Communication and signalling Flashcards
How do multicellular organisms signal between cells?
extracellular signalling molecules
What are receptor molecules of target cells?
proteins with a binding site for a specific signal molecule
What does binding of a receptor do?
Binding changes the conformation of the receptor, which initiates a response within the cell
Different cell types produce specific signals which means?
that can only be detected and responded to by
cells with the specific receptor
In a multicellular organism, different cell types may show what kind of response to the same signal?
tissue specific
Where are the receptors for hydrophobic signals and why?
Hydrophobic signalling molecules can diffuse directly through the phospholipid bilayers of membranes, and so bind to intracellular receptors
What are the receptors for hydrophobic signalling molecules?
transcription factors
Example of hydrophobic signalling molecule?
steroid hormones oestrogen and testosterone
Where in the cell are receptors for steroid hormones?
cytosol or nucleus
What is the hormone receptor complex?
When a steroid hormone enters a cell it binds to and activates a specific receptor in the cytosol (or nucleus) forming a hormone-receptor complex
Pathway for steroid hormone?
1) passes through membrane
2) binds to receptor protein which activates it
3) hormone receptor complex binds to specific sites on DNA (hormone response element)
4) Binding at HRE affects gene expression
Where are receptors for hydrophilic signalling molecules?
bind to transmembrane receptors and do not enter the cytosol
What happens to transmembrane proteins when the ligand binds to extracellular face?
the signal molecule does not enter the cell, but the signal is transduced across the plasma membrane which amplifies the signal
What acts as signal transducers and how?
Transmembrane receptors act as signal transducers by converting the extracellular ligand binding event into intracellular signals, which alters the behaviour of the cell
Transduced hydrophilic signals often involve?
G-proteins or cascades of phosphorylation by kinase enzymes
Function of phosphorylation cascades?
Phosphorylation cascades allow more than one intracellular signalling pathway to be activated
Steps for hydrophilic signalling?
Stage 1 - reception
Transmembrane receptors change conformation (shape) when the ligand (signalling molecule) binds to the extracellular face. The signal molecule does not enter the cell, but the signal is transduced across the membrane of the cell.
Stage 2 - transduction
Transmembrane receptor proteins act as signal transducers by convert an extracellular ligand-binding event to a specific intracellular response through a signal transduction pathway. Transduced hydrophilic signals often involve cascades of G-proteins or phosphorylation by kinase enzymes.
Stage 3 - response
The response of the cell will vary depending on the signal. For example, hormone binding may trigger recruitment of a channel protein to the surface of the cell. A hormone binding to a GPCR may result in the opening of an ion channel, while a hormone binding to a RTK may bring about transcription of certain genes
Binding of the peptide hormone insulin to its receptor results in?
an intracellular signalling cascade that triggers recruitment of GLUT4 glucose transporter proteins to the cell membrane of fat and muscle cells
How does type 1 and type 2 diabetes occur?
Diabetes mellitus can be caused by failure to produce insulin (type 1) or loss of receptor
function (type 2)
Type 2 is generally associated with obesity
How to help diabetes?
Exercise also triggers recruitment of GLUT4, so can improve uptake of glucose to fat and
muscle cells in subjects with type 2
What is resting membrane potential?
a state where there is no net flow of ions across the membrane
The transmission of a nerve impulse requires changes in what?
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
Where do neurotransmitters initiate a response?
binding to their receptors at a synapse
What happens as a result of entry of positive ions?
Depolarisation of the plasma membrane and triggers the
opening of voltage-gated sodium channels, and further depolarisation occurs
How to restore the resting membrane potential?
Inactivation of the sodium channels and the opening of potassium channels
What happens to neighbouring regions of the membrane as depolarisation occurs?
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 action potential reaches the end of the neuron?
it causes vesicles containing neurotransmitter to fuse with the membrane — this releases neurotransmitter, which stimulates a response in a connecting cell
Restoration of the resting membrane potential 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
What is re-established by the sodium potassium pump?
Ion concentration gradients are re-established by the sodium-potassium pump, which actively transports excess ions in and out of the cell
Generation of a nerve impulse summary
1)Binding of a neurotransmitter triggers the opening of ligand-gated ion channels at a synapse.
2)Ion movement occurs and there is depolarisation of the plasma membrane.
3)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 down their electrochemical gradient.
4)This leads to a rapid and large change in the membrane potential.
5)A short time after opening, the sodium channels become inactivated.
6)Voltage-gated potassium channels then open to allow potassium ions to move out of the cell to restore the resting membrane potential.
7)Ion concentration gradients are re-established by the sodium-potassium pump, which actively transports excess ions in and out of the cell.
What is a membrane potential?
an electrical potential difference
What is the retina?
area within the eye that detects light and contains two types of
photoreceptor cells
What are the photoreceptor cells?
rods and cones
In animals what combines to form the photoreceptors of the eye
light-sensitive molecule retinal is combined with a membrane protein, opsin,
In rod cells the retinal-opsin complex is called?
rhodopsin
Rhodopsin diagram?
retinal in middle and opsin surrounds it
Initiation of a nerve impulse in response to an environmental stimulus is brought about when?
1)Retinal absorbs a photon of light and rhodopsin changes conformation to photoexcited rhodopsin
2) A cascade of proteins amplifies the signal
3) Photoexcited rhodopsin activates a G-protein, called transducin, which activates the enzyme phosphodiesterase (PDE)
4) PDE catalyses the hydrolysis of a molecule called cyclic GMP (cGMP)
5) This results in the closure of ion channels in the membrane of the rod cells, which triggers nerve impulses in neurons in the retina
Why can rod cells respond to low intensity of light?
A very high degree of amplification
Nocturnal animals have greater proportion of what to allow them to see better at night?
rod cells
Why are cone cells sensitive to specific colours (wavelengths)
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
Cone cells allow animals to have colour vision and only function in bright light
