1.4 Communication and Signalling

Question 1

How do cells Communicate?

Answer 1

cells use extracellular signalling molecules such as steroid hormones, peptide hormones and neurotransmitters.

Question 2

Describe the process of cell signalling

Answer 2

Firstly, the target cell must have receptor protein molecules with a binding sight that is specific to the signalling molecule.
When binding occurs, it changes the conformation of the receptor in some way that changes the behaviour of the cell (response).

Question 3

How does cell signalling differ between cell types?

Answer 3

Different cell types produce specific signals that can only be detected and responded to by cells with the specific receptor. Signalling molecules may have different effects on different target cell types due to differences in the intracellular signalling molecules and pathways that are involved. In multicellular organisms, different cell types may show a tissue-specific response to the same signal.

Question 4

How do hydrophobic signalling molecules travel through plasma membranes? and what does this mean for their proteins?

Answer 4

Hydrophobic signalling molecules can diffuse directly through plasma membranes because the tails of the phospholipids in the membrane are also hydrophobic. This means that their receptor proteins can be deep in the cytoplasm or in the nucleus itself (no receptors on membrane surface).

Question 5

Give an example of a hydrophobic signalling molecule

Answer 5

The steroid hormones

Question 6

What is a transcription factor?

Answer 6

A transcription factor is a protein that binds to DNA and controls the rate of transcription either by stimulating or inhibiting it (they do this by enhancing or blocking the binding of RNA polymerase)

Question 7

The receptor proteins for steroid hormones (such as testosterone and oestrogen) are…….

Answer 7

transcription factors

Question 8

What are two examples of steroid hormones?

Answer 8

oestrogen and testosterone

Question 9

Where do Steroid hormones bind?

Answer 9

transcription factors

Question 10

What happens when a steroid hormone binds to the receptor protein

Answer 10

When a steroid hormone binds to the receptor protein in the cytosol or nucleus, the conformational change that occurs forms a hormone-receptor complex that is then able to bind to specific sites on DNA and affects gene expression. The hormone-receptor complex binds to 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 11

Where are receptor molecules for hydrophilic signals required to be and why?

Answer 11

Hydrophilic signalling molecules, such as peptide hormones and neurotransmitters, require receptor molecules to be at the surface of the cell because they are not capable of passing across the hydrophobic plasma membrane.

Question 12

Hydrophilic signalling molecules do not enter the…..

and this means……

Answer 12

cytosol

This means that they require the signal to be transduced within the cell

Question 13

How many stages are involved in transducing a cell and what are they called?

Answer 13

3 stages

• reception
• transduction (of which there are two types)
• response.

Question 14

Describe stage 1 of transducing a cell

Answer 14

• reception

When the signalling molecule (acting as a ligand) binds to the transmembrane receptor on the outside of the cell, it causes a conformational change in this protein.

Question 15

What are the two types of transduction

Answer 15

Transduced signals can involve either G-proteins or cascades of phosphorylation by kinase enzymes.

Question 16

Describe what happens in stage 2 of transducing a cell

Answer 16

Receptor proteins convert the extracellular lignand binding event into an intracellular signal through a signal transduction pathway.

Question 17

Describe Transduction by G-proteins

Answer 17

G-proteins relay signals from activated receptors to target proteins such as enzymes and ion channels

Question 18

Describe Transduction by phosphorylation

Answer 18

Phosphorylation cascades allow more than one intracellular signalling pathway to be activated. Phosphorylation cascades involve a series of events with one kinase activating the next in the sequence and so on. Phosphorylation cascades can result in the phosphorylation of many proteins as a result of the original signalling event.

Question 19

Describe what happens when insulin binds to its receptors

Answer 19

The binding of insulin (a peptide hormone) to its receptor causes a conformational change that triggers the phosphorylation of the receptor.

This starts a phosphorylation cascade inside the cell that triggers the recruitment of GLUT4 glucose transporters (found in vesicles in the cytoplasm) to the cell membrane of fat and muscle cells.

Glucose can then be taken into the cells which lowers the blood glucose level.

Question 20

What is the role of insulin?

Answer 20

Insulin plays an important role in allowing fat tissue and skeletal muscles to absorb glucose from the bloodstream.

Question 21

What is Diabetes mellitus

Answer 21

A medical condition that results from the failure of some stage in the insulin-signalling pathway.

Question 22

Describe Type 1 Diabetes and its treatment

Answer 22

Type 1 diabetes is caused by the failure to produce insulin in the pancreas. It is treated by regular injections of insulin throughout the day.

