmembrane protein-enzyme that add phosphate group to protein where OH- group is
such hydroxyl groups only found in tyrosine, serine, threonine
Receptor tyrosine kinase (RTK) steps
- RTK receives signal by signaling molecule
- RTK forms dimer: 2 RTKs associate forming pair (dimer)
- RTK activated by autophosphorylation: on inner surface or membrane, each RTK in dimer phosphorylates the other using phosphate groups form ATP
- Relay protein is phosphorylated
- Relay protein initiates signal transduction pathway: activated by phosphorylation relay protein can activate chemical signal
- RTK pathway deactivated by dephosphorylation
Differences between RTK and GPCR (2)
- RTK directly responsible for initiating STP while GPCR indirectly activates STP via G protein and effector protein
- RTK can trigger many transduction pathways, directing host of coordinated responses while GPCR triggers single STP for a specific response
RTK pathway example (Insulin signal transduction)
insulin (signaling molecule) secreted into blood as result of excess glucose and binds to insulin receptor. insulin receptor conformationally changes and forms RTK dimer. then complex phosphorylates insulin response protein to initiate many signaling cascades. in muscles, glycogen synthesis for short term energy storage and transport of glucose into cell
Intracellular receptor
positioned in cytoplasm/ nucleus
ligands are small/ lipid-soluble nonpolar molecules that can passively diffuse across plasma membrane
IP3
Intracellular receptor pathway steps
- Signaling molecule enters cytoplasm
- Signaling molecule binds to intracellular receptor, activating it: may be in cytoplasm or nucleus
- Receptor-signaling molecule complex acts as transcription factor: binds to DNA to promote/ suppress gene transcription
- Deactivation of pathway occurs when signaling molecules/ receptor proteins enzymatically degraded: can happen by (-) feedback mechanism
Ligands that bind to intracellular receptors
steroid hormones like testosterone/ estrogen
Diseases as a result of distorted STP
Cholera: waterborne bacteria caused disease that disrupts GCPR activity in intestines
Cancer: uncontrolled cell division by improper cell control and response to growth factors
