11 Cell Communication Flashcards
How do bacteria measure the number of bacteria surrounding it?
Each bacteria cell secretes a certain chemical. Therefore the concentration of this can be used by each bacterium to work out its density.
This is known as ‘quorum sensing’
How does ‘quorum sensing’ affect bacterial behaviour?
If they sense that they are in a high density area they begin to secrete substances so that a biofilm is formed.
What are the types of cell singling?
Communication by direct contact, local signalling and long-distance signalling.
What are the forms of direct signalling?
Cell junctions and cell-cell recognition
What is cell junction chemical messaging?
Chemical molecules are passed through the gap junctions/plasmodesmata of neighbouring cells
What is cell-cell recognition?
One cell has molecules on the outside of its plasma membrane that can be detected by receptors on the other cell’s plasma membrane.
What are the forms of local signalling?
Paracrine signalling and synaptic signalling
What is paracrine signalling?
A cell secretes a ‘local regulator’ i.e. a growth factor into the extracellular fluid which acts on neighbouring cells.
What is synaptic signalling?
The axon of a nerve cell secretes neurotransmitters which cross the synapse and trigger the continuation of an action potential.
What are the forms of long-distance signalling?
Endocrine (hormonal signalling)
What is Endocrine signalling?
Specialised endocrine cells secrete hormones into the body fluids i.e. blood which circulate and act on specific cell.
How do hormones act on specific cells?
Only certain cells have receptors for that specific hormone and thus only those respond.
How does long distance signalling occur in plants?
They also have hormones although they are often called ‘plant growth regulators’.
These either travel through vascular bundles, between cells through plasmodesmata or diffuse through the organism as a gas.
What is a common plant gas hormone?
Ethylene (C2H4)
What does epinephrine trigger?
The breakdown of glycogen in the liver.
How does the breakdown of glycogen provide energy?
It is broken down into glucose 1-phosphate which the cells convert to glucose 6-phosphate which is used as an early intermediate of glycolysis
What enzyme breaks down glycogen?
Glycogen phosphorylase
What are the basic steps of responding to a chemical messenger?
1) Reception
2) Transduction
3) Response
How does reception typically occur during chemical messenger?
A receptor, typically on the plasma membrane, binds to the ligand and is activated.
How does transduction typically occur during chemical messenger?
The receptor releases a relay molecule which triggers another receptor which intern releases another relay molecule and so on.
This is known as a ‘signal transduction pathway’
How does response typically occur during chemical messenger?
The final receptor in the signal transaction pathway releases a chemical that triggers an enzyme etc. to perform the intended action or the transcription of specific genes
How does ligand binding generally trigger a response?
It changes the shape of the receptor, causing something to happen.
What is a ligand?
The chemical that triggers a specific receptor.
What type of signalling molecules tends to bind to cell surface receptors?
Water soluble ones as they can not easily cross the plasma membrane as they are hydrophilic
What are the types of cellular receptors as distinguished by location? Where is each?
Cell surface receptors- one the plasma membrane
Intracellular receptors - in the cytoplasm or the nucleus
What are the classes of cell surface receptors?
G Proteins-Coupled Receptors, Receptor Tyrosine Kinases and Ligand-gated ion channels.
What is the structure of a G Protein-Coupled receptor?
A single ribbon coiled into 7 α-helixes
Where are G Protein-Coupled receptors used?
In embryonic development, vision, smell, taste and the reception of epinephrine and neurotransmitters.
How many steps are there of signal reception by G-Protein coupled receptors?
4
What is step 1 of G-Protein coupled reception?
A G-protein is loosely attached to the inside of the plasma membrane. It is bound to a GDP (not GTP) molecule so it is inactive and thus will not trigger an associated enzyme.
What is step 2 of G-Protein coupled reception?
When the appropriate ligand binds it activates the receptor and changes its shape, causing an inactive G-protein to also bind to it.
The G-protein is then activated by displacing its inactivating GDP molecules and replacing it with a GTP. This activates the G-protein
What is step 3 of G-Protein coupled reception?
The activate G protein dissociates and diffuses to a near enzyme and binds to it. This activates the enzyme causing a cellular response.
What is step 4 of G-Protein coupled reception?
The activated G-protein also acts as a GTPase enzyme so breaks down its attached GTP to GDP. This deactivates it and thus the enzyme causing the G-protein to detach so that it can be reused.
Why does the fact the G-protein acts a GTPase important?
It ensures that it does not activate the enzyme for too long and thus the response does not continue indefinitely.
