Communication and signalling in multicellular organisms

Question 1

How do multicellular organisms signal between cells?

Answer 1

By way of extracellular signalling molecules such as steroid hormones, peptide hormones, and neurotransmitters.

Question 2

What is a receptor molecule?

Answer 2

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

Question 3

What happens when the extracellular (from outside the cell) signalling mollecule binds to the receptor?

Answer 3

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

Question 4

Signalling mollecules are specific, what does this mean?

Answer 4

That they can only be detected and responded to

by cells with the specific receptor, not just a cell with any receptor.

Question 5

Can the same signalling mollecule have different effects on different target cell types? If yes, why?

Answer 5

Signalling molecules may have different
effects on different target cell types due to
differences in the intracellular signalling
molecules and pathways that are involved.
(There may be a tissue specific response to the same signal).

Question 6

Can hydrophobic signalling mollecules diffuse through the phospholipid bilayer? And why/why not?

Answer 6

Hydrophobic signalling molecules can diffuse
directly through the phospholipid bilayers of
membranes because they are not repelled by the non polar tails of the phospholipids.

Question 7

Where are the receptors for hydrophobic signalling mollecules found and what is the word used to describe this location?

Answer 7

In the cytosol, usually or the nucleas - intracellular - Inside the cell.

Question 8

What kind of protein is the receptor for hydrophobic signalling mollecules?

Answer 8

Transcription factors.

Question 9

What is the function of a transcription factor?

Answer 9

Transcription factors are proteins that when
bound to DNA can either stimulate or inhibit
initiation of transcription and so effecting gene expression.

Question 10

Steroid hormones are hydrophobic signalling molecules. Give 2 examples of steroid hormones.

Answer 10

Oestrogen and testosterone.

Question 11

Describe the complete process of how a steroid hormone affects gene expression.

Answer 11

The steroid hormone diffuses across the phospholipid bilayer and binds to a specific receptor (a transcription factor) in the cytosol or the nucleus. The hormone receptor complex then enters the nucleus (if not already inside) across the nuclear membrane and 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 12

Can hydrophilic signalling mollecules diffuse through the phospholipid bilayer? And why/why not?

Answer 12

Hydrophilic signalling mollecules do not pass across the phospholipid bilayer as they are repelled by the hydrophobic phospholipid tails.

Question 13

Where are the receptors for hydrophilic signalling mollecules found?

Answer 13

Embedded in the cell membrane.

Question 14

Name 2 main types of hydrophobic signals.

Answer 14

Peptide hormones and neurotransmitters.

Question 15

Describe reception in a transmembrane receptor.

Answer 15

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 16

How do transmembrane receptors act as signal

transducers?

Answer 16

They convert the extracellular ligand-binding event into intracellular signals, which alters the behaviour of the cell.

Question 17

Name 2 common processes in hydrophilic signal transduction.

Answer 17

Transduction by G-proteins or cascades of phosphorylation by kinase enzymes.*

Question 18

What do G-proteins do?

Answer 18

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

Question 19

Briefly describe what a phosphorylation cascade is and what it results in.

Answer 19

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.

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

Question 20

What effect does the binding of insulin to its receptor have?

Answer 20

Binding of the peptide hormone 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 membranes of fat and muscle cells.

Question 21

What are the 2 causues of diabeties (one for each type)?

Answer 21

Type 1 is caused by failure to produce insulin.

Type 2 is caused by loss of receptor function, usually caused in part by obesity.

Question 22

What else can trigger recruitment of GLUT4 and so can be used to help treat type 2 diabeties?

Answer 22

Excercise.

Question 23

What is meant by the term resting membrane poteintial?

Answer 23

Resting membrane potential is a state where

there is no net flow of ions across the membrane.

Question 24

What is required for the transmission of a nerve impulse?

Answer 24

Rapid changes in the membrane potential of the

neuron’s plasma membrane caused by depolarisation cascades.

Question 25

What is an action potential?

Answer 25

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

Question 26

What is the receptor for a neurotransmitter and where is it found?

Answer 26

Neurotransmitter receptors are ligand-gated

ion channels found in the synapses of nerve vells.

Question 27

Describe how a nerve impulse (electrical signal) is transmitted along a neuron in as much detail as possible.

Answer 27

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. A sodium potassium pump re-establishes original concentration and electrical gradients. This process is self perpetuating along a chain of neurons as one neuron triggers the next.

Question 28

What is meant by depolarisation of the plasma membrane?

Answer 28

Depolarisation is a change in the membrane

potential to a less negative value inside.

Question 29

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

Answer 29

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 30

How are ion concentration gradients reestablished.

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

What does restoration of the resting membrane potential result in?

Answer 31

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.

Question 32

What restores the sodium and potassium ions gradients back to resting levels? (No net ion flow.)

Answer 32

The sodium-potassium pump restores the sodium and potassium ions back to resting potential levels.

Question 33

What is the retina?

Answer 33

The area within the eye that detects light.

Question 34

Name the 2 types of photoreceptors cells and what each of them is specialised to do, and how they differ.

Answer 34

Rod cells which absorb all frequencies of visible light and can allow sight at very low light levels but lack colour perception. And come cells which are sensitive to different wavelengths (red, green and blue) but require greater light levels to function.

Question 35

What forms the photoreceptors of the eye?

Answer 35

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

Question 36

In rod cells what is the retinal opsin complex called?

Answer 36

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

Question 37

What happens when retinal absorbs a photon of light?

Answer 37

Rhodopsin changes conformation to photo excited rhodopsin. A cascade amplifies the signal.

Question 38

What does photoexcited rhodopsin activate?

Answer 38

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

Question 39

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

Answer 39

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

Question 40

How are different ranges of wavelengths of light detected?

Answer 40

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.