Question 23

Describe Type 2 Diabetes and its treatment

Answer 23

Type 2 diabetes is caused by a loss of insulin receptor function. This type of diabetes is usually associated with obesity. It may be treated with medications to lower blood glucose levels along with lifestyle changes.

Exercise also triggers the recruitment of GLUT4 so can improve uptake of glucose to fat and muscle cells.

Question 24

When is a membrane potential created?

Answer 24

A membrane potential (an electrical potential difference) is created when there is a difference in electrical charge on the two sides of the membrane.

Question 25

What is a resting membrane potential?

Answer 25

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

Question 26

What is depolarisation?

Answer 26

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

Question 27

What is a nerve transmission?

Answer 27

Nerve transmission is a wave of depolarisation of the resting potential of a neuron.

Question 28

Depolarisation is a result of…..

and this is triggered by……..

Answer 28

a wave of electrical excitation along a neuron’s plasma membrane called an action potential.

A neurotransmitter molecule at the synapse between neurons.

Question 29

Describe the process of depolarisation

Answer 29

1. The neurotransmitter binds to a transmembrane receptor protein on the surface of the neuron.
2. This receptor is a ligand-gated Na+ channel and the binding causes a conformational change, which in turn opens the channel.
3. Positively charged Na+ ions enter the neuron and the voltage (membrane potential) changes and the membrane is considered depolarised.
4. 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.
5. This then leads to a domino effect as the triggering of one voltage-gated channel depolarises another part of the membrane and triggers the next channel and so on resulting in a large change in the membrane potential. This effect travels along the length of the neuron as a wave of depolarisation.

Question 30

Describe the process of resetting the resting potential

Answer 30

After the wave passes, the resting potential has to be reset, ready for the next impulse.

1. To allow this the Na+ channels close after a short period of time and K+ channels open.
2. The K+ ions diffuse out of the neuron in the opposite direction from the Na+ ions.
3. This restores the resting potential by repolarising the membrane. The K+ channels will also then close.

Question 31

What is hyperpolarisation?

Answer 31

When the K+ channels are open in order to reset the resting potential they can often let too many K+ ions leave the neuron, in this case we would say the membrane is hyperpolarised.

Question 32

How do you correct hyperpolarisation?

Answer 32

This is corrected by the action of the sodium potassium pump.

Question 33

What does the retina do?

Answer 33

detects light

Question 34

The retina contains……

Answer 34

two classes of photoreceptor cells, called rods and cones.

Question 35

In animals the light-sensitive molecule retinal is combined with…..

Answer 35

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

Question 36

What is the retinal-opsin complex in rod cells called?

Answer 36

rhodopsin

Question 37

What is the retinal-opsin complex

Answer 37

Retinal is a prosthetic group, which is covalently bound to a membrane protein called opsin. This retinal-opsin complex is embedded in membranes inside photoreceptor cells in the retina.

Question 38

Describe the stages in the generation of a nerve impulse from light

Answer 38

1. Retinal absorbs a photon of light and rhodopsin changes conformation to photoexcited rhodopsin
2. A cascade of proteins amplifies the signal, photoexcited rhodopsin activates a G-protein called transducin (a single photoexcited rhodopsin activates hundreds of molecules of transducin)
3. Transducin activates the enzyme phosphodiesterase (PDE) (each activated molecule of transducin activates one molecule of PDE);
4. PDE catalyses the hydrolysis of a molecule called cyclic GMP (cGMP) (each active PDE molecule breaks down thousands of cGMP molecules per second)
5. The reduction in cGMP concentration as a result of its hydrolysis causes the closure of ion channels in the membrane of the rod cells
6. The inward leakage of positive ions (Na+ and Ca+) is halted so the membrane potential increases; hyperpolarisation (increasing charge) triggers nerve impulses in neurons in the retina.

Question 39

What is the difference between rod cells and cones

Answer 39

Rod cells are sensitive to changes in light intensity and are particularly useful for vision in areas of low light intensity. Nocturnal animals have a greater proportion of rod cells in their retina which gives them better vision at night. This sensitivity to low light intensities is due to the fact that rhodopsin in rod cells can exhibit a greater degree of amplification from a single photon of light

In cone cells different forms of opsin combine with retinal to give different photoreceptor proteins. They are not as sensitive to light as rod cells; however, they are particularly sensitive to specific wavelengths of light: green, red, blue and (in some animals) UV. Different forms of opsin give sensitivity to the different wavelengths of light. Cone cells allow animals to have colour vision. People who are colour blind lack a particular type of cone cell in their retina.