What does kinase refer to in an enzyme?
The fact that it catalyses the transfer of phosphates from ATP to its substrate.
What is the structure of a receptor tyrosine kinase protein?
An extracellular ligand binding site, an α-helix through the plasma membrane and a intracellular tail containing many tyrosine’s.
How many stages are there of tyrosine kinase reception?
3
What is step 1 of tyrosine kinase reception?
Two signalling molecules bind to 2 separate receptor tyrosine kinase protein monomers.
This triggers these two proteins to come together an form a dimer. This is called ‘dimerisation’
What is step 2 of tyrosine kinase reception?
Dimerisation triggers each tyrosine molecule of the intracellular tail to have a phosphate group added from ATP.
What is step 3 of tyrosine kinase reception?
The phosphorylated tyrosine molecules remain attached as specific relay proteins bind to them. This activates the relay proteins, causing a transduction pathway.
What is the significance of the receptor tyrosine kinases having multiple tyrosine molecules?
Each can trigger a different relay protein and thus multiple transduction pathways can be triggered, leading to multiple cellular responses from a single ligand.
How does a ligand-gate ion channel operate?
When the ligand binds to the receptor it opens the ion channel, allowing them to diffuse in.
Eventually the ligand dissociates, causing the channel to close.
What are some ion channels triggered by other than ligands?
Membrane potential i.e. the voltage-gated ion channels that conduct action potentials.
What classes of chemicals acts as ligands for intracellular receptors?
Lipid soluble chemicals and small gases as they can easily cross the plasma membrane.
What class of chemicals are steroids and thus which signal receptors do they activate?
Lipids so intracellular receptors.
How may step are there to intracellular reception?
3
What does intracellular reception often trigger?
The expression of specific genes.
What is step 1 of intracellular reception?
The hormone enters the plasma membrane and binds to a receptor protein in the cytoplasm.
This forms a ‘Hormone-receptor complex’
What is step 2 of intracellular reception?
The Hormone-receptor complex enters the nucleus and binds to a specific gene.
What is step 3 of intracellular reception?
The hormone-receptor complex acts as a transcription factor and stimulates the transcription of the gene into mRNA and thus the desired protein.
How is gene expression controlled by signalling molecules?
By activating transcription factors which transcribe DNA to mRNA.
What are the basic forms of signal transduction?
Protein phosphorylation and dephosphorylation
and small molecules/ions as second messengers.
What do cytoplasmic protein kinases phosphorylate specifically?
Serine or throning (both amino acids)
How many steps are there of protein phosphorylation and dephosphorylation?
5
What is step 1 of protein phosphorylation and dephosphorylation
A relay molecule released by the original signal receptor activates the first protein kinase. ( protein kinase 1)
What is step 2 of protein phosphorylation and dephosphorylation?
The activated protein kinase 1 then transfers a phosphate from an ATP to an inactive kinase 2 and thus activates it.
What is step 3 of protein phosphorylation and dephosphorylation?
The activated kinase 2 then triggers the phosphorylation and thus activation of kinase 3.
This continues as a chain known as the phosphorylation cascade.
What is step 4 of protein phosphorylation and dephosphorylation?
The final protein kinase is activated. This triggers to phosphorylate an inactive protein. This now active protein will trigger the cellular response.
What is step 5 of protein phosphorylation and dephosphorylation?
Enzymes called protein phosphatases (PP) catalyze the removal of the phosphate groups from the proteins, making them inactive and available for reuse.
Which enzyme dephosphorylates and thus deactivate the protein kinases?
Protein phosphotases.
What substances are commonly used as second messengers for transduction?
Cyclic AMP and Calcium ions that work with IP3
What is cyclic AMP abbreviated to?
cAMP
How is cAMP formed?
The enzyme adenylyl cyclase is embedded in the plasma membrane and catalyses the conversion of ATP to cAMP
How is cAMP deactivated?
The enzyme phosphodiesterase converts it to AMP
Specifically what second messenger does epinephrine activate?
cAMP
What is the cAMP pathway?
An activated G protein triggers adenylyl to produce cAMP. This cAMP then activates a protein kinase causing a phosphorylation cascade and eventually a cellular response.
What receptors may use Calcium ions as second messengers?
Both G-protein coupled an tyrosine kinases
What chemicals often act with calcium ions in signal transaction?
IP3 (inositol triphosphate) and DAG (diacylglycerol)
Where are the calcium ions for transaction sourced and how is a diffusion gradient maintained?
