Lecture 4

Question 1

physiological signals include

Answer 1

• electrical signals -> changes in the membrane potential of a cell
• chemical signals -> molecules secreted by cells into ECF
  -> bind to specific receptor on or in target cells

Question 2

what are the methods of local cell-cell communication

Answer 2

• gap junctions (direct cytoplasmic connections between adjacent cells) -> depolarization of one cell causes depolarization of neighboring cell
• contact dependent signals (require interaction between membrane molecules on two cells)-> synapses
• autocrine signals (act on the same cell that secreted them)
• paracrine signals (secreted by one cell and diffuse to adjacent cells) -> releases signal into interstitial fluid

Question 3

explain how hormones are an example of long distance communication (insulin)

Answer 3

• hormones are secreted by endocrine glands or cells into the blood
• only target cells with receptors for the hormone will respond to the signal
• insulin is secreted from the pancreatic beta cells and binds to insulin receptors on cells throughout the body, enabling glucose to enter these cells

Question 4

explain how neurotransmitters are an example of long distance communication (acetylcholine)

Answer 4

• neurotransmitters are chemicals secreted by neurons that diffuse across a small gap to the target cell
• acetylcholine is a neurotransmitter that can be used to elicit skeletal muscle contraction

Question 5

explain how neurohormones are an example of long distance communication (oxytocin)

Answer 5

• neurohormones are chemicals released by neurons into the blood for actions at distant targets
• oxytocin is released by neurons in the posterior pituitary gland into the blood and stimulates uterine contractions during labor and delivery

Question 6

what are the major classes and examples of chemical signaling molecules

Answer 6

• amino acids ->glutamate, glycine, GABA (neurotransmitters)
• amines -> dopamine, epinephrine, norepinephrine, histamine, serotonin
• peptides or proteins -> insulin, glucagon
• steroids -> derived from cholesterol
  -> estrogen, testosterone
• Ca2+, acetylcholine, nitric oxide

Question 7

explain intracellular signal receptors

Answer 7

• intracellular signal receptors
• lipophilic signal molecules diffuse through the cell membrane
• receptor in cytosol and in nucleus
• binding to cytosolic or nuclear receptors triggers slower responses related to changes in gene activity

Question 8

explain cell membrane receptors

Answer 8

• extracellular signal molecule binds to a cell membrane receptor
• binding triggers rapid cellular responses
• not membrane permeable

Question 9

what are the 4 categories of membrane receptors

Answer 9

• receptor channel -> ligand binding opens or closes the channel
• receptor enzyme -> ligand binding to a receptor enzyme activates an intracellular enzyme
• G protein coupled receptor -> ligand binding to a G protein coupled receptor open an ion channel or alters enzyme activity
• integrin receptor -> ligand binding to integrin receptors alters the cytoskeleton

Question 10

explain the signal transduction overview

Answer 10

• once a signaling molecule binds to its receptor the stimulus is transduced resulting in a series of intracellular responses that lead to a specific response
• whether a cell responds to a signal depends on presence of a receptor
• specific response of a cell depends on the receptors associated signal transduction pathway
• receptors and transduction pathways can vary in different cells/tissues

Question 11

what do kinases do

Answer 11

phosphorylate proteins
- add phosphate = turning it on

Question 12

what do phosphatases do

Answer 12

• dephosphorylate proteins
• remove phosphate = turning protein off

Question 13

what is signal amplification

Answer 13

• receptor ligand complex activates an amplifier enzyme
• one ligand is amplified into many intracellular molecules

Question 14

what is the signal transduction cascade

Answer 14

• prevents unwanted activation
• conversion of substrate to product is the final step of the cascade

Question 15

what are enzyme linked receptors

Answer 15

• ligand binds to receptor, which activates associated enzyme
  ex. tyrosine kinase transfers a phosphate group from ATP to a tyrosine (amino acid) of a protein

Question 16

what are GPCRs

Answer 16

• ligand binds to a receptor that is physically coupled to a guanosine nucleotide binding (G) protein
• largest family of cell surface receptors
• bind to and activate G proteins (made up of 3 subunits alpha, beta, gamma)
• when G proteins are activated they may -> directly alter ion channel gating
  -> alter the activity of another enzyme
• the type of g protein involved will determine which intracellular cascade is initiated

Question 17

explain G protein coupled receptors and adenylyl cyclase

Answer 17

• ligand binds to GPCR
• G protein binds to GPCR then Gs binds to adenylyl cyclase
• activation of AC triggers production of cAMP
• cAMP activates protein kinase A
• PKA phosphorylates to mediate response

Question 18

explain G protein coupled receptors and phospholipase C

Answer 18

• signal molecule activates receptor and associated G protein
• Galphaq activates phospholipase C (PLC)
• PLC cleaves PIP2(membrane phospholipid) into IP3 (inositol triphosphate) and DAG (diacylglycerol)
• DAG activates PKC which phosphorylates proteins
• IP3 causes release of Ca2+ from organelles creating a Ca2+ signal
  -> increase in cytosol Ca2+ levels

Question 19

what are ligand gated channels

Answer 19

• signaling molecule binds to its receptor
• the receptor is an ion channel that is gated by ligand binding

Question 20

what are 3 different signal transduction pathways using ion channels

Answer 20

1. receptor channels open or close in response to signal molecule binding
2. some channels are directly linked to G proteins
3. other channels respond to intracellular second messengers

Question 21

explain Ca2+ signaling

Answer 21

Ca2+ = 1.8 (outside the cell)
Ca2+ = <0.001 (inside the cell)
- it can enter the cell via voltage-gated, ligand-gated or mechanically gated channels
- can also be released from stores by second messengers
- increased Ca2+ interacts with Ca2+ binding proteins to exert effects (calmodulin or other binding proteins)
- can alter protein activity, exocytosis, or movement