1.4 Communication and Signalling Flashcards
how do multicellular organisms signal between cells
using extracellular signalling molecules
what are examples of extracellular signalling molecules
steroid hormones
peptide hormones
neurotransmitters
what are receptor molecules of target cells
proteins with a binding for a specific signal molecule
why does binding change the conformation of the receptor
initiates a response within the cell
different cell types produce specific signals that can only
be detected and responded to by cells with the specific receptor
why can signalling molecules have different effects on different target cell types
due to differences in the intracellular signalling molecules and pathways that are involved
in a multicellular organism, different cell types may show
a tissue-specific response to the same signal
how do hydrophobic signalling molecules transport through the membrane
they can diffuse directly through the phospholipid bilayers of membranes and so bind to intracellular receptors
what are the receptors for hydrophobic molecules
transcription factors
what are transcription factors
proteins that when bound to DNA can either stimulate or inhibit initiation of transcription
what are examples of hydrophobic signalling molecules
steroid hormones
oestrogen
testosterone
what do steroid hormones bind to
specific receptors in the cytosol or the nucleus
where does the hormone-receptor complex move to
the nucleus where it binds to specific sites on DNA and affects gene expression
what do hormone-receptor complexes bind 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
what do hydrophilic signalling molecules bind to
transmembrane receptors and do not enter the cytosol
what are examples of hydrophilic extracellular signalling molecules
peptide hormones
neuritransmitters
what happens when the ligand binds to the extracellular face
transmembrane receptors change conformation when when the ligand binds to the extracellular face
the signaling molecule does not enter the cell, but the signal is transduced across the plasma membrane
how do transmembrane receptors act as signal transducers
by converting the extracellular ligand-binding event into intracellular signal, which alters the behaviour of the cell
what do transduced hydrophilic signals often involve
G-proteins or cascades of phosphorylation by kinase enzymes
G-proteins relay signals from
activated receptors (receptors that have bound a signalling molecule) to target proteins such as enzymes and ion channels
what do phosphorylation cascades allow
more than one intracellular signalling pathway to be activated
what do phosphorylation cascades involve
a series of events with one kinase activating the next in the sequence and so on
what can phosphorylation cascades result in
the phosphorylation of many proteins as a result of the original signalling event
what does binding of the peptide hormone insulin to its receptor result in
an intracellular signalling cascade that triggers recruitment of GLUT4 glucose transporter proteins to the cell membrane of fat and muscle cells
what does binding of insulin to its receptor cause
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
what can diabetes mellitus be caused by
type 1 or type 2 diabetes
what is type 1 diabetes
failure to produce insulin
what is type 2 diabetes
loss of insulin receptor function
what is type 2 diabetes associate with
obesity
what triggers the recruitment of GLUT4
exercise
increase uptake of glucose to fat and muscle cells in subject with type 2
what is the resting membrane potential
a state where there is no net flow of ions across the membrane
what does the transmission of a nerve impulse require
changes in the membrane potential of the neuron’s plasma membrane
what is an action potential
a wave of electrical excitation along a neuron’s plasma membrane
what do neurotransmitters do
initiate a response by binding to their receptors at a synapse
what are neurotransmitters receptors
ligand-gated ion channels
what does the depolarisation of the plasma membrane as a result of the entry of positive ions trigger
the opening of voltage-gated sodium channels and further depolarisation occurs
what is depolarisation
a change in the membrane potential to a less negative value inside
what does inactivation of the sodium channels and the opening of potassium channels do
restores the resting membrane potential
stages of restoring membrane potential
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 done 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
what does depolarisation of a patch of membrane cause
neighbouring regions of membrane to depolarise and go through the same cycle, as adjacent voltage-gated sodium channels are opened
what happens when the action potential reaches the end of the neuron
it causes vesicles containing neurotransmitters to fuse with the membrane
this releases neurotransmitter which stimulates a response in a connecting cell
what does restoration of the resting membrane potential allow
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
how are ion concentration gradients re-established
by sodium-potassium pumps.
this actively transports excess ions in and out of the cell
what follows depolarisation
the sodium and potassium ion concentration gradients are reduced.
the sodium-potassium pump restores the sodium and potassium ions back to resting potential
what is the retina
the area within the eye that detects light and contains two types of photoreceptor cells
what are the types of photoreceptor cells that the retina contains
rods
cones
what are rods
they function in dim light but do not allow colour perception
what are cones
responsible for colour vision and only function in bright light
what is the light-sensitive molecule retinal combines with
a membrane protein, opsin, to form the photoreceptors of the eye
what is the retinal-opsin complex called in rod cells
rhodopsin
what happens when retinal absorbs a photon of light
rhodopsin changes conformation to photoexcited rhodopsin
what amplifies the signal
a cascade of proteins
what does photoexcited rhodopsin activate
G-proteins called transducin, which activates the enzyme phosphodiesterase (PDE)
how many molecules of G-protein does a single photoexcited rhosdopsin activate
a single photoexcited rhodopsin activated hundreds of molecules of G-protein.
each activated G-protein activates one more of PDE
what does PDE catalyse
the hydrolysis of a molecule called cyclic GMP (cGMP)
what does each active PDE molecule do
breaks down thousands of cGMP molecules per second
what does the reduction in cGMP concentration affect
the reduction in cGMP concentration as a
result of its hydrolysis affects the function of
ion channels in the membrane of rod cells.
what does the reduction in cGMP concentration result in
the closure of ion channels in the membrane of the rod cells, which triggers nerve impulses in neurons in the retina
what does a high degree of amplification result in
rod cells being able to respond to low intensities of light
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
