1.4 Communication and Signalling

Question 1

how do multicellular organisms signal between cells

Answer 1

using extracellular signalling molecules

Question 2

what are examples of extracellular signalling molecules

Answer 2

steroid hormones
peptide hormones
neurotransmitters

Question 3

what are receptor molecules of target cells

Answer 3

proteins with a binding for a specific signal molecule

Question 4

why does binding change the conformation of the receptor

Answer 4

initiates a response within the cell

Question 5

different cell types produce specific signals that can only

Answer 5

be detected and responded to by cells with the specific receptor

Question 6

why can signalling molecules have different effects on different target cell types

Answer 6

due to differences in the intracellular signalling molecules and pathways that are involved

Question 7

in a multicellular organism, different cell types may show

Answer 7

a tissue-specific response to the same signal

Question 8

how do hydrophobic signalling molecules transport through the membrane

Answer 8

they can diffuse directly through the phospholipid bilayers of membranes and so bind to intracellular receptors

Question 9

what are the receptors for hydrophobic molecules

Answer 9

transcription factors

Question 10

what are transcription factors

Answer 10

proteins that when bound to DNA can either stimulate or inhibit initiation of transcription

Question 11

what are examples of hydrophobic signalling molecules

Answer 11

steroid hormones
oestrogen
testosterone

Question 12

what do steroid hormones bind to

Answer 12

specific receptors in the cytosol or the nucleus

Question 13

where does the hormone-receptor complex move to

Answer 13

the nucleus where it binds to specific sites on DNA and affects gene expression

Question 14

what do hormone-receptor complexes bind to

Answer 14

specific DNA sequences called hormone response elements (HREs).
binding at these sites influences the rate of transcription, with each steroid hormone affecting the gene expression of many different genes

Question 15

what do hydrophilic signalling molecules bind to

Answer 15

transmembrane receptors and do not enter the cytosol

Question 16

what are examples of hydrophilic extracellular signalling molecules

Answer 16

peptide hormones

neuritransmitters

Question 17

what happens when the ligand binds to the extracellular face

Answer 17

transmembrane receptors change conformation when when the ligand binds to the extracellular face
the signaling molecule does not enter the cell, but the signal is transduced across the plasma membrane

Question 18

how do transmembrane receptors act as signal transducers

Answer 18

by converting the extracellular ligand-binding event into intracellular signal, which alters the behaviour of the cell

Question 19

what do transduced hydrophilic signals often involve

Answer 19

G-proteins or cascades of phosphorylation by kinase enzymes

Question 20

G-proteins relay signals from

Answer 20

activated receptors (receptors that have bound a signalling molecule) to target proteins such as enzymes and ion channels

Question 21

what do phosphorylation cascades allow

Answer 21

more than one intracellular signalling pathway to be activated

Question 22

what do phosphorylation cascades involve

Answer 22

a series of events with one kinase activating the next in the sequence and so on

Question 23

what can phosphorylation cascades result in

Answer 23

the phosphorylation of many proteins as a result of the original signalling event

Question 24

what does binding of the peptide hormone insulin to its receptor result in

Answer 24

an intracellular signalling cascade that triggers recruitment of GLUT4 glucose transporter proteins to the cell membrane of fat and muscle cells

Question 25

what does binding of insulin to its receptor cause

Answer 25

a conformational change that triggers phosphorylation of the receptor this starts a phosphorylation cascade inside the cell, which eventually leads to GLUT4-containing vesicles being transported to the cell membrane

Question 26

what can diabetes mellitus be caused by

Answer 26

type 1 or type 2 diabetes

Question 27

what is type 1 diabetes

Answer 27

failure to produce insulin

Question 28

what is type 2 diabetes

Answer 28

loss of insulin receptor function

Question 29

what is type 2 diabetes associate with

Answer 29

obesity

Question 30

what triggers the recruitment of GLUT4

Answer 30

exercise | increase uptake of glucose to fat and muscle cells in subject with type 2

