Communication and signalling

Question 1

How do multicellular organisms signal between cells?

Answer 1

extracellular signalling molecules

Question 2

What are receptor molecules of target cells?

Answer 2

proteins with a binding site for a specific signal molecule

Question 3

What does binding of a receptor do?

Answer 3

Binding changes the conformation of the receptor, which initiates a response within the cell

Question 4

Different cell types produce specific signals which means?

Answer 4

that can only be detected and responded to by
cells with the specific receptor

Question 5

In a multicellular organism, different cell types may show what kind of response to the same signal?

Answer 5

tissue specific

Question 6

Where are the receptors for hydrophobic signals and why?

Answer 6

Hydrophobic signalling molecules can diffuse directly through the phospholipid bilayers of membranes, and so bind to intracellular receptors

Question 7

What are the receptors for hydrophobic signalling molecules?

Answer 7

transcription factors

Question 8

Example of hydrophobic signalling molecule?

Answer 8

steroid hormones oestrogen and testosterone

Question 9

Where in the cell are receptors for steroid hormones?

Answer 9

cytosol or nucleus

Question 10

What is the hormone receptor complex?

Answer 10

When a steroid hormone enters a cell it binds to and activates a specific receptor in the cytosol (or nucleus) forming a hormone-receptor complex

Question 11

Pathway for steroid hormone?

Answer 11

1) passes through membrane
2) binds to receptor protein which activates it
3) hormone receptor complex binds to specific sites on DNA (hormone response element)
4) Binding at HRE affects gene expression

Question 12

Where are receptors for hydrophilic signalling molecules?

Answer 12

bind to transmembrane receptors and do not enter the cytosol

Question 13

What happens to transmembrane proteins when the ligand binds to extracellular face?

Answer 13

the signal molecule does not enter the cell, but the signal is transduced across the plasma membrane which amplifies the signal

Question 14

What acts as signal transducers and how?

Answer 14

Transmembrane receptors act as signal transducers by converting the extracellular ligand binding event into intracellular signals, which alters the behaviour of the cell

Question 15

Transduced hydrophilic signals often involve?

Answer 15

G-proteins or cascades of phosphorylation by kinase enzymes

Question 16

Function of phosphorylation cascades?

Answer 16

Phosphorylation cascades allow more than one intracellular signalling pathway to be activated

Question 17

Steps for hydrophilic signalling?

Answer 17

Stage 1 - reception

Transmembrane receptors change conformation (shape) when the ligand (signalling molecule) binds to the extracellular face. The signal molecule does not enter the cell, but the signal is transduced across the membrane of the cell.

Stage 2 - transduction

Transmembrane receptor proteins act as signal transducers by convert an extracellular ligand-binding event to a specific intracellular response through a signal transduction pathway. Transduced hydrophilic signals often involve cascades of G-proteins or phosphorylation by kinase enzymes.

Stage 3 - response

The response of the cell will vary depending on the signal. For example, hormone binding may trigger recruitment of a channel protein to the surface of the cell. A hormone binding to a GPCR may result in the opening of an ion channel, while a hormone binding to a RTK may bring about transcription of certain genes

Question 18

Binding of the peptide hormone insulin to its receptor results in?

Answer 18

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

Question 19

How does type 1 and type 2 diabetes occur?

Answer 19

Diabetes mellitus can be caused by failure to produce insulin (type 1) or loss of receptor
function (type 2)
Type 2 is generally associated with obesity

Question 20

How to help diabetes?

Answer 20

Exercise also triggers recruitment of GLUT4, so can improve uptake of glucose to fat and
muscle cells in subjects with type 2

Question 21

What is resting membrane potential?

Answer 21

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

Question 22

The transmission of a nerve impulse requires changes in what?

Answer 22

the membrane potential of the neuron’s plasma membrane

Question 23

What is an action potential?

Answer 23

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

Question 24

Where do neurotransmitters initiate a response?

Answer 24

binding to their receptors at a synapse

Question 25

What happens as a result of entry of positive ions?

Answer 25

Depolarisation of the plasma membrane and triggers the
opening of voltage-gated sodium channels, and further depolarisation occurs

Question 26

How to restore the resting membrane potential?

Answer 26

Inactivation of the sodium channels and the opening of potassium channels

Question 27

What happens to neighbouring regions of the membrane as depolarisation occurs?

Answer 27

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

Question 28

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

Answer 28

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

Question 29

Restoration of the resting membrane potential allows?

Answer 29

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 30

What is re-established by the sodium potassium pump?

Answer 30

Ion concentration gradients are re-established by the sodium-potassium pump, which actively transports excess ions in and out of the cell

Question 31

Generation of a nerve impulse summary

Answer 31

1)Binding of a neurotransmitter triggers the opening of ligand-gated ion channels at a synapse.
2)Ion movement occurs and there is depolarisation of the plasma membrane.
3)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 down their electrochemical gradient.
4)This leads to a rapid and large change in the membrane potential.
5)A short time after opening, the sodium channels become inactivated.
6)Voltage-gated potassium channels then open to allow potassium ions to move out of the cell to restore the resting membrane potential.
7)Ion concentration gradients are re-established by the sodium-potassium pump, which actively transports excess ions in and out of the cell.

Question 32

What is a membrane potential?

Answer 32

an electrical potential difference

Question 33

What is the retina?

Answer 33

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

Question 34

What are the photoreceptor cells?

Answer 34

rods and cones

Question 35

In animals what combines to form the photoreceptors of the eye

Answer 35

light-sensitive molecule retinal is combined with a membrane protein, opsin,

Question 36

In rod cells the retinal-opsin complex is called?

Answer 36

rhodopsin

Question 37

Rhodopsin diagram?

Answer 37

retinal in middle and opsin surrounds it

Question 38

Initiation of a nerve impulse in response to an environmental stimulus is brought about when?

Answer 38

1)Retinal absorbs a photon of light and rhodopsin changes conformation to photoexcited rhodopsin
2) A cascade of proteins amplifies the signal
3) Photoexcited rhodopsin activates a G-protein, called transducin, which activates the enzyme phosphodiesterase (PDE)
4) PDE catalyses the hydrolysis of a molecule called cyclic GMP (cGMP)
5) This results in the closure of ion channels in the membrane of the rod cells, which triggers nerve impulses in neurons in the retina

Question 39

Why can rod cells respond to low intensity of light?

Answer 39

A very high degree of amplification

Question 40

Nocturnal animals have greater proportion of what to allow them to see better at night?

Answer 40

rod cells

Question 41

Why are cone cells sensitive to specific colours (wavelengths)

Answer 41

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
Cone cells allow animals to have colour vision and only function in bright light