Communication And Signalling Flashcards
(34 cards)
How is communication achieved in a multicellular organism?
Communication is achieved with extra cellular signalling molecules and complimentary receptor proteins. Such as peptide hormones, steroid hormones and neurotransmitters.
Describe the series of events that occur when a signal is received.
First: the target cells must have receptor molecules with binding sites for the specific signalling molecule.
Then: when binding occurs it changes the conformation of the receptor, which changes the behaviour of the target cell, initiating a response.
Why may signalling molecules have different effects on different target cell types?
Signalling molecules may have different effects on different target cell types due to their different intracellular signalling molecules and pathways that are involved.
What are hormones? How do they function?
Hormones are extra cellular signalling molecules that are secreted by particular tissues into the blood.
The hormones will circulate the bloodstream until it reaches its receptor or is broken down. They are either hydrophobic (oestrogen/testosterone) or hydrophilic (peptide hormones).
What are neurotransmitters?
Neurotransmitters are signals that are released into the synaptic gap between a nerve cell and its neighbours.
Neuron communication is very specific and rapid due to intimate association between signal and its target. Neurotransmitters are hydrophilic !
Where are receptor proteins located?
Hydrophobic signalling molecules can pass through membranes so their receptor molecules are located within the cytoplasm or nucleus of the target cell. Hydrophilic signalling molecules can’t pass through membranes so they require integral cell-surface receptor proteins.
What are hydrophobic signals? Describe their function.
Hydrophobic signalling molecules are lipid soluble, so they are able to diffuse through the phospholipid bilayer of membranes and bind to intracellular receptors.
The receptors for steroid hormones bind to specific DNA sequences called hormone receptor elements which then influences the rate of transcription.
How do hydrophobic signals directly affect the transcription of genes?
- The steroid hormone passes through cell membrane
- It binds with the receptor protein in the cell to form the hormone-receptor complex.
- This complex is a transcription factor that now binds to hormone response elements on DNA and alters the rate of change transcription
Name three types of major hydrophilic signal receptor
- ion-channel linked
- Kinase linked
- G-protein linked
Describe the process of signal transduction
NOTE THAT hydrophilic signalling molecules are NOT lipid soluble therefore cannot pass the hydrophobic part of the plasma membrane.
- They must instead bind to transmembrane receptor molecules and do not enter the cytosol. These transmembrane proteins change conformation when the signal ligand binds to the extra cellular surface.
- The signal molecule does not enter the cell but the signal is transduced across the plasma membrane. This means that the behaviour of the cell changes in response to the external binding of the signal molecule to the receptor molecule.
- The extra cellular ligand binding event is converted into intracellular signals.
What do transduced signals often involve?
Transduced signals often involve G-proteins or cascades of phosphorylation by kinase enzymes.
The G-proteins relay signals from activated receptors to target proteins such as enzymes and ion channels.
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 signalling extent.
What are peptide hormones?
Peptide hormones are small, hydrophilic proteins. Well known examples include insulin, glucagon and ADH.
Only their target cells have the appropriate receptors on their surface, therefore the function of these peptide hormones are very specific.
Examples of neurotransmitters and their function.
Examples of neurotransmitters include acetylcholine and noradrenaline. Both are hydrophilic peptides.
ACh is the transmitter at the neuromuscular junction connecting motor nerves to muscles.
Noradrenaline has a role in the central nervous system and the sympathetic nervous system.
Describe the differences between hydrophilic and hydrophobic signalling molecules.
Hydrophilic: lipid insoluble, does not pass through phospholipid bilayer, receptor proteins are transmembrane proteins, example includes peptide hormone.
Hydrophobic: lipid soluble, passes through the phospholipid bilayer, receptor proteins are in the cytosol or nucleus, examples include steroid hormones.
Discuss insulin and the recruitment of GLUT4
Blood glucose must be kept within narrow limits. Peptide hormones insulin and glucagon interact in a negative feedback system to control blood glucose. If blood glucose rises, the pancreas detects this increase and causes it to increase secretion of insulin!