Calcium ions are actively pumped from the cytoplasm to the ER and also outside the cell.
This provides a concentration gradient so the Ca ions can be released from the ER
How does Calcium ion transduction begin?
A G-protein couple receptor is triggered. The activated G-protein activates the enzyme Phospholipase C. This enzyme cleaves the molecule PIP2 into DAG and IP3.
After DAG and IP3 have been formed what happens during calcium ion transduction?
The DAG diffuses and acts as second messenger to other pathways.
The IP3 diffuses and activates IP3-gated calcium channels on the ER membrane causing CA2+ ions to be released.
The rising calcium ion concentration in the cytosol triggers various proteins which intern trigger cellular responses.
What is an example of a third messenger.
There aren’t any: even though calcium ions are triggered by a second messenger they are still considered second messengers
What are the fundamental ways a signal can affect the cell?
It can either cause the synthesis of proteins like enzymes or increase the activity of enzymes that are already present.
What are the advantages of transduction pathways as opposed to receptors triggering response?
They enable fine-tuning of the signal.
For example it allows one signal to activate multiple responses, it allows the inhibition/activation of signals, allows one ligand to trigger different responses in different cells and amplification
How does signal transduction allow one signal to activate multiple responses?
The transaction cascade can be branched i.e. protein kinase 1 might active protein kinase 2 and kinase 3, each of which lead to different phosphorylation cascades and thus responses.
Similarly Ca2+ and cAMP molecules release may activate multiple different proteins.
How does signal transduction allow the inhibition/activation of signals?
Two receptors of different ligands could ‘cross talk’
For example one receptor could trigger an allosteric regulator for an enzyme of the other receptor’s phosphorylation cascade. Thus the other receptor inhibit or activates the signal of the other receptor.
How does signal transduction allow one ligand to trigger different responses in different cells?
Different cells can have different phosphorylation cascades etc. for the same receptor. Therefore different cells (i.e. liver vs. brain) will respond differently to the same hormone etc.
How does signal transduction allow amplification of the signal?
Each stage of the transduction pathway could activate multiple others. For example one activated adenylyl cyclase could form many cAMPs and thus a large signal can be formed from a single ligand.
What is a major inefficiency of transaction pathways?
Many relay molecules are large proteins that slowly diffuse between the various stages i.e. protein kinases.
How can the slow diffusion between the transduction pathway be rectified?
Scaffolding proteins hold many kinases etc. of the same pathway together. Therefore the relay molecules have to travel less distance.
How can the intensity of a chemical signal be increased?
The intensity of each ligand can not be increased. However releasing more ligands would activate more receptors and thus a larger response would be yielded. Similarly the ligands could be released for a greater period or ligands that are allosteric to the main pathway could be released.
What condition trigger a cell to commit apoptosis?
The cell being infected, damaged or just old.
What occurs during apoptosis?
The organelles are broken down. The cell shrinks and its contents form lobes that are shed (known as ‘blebbing’)
Why is apoptosis important?
It prevents the cell from suddenly rupturing and releasing its digestive enzymes which would damage neighbouring cells.
If the cell is infected it prevent the spread as the virus etc. often ‘dies’ with the cell.
In nematodes what triggers apoptosis?
Ced-9 is a protein that is normally active, causing it to inactive Ced-4 and Ced-3.
If a death signalling molecule is received from another cell, Ced-9 is inactivated causing Ced-4 and Ced2 to be active. This triggers a cascade that eventually leads to the activation of enzymes that breakdown the cell.
In many mammals what results in apoptosis?
The mitochondria triggers proteins causing pores in the mitochondria to form. This allows proteins to leak out that trigger apoptosis.
Where can the signal to perform apoptosis originate?
It often originates from outside the cell i.e. neighbouring cells. This
However the nucleus can also trigger it if the DNA is significantly damaged. The ER can also trigger ti if excessive protein misfolding occurs
What can incorrect apoptosis cause?
If apoptosis fails to occur cancer may start
Excess apoptosis also leads to Parkinson’s and Alzheimer’s
How do yeast mate?
There are two genders of yeast: a and α.
The a yeast release a factor and the α yeast release α factor. Each yeast cell has receptors for the opposite gender’s factor. When these receptors on the plasma membrane detect these factors the yeast change shape and move together allowing mating to occur.
This mating forms a fused cell that includes all the genes from the a yeast and the α yeast. This is know as a a/α yeast.
What does AMP stand for?
Adenosine monophosphate.
