Ch. 6: Cell Communication

Question 1

How do cells become different?

Answer 1

gene expression varies among cells; in some cells genes are turned off, while in others those genes are turned on, influenced by the environment

Question 2

How do cells receive signals from other cells?

Answer 2

direct contact (gap junctions, plasmodesmata), synaptic signaling, paracrine signaling, endocrine signaling

Question 3

Direct contact signaling

Answer 3

allows proteins, carbs, and lipids of plasma membrane to transmit info., common among cells of early development
2 ways: gap junctions, plasmodesmata

Question 4

Gap junctions

Answer 4

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

Question 5

Plasmodesmata

Answer 5

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

Question 6

Synaptic signaling

Answer 6

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

Question 7

Paracrine signaling

Answer 7

mechanism for local communication

cells secrete substances, like growth factors, that will affect only nearby cells who will readily absorb the hormones

Question 8

Endocrine signaling

Answer 8

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

Question 9

Signal transduction pathway

Answer 9

sequence of molecular interactions that transforms an extracellular signal into a specific cellular response
signal (1st messenger) –> receptor –> proteins/ other 2nd messengers –> cellular response

Question 10

Signaling molecules (ligands)
2 types

Answer 10

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

Question 11

Receptor proteins

2 types

Answer 11

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

Question 12

Second messengers

Answer 12

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

Question 13

Signaling cascade

Answer 13

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

Question 14

Kinase cascade (phosphorylation cascade)

Answer 14

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

Question 15

Scaffold proteins

Answer 15

improve efficiency of signaling cascade by holding all participating enzymes close to each other
also keep different signaling cascades apart

Question 16

Protein phosphatase

Answer 16

enzyme that dephosphorylates its substrate, so dephosphorylate kinases in kinase cascade to stop signaling response

Question 17

Advantages of complex STP (3)

Answer 17

1. Amplification: amplify effect of signaling molecule
2. 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
3. Multiplicity: single signaling molecule can activate multiple cytoplasmic proteins, each generating a dif. response so many processes can be coordinated to produce one response

Question 18

Gated ion receptor

Answer 18

transmembrane protein containing gated channel that opens in response to ligand (ligand-gated ion receptor) or voltage (voltage-gated ion receptor)

Question 19

Ligand-gated- ion receptor (steps)

Answer 19

1. Ligand-gated ion receptor receives signal: messenger binds
2. Receptor channel opens and ions pass through: 3D shape changes
3. Ions initiate chemical response: in cytoplasm
4. Ligand-gated ion receptor deactivates when ligand detaches from receptor: broken, blocked allosterically, obstructed by blocker

Question 20

Ligand-gated ion example (nerve impulse)

Answer 20

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

Question 21

G protein-coupled receptor (GPCR)

Answer 21

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

Question 22

GPCR steps

Answer 22

1. GPCR receives signal: messenger ligand binds
2. GPCR activates G protein by making GDP –> GTP
3. G protein binds to effector protein
4. 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+
5. GPCR signaling is deactivated when GTP is hydrolyzed: GTP –> GDP + Pi

Question 23

cAMP signaling pathway example (Glycogen breakdown)

Answer 23

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

Question 24

Protein kinase receptor

Answer 24

transmembrane protein-enzyme that add phosphate group to protein where OH- group is
such hydroxyl groups only found in tyrosine, serine, threonine

Question 25

Receptor tyrosine kinase (RTK) steps

Answer 25

1. RTK receives signal by signaling molecule
2. RTK forms dimer: 2 RTKs associate forming pair (dimer)
3. RTK activated by autophosphorylation: on inner surface or membrane, each RTK in dimer phosphorylates the other using phosphate groups form ATP
4. Relay protein is phosphorylated
5. Relay protein initiates signal transduction pathway: activated by phosphorylation relay protein can activate chemical signal
6. RTK pathway deactivated by dephosphorylation

Question 26

Differences between RTK and GPCR (2)

Answer 26

1. RTK directly responsible for initiating STP while GPCR indirectly activates STP via G protein and effector protein
2. RTK can trigger many transduction pathways, directing host of coordinated responses while GPCR triggers single STP for a specific response

Question 27

RTK pathway example (Insulin signal transduction)

Answer 27

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

Question 28

Intracellular receptor

Answer 28

positioned in cytoplasm/ nucleus
ligands are small/ lipid-soluble nonpolar molecules that can passively diffuse across plasma membrane
IP3

Question 29

Intracellular receptor pathway steps

Answer 29

1. Signaling molecule enters cytoplasm
2. Signaling molecule binds to intracellular receptor, activating it: may be in cytoplasm or nucleus
3. Receptor-signaling molecule complex acts as transcription factor: binds to DNA to promote/ suppress gene transcription
4. Deactivation of pathway occurs when signaling molecules/ receptor proteins enzymatically degraded: can happen by (-) feedback mechanism

Question 30

Ligands that bind to intracellular receptors

Answer 30

steroid hormones like testosterone/ estrogen

Question 31

Diseases as a result of distorted STP

Answer 31

Cholera: waterborne bacteria caused disease that disrupts GCPR activity in intestines
Cancer: uncontrolled cell division by improper cell control and response to growth factors