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3.1.10 · D5Hydrogen and s-Block

Question bank — Biological importance of Na, K, Ca, Mg

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Before you start, three words you must own (from the parent note):

  • Extracellular = outside the cell (blood, fluid between cells). Home of Na⁺.
  • Intracellular = inside the cell. Home of K⁺ and (mostly) Mg²⁺.
  • Gradient = a difference in concentration across the membrane — like water held behind a dam, ready to flow.

Figure 1 — the membrane stage. The picture below is the stage on which every trap plays out. Read it left-to-right: the shaded violet strip in the middle is the cell membrane; everything to its left is OUTSIDE, everything to its right is INSIDE. Each ion appears twice, with its concentration on each side — notice Na⁺ is high outside (145 mM) but low inside (12 mM), while K⁺ is the mirror image (4 outside, 140 inside) and Ca²⁺ is a staggering 10 000× lower inside. The coloured arrows show the natural direction each ion would flow if a door opened (down its gradient). The navy box in the middle is the Na/K pump, which spends 1 ATP to push 3 Na⁺ out and 2 K⁺ in — the machine that maintains those lopsided numbers. Keep this whole scene in your head as you answer.

Figure — Biological importance of Na, K, Ca, Mg

The Nernst equation — every letter defined

Several traps hinge on one formula, so let's disarm its symbols first. The Nernst equation answers: "if an ion has a concentration gradient, what membrane voltage exactly cancels its urge to flow?"

Figure 2 — reading the Nernst curve. The graph below plots (vertical axis, mV) against the concentration ratio (horizontal axis). Three features tell the whole story:

  1. The curve crosses zero at ratio = 1. When outside equals inside, , so there is no driving voltage — the ion is already balanced.
  2. The shape is a logarithm, not a straight line. Because , doubling the ratio always adds the same fixed step ( mV), so the curve rises fast at small ratios then flattens — big concentration changes buy less and less voltage.
  3. Sign follows the ratio. K⁺ sits at ratio (out ≪ in), which is left of 1, so is negative → the orange point lands at about mV. Na⁺ sits at ratio (out ≫ in), right of 1, so is positive → the magenta point lands near mV. The dashed navy line marks the measured resting mV, which sits just above pure because a little Na⁺ leaks in and tugs it upward.
Figure — Biological importance of Na, K, Ca, Mg

Figure 3 — why +1 ions switch fast and +2 ions stick. This schematic plots binding energy (vertical axis; more negative = held more tightly) against distance from a negative binding site (horizontal axis) for an ion sitting near, say, a phosphate group. An ion sits in a potential-energy well: the deeper the well, the more energy it must gather (from random thermal kicks) to climb out and leave. A +2 ion (magenta, deep well) is pulled twice as hard by the negative site, so its well is roughly twice as deep — it climbs out rarely, meaning slow off-rates → it clings and acts as structural "glue." A +1 ion (orange, shallow well) barely dips, so thermal energy pops it in and out constantly → fast on/off rates, exactly what an electrical switch needs. The width of the well is similar; it is the depth set by charge that decides the speed. This is why Nature reserved Na⁺/K⁺ for signalling and Mg²⁺/Ca²⁺ for gripping.

Figure — Biological importance of Na, K, Ca, Mg

True or false — justify

A +2 ion binds a phosphate group more tightly than a +1 ion.
True — charge doubles the electrostatic pull, so Mg²⁺/Ca²⁺ act as "molecular glue" on phosphates and –COO⁻, whereas Na⁺/K⁺ slip on and off easily. See Alkaline Earth Metals (Mg, Ca).
Na⁺ is the main cation inside the cell because sodium is abundant in the body.
False — the pump throws Na⁺ out; inside is K⁺-rich (~140 mM K⁺ vs ~12 mM Na⁺). Abundance in blood ≠ location inside the cell.
The Na⁺/K⁺-ATPase moves equal numbers of Na⁺ and K⁺.
False — it ejects 3 Na⁺ out and imports 2 K⁺ in per ATP. The unequal 3:2 stoichiometry is itself electrogenic (net +1 charge leaves).
Calcium's only real job is building bones.
False — bones store ~99 percent of it, but its dynamic roles (muscle contraction, blood clotting as Factor IV, nerve/hormone signalling) are equally life-critical.
Chlorophyll and haemoglobin use the same central metal.
False — both use a porphyrin ring (see Coordination Chemistry — Porphyrins), but chlorophyll's centre is Mg²⁺ and haemoglobin's is Fe²⁺.
The resting membrane potential is set mostly by Na⁺.
False — it is set mostly by K⁺, whose Nernst potential (about −90 mV at 310 K) sits closest to the measured −70 mV. The membrane at rest is far more permeable to K⁺.
Mg²⁺ is the true partner of ATP inside cells, not free ATP.
True — the active fuel is the Mg-ATP complex; Mg²⁺ neutralises the negative phosphate charges so enzymes can bind and act on it. See ATP and Bioenergetics.
The cell keeps intracellular Ca²⁺ high so it is always available.
False — it keeps it insanely low (about 10⁻⁷ M, roughly 10 000× below outside). Low baseline is what makes a channel opening a sharp, unmistakable ON-signal.

