Prelim Steps of Questions (Unit 1) Flashcards
Explain the action of the Sodium Potassium Pump
1) The pump has high affinity for sodium ions inside the cell; binding of 3 sodium ions occurs at binding sites.
2) Stage 1 stimulates phosphorylation by ATP; addition of phosphate group causes conformational change which opens protein to extracellular side.
3) Second conformation has a decreased affinity for sodium ions; sodium ions released outside of the cell.
4) Second conformation has a higher affinity for potassium ions, 2 K+ ions bind to specific sites exposed outside the cell.
5) Stage 4 causes dephosphorylation of the transporter protein; conformation changes back again - opening on intracellular side.
6) Original conformation has low affinity for potassium ions so are taken into cell cytosol; affinity for sodium returns to start.
The pump (Na/K ATPase) has two stable conformational states; one has high affinity for sodium ions inside the cell, one has high affinity for potassium ions outside the cell.
Describe the role of the sodium-potassium pump in the epithelial cells lining the small intestine
In the small intestine, the sodium gradient created by the sodium-potassium pump drives the active transport of glucose.
-In intestinal epithelial cells the sodium-potassium pump generates a sodium ion gradient across the plasma membrane, actively transports sodium ions into bloodstream so creates low concentration in cytosol. (Also pumps K+ in, not relevant)
-Sodium ions enter the cell down their concentration gradient; this is coupled to and simultaneous with the transport of glucose being pumped into the cell against its concentration gradient (AT), from the intestine lumen into the epithelial cells, by a glucose symporter protein .
-The glucose transporter responsible for this glucose symport transports sodium ions and glucose at the same time and in the same direction
-A GLUT glucose transporter removes glucose from the epithelial cell cytosol into the bloodstream (by diffusion)
Explain hydrophobic signals and control of transcription
1) Hydrophobic signalling molecules can diffuse directly through the phospholipid bilayers of membranes, and so bind to specific intracellular receptors, in the cytosol or the nucleus
2) The receptors for hydrophobic signalling molecules are transcription factors; proteins that when bound to DNA can either stimulate or inhibit initiation of transcription.
3) The hormone-receptor complex moves to the nucleus where it binds to specific sites on DNA and affects gene expression
4) The hormone-receptor complex binds to specific DNA sequences called hormone response elements (HREs). Binding at these sites influences the rate of transcription, with each steroid hormone affecting the gene expression of many different genes.
Describe hydrophilic signal transduction ?
1) Hydrophilic signalling molecules bind to transmembrane receptors and do not enter the cytosol
2) 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 across the plasma membrane
3) Transmembrane receptors act as signal transducers by converting the extracellular ligand-binding event into intracellular signals, which alters the behaviour of the cell
4) Transduced hydrophilic signals often involve G-proteins or cascades of phosphorylation by kinase enzymes
How does insulin promote cell glucose uptake ?
Triggers recruitment of GLUT4 (does facilitated diffusion)
-Binding of the peptide hormone insulin (hydrophillic) to its receptor causes a conformational change which tirggers phosphorylation of the receptor. This causes an intracellular phosphorylation signalling cascade, which eventually leads to GLUT4-containing vesicles being transported to the cell membrane of fat and muscle cells.
How is a nerve impulse transmitted ?
1) Neurotransmitters initiate a response by binding to their neurotransmitter receptors at a synapse, these receptors are ligand-gated ion channels, causing receptor to open and allowing positively charged Na+ to enter.
2) Binding of a neurotransmitter triggers the opening of ligand-gated ion channels at a synapse.
3) Ion movement occurs and there is depolarisation of the plasma membrane.
4) 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. This leads to a rapid and large change in the membrane potential (causing further depolarisation)
5) A short time after opening, the sodium channels become inactivated and voltage-gated potassium channels then open to allow potassium ions to move out of the cell to restore the resting membrane potential.
6) 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
7) 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
8) 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
9) Following repolarisation the sodium and potassium ion concentration gradients are reduced. Ion concentration gradients are restored by the sodium-potassium pump, which actively transports excess ions in and out of the cell, back to resting potential levels
Describe the stages of initiation of a nerve impulse in the eye (to turn off rod cells)
1) Retinal absorbs a photon of light and rhodopsin changes conformation to photoexcited rhodopsin
2) A cascade of proteins amplifies the signal (allows vision in very low light levels, lets rods see)
3) Photoexcited rhodopsin activates a G-protein, called transducin, which activates the enzyme phosphodiesterase (PDE). A single photoexcited rhodopsin activates hundreds of molecules of G-protein. Each activated G-protein activates one molecule of PDE.
4) PDE catalyses the hydrolysis of a molecule called cyclic GMP (cGMP) to GMP. Each active PDE molecule breaks down thousands of cGMP molecules per second. The sufficient reduction in cGMP concentration as a result of its hydrolysis affects the function of ion channels in the membrane of rod cells (low cGMP concentration turns off rods)
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
