Q: Which ion has a higher concentration outside the axonal membrane at resting membrane potential?  

Answer: A

At resting membrane potential, the axonal membrane of a neuron is selectively permeable, meaning it allows certain ions to pass through while preventing others from doing so. The main ion that cannot pass through the membrane at resting potential is sodium ions (Na+), as the sodium channels are closed. Potassium ions (K+) are able to move freely through the membrane because there is a higher concentration of K+ inside the cell compared to outside, creating a driving force that pushes K+ out of the cell. Calcium ions (Ca2+) and chloride ions (Cl-) also cannot pass through the membrane at resting potential.
Because sodium ions are unable to cross the membrane at resting potential, their concentration remains higher outside the cell compared to inside. This concentration gradient creates a driving force that pushes Na+ into the cell if the channels were open. This gradient is maintained by the sodium-potassium pump, a protein that actively transports Na+ out of the cell and K+ into the cell, using ATP as energy. This pump plays a critical role in maintaining the resting membrane potential of the neuron.
In summary, at resting membrane potential, the concentration of sodium ions is higher outside the axonal membrane of a neuron compared to inside, due to the selective permeability of the membrane.

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