Key Area 4 - Communication & Signalling

Question 1

Co-ordination

Question 2

Multicellular organisms signal between cells using what molecules?

Answer 2

Multicellular organisms signal between cells using extracellular signalling molecules.

Question 3

Give examples of extracellular signalling molecules

Answer 3

Steroid hormones, peptide hormones, and
neurotransmitters are examples of
extracellular signalling molecules.

Question 4

What are target cells for receptor molecules?

Answer 4

Receptor molecules of target cells are
proteins with a binding site for a specific
signal molecule.

Question 5

What does binding change and initiate?

Answer 5

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

Question 6

What do different cell types types produce?

Answer 6

Different cell types produce specific signals
that can only be detected and responded to by cells with the specific receptor.

Question 7

Why may signalling molecules have different effects on different target cell types?

Answer 7

Signalling molecules may have different
effects on different target cell types due to
differences in the intracellular signalling
molecules and pathways that are involved.

Question 8

In a multicellular organism, what may different cell types show?

Answer 8

In a multicellular organism, different cell
types may show a tissue-specific response to the same signal.

Question 9

Hydrophobic Signals and Control of
Transcription

Question 10

What can hydrophobic signalling molecules diffuse through?

Answer 10

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

Question 11

What are receptors for hydrophobic signalling molecules?

Answer 11

The receptors for hydrophobic signalling
molecules are transcription factors.

Question 12

What are transcription factors?

Answer 12

Transcription factors are proteins that when bound to DNA can either stimulate or inhibit initiation of transcription.

Question 13

Give examples of hydrophobic
signalling molecules

Answer 13

The steroid hormones oestrogen and
testosterone are examples of hydrophobic
signalling molecules.

Question 14

What and where do steroid hormones bind to?

Answer 14

Steroid hormones bind to specific receptors in the cytosol or the nucleus.

Question 15

What happens to the hormone-receptor complex?

Answer 15

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

Question 16

What do transduced hydrophilic signals involve?

Answer 16

Transduced hydrophilic signals often involve G-proteins or cascades of phosphorylation by kinase enzymes.

Question 16

Explain in detail what happens to the hormone-receptor complex

Answer 16

The hormone-receptor complex binds to
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 16

What do hydrophilic signalling molecules bind to?

Answer 16

Hydrophilic signalling molecules bind to
transmembrane receptors and do not enter the cytosol.

Question 16

How do transmembrane receptors act as signal transducers?

Answer 16

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

Question 16

Give examples of hydrophilic extracellular
signalling molecules

Answer 16

Peptide hormones and neurotransmitters are examples of hydrophilic extracellular
signalling molecules.

Question 16

What do G-proteins do?

Answer 16

G-proteins relay signals from activated
receptors (receptors that have bound a
signalling molecule) to target proteins such
as enzymes and ion channels.

Question 16

What do phosphorylation cascades allow?

Answer 16

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

Question 17

Hydrophilic Signals and Transduction

Question 17

When do transmembrane receptors change conformation?

Answer 17

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

Question 18

What do phosphorylation cascades involve?

Answer 18

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

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

Answer 19

Binding of the peptide hormone insulin to its receptor results in an intracellular signalling cascade that triggers recruitment of GLUT4 glucose transporter proteins to the cell membrane of fat and muscle cells.

Question 20

What does binding of insulin to its receptor cause?

Answer 20

Binding of insulin to its receptor causes 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 21

What can diabetes mellitus be caused by?

Answer 21

Diabetes mellitus can be caused by failure to produce insulin (type 1) or loss of receptor function (type 2).

Question 22

Explain the cause, age of onset, nature of defect and treatment of Type 1 Diabetes

Answer 22

Destruction of beta-cells in pancreas by immune system.

Occurs in childhood.

Pancreas does not produce any insulin.

Daily insulin injections and management of diet to control blood glucose concentration.

Question 22

Explain the cause, age of onset, nature of defect and treatment of Type 2 Diabetes

Answer 22

Exact cause is unknown.
Obesity is a risk factor.