Question 31

what is the resting membrane potential

Answer 31

a state where there is no net flow of ions across the membrane

Question 32

what does the transmission of a nerve impulse require

Answer 32

changes in the membrane potential of the neuron's plasma membrane

Question 33

what is an action potential

Answer 33

a wave of electrical excitation along a neuron's plasma membrane

Question 34

what do neurotransmitters do

Answer 34

initiate a response by binding to their receptors at a synapse

Question 35

what are neurotransmitters receptors

Answer 35

ligand-gated ion channels

Question 36

what does the depolarisation of the plasma membrane as a result of the entry of positive ions trigger

Answer 36

the opening of voltage-gated sodium channels and further depolarisation occurs

Question 37

what is depolarisation

Answer 37

a change in the membrane potential to a less negative value inside

Question 38

what does inactivation of the sodium channels and the opening of potassium channels do

Answer 38

restores the resting membrane potential

Question 39

stages of restoring membrane potential

Answer 39

binding of a neurotransmitter triggers the opening of ligand-gated ion channels at a synapse. ion movement occurs and there is depolarisation of the plasma membrane. if sufficient ion movement occurs, and the membrane is depolarised beyond a threshold value, the opening of voltage-gated sodium channels is triggered and sodium ions enter the cell done their electrochemical gradient. this leads to a rapid and large change in the membrane potential. a short time after opening, the sodium channels become inactivated. voltage-gated potassium channels then open to allow potassium ions to move out of the cell to restore the resting membrane potential

Question 40

what does depolarisation of a patch of membrane cause

Answer 40

neighbouring regions of membrane to depolarise and go through the same cycle, as adjacent voltage-gated sodium channels are opened

Question 41

what happens when the action potential reaches the end of the neuron

Answer 41

it causes vesicles containing neurotransmitters to fuse with the membrane this releases neurotransmitter which stimulates a response in a connecting cell

Question 42

what does restoration of the resting membrane potential allow

Answer 42

allows the inactive voltage-gated sodium channels to return to a conformation that allows them to open again in response to depolarisation of the membrane

Question 43

how are ion concentration gradients re-established

Answer 43

by sodium-potassium pumps. | this actively transports excess ions in and out of the cell

Question 44

what follows depolarisation

Answer 44

the sodium and potassium ion concentration gradients are reduced. the sodium-potassium pump restores the sodium and potassium ions back to resting potential

Question 45

what is the retina

Answer 45

the area within the eye that detects light and contains two types of photoreceptor cells

Question 46

what are the types of photoreceptor cells that the retina contains

Answer 46

rods | cones

Question 47

what are rods

Answer 47

they function in dim light but do not allow colour perception

Question 48

what are cones

Answer 48

responsible for colour vision and only function in bright light

Question 49

what is the light-sensitive molecule retinal combines with

Answer 49

a membrane protein, opsin, to form the photoreceptors of the eye

Question 50

what is the retinal-opsin complex called in rod cells

Answer 50

rhodopsin

Question 51

what happens when retinal absorbs a photon of light

Answer 51

rhodopsin changes conformation to photoexcited rhodopsin

Question 52

what amplifies the signal

Answer 52

a cascade of proteins

Question 53

what does photoexcited rhodopsin activate

Answer 53

G-proteins called transducin, which activates the enzyme phosphodiesterase (PDE)

Question 54

how many molecules of G-protein does a single photoexcited rhosdopsin activate

Answer 54

a single photoexcited rhodopsin activated hundreds of molecules of G-protein. each activated G-protein activates one more of PDE

Question 55

what does PDE catalyse

Answer 55

the hydrolysis of a molecule called cyclic GMP (cGMP)

Question 56

what does each active PDE molecule do

Answer 56

breaks down thousands of cGMP molecules per second

Question 57

what does the reduction in cGMP concentration affect

Answer 57

the reduction in cGMP concentration as a result of its hydrolysis affects the function of ion channels in the membrane of rod cells.

Question 58

what does the reduction in cGMP concentration result in

Answer 58

the closure of ion channels in the membrane of the rod cells, which triggers nerve impulses in neurons in the retina

Question 59

what does a high degree of amplification result in

Answer 59

rod cells being able to respond to low intensities of light

Question 60

in cone cells, different forms of opsin combine with

Answer 60

retinal to give different photoreceptor proteins, each with a maximal sensitivity to specific wavelengths: red, green, blue or UV