1.4 communication and signalling

Question 1

how do multicellular organisms achieve coordination and communication?

Answer 1

extracellular signalling molecules, receptors and responses

Question 2

how does the binding of a signalling molecule change a receptor?

Answer 2

changes its conformation, altering/initiating the response in the cell

Question 3

why can different cell types show a tissue-specific response to the same signal?

Answer 3

due to differences in intracellular pathways and signalling molecules

Question 4

how do hydrophobic signals work?

Answer 4

by passing directly through phospholipid bilayers and binding to intracellular receptors

Question 5

why can hydrophobic signals diffuse through the bilayer?

Answer 5

the tails of the phospholipids in the PM are also hydrophobic

Question 6

how can hydrophobic signals influence the transcription of genes?

Answer 6

by binding to transcription factors

Question 7

what are transcription factors?

Answer 7

a protein that when bound to DNA can either stimulate or inhibit transcription

Question 8

name two steroid hormones

Answer 8

oestrogen and testosterone

Question 9

what type of signalling molecules are oestrogen and testosterone?

Answer 9

hydrophobic

Question 10

describe the process of a hydrophobic signalling molecule affecting transcription

Answer 10

• the hormone binds to a receptor protein in the cytosol/nucleus
• this forms a hormone/receptor complex
• this binds to a specific DNA sequence called HREs
• this influences the rate of transcription by affecting the expression of several different genes

Question 11

what does HREs stand for?

Answer 11

hormone response elements

Question 12

how do hydrophilic signals work?

Answer 12

signal molecules bind to proteins on the plasma membrane that act as receptors

Question 13

name two examples of hydrophilic signal molecules

Answer 13

neurotransmitters and peptide hormones

Question 14

how do transmembrane receptors proteins work?

Answer 14

they change conformation when the ligand binds to the extracellular face of the protein, triggering a transduction pathway

Question 15

how does a signal transduction pathway work?

Answer 15

it links an extracellular chemical stimulus to a specific cellular response, altering the behaviour of the cell

Question 16

what is a specific cellular response?

Answer 16

a cascade of biochemical reactions inside the cell that eventually reach the target molecule or reaction

Question 17

an enzyme needs to be activated to bring out a response within the cell, but the receptor cannot directly interact with it. how is this resolved?

Answer 17

a G-protein is a relay protein which can relay the signal and activate the enzyme

Question 18

what does the active G-protein do?

Answer 18

stimulates an enzyme, leading to a response in the cell

Question 19

why is the response from a G-protein only temporary?

Answer 19

because the G protein also acts as a GTPase and hydrolysis the bound GTP to GDP, making the G protein inactive again

Question 20

what is a phosphorylation cascade?

Answer 20

a series of events activated by one kinase, wich activates the next kinase etc

Question 21

what do phosphorylation cascades result in?

Answer 21

the phosphorylation of many proteins from one original signal

Question 22

what happens when insulin, a peptide hormone, binds to its receptor?

Answer 22

change in conformation, which causes a phosphorylation cascade
this causes the recruitment of GLUT4 to the plasma membrane

Question 23

exercise can trigger GLUT4, therefore…

Answer 23

improving the uptake of glucose to fat in muscle cells in people with type 2 diabetes

Question 24

what is the resting potential?

Answer 24

the membrane potential of a neuron that is not transmitting signals

Question 25

what does it mean if a neuron is not transmitting signals?

Answer 25

there is no net flow of ions across the membrane

Question 26

what is an action potential?

Answer 26

a wave of electrical excitation along the PM of a neuron

Question 27

what does the transmission of a nerve impulse require?

Answer 27

the change of membrane potential to action potential

Question 28

what type of channel proteins are neurotransmitter receptors?

Answer 28

ligand-gated ion channels

Question 29

what occurs when neurotransmitters bind to their complementary receptors at a synapse?

Answer 29

depolarisation of the PM (the membrane potential becomes less -ve inside due to the entry of +ve ions)

Question 30

after the binding of neurotransmitters depolarises the plasma membrane, what does this change in voltage trigger?

Answer 30

the opening of voltage-gated sodium channels, resulting in further depolarisation

Question 31

what happens once the membrane becomes depolarised beyond a threshold?

Answer 31

sodium ions are able to enter down their electrochemical gradient, which results in a large change in membrane potential

Question 32

what happens when the channels become inactivated? (neurons)

Answer 32

voltage-gated potassium channels open, causing ions to move out which restores the resting membrane potential

Question 33

what happens once a patch of membrane has become depolarised?

Answer 33

neighbouring regions of the membrane go through the same cycle due to adjacent sodium channels opening

Question 34

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

Answer 34

it causes the vesicles containing the neurotransmitter to fuse with the membrane and release the chemical, stimulating a response in the connected cell

Question 35

how are ion concentration gradients re-established? (neuron)

Answer 35

by the Na/K pump as excess ions are actively transported either in or out

Question 36

what happens following repolarisation?

Answer 36

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

Question 37

what is the retina?

Answer 37

the area of the eye that detects light

Question 38

what are the two types of photoreceptor cell in the retina?

Answer 38

rods and cones

Question 39

describe rods

Answer 39

function in dim light, no colour perception

Question 40

describe cones

Answer 40

function in bright light, colour vision

Question 41

what is retinal?

Answer 41

a light-sensitive molecule found in the eye of animals

Question 42

how does retinal form photoreceptors?

Answer 42

by combining with a membrane protein called opsin

Question 43

what is retinal-opsin called in rod cells?

Answer 43

rhodopsin

Question 44

how does retinal become activated?

Answer 44

is absorbs a photon of light and changes conformation to become photoexcited rhodopsin

Question 45

describe the first step in the cascade of reactions occurring after the retinal becomes photoexcited

Answer 45

one photoexcited rhodopsin activates hundreds of G protein (transduction) molecules

Question 46

describe the second step in the cascade of reactions occurring after retinal becomes photoexcited

Answer 46

each G-protein activates one molecule of PDE (phosphodiesterase enzyme)

Question 47

describe the third step in the cascade of reactions occurring after retinal becomes photoexcited

Answer 47

PDE catalyses the hydrolysis of cyclic GMP (cGMP)

Question 48

how many cGMP molecules are broken down by 1 PDE per second?

Answer 48

thousands

Question 49

describe the fourth step in the cascade of reactions occurring after retinal becomes photoexcited

Answer 49

the decrease the cGMP concentration closes ion channels, triggering nerve impulses in the retina

Question 50

the greater the amplification of the impulse... (eye)

Answer 50

the better the rod cells are able to respond to low intensities of light

Question 51

what happens in cone cells?

Answer 51

different forms of opsin combine with retinal to give different receptor proteins. each of these proteins have a maximal sensitivity to specific wavelengths; red, green, blue or UV