1.4 communication and signalling Flashcards
how do multicellular organisms achieve coordination and communication?
extracellular signalling molecules, receptors and responses
how does the binding of a signalling molecule change a receptor?
changes its conformation, altering/initiating the response in the cell
why can different cell types show a tissue-specific response to the same signal?
due to differences in intracellular pathways and signalling molecules
how do hydrophobic signals work?
by passing directly through phospholipid bilayers and binding to intracellular receptors
why can hydrophobic signals diffuse through the bilayer?
the tails of the phospholipids in the PM are also hydrophobic
how can hydrophobic signals influence the transcription of genes?
by binding to transcription factors
what are transcription factors?
a protein that when bound to DNA can either stimulate or inhibit transcription
name two steroid hormones
oestrogen and testosterone
what type of signalling molecules are oestrogen and testosterone?
hydrophobic
describe the process of a hydrophobic signalling molecule affecting transcription
- the hormone binds to a receptor protein in the cytosol/nucleus
- this forms a hormone/receptor complex
- this binds to a specific DNA sequence called HREs
- this influences the rate of transcription by affecting the expression of several different genes
what does HREs stand for?
hormone response elements
how do hydrophilic signals work?
signal molecules bind to proteins on the plasma membrane that act as receptors
name two examples of hydrophilic signal molecules
neurotransmitters and peptide hormones
how do transmembrane receptors proteins work?
they change conformation when the ligand binds to the extracellular face of the protein, triggering a transduction pathway
how does a signal transduction pathway work?
it links an extracellular chemical stimulus to a specific cellular response, altering the behaviour of the cell
what is a specific cellular response?
a cascade of biochemical reactions inside the cell that eventually reach the target molecule or reaction
an enzyme needs to be activated to bring out a response within the cell, but the receptor cannot directly interact with it. how is this resolved?
a G-protein is a relay protein which can relay the signal and activate the enzyme
what does the active G-protein do?
stimulates an enzyme, leading to a response in the cell
why is the response from a G-protein only temporary?
because the G protein also acts as a GTPase and hydrolysis the bound GTP to GDP, making the G protein inactive again
what is a phosphorylation cascade?
a series of events activated by one kinase, wich activates the next kinase etc
what do phosphorylation cascades result in?
the phosphorylation of many proteins from one original signal
what happens when insulin, a peptide hormone, binds to its receptor?
change in conformation, which causes a phosphorylation cascade
this causes the recruitment of GLUT4 to the plasma membrane
exercise can trigger GLUT4, therefore…
improving the uptake of glucose to fat in muscle cells in people with type 2 diabetes
what is the resting potential?
the membrane potential of a neuron that is not transmitting signals
what does it mean if a neuron is not transmitting signals?
there is no net flow of ions across the membrane
what is an action potential?
a wave of electrical excitation along the PM of a neuron
what does the transmission of a nerve impulse require?
the change of membrane potential to action potential
what type of channel proteins are neurotransmitter receptors?
ligand-gated ion channels
what occurs when neurotransmitters bind to their complementary receptors at a synapse?
depolarisation of the PM (the membrane potential becomes less -ve inside due to the entry of +ve ions)
after the binding of neurotransmitters depolarises the plasma membrane, what does this change in voltage trigger?
the opening of voltage-gated sodium channels, resulting in further depolarisation
what happens once the membrane becomes depolarised beyond a threshold?
sodium ions are able to enter down their electrochemical gradient, which results in a large change in membrane potential
what happens when the channels become inactivated? (neurons)
voltage-gated potassium channels open, causing ions to move out which restores the resting membrane potential
what happens once a patch of membrane has become depolarised?
neighbouring regions of the membrane go through the same cycle due to adjacent sodium channels opening
what happens when the action potential reaches the end of the neuron?
it causes the vesicles containing the neurotransmitter to fuse with the membrane and release the chemical, stimulating a response in the connected cell
how are ion concentration gradients re-established? (neuron)
by the Na/K pump as excess ions are actively transported either in or out
what happens following repolarisation?
sodium and potassium ion concentration gradients are reduced. the sodium-potassium pump restores the sodium and potassium ions back to resting potential levels.
what is the retina?
the area of the eye that detects light
what are the two types of photoreceptor cell in the retina?
rods and cones
describe rods
function in dim light, no colour perception
describe cones
function in bright light, colour vision
what is retinal?
a light-sensitive molecule found in the eye of animals
how does retinal form photoreceptors?
by combining with a membrane protein called opsin
what is retinal-opsin called in rod cells?
rhodopsin
how does retinal become activated?
is absorbs a photon of light and changes conformation to become photoexcited rhodopsin
describe the first step in the cascade of reactions occurring after the retinal becomes photoexcited
one photoexcited rhodopsin activates hundreds of G protein (transduction) molecules
describe the second step in the cascade of reactions occurring after retinal becomes photoexcited
each G-protein activates one molecule of PDE (phosphodiesterase enzyme)
describe the third step in the cascade of reactions occurring after retinal becomes photoexcited
PDE catalyses the hydrolysis of cyclic GMP (cGMP)
how many cGMP molecules are broken down by 1 PDE per second?
thousands
describe the fourth step in the cascade of reactions occurring after retinal becomes photoexcited
the decrease the cGMP concentration closes ion channels, triggering nerve impulses in the retina
the greater the amplification of the impulse… (eye)
the better the rod cells are able to respond to low intensities of light
what happens in cone cells?
different forms of opsin combine with retinal to give different receptor proteins. each of these proteins have a maximal sensitivity to specific wavelengths; red, green, blue or UV