Spot the error

"Firing a nerve requires the pump to actively push ions during the spike."
Error — the spike itself is the battery discharging (ions flowing down their gradient through open channels). The pump did its work earlier, charging the battery; it merely restores the gradient afterward.
"K⁺ sets the resting potential, so its Nernst value should be exactly −70 mV."
Error — the K⁺ Nernst value is about −90 mV (at 310 K, 4/140 mM). Resting is −70 mV because the membrane also leaks a little Na⁺, which drags the potential slightly upward from pure E_K.
"Na⁺ controls blood pressure by exerting an electrical push on vessel walls."
Error — Na⁺ controls blood volume/pressure osmotically: water follows salt, so more Na⁺ retained means more water retained and higher volume. See Osmosis and Fluid Balance.
"In the Nernst equation, using z = +2 for K⁺ is fine since it's an s-block ion."
Error — z is the actual charge, not the group. K⁺ has z = +1; using z = +2 would halve the predicted voltage. See Nernst Equation.
"Mg²⁺ is used for structure like Ca²⁺, so Mg²⁺ builds bones too."
Error — bone mineral is calcium hydroxyapatite, Ca10(PO4)6(OH)2. Mg²⁺'s gripping role is molecular (chlorophyll, ATP, enzyme cofactor), not skeletal.
"Iron sits in the green pigment of leaves."
Error — the green pigment is chlorophyll, centred on Mg²⁺. Iron lives in the red pigment haemoglobin. Same porphyrin frame, different metal, different colour.

Why questions

Why did Nature pick +1 ions (Na⁺, K⁺) for electrical signalling?
Because a single charge sits in a shallow energy well (see Figure 3), so the ion slips in and out of channels fast without sticking to the many negative groups in proteins — speed is exactly what a nerve impulse needs.
Why is the Na⁺/K⁺ pump's 3:2 ratio important beyond just moving ions?
Net charge leaves the cell each cycle (3 out, 2 in), so the pump directly contributes to the negative interior — it is electrogenic, not just a housekeeper.
Why does a huge 10 000-fold Ca²⁺ gradient make a sharp signal?
With such a steep drop, even a tiny channel opening lets Ca²⁺ flood in as a large, sudden spike — a clean, high-contrast ON pulse that stands far above the near-zero baseline.
Why must Ca²⁺ be pumped out immediately after a signal?
To reset the switch: restoring the near-zero baseline means the next channel opening produces an equally sharp spike. Cheap, fast, reusable.
Why does Mg²⁺ specifically (not Ca²⁺) sit at the heart of chlorophyll and ATP?
Mg²⁺ is smaller and higher charge-density, so it grips the electron-rich porphyrin/phosphate tightly yet stays kinetically nimble — Ca²⁺ is larger and reserved for triggering, not fixed catalytic seats.

Edge cases

If the cell's ATP supply drops to zero, what happens to the Na⁺/K⁺ gradient?
The pump stalls, gradients slowly leak away toward equal concentrations, the resting potential collapses toward 0 mV, and the cell can no longer fire — energy is the price of the battery.
If K⁺ outside is raised (hyperkalemia) so it approaches K⁺ inside, what happens to E_K?
The ratio out/in approaches 1, so ln(...) approaches 0 and E_K approaches 0 mV — the resting potential rises (depolarises) toward zero, dangerously disrupting excitability.
For an ion with equal inside and outside concentrations, what does the Nernst equation predict?
E = (RT/zF) ln(1) = 0 mV — with no concentration gradient there is no driving voltage; the ion is already at equilibrium with zero membrane potential.
If Mg²⁺ were completely removed from a cell, name the TWO priority systems that fail first.
(1) Photosynthesis — no Mg centre in chlorophyll; and (2) all ATP-dependent chemistry — no Mg-ATP complex. These two are the headline failures (many enzymes and ribosome stability also suffer, but these two are the priority examples).
What would happen to a nerve if intracellular Ca²⁺ never returned to baseline after a signal?
The next signal could not stand out against the raised background, so signalling loses its sharp ON/OFF contrast — the alarm bell that never resets can no longer ring clearly.
Recall One-line survival summary

Na⁺ out, K⁺ in (3:2 pump) → K⁺ sets rest (about −90 mV Nernst at 310 K); Mg²⁺ = chlorophyll + Mg-ATP (small, grippy); Ca²⁺ = bones AND the low-baseline alarm bell for muscle/clotting/signals. Fe (not Mg) is in haemoglobin.

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