QUESTION 8 Which intervention will disrupt the function of the Na /K ATPase pump

Question

QUESTION 8 Which intervention will disrupt the function of the Na /K ATPase pump? Removal of extracellular K+ Removal of extracellular NaT Removal of extracellular ATP Removal of cytoplasmic Ca 2+ QUESTION 9 The most important factor determining whether a ligand gated ion channel is inhibitory or excitatory is the ligand-binding properties of the receptor. whether the ion that passes through it has a reversal potential that is positive or negative to the current membrane potential whether the ion that passes through it is positively or negatively charged. the number of different ions that can pass through the receptor. QUESTION 10 Which statement about Na* permeability during an action potential is most accurate? It is responsible for the falling phase of the action potential. It is responsible for the rising phase of the action potential. It is long lasting It restores the membrane potential to its usual level following the action potential

Explanation / Answer

Question 8. Option 1: Removal of extracellular K+

The Na+/K+ ATPase pump is a solute pump, which pumps 3 sodium ions out of the cell for every 2 potassium ions pumped into the cell. While the intracellular K+ concentration is relatively higher than the extracellular K+ concentration, the Na+ concentration is higher outside the cell than its inside. This indicates that the Na+K+ ATPase pumps ions against their concentration gradient, forcefully, mediated by active transport in presence of ATP. The pump binds intracellular ATP and then binds 3 intracellular Na+ ions. ATP hydrolysis induces a conformational change , exposing the Na+ ions outside the cell. The pump then binds 2 K+ ions, dephosphorylates and releases the K+ ions. So, in this mechanism,i) removal of extracellular K+ will disrupt functioning of the pump as their will be no K+ ions to pump inside the cell, the pump won't be able to dephosphorylate and initiate the next round of activity. As the pump will remain phosphorylated it will not be able to bind Na+, as the unphosphorylated pump has greater affinity for Na+, ii) the Na+/K+ ATPase pumps against the Na+ gradient, so removal of extracellular Na+ will not affect, since the pump will still actively pump out the ions, iii) presence or removal extracellular ATP will not affect the pump because intracellular ATP is used,  iv) Ca2+ directly does not use this pump, so there will be no effect; however, a carrier enzyme (Na+-Ca2+ translocator) that pumps Ca2+ outside cell to maintain resting intracellular Ca2+ concentration uses the Na+ gradient caused by the pump.

Question 9. Option 2: whether the ion that passes through it has a reversal potential that is positive or negative to the current membrane potential.

Ligand gated ion channels undergo a conformational change upon ligand binding openning the ion channels, which triggers flow of ions across the cell membrane. This flow of ions can either result in depolarization of the membrane leading to an excitatory response or hyperpolarization of the membrane leading to an inhibitory response. Hyperpolarization makes the cell's membrane potential more negative, which can be achieved in 2 ways : influx of anions into the cell (eg. Cl- ions by chlroride channels) or efflux of cations outside of the cell (eg. K+ through K+ channels). Hyperpolarization is opposite to depolarization. So this excitatory/ inhibitory response is solely determined by whether the ion can reverse the membrane potential (option 2). The ligand binding properties of the channel, the charge on the ion or number of ions transferred do not determine the response.

Question 10. Option 2: It is responsible for the rising phase of the action potential

Membrane potential is determined by the potential at equilibrium and the relative permeabilities of the ions across the membrane. At resting membrane potential, [K+] is higher inside the cell while [Na+] is higher outside the cell. But in this resting state, the membrane is more permeable to K+, leading to efflux of K+ outside the cell. During an action potential, the permeability of the membrane to Na+ is dramatically increased due to opening of Na+ channels. This leads to an influx of Na+ ions into the cell, depolarizing the membrane and changing the membrane potential from -70 mV (resting membrane potential) to -55mV. This entire chain of events occurs rapidly to reach the threshhold and then to reach the peak of the action potential (rules out option 3). The falling phase of the action potential is caused by an increase of membrane permeability to K+ ions (rules out option 1) and K+ flows outside the cell until the membrane potential is reduced from +40mV to its resting membrane potential (rules out option 4).

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