Communication & Signalling Flashcards
How do multicellular organisms signal between cells?
Using extracellular signalling molecules
Examples of extracellular signalling molecules
Steroid hormones
Peptide hormones
Neurotransmitters
What are receptor molecules of target cells?
Proteins with a binding site for a specific signal molecule
Different cell types produce…
Specific signals that can only be detected by cells with the specific receptor
In multicellular organisms different cell types may show…
Tissues specific response to the same signal
Action of extracellular signalling molecules
- Signal molecule released from cell
- Signal molecule travels to target cell
- Signal molecule binds to receptor on target cell causing it to change conformation which initiates a response within the cell
Why do signalling molecules have different effects on different target cell types?
Due to differences in intracellular signalling molecules involved
Hydrophobic signalling molecules are… so can…
Lipid soluble, diffuse directly through the phospholipid bilayers of membranes
Hydrophobic signalling molecules bind to…
Intracellular receptors
Receptors for hydrophobic signalling molecules are…
Transcription factors
What are transcription factors?
Proteins that when bound to DNA can either stimulate or inhibit the initiation of transcription
Examples of hydrophobic signalling molecules
Steroid hormones such as oestrogen and testosterone
Where do steroid hormones bind?
Specific receptors in the cytosol or nucleus forming hormone-receptor complex
Hormone-receptor complex moves to…
Nucleus where it binds to specific sites on DNA (HREs)
What are hormone-response elements (HREs)?
Specific DNA sequences that hormone-receptor complex binds to
Binding at HREs…
Influences the rate of transcription, with each steroid hormone affecting the gene expression of many different genes
Hydrophilic signalling molecules are…
Not lipid soluble so cannot pass across the hydrophobic part of the plasma membrane
Where do hydrophilic signalling molecules bind?
Transmembrane receptors (do not enter cytosol)
Examples of hydrophilic extracellular signalling molecules
Peptide hormones
Neurotransmitters
Transduced hydrophilic signals often include…
G-proteins
Phosphorylation cascades
What do G-proteins do?
Relay signals from activated receptors (have bound signalling molecule) to target proteins such as enzymes and ion channels
What does a phosphorylation cascade involve?
Series of events with one kinase activating the next in the sequence and so on
What is a phosphorylation cascade?
Phosphorylation of many proteins as a result of the original signalling event , allows more than one intracellular signalling pathway to be activated
What is blood glucose controlled by?
Peptide hormones, insulin and glucagon
If blood glucose increases…
Secretion of insulin increases
Stages in the recruitment of GLUT-4
- Insulin binds to its receptor
- Receptor undergoes a conformational change that triggers phosphorylation of the receptor
- Phosphorylation cascade is started inside cell
- Vesicles containing GLUT4 are transported to cell membrane (of fat and muscle cells)
- Glucose passes through GLUT4 transporters
What is GLUT4?
Transporter protein that allows glucose to pass into cell by facilitated diffusion
What causes type 1 diabetes mellitus?
Failure to produce insulin
What causes type 2 diabetes mellitus?
Loss of receptor function, generally associated with obesity
What can improve the uptake of glucose to fat and muscle cells in subjects with type 2?
Exercise as it triggers recruitment of GLUT4
What is resting membrane potential?
State where there is no net flow of ions. across the membrane e.g. neutron that is not transmitting signals
What does transmission of a nerve impulse require?
Changes in the membrane potential of neuron’s plasma membrane
What is action potential?
A wave of electrical excitation along a neuron’s plasma membrane, triggered if change in membrane potential is big enough
What describes nerve transmission?
Wave of depolarisation
How do neurotransmitters initiate a response?
By binding to their receptors at a synapse (ligand-gated ion channels)
Process of nerve impulse transmission…
- Neurotransmitter bind to receptor on the surface of neuron, triggering the opening of ligand-gated ion channels at the synapse
- This allows sodium ions to diffuse into neuron, ion movement causes depolarisation of the plasma membrane
- If sufficient ion movement occurs and membrane is depolarised beyond a threshold value, this triggers the opening of voltage gated sodium channels so sodium ions enter the cell down their electrochemical gradient
- Leads to a rapid and large change in membrane potential
- A short time after opening, the sodium channels become inactive, wave of depolarisation continues along the neuron
- Voltage gated potassium ion channels then open to allow potassium ions to move out of cell to restore the resting membrane potential
What is depolarisation?
Change in the membrane potential to a less negative value inside
What is the effect of depolarisation on a patch of membrane?
Neighbouring regions of membrane depolarise and go through the same cycles, as adjacent voltage-gated sodium channels are opened
Restoring the membrane potential allows…
Inactive voltage-gated sodium channels to return to a conformation that allows them to open again in response to another depolarisation of the membrane
What is reduced after repolarisation?
Sodium and potassium ion concentration gradients
What restores the sodium and potassium ions back to resting potential levels?
Sodium-potassium pump (resets electrochemical gradient)
How does the sodium-potassium pump reset the electrochemical gradient?
By actively transporting excess ions in and out of cell
What is the retina?
Area with eye that detects light
Retina contains 2 types of…
Photoreceptor cells
What are the 2 types of photoreceptors cells?
Rods- function in dim light, do not allow colour perception
Cones- only function in bright light, responsible for colour vision
What combines to form the photoreceptors of the eye?
Light-sensitive molecule retinal and membrane protein opsin
What is the retinal-opsin complex called in rod cells?
Rhodopsin
In cone cells…
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)
Why can rod cells function in dim light?
Higher degree of amplification of signal
Process of initiation of nerve impulse (involving rhodopsin)
- Retinal absorbs a photo of light and rhodopsin changes conformation to photoexcited rhodopsin
- Photoexcited rhodopsin activates a G-protein called transducin, a single photo excited rhodopsin activated hundred of molecules of G-protein
3.Each active G-protein/transducin activated one molecule of phosphodiesterase (PDE) - Each active PDE breaks down thousands of cyclic GMP (cGMP) molecules per second
- This results in closure of ion channels in the membrane of rod cells
What is phosphodiesterase (PDE)?
Enzymes that catalyses the hydrolysis of cyclic GMP (cGMP) molecule
What does the reduction of cGMP concentration (as a result of its hydrolysis) affect?
Function of ion channels in membrane of rod cells