Occurs in adulthood.

Target cells develop insulin resistance.
Loss of receptor function.

Eat less sugar and saturated fats.
Regular exercise - triggers recruitment of GLUT4, so can improve uptake of glucose to fat and muscle cells in type 2.
Medication to lower blood glucose concentration.

Question 23

Nerve Impulse Transmission

Generation of a Nerve Impulse

Question 24

What is resting membrane potential?

Answer 24

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

Question 25

What does the transmission of a nerve impulse require?

Answer 25

The transmission of a nerve impulse requires changes in the membrane potential of the neuron’s plasma membrane.

Question 26

What is an action potential?

Answer 26

An action potential is a wave of electrical
excitation along a neuron’s plasma membrane.

Question 27

What are neurotransmitter receptors?

Answer 27

Neurotransmitter receptors are ligand-gated ion channels.

Question 27

How do neurotransmitters initiate a response?

Answer 27

Neurotransmitters initiate a response by
binding to their receptors at a synapse.

Question 28

What does depolarisation of the plasma membrane trigger?

Answer 28

Depolarisation of the plasma membrane as a result of the entry of positive ions triggers the opening of voltage-gated sodium channels, and further depolarisation occurs.

Question 29

What is depolarisation?

Answer 29

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

Question 29

What restores the resting membrane potential?

Answer 29

Inactivation of the sodium channels and the opening of potassium channels restores the resting membrane potential.

Question 30

Explain what a neurotransmitter triggers and what happens after

Answer 30

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 down 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 31

What does depolarisation of a patch of membrane cause?

Answer 31

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 32

What is caused when the action potential reaches the end of the neuron?

Answer 32

When the action potential reaches the end of the neuron it causes vesicles containing
neurotransmitter to fuse with the membrane — this releases neurotransmitter, which stimulates a response in a connecting cell.

Question 33

What does restoration of the resting membrane potential allow?

Answer 33

Restoration of the resting membrane
potential 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. Ion concentration gradients are reestablished by the sodium-potassium pump, which actively transports excess ions in and out of the cell.

Question 34

What happens following repolarisation?

Answer 34

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

Question 35

Initiation of a Nerve Impulse in Response to an Environmental Stimulus: the Vertebrate Eye

Question 36

What is the retina and what does it contain?

Answer 36

The retina is the area within the eye that
detects light and contains two types of
photoreceptor cells: rods and cones.

Question 37

How are photoreceptors of the eye formed in animals?

Answer 37

In animals the light-sensitive molecule retinal is combined with a membrane protein, opsin, to form the photoreceptors of the eye.

Question 38

Explain the functions of rods and cones

Answer 38

Rods function in dim light but do not allow
colour perception.

Cones are responsible for colour vision and only function in bright light.

Question 39

In rod cells what is the retinal-opsin complex called?

Answer 39

In rod cells the retinal-opsin complex is called rhodopsin.

Question 40

What does photoexcited rhodopsin activate?

Answer 40

Photoexcited rhodopsin activates a G-protein, called transducin, which activates the enzyme phosphodiesterase (PDE).

A single photoexcited rhodopsin activates
hundreds of molecules of G-protein. Each
activated G-protein activates one molecule of PDE.

Question 41

What does the retinal absorb?

Answer 41

Retinal absorbs a photon of light and
rhodopsin changes conformation to
photoexcited rhodopsin.

Question 41

What amplifies a signal?

Answer 41

A cascade of proteins amplifies the signal.

Question 42

What does PDE catalyse?

Answer 42

PDE catalyses the hydrolysis of a molecule
called cyclic GMP (cGMP).

Each active PDE molecule breaks down
thousands of cGMP molecules per second.
The reduction in cGMP concentration as a
result of its hydrolysis affects the function of ion channels in the membrane of rod cells.

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 43

What does a very high degree of amplification result in?

Answer 43

A very high degree of amplification results in rod cells being able to respond to low
intensities of light.

Question 44

What do different forms of opsin combine with and form in cone cells?

Answer 44

In cone cells, 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.