GATE 2026 BT Q.19: solution and answer

Official answer: BSource: Official answer key, GATE 2026 BT (IIT Guwahati)

1. How gel filtration (size-exclusion chromatography) works: the column is packed with porous beads. Small molecules can enter the pores, so they have more volume to pass through and come out later. Large molecules are kept out of the pores, travel only through the space between the beads, and come out earlier. So the elution volume $V_e$ (the volume of buffer collected before a protein comes out) goes down as molecular mass goes up.

2. For native globular proteins inside the column's fractionation range, size follows molecular mass $M$, and a calibrated column gives the empirical relation $V_e\approx a-b\log M$, where $a$ and $b>0$ are column constants. The heaviest protein therefore has the smallest $V_e$ and comes out first.

3. Sort the four proteins by molecular mass, largest first: Albumin ($66\text{kDa}$) > Thrombin ($37\text{kDa}$) > Myoglobin ($17\text{kDa}$) > Insulin ($6\text{kDa}$).

4. So they elute in this order: 1st Albumin, 2nd Thrombin, 3rd Myoglobin, 4th Insulin.

5. The protein that elutes third is Myoglobin ($17\text{kDa}$), option (B).