1.4 Cells and Proteins: Communication and Signalling

Question 1

how do multicellular organisms signal between cells?

Answer 1

they use extracellular signalling molecules

Question 2

what are three examples of signalling molecules?

Answer 2

steroid hormones
peptide hormones
neurotransmitters

Question 3

What are receptor molecules?

Answer 3

proteins with a binding site for a specific to the receptor molecules of the target cells

Question 4

what occurs after the binding to a receptor?

Answer 4

changes the conformation of the receptor which initiates a response within the target cell

Question 5

Why do only certain cells respond to specific signal molecules?

Answer 5

different cell types produce specific signals that can only be detected and responded to by cells that have the specific receptor protein

Question 6

why might signalling molecules have a have different effects on different target cell types?

Answer 6

there may be differences in the intracellular signalling molecules and pathways that are involved

Question 7

what may different cell types show in response to the same signal in a multicellular organism?

Answer 7

tissue specific response

Question 8

what are hydrophobic signalling molecules?

Answer 8

signalling molecules that can diffuse directly through the phospholipid bilayer of membranes and so bind to intracellular receptors located either in the cytosol or nucleus

Question 9

what are the receptors for hydrophobic signalling molecules?

Answer 9

transcription factors

Question 10

what are transcription factors?

Answer 10

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

Question 11

what are two examples of hydrophobic signalling molecules?

Answer 11

(steroid hormones)
oestrogen and testosterone

Question 12

where do steroid hormones bind to their specific receptors?

Answer 12

in the cytosol or in the nucleus

Question 13

what happens when a signalling molecule binds with its receptor in the cytosol?

Answer 13

forms a hormone-receptor complex and moves to the nucleus where it binds to specific sites on the DNA and affects gene expression

Question 14

what are the specific binding sites on DNA called?

Answer 14

hormone response elements

Question 15

what occurs after binding at these sites?

Answer 15

affects the rate of gene expression

Question 16

describe how hydrophobic signalling molecules can enter cells:

Answer 16

the can diffuse directly through the bilayers of membranes

Question 17

describe how hydrophobic signal molecules can affect gene expression:

Answer 17

hydrophobic signalling molecules enter the cell and bind with their specific receptor in the cytosol or the nucleus, they form a hormone-receptor complex, this binds to specific binding site on DNA called hormone response elements, affects the rate of transcription

Question 18

what are hydrophilic signalling molecules?

Answer 18

bind to transmembrane receptors as ligands and do not enter the cytosol

Question 19

what are two examples of hydrophilic signalling extracellular signalling molecules?

Answer 19

peptide hormones and neurotransmitters

Question 20

where specifically do extracellular signalling molecules bind?

Answer 20

extracellular face of transmembrane receptors

Question 21

what happens when the ligands bind to the extracellular face?

Answer 21

receptors change conformation and act as signal transducers

Question 22

if hydrophilic signalling molecules do not enter the cell, then how does the signal travel across?

Answer 22

the signal is transduced across the plasma membrane and the extracellular ligand-binding event is converted into intracellular signals which can then alter the behaviour of the cell

Question 23

what do transduced hydrophilic signals often involve?

Answer 23

G-proteins
cascades
kinase enzymes

Question 24

what is the role of G-proteins?

Answer 24

they relay signals from activated receptors that have bound to a signalling molecule to target proteins such as enzymes and ion channels

Question 25

what is the role of phosphorylation cascades?

Answer 25

allows more than one intracellular signalling pathway to be activated, phosphorylation cascades involve a series of events with one kinase enzyme activating the next in the sequence, and so on

Question 26

describe how hydrophilic signalling molecules act specifically:

Answer 26

signal molecule acts as a ligand and binds to specific receptor on the cell membrane

Question 27

describe the process of signal transduction:

Answer 27

when signal bound, the receptor causes a G-protein or kinase cascade to activate a cellular response

Question 28

describe the control of blood glucose by the action of insulin:

Answer 28

increase in blood glucose level causes insulin to be released from pancreas, insulin binds to receptor in membrane of fat or muscle cells, this causes a conformational change and triggers phosphorylation of the receptor, this initiates a phosphorylation cascade within the cell, vesicles containing GLUT4 and translocated and recruited to the cell membrane, GLUT4 is inserted into the membrane and glucose can enter the cell to be converted into glycogen or used in glycolysis

Question 29

what is type 1 diabetes?

