2. You now create a different artificial cell in a different test tube. This art

Question

2. You now create a different artificial cell in a different test tube. This artificial cell consists of a cytoplasm (i.e. the inside of the cell), a lipid bilayer, and the outside. The cytoplasm contains 150 mM NaCl, whereas the outside solution contains mM NaCl. The only kind of proteins present in the membrane are H+/Na+ pumps, which simultaneously pump one H+ outside the cell for every one Na+ it pumps inside the cell, but these pumps are inactive. (Assume there is an excess of ATP both inside and outside of the cell that allows the pump to function. Also assume the NaC1 is dissolved in water, which is where the H+ ions are coming from.) Now assume that, all of the sudden, the pumps in the cell membrane become active. a. Are any ions flowing across the cell membrane? If so, which ions are flowing and are they flowing inward or outward? b. Is there a membrane potential across the cell membrane? If so, is the membrane potential positive on the inside or negative on the inside? c. Are there concentration gradients across the cell membrane?? If so, for which ions are there concentration gradients, and are those ions at higher concentrations or lower concentrations inside the cell? 4. After depolarization occurs during a normal action potential, is the concentration of sodium inside the cell high or low with respect to the concentration of sodium outside the cell? 5. Do you think that action potentials in a neuron would cease immediately after one exposes the neuron to a drug that inhibits the Na+/K+ pump?

Explanation / Answer

2a. The lipid bilayer is selectively permeable, i.e. it allows only selective ions of H+ and Na+ to pass through it. Although, Solvent concentration is higher outside because solute concentration (15mM NaCl) is less in comparison to inside (150mM), but When the ion channels become active, H+ ions will move from inside (higher concentration) to outside (lower concentration). This is so because, as given in the question- proteins present in the membrane are Na+/H+ pumps. For each H+ moved outside , one Na+ will be moved inside. Energy in form of ATP will be consumed in the process.

b. yes, there is a membrane potential. Due to excess Na+ ions, there is a positive charge on the inside of the membrane.

c. yes, there is a concentration gradient also. For Na+ ions; These sodium ions are at higher concentration inside the cell ; similarly H+ ions are at higher concentration outside the cell.

4. After depolarisation, sodium will move out. So, the concentration of sodium will be higher outside the cell in comparison to inside.

5. No, this is not the case. The action of Na+/K+ pump is not solely responsible for action potential. The selective permeability of cell's plasma membrane for different ions also plays an important role. Potassium ions, chloride ions, bicarbonate ions, and others are also responsible. Active and passive transport go hand in hand. Diffusion and facilitated diffusion also occurs.

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