1. Key Area 4- Communication and Signalling Flashcards
How do multicellular organisms signal between cells?
Using extracellular signalling molecules
What are 3 examples of extracellular signalling molecules?
Steroid hormones, peptide hormones and neurotransmitters
What are receptor molecules of target cells?
They are proteins with a binding site for a specific signal molecule
What effect does binding have on the receptor molecule?
Changes the conformation of the receptor which initiates a response within the cell
Why might signalling molecules have different effects on different target cell types?
Due to differences in the intracellular signalling molecules and pathways that are involved
In multicellular organisms, what might different cell types show?
A tissue specific response to the same signal
What are hydrophobic signalling molecules?
They can diffuse through the phospholipid bilayers of membranes and so bind to intracellular receptors
What are the receptors for hydrophobic signalling molecules?
Transcription factors
What are transcription factors?
They are proteins that when bound to DNA can either stimulate or inhibit initiation of transcription
What are 2 examples of hydrophobic signalling molecules?
The steroid hormones: oestrogen and testosterone
Where do steroid hormones bind to specific receptors?
Steroid hormones bind to specific receptors in the cytosol or the nucleus.
Describe the hormone-steroid complex in terms of steroid hormones?
The hormone-receptor complex moves to the nucleus where it binds to specific DNA sequences called hormone response elements (HREs) and affects gene expression. Binding at these sites influences the rate of transcription, with each steroid hormone affecting the gene expression of many different genes
Where do hydrophilic signalling molecules bind to?
They bind to transmembrane receptors and do not enter the cytosol
What are 2 examples of hydrophilic signalling molecules?
peptide hormones and neurotransmitters
When do 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 (converted into another form) across the plasma membrane
How do transmembrane receptors act as signal transducers?
By converting the extracellular ligand-binding event into intracellular signals which alters the behaviour of the cell
What do transduced hydrophilic signals often involve?
G-proteins or cascades of phosphorylation by kinase enzymes
What is the function of G proteins?
Relay signals from activated receptors to target proteins such as enzymes and ion channels
What is an advantage of phosphorylation cascades?
They allow more than one intracellular signalling pathway to be activated
What does phosphorylation cascades involve?
They involve a series of events with one kinase activating the next in the sequence and so on which can result in the phosphorylation of many proteins as a result of the original signalling event
What does phosphorylation cascades involve?
They involve a series of events with one kinase activating the next in the sequence and so on which can 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?
Binding of the peptide hormone insulin to its receptor causes a conformational change that triggers phosphorylation of the receptor
What does phosphorylation at the receptor of the peptide hormone insulin create?
It starts a phosphorylation cascade inside the cell, which eventually leads to GLUT4 containing vesicles being transported to the cell membrane of fat and muscle cells.
Where are GLUT4 containing vesicles transported to in a phosphorylation cascade?
fat and muscle cells
What is diabetes mellitus caused by?
It can be caused by failure to produce insulin (type 1) or loss of receptor function (type 2)
What is type 2 diabetes normally associated with?
Obesity
What is the relationship between exercise and GLUT4 production?
Exercise triggers the recruitment of GLUT4 so can improve uptake of glucose to fat and muscle cells in subjects with type 2
What is resting membrane potential?
Is a state where there is no net flow of ions across the membrane
What does the transmission of a nerve impulse require?
It requires changes in the membrane potential of the neurons’ plasma membrane
What is action potential?
An active potential is a wave of electrical excitation along a neuron’s plasma membrane
How do neurotransmitters initiate a response?
By binding to their receptors which are ligand-gated ion channels at a synapse
What does the binding of a neurotransmitter trigger?
It triggers the opening ligand-gated ion channels at a synapse
What is depolarisation?
A change in the membrane potential to a less negative value inside
What does ion movement occurring lead to?
The depolarisation of the plasma membrane
How is the opening of voltage-gated sodium channels triggered?
If sufficient ion movement occurs and the membrane is depolarised beyond a threshold value the opening of voltage-gated sodium channels is triggered thus allowing sodium ions to enter the cell down their electrochemical gradient which leads to a rapid and large change of the membrane potential
What does the inactivation of the sodium channels and the opening of potassium channels restore?
The resting membrane potential
What does depolarisation of a patch of membrane cause/ what effect does it have on neighbouring regions of membrane?
Depolarisation of a patch of membrane causes neighbouring regions of membrane to depolarise and go through the same cycle as adjacent voltage gated sodium channels are opened
What happens when the activation potential reaches the end of the neuron?
It causes vesicles containing neurotransmitter to fuse with the membrane which releases neurotransmitter and stimulates a response in a connecting cell.
What does restoration of the resting membrane potential allow?
It allows the inactive voltage-gated sodium channels to return to a conformation that allows them to open up again in response to depolarisation of the membrane
How are ion concentration gradients re-established?
By the sodium-potassium pump which actively transports excess ions in and out of the cell.
What happens to the sodium and potassium ion gradients following repolarisation?
The sodium and potassium ion concentration is reduced
What is the effect of the sodium-potassium pump on the restoration of the resting membrane potential?
The sodium potassium pump restores the sodium and potassium ions back to resting potential levels
Which area within the eye detects light?
The retina
What are the two types of photoreceptor cells contained in the retina?
Rod and cone cells
When do rod cells function?
Rod cells function in dim light but do not allow for colour perception
When do cone cells function?
Only function in bright light and responsible for colour vision
In animals what is the light sensitive molecule retinal combined with?
The light sensitive molecule retinal is combined with a membrane protein opsin to form the photoreceptors of the eye
In rod cells, what is the retinal-opsin complex called?
Rhodopsin
What is the function of retinal?
Absorbs a photon of light
What is the function of rhodopsin?
Changes conformation to photoexcited rhodopsin
What amplifies the signal?
A cascade of proteins
What does photoexcited rhodopsin activate?
The G protein called transducin which activates the enzyme phosphodiesterase (PDE)
What does phosphodiesterase (PDE) catalyse?
The hydrolysis of the molecule called cyclic GMP (cGMP)
How many molecules does a single photoexcited rhodopsin activate?
Hundreds of molecules of G protein
How many molecules foes each active phosphodiesterase (PDE) break down?
Thousands of cGMP molecules per second
What effect does the reduction of cGMP concentration have as a result of its hydrolysis?
The reduction in cGMP concentration as a result of its hydrolysis results in the closure of ion channels in the membranes of rod cells, which triggers nerve impulses in neurons in the retina.
What allows rod cells to be able to respond to low intensities of light?
A very high degree of amplification
Where does this take place:
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
In cone cells