Answer 29

present from birth and is the failure to produce the hormone insulin, treated by injecting insulin

Question 30

what is type 2 diabetes?

Answer 30

generally associated with obesity, caused by the loss of the insulin receptor function, can be treated with drugs or by change in diet and exercise

Question 31

when is a neuron in a state of resting membrane potential?

Answer 31

when there is nervous impulse being transmitted

Question 32

what is required in order for the transmission of a nerve impulse?

Answer 32

a change in the membrane potential of the neuron's plasma membrane

Question 33

describe neurotransmitter receptors:

Answer 33

ligand-gated ion channels in the synaptic region at one end of a neuron

Question 34

describe how an action potential is achieved in neurons:

Answer 34

neurotransmitter binds to receptor allowing sodium ions to enter, the change in potential opens the voltage gated channel, allowing more sodium ions to enter, this takes membrane to threshold

Question 35

describe how the resting membrane potential is restored:

Answer 35

membrane reaches threshold and sodium channels close, potassium channels open and potassium ions leave the cell, the sodium potassium pump is activated and the pump causes net loss of sodium ions

Question 36

how is a nerve impulse transmitted?

Answer 36

the series of changes is repeated region after region along the neuron, causing the transmission of the impulse followed by restoration of the resting membrane potential

Question 37

what part of the vertebrate eye detects light?

Answer 37

the retina

Question 38

what are the two types of photoreceptor cells called?

Answer 38

rod cells cone cells

Question 39

describe rod cells:

Answer 39

function in dim light but not allow colour perception

Question 40

describe cone cells:

Answer 40

only function in bright light and are responsible for colour vision

Question 41

where is the retina?

Answer 41

at the back of the eye

Question 42

what is the light sensitive molecule?

Answer 42

retinal

Question 43

what is the membrane protein required to make photoreceptors?

Answer 43

opsin

Question 44

what is the retinal-opsin complex called?

Answer 44

rhodopsin

Question 45

what is the role of rhodopsin?

Answer 45

retinal absorbs a photon of light and rhodopsin changes conformation to form a photo-excited rhodopsin

Question 46

what does photo excited rhodopsin do?

Answer 46

activates hundreds of G-protein molecules called transducins

Question 47

once the G-protein transducin is produced, what is its role?

Answer 47

activates one molecule of the enzyme PDE

Question 48

what does PDE enzyme do?

Answer 48

catalyses the hydrolysis of a molecule called cyclic GMP (cGMP), each PDE molecule breaks down thousands of cGMP molecules per second

Question 49

what happens when cGMP concentration is then reduced?

Answer 49

affects the function of ion channels in the membrane of rod cells, which close to trigger nerve impulses in neurons in the retina

Question 50

How do cone cells detect different colours of light?

Answer 50

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

Question 51

describe the structure of rhodopsin:

Answer 51

consists of the light-sensitive molecule retinal bound to the membrane protein opsin

Question 52

describe how a photon of light has its affect amplified in a rod cell to produce a nervous impulse:

Answer 52

photon of light is absorbed by retinal in rhodopsin and changes conformation to produce photoexcited rhodopsin, photoexcited rhodopsin activates hundreds of G-proteins called transducins, each transducin activates one molecule of the enzyme PDE, PDE catalyses the hydrolysis of cGMP, lower conc. of cGMP closes the ion channels in the membrane of rod cells, the change in action potential triggers a nerve impulse in the neurons in the retinal

Question 53

rod cells amplify the effects of light, state why amplification is an advantage to vertebrates:

Answer 53

allows for vision in dim lights

Question 54

state how photoreceptor molecules of cone cells differ from those in rod cells:

Answer 54

they have different opsins

Question 55

state what is meant by the threshold:

Answer 55

the potential at which the neuron will transmit the impulse

Question 56

explain how repolarisation is achieved:

Answer 56

sodium channels close and potassium channels open, potassium moves out of the cell, the sodium-potassium pump restores the resting membrane potential

Question 57

explain why it is important that the neuron is restored to resting potential following the transmission of a nervous impulse:

Answer 57

allows the system/neuron to remain sensitive to further stimulation