NCERT Class 11 Science — Chapter 8: Neural Control and Coordination

The correct option is

A is true but R is false

.

Explanation

The question addresses the electrochemical properties of a neuron during its resting state, specifically focusing on the resting membrane potential and the mechanism of the sodium-potassium pump.

Analysis of Assertion (A):

Correct. In a resting neuron (not conducting an impulse), the axonal membrane is polarized. The exterior of the membrane possesses a net positive charge, while the interior (axoplasm) possesses a net negative charge. This potential difference across the plasma membrane is known as the resting potential, typically ranging around -70 mV.

Analysis of Reason (R):

Incorrect. The sodium-potassium pump (Na⁺-K⁺ ATPase) is an active transport mechanism that maintains the ionic gradients across the membrane. However, the direction of transport stated in the reason is reversed. The pump actively transports 3 Na⁺ outwards (out of the cell) for every 2 K⁺ inwards (into the cell). The statement incorrectly claims that Na⁺ is transported into the cell.

Conclusion:

Since the Assertion describes the correct charge distribution but the Reason provides a factually incorrect description of the ion pump's mechanism, the Assertion is true and the Reason is false.

Key Takeaway:

The resting membrane potential is characterized by a positive exterior and a negative interior. The Na⁺-K⁺ pump contributes to this by expelling 3 Na⁺ ions for every 2 K⁺ ions it imports, creating an electrochemical gradient.