The pump flips between two shapes: E1 (open to inside, loves Na⁺) and E2 (open to outside, loves K⁺).
3 Na⁺ bind from inside the cell to the E1 form (high affinity for Na⁺).
Why? Inside Na⁺ is low, so binding sites must be "sticky" to grab them.
ATP is hydrolysed; a phosphate group attaches to the pump (phosphorylation).
Why? This is the energy input — it forces a shape change.
The pump changes to E2, opening to the outside and losing affinity for Na⁺ → 3 Na⁺ released outside.
Why? Low Na⁺ affinity means it lets go even though outside Na⁺ is already high.
2 K⁺ bind from outside (E2 has high affinity for K⁺).
The pump is dephosphorylated (phosphate falls off).
Why? Removing phosphate lets it snap back to E1.
Pump returns to E1, opening inward and losing K⁺ affinity → 2 K⁺ released inside. Cycle repeats.
What energy source powers primary active transport?
Direct hydrolysis of ATP at the transport protein (an ATPase).
How many Na⁺ and K⁺ move per cycle of the Na⁺/K⁺ pump?
3 Na⁺ out, 2 K⁺ in.
Why is the Na⁺/K⁺ pump described as electrogenic?
It moves net +1 charge out per cycle (3 out − 2 in), helping make the inside negative.
What chemical event drives the conformational change in the pump?
Phosphorylation — phosphate from ATP attaches to the pump.
Name three jobs of the Na⁺/K⁺ pump.
Build Na⁺/K⁺ gradients; control cell volume; maintain resting membrane potential.
Difference between primary and secondary active transport?
Primary uses ATP directly; secondary uses a gradient (e.g. Na⁺) already built by a primary pump.
Which form (E1/E2) has high affinity for Na⁺, and where does it open?
E1 — opens to the inside, high Na⁺ affinity.
What happens to a cell if the pump is poisoned?
Na⁺ accumulates inside, water enters by osmosis, cell swells and may burst.
Is the Na⁺/K⁺ pump a channel?
No — it's a carrier protein (and an enzyme) that changes shape; it never opens a continuous pore.
What is the ion-to-ATP ratio of the pump?
3 Na⁺ + 2 K⁺ moved per 1 ATP hydrolysed.
Recall Feynman: explain to a 12-year-old
Your cell is like a tiny submarine. It needs lots of "salt A" (potassium) inside and lots of "salt B" (sodium) outside — but salts always like to spread out evenly, so they keep leaking the wrong way. So the submarine has a little pump that runs on battery juice (ATP). Every push of the button it throws 3 sodiums overboard and pulls 2 potassiums in. It costs energy because it's pushing them the way they don't want to go — like rolling balls uphill. Doing this keeps the submarine from filling with water and bursting, and it lets the cell send electric signals later, like sliding the balls back downhill when needed.
Dekho, sodium-potassium pump cell membrane ka ek special protein hai jo direct ATP todke kaam karta hai — isiliye ise primary active transport bolte hain. Iska kaam simple yaad rakho: 3 sodium (Na⁺) bahar, 2 potassium (K⁺) andar, har ek cycle mein, aur dono ions apne natural gradient ke ulta (uphill) move karte hain. Gradient ke khilaf push karna free nahi hota, isliye ATP ka coin lagta hai.
ATP jab toot-ta hai, uska phosphate pump pe chipak jaata hai (phosphorylation), aur isse pump ka shape badalta hai — pehle andar khulta tha (E1, Na⁺ ko pakad'ta), phir bahar khulta hai (E2, Na⁺ chhodta aur K⁺ pakad'ta). Yahi shape badalna ions ko paar le jaata hai. Channel nahi hai bhai — yeh carrier hai jo "revolving door" ki tarah ghoomta hai.
Yeh itna important kyun? Teen reasons: (1) gradients banata hai jo nerve impulse aur muscle ke liye chahiye, (2) cell volume control karta hai — agar pump band ho jaaye to Na⁺ andar bharta hai, paani osmosis se aata hai, aur cell phool ke phat sakta hai, (3) resting membrane potential maintain karta hai kyunki net +1 charge bahar nikalta hai (3 minus 2), isiliye andar negative banta hai — ise electrogenic kehte hain.
Exam tip: ratio yaad rakho 3:2:1 (3 Na⁺ out, 2 K⁺ in, 1 ATP). Aur galti mat karna — log sochte hain barabar ions move hote hain, par nahi, unequal hai, aur yahi unequal hona hi pump ko electrogenic banata hai.
