Which one of the following elements is needed in the human body to transfer electrical signals by nerve cells ?
- (a)Lithium
- (b)Sodium
- (c)Rubidium
- (d)Caesium
Correct — B, Sodium. A nerve cell at rest holds a voltage across its membrane, kept there by a pump that pushes sodium ions out of the cell and potassium ions in. When the cell is stimulated, gates in the membrane open and sodium ions rush inwards down that gradient; the inrush of positive charge flips the voltage locally, and the flip propagates along the fibre. That travelling reversal of membrane voltage is the nerve impulse. All four options are alkali metals, but sodium is the only one of them the human body actually uses in this process — the others have no physiological role in nerve conduction at all.
- (a)Lithium — Lithium is not a nutrient and has no part in normal nerve conduction. It does appear in medicine, as lithium carbonate used in the treatment of bipolar disorder, and that association is probably why it is placed first among the options.
- (c)Rubidium — Rubidium has no known essential role in the human body. Being chemically similar to potassium it can enter cells in traces, but it carries no signalling function and is not required in the diet.
- (d)Caesium — Caesium likewise has no biological role. It is best known in exam terms for the caesium atomic clock and, in its radioactive form, as a fallout product — neither of which is a physiological function.
The sodium-potassium pump sits in the membrane of every animal cell and moves three sodium ions out for every two potassium ions it brings in, using energy from ATP. The result is a cell whose inside is negative relative to the outside and which holds sodium out and potassium in against their concentration gradients. Nerve and muscle cells exploit that stored gradient: opening sodium gates lets the ions flood in and generates the action potential, and potassium then flows out to restore the resting state. The two ions work as a pair, which is why physiology textbooks always name them together.
Because the two ions work together, the honest answer to 'which element carries nerve signals' is sodium and potassium — and NDA itself keyed exactly that in 2022, when both were on offer along with a combined option. Here potassium is not among the choices, so the question reduces to picking the one alkali metal with any role in the body, and sodium is the only candidate. Reading an options list before committing to an answer is the practical lesson: the same underlying fact can have different correct expressions depending on what the paper puts in front of you. On the wider picture, sodium's other jobs are worth carrying too — it is the main cation of the fluid outside cells, it governs the body's water balance and blood volume, and the sodium in common salt is why excessive salt intake is linked to raised blood pressure.
- The sodium-potassium pump moves three sodium ions out of a cell for every two potassium ions in, using ATP.
- A nerve impulse is a travelling reversal of membrane voltage driven by sodium ions rushing into the cell.
- Potassium ions moving outwards restore the resting voltage after the impulse has passed.
- Sodium is the principal cation of the fluid outside cells and governs the body's water and blood-volume balance.
- Lithium, rubidium and caesium are alkali metals with no essential role in human physiology.
- Assuming every alkali metal has some biological role because they all resemble sodium chemically.
- Naming lithium because of its use as a medicine for bipolar disorder, which is a drug effect and not a nutrient function.
- Forgetting that potassium is an equal partner in nerve conduction when a paper offers it alongside sodium.
NDA sets this either as a single-element question, as here, or with a combined 'sodium and potassium' option, in which case the combined option is the one keyed.
Living organisms require at least 27 elements, of which 15 are metals. Among these, those required in major quantities include
- (a) Potassium, Manganese, Molybdenum & Calcium
- (b) Potassium, Molybdenum, Copper & Calcium
- (c) Potassium, Sodium, Magnesium & Calcium
- (d) Sodium, Magnesium, Copper & Manganese
Answer(c) Potassium, Sodium, Magnesium & Calcium
UPSC's keyed list of the metals the body needs in quantity contains sodium and potassium and excludes everything else in the NDA options — which is exactly the reasoning that rules out lithium, rubidium and caesium.
The transfer of electrical signals by nerve cells in human body is enabled by
- (a) sodium
- (b) potassium
- (c) iron
- (d) sodium and potassium
Answer(d) sodium and potassium
Almost the same sentence four years later, with potassium added to the options — and NDA then keyed the pair. A clean illustration of why the options list, not just the fact, decides the answer.
Which one of the following depicts the correct circuit of a reflex arc?
- (a) Effector—sensory neuron—spinal cord—motor neuron—receptor
- (b) Receptor—sensory neuron—spinal cord—motor neuron—effector
- (c) Receptor—sensory neuron—brain—motor neuron—effector
- (d) Sensory neuron—receptor—brain—effector—motor neuron
Answer(b) Receptor—sensory neuron—spinal cord—motor neuron—effector
The route the signal takes, set in the same year's second sitting. Together the two items cover both halves of nerve conduction — the ion that carries the impulse and the circuit it runs through.
- practice — not a real PYQ
The sodium-potassium pump in a cell membrane works by
- (a)letting both ions diffuse freely down their gradients
- (b)moving sodium out and potassium in, using energy from ATP
- (c)moving sodium in and potassium out, without using energy
- (d)exchanging sodium for calcium in equal numbers
Answer(b) moving sodium out and potassium in, using energy from ATP — which sets up the gradient a nerve impulse then spends.
- practice — not a real PYQ
The principal cation of the fluid outside the body's cells is
- (a)potassium
- (b)sodium
- (c)calcium
- (d)magnesium
Answer(b) sodium — potassium is the principal cation inside cells, and the contrast between the two is what the pump maintains.