The insulin receptor is a kinase linked receptor found in the cell membrane of fat and muscle cells.
Once peptide hormone insulin binds to its receptor, the signal is transduced and a series of phosphorylation events that triggers the recruitment of GLUT4 glucose transporters to the cell membrane.
These transporters allow glucose to enter the cell for further metabolism and therefore the blood glucose concentration is controlled.
Describe in detail, diabetes mellitus.
Diabetes mellitus is a condition caused by deficiency in the effect of insulin and the loss of control of blood glucose levels. This can be caused by failure to produce insulin or the loss of receptor function. Type 1 diabetes is treated with insulin injections, however Type 2 diabetes cells are no longer sensitive to insulin and this disease is generally associated with obesity.
Excessive reduces the impact of type 2 diabetes as it triggers recruitment of GLUT4 through other metabolic pathways; exercise improves the uptake of glucose to fat and muscle cells in subjects with Type 2 diabetes.
PLEASE REFER TO DIAGRAM IN NOTEBOOK.
What is resting membrane potential?
Resting membrane potential is the state where there is no net flow of ions across the membrane.
Why is change in the membrane potential of a neurons plasma membrane required?
The sodium potassium pump moves 3 Na+ ions out of a neuron and 2 K+ ions into the neuron cell. The neuron membrane also has potassium channels which let some of the K+ ions leak back out of the cell, and the result is a positive charge outside of the cell compared to the inside of the cell.
This means the transmission of a nerve impulse along a neuron requires changes in the membrane potential of the neurons plasma membrane.
Define ‘action potential.’
An action potential is the wave of electrical excitation along a neurons plasma membrane.
Describe in detail how an action potential is triggered.
Depolarisation of the resting potential can be triggered by neurotransmitter molecules. The neurotransmitter binds to a transmembrane receptor protein on the surface of the next neuron.
This receptor is a ligand gated ion channel so the binding of the neurotransmitter causes a change in conformation, making the channel open and letting Na+ ions diffuse through.
If sufficient ion movement occurs, then the voltage change across the membrane reaches a critical level and a patch of membrane is depolarised. This voltage change causes neighbouring voltage-gated Na+ channel to open, which depolarises neighbouring regions of membrane and causes a domino effect:
- the triggering of one voltage gated channel depolarises the membrane so triggering the adjacent voltage gated sodium channel and so on.
Define ‘depolarisation’.
Depolarisation is the change in the membrane potential to a less negative value inside the cell.
Summarise how a neurotransmitter triggers a nerve impulse.
STEP 1 - the neurotransmitter binds to receptor protein (ligand gated channel) at synapse.
STEP 2 - the ligand gated channel opens so Na+ ions can diffuse through and into the neuron.
STEP 3 - Na+ ion movement causes depolarisation of the plasma membrane.
STEP 4 - depolarisation reaches critical threshold level.
STEP 5 - voltage gated channels open so Na+ ions can diffuse into the neuron down an electrochemical gradient.
Describe in detail how a resting potential is reset
After the wave of depolarisation passes along the neuron, there has to be a process to re-establish the resting potential ready for the next impulse.
When voltage reaches a critically high level, the voltage gated Na+ channels are inactivated and voltage gated K+ channels open. This allows the K+ ions to diffuse out of the neuron in the opposite direction of the Na+ ions, restoring the resting potential.
Once resting potential is reached — where there is no net flow of ions across the membrane — the K+ channel closes again and Na+ channel returns to conformation that allows them to respond again.
Through all this, the sodium potassium pump continues to work and the essential ion gradient is quickly reset through the active transport of excess ions in and out of the cell.
Summarise the resetting of a resting potential
STEP 1 - voltage gated channels open so Na+ diffuse into neurons
STEP 2 - voltage builds up to critical threshold so Na+ channels close and K+ channel opens.
STEP 3 - resting potential is restored so K+ channels close and Na+ channels are ready again.
STEP 4 - the sodium potassium pump resets the electrochemical gradient.
