Explain secondary active transport (co-transport)

WHAT is it?

Two flavours:

Symport (co-transport): both solutes move in the same direction (e.g. $Na^+$ –glucose, SGLT1).
Antiport (counter-transport): they move in opposite directions (e.g. $Na^+/Ca^{2+}$ exchanger, $Na^+/H^+$ exchanger).

WHY does it need a pre-existing gradient?

If you poison the $Na^+/K^+$ ATPase (e.g. with ouabain), $Na^+$ leaks in, the gradient collapses, and glucose uptake by SGLT also stops — even though SGLT never touched ATP. This is the key proof that it is secondary.

HOW it works — derive the energy budget from first principles

We want to know: how much "downhill push" does one ion give, and is it enough to drag glucose uphill?

Step 1 — Energy stored in a concentration difference. For an uncharged solute, moving 1 mole from concentration $C_1$ to $C_2$ : $\Delta G_{conc} = RT\ln\frac{C_2}{C_1}$ Why? This is the standard free-energy of dilution from chemical potential $\mu = \mu^\circ + RT\ln C$ . Moving from high to low ( $C_2<C_1$ ) gives $\Delta G<0$ = energy released.

Step 2 — Add the electrical part (ions are charged). A charged ion also feels the membrane voltage $V_m$ . Total electrochemical free energy per mole: $\boxed{\Delta G_{ion} = RT\ln\frac{[Na^+]_{in}}{[Na^+]_{out}} + zFV_m}$ Why? $z$ = charge (+1 for $Na^+$ ), $F$ = Faraday constant (charge per mole), $V_m$ = $V_{in}-V_{out}$ (typically $\approx -70\,mV$ , inside negative). Both terms favour $Na^+$ entering the cell: it's more concentrated outside and the inside is negative (attracts +).

Step 3 — Energy needed to push glucose uphill. $\Delta G_{glu} = RT\ln\frac{[glu]_{in}}{[glu]_{out}}$ To pump glucose in against its gradient, $[glu]_{in}>[glu]_{out}$ , so $\Delta G_{glu}>0$ (costs energy).

Step 4 — The coupling rule (the whole secret). The transporter physically links the two: glucose only moves if $Na^+$ moves with it. So they share one energy budget. For $n$ $Na^+$ ions per glucose: $\Delta G_{total} = \underbrace{n\,\Delta G_{Na}}_{\text{released (}<0)} + \underbrace{\Delta G_{glu}}_{\text{costs (}>0)}$ Transport proceeds only if $\Delta G_{total} < 0$ : $\boxed{\,n\,\Delta G_{Na} + \Delta G_{glu} < 0\,}$

Figure — Explain secondary active transport (co-transport)

Worked Examples

Worked example Example 1 — Is the gradient enough to move glucose?

Given: $[Na^+]_{out}=145$ , $[Na^+]_{in}=12\,mM$ ; $V_m=-70\,mV$ ; $T=310\,K$ ; $RT=2.58\,kJ/mol$ ; $F=96485\,C/mol$ . Energy from one Na⁺ entering: $\Delta G_{Na}=2.58\ln(12/145) + (+1)(96485)(-0.070)/1000$ $=2.58(-2.49) + (-6.75) = -6.43 - 6.75 = -13.2\,kJ/mol$ . Why this step? First term: concentration favours entry. Second: voltage (inside −) also favours +entry. Both negative → big push inward. With $n=2$ : total push $=2(-13.2)=-26.4\,kJ/mol$ . That's enough to pump glucose up a ratio where $RT\ln([glu]_{in}/[glu]_{out})=+26.4$ , i.e. ratio $=e^{26.4/2.58}=e^{10.2}\approx 27000$ -fold. Massive concentrating power. ✔

Common Mistakes

Recall Feynman: explain to a 12-year-old

Imagine a water wheel. Someone (the ATP pump) lifts buckets of water up to a high tank — that takes effort. Now that water rushes back down and turns a wheel, and the wheel lifts a heavy bag (glucose) up to a shelf. The wheel never touched ATP — it just used the falling water. Stop filling the tank and the wheel stops, and the bag never reaches the shelf. That falling-water-lifts-bag combo is co-transport.

