Modeling Active Transport - Action Potential.docx



Modeling Active Transport - Action PotentialStandard 2.5: Growth and dynamic homeostasis are maintained by the constant movement of molecules across membranes.Performance Indicator: a.Use representations and models to analyze situations or solve problems qualitatively and quantitatively to investigate whether dynamic homeostasis is maintained by the active movement of molecules across membranes.Active transport across a membrane requires the use of energy by the cell. Normally this energy is in the form of ATP but in some cases (like in cellular respiration), active transport can be completed by charged particles supplying the energy. Active transport moves substances against their concentration gradient from a low concentration to a high concentration - the reverse of passive transport. Part 1: Modeling Active Transport With the Na+ and K+ PumpReview the basics of active transport and the Na+ and K+pump HERE. Create a yellow plasma membrane insert the sodium-potassium pump protein into the membrane of the model. Place 7 sodium ions (round) and 8 potassium ions (square) inside of the cell to simulate the intracellular environment ion concentrations. Place 8 sodium ions and 7 potassium ions outside of the cell to simulate the extracellular ion concentrations.A typical animal cell has a much higher concentration of potassium ions (K+) and a much lower concentration of sodium ions (Na+) on the inside of the cell than the outside. The sodium-potassium pump uses energy in the form of ATP to move these ions against their concentration gradients and establish the “normal” intracellular ion concentrations. Set the sodium-potassium pump so that it is open to the inside of the cell. Record the initial concentrations of Na+ and K+in the data table on your answer sheet.Follow these steps:Step 1: Bind three three intracellular sodium ions to the appropriate spots in the protein.-23812466675Step 2: Bring the ATP in close proximity to the pump. Step 3: Sodium ion binding stimulates phosphorylation of the pump protein by ATP. In other words, a phosphate group is added to the sodium-potassium pump from the ATP molecule. (You will not be able to demonstrate this step with the model).Step 4: Phosphorylation causes a change in the shape of the protein. You can demonstrate this by “swinging” the sides of the protein so that it opens to the outside of the cell.Step 5: The shape change reduces the protein’s binding affinity for sodium ions and increases the binding affinity for potassium ions. Remove the sodium ions from the protein and deposit them outside the cell and bind two potassium ions to the appropriate spots in the protein.Step 6: Potassium ion binding triggers the release of the phosphate group from the protein. (Again, you will not be able to demonstrate this step with the model).Step 7: Loss of the phosphate group results in the restoration of the protein’s original shape which then releases the potassium ions. Swing the sides of the protein back so that they open to the inside of the cell and deposit the potassium ions.Step 8: Repeat this process one more time.Record the ion concentrations in your data table after completing the first cycle of the action of the sodium-potassium pump.The firing of a neuron is based on a charge difference across the cell membrane. The initial potential energy is not zero but -71 mV inside the cell. This charge (resting potential) is maintained through homeostasis by limiting the diffusion of Na+ and K+. When a nerve fires, an electrical gradient change occurs by ions passing through the membrane by facilitated diffusion. The membrane reverses polarity (+in/- out) briefly as a wave down the neuron. This occurs by the influx of ions into the cell. Once past, the cell must re-establish the resting potential. A review of this process can be found HERE. Part 2: Modeling Action PotentialUsing your kit, set up a linear axon membrane as outlined in the video above and the snapshot to the right. You should have membrane, Na+ and K+ transporters, and the Na+ and K+ pump embedded in the membrane. Using the video as a guide, walk through the process of passive and active transport as they relate to action potential. 1504950400050Part 3: Exocytosis in the SynapseUsing two of the membrane kits, set up your model to look like the animation below. You should have the following:Post-Synaptic cell with two transporters Vesicle with round neurotransmitters insidePre-Synaptic membrane with a calcium transporterSquare sodium ions in between43815095250369570095250Using the animation found HERE, walk through the model the exocytosis of neurotransmitters across the synaptic cleft. Make sure you understand how both passive and active transport are involved with signal transmission both the Part 2 and 3. ................
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