Recall — Active Flashcards

What energy source directly powers secondary active transport?

The electrochemical gradient of another ion (usually Na⁺), NOT direct ATP hydrolysis.

What establishes the Na⁺ gradient that co-transporters exploit?

Primary active transport by the Na⁺/K⁺ ATPase (which uses ATP).

Define symport vs antiport.

Symport = both solutes move the same direction; Antiport = they move in opposite directions.

Why does ouabain (Na⁺/K⁺ pump inhibitor) stop glucose uptake by SGLT even though SGLT uses no ATP?

It collapses the Na⁺ gradient that SGLT depends on; no gradient = no driving energy.

Write the electrochemical free energy of moving Na⁺ into the cell.

\Delta G = RT\ln([Na^+]_{in}/[Na^+]_{out}) + zFV_m

What is the condition for co-transport to proceed for n driver ions?

n\Delta G_{driver} + \Delta G_{cargo} < 0

How does glucose leave the intestinal cell into blood, and is it active?

Via GLUT2, by facilitated diffusion (passive, downhill — no energy).

Why is carrying 2 Na⁺ per glucose advantageous?

It doubles the energy, squaring the maximum glucose concentration ratio the cell can build up.

Difference between facilitated diffusion and symport?

Facilitated diffusion is passive (down-gradient); symport is active (drags a solute up-gradient using a coupled ion).

Connections

Primary Active Transport — pays the ATP that builds the gradient.
Na+/K+ ATPase — the master pump behind almost all $Na^+$ -driven co-transport.
Facilitated Diffusion — passive partner (e.g. GLUT2) that completes transcellular transport.
Electrochemical Gradient — the stored energy currency.
Membrane Potential — the $zFV_m$ term in the energy equation.
Gibbs Free Energy — first-principles basis of the $\Delta G$ logic.
Glucose Absorption in Intestine — the headline physiological example.

Concept Map

Hinglish (regional understanding)

Intuition Hinglish mein samjho

Dekho, secondary active transport ka funda simple hai: yahan koi protein directly ATP nahi todta, lekin transport phir bhi "active" (uphill) hota hai. Trick yeh hai ki pehle Na+/K+ ATPase ne ATP kharch karke sodium ka gradient bana diya — bahar Na+ zyada, andar kam. Yeh stored energy bilkul bank mein paisa jaisa hai. Ab ek doosra protein (jaise SGLT1) us Na+ ko wapas andar girne deta hai, aur us girne wali energy se glucose ko gradient ke against (uphill) andar kheech leta hai. Matlab Na+ neeche giri, aur uss "push" se glucose upar chadh gaya.

Agar dono cheezein same direction mein jaayein to usse symport (co-transport) kehte hain — jaise Na+ aur glucose dono andar. Agar opposite direction mein jaayein to antiport — jaise Na+/Ca2+ exchanger, jisme Na+ andar aur Ca2+ bahar. Energy ka logic dono mein same hai: driver ion ka downhill push, cargo ke uphill cost se bada hona chahiye, tabhi $\Delta G_{total}<0$ aur transport chalega.

Yeh important kyun hai? Exam mein ek classic trap: log bolte hain "secondary transport energy use nahi karta." Galat! Energy use hoti hai, bas udhaar li gayi hoti hai — ATP toh pehle Na+/K+ pump ne kharch kiya tha. Iska proof: agar tum ouabain se Na+/K+ pump band kar do, toh Na+ gradient khatam, aur glucose uptake bhi ruk jaata hai — chahe SGLT ne ATP ko haath bhi nahi lagaya. Real life example: tumhari intestine glucose ko khoon mein isi tarah absorb karti hai.

Test yourself — Cell Membrane & Transport

Connections