Question

The graph shows the relative filterability of molecules across the glomerular capillary wall plotted against effective molecular radius, for three curves A, B and C carrying different charges. Based on charge, which molecule is filtered most readily?
(Refer to the figure; for a given molecular radius, filterability order A > B > C, with C marked as anions.)

• A. A (cation)
• B. B (neutral)
• C. C (anion)
• D. All are filtered equally

Hint:

The filtration barrier is negatively charged, so it lets positive molecules through most easily.

Answer 1

The Correct Option is A

Step 1: Recall the glomerular filtration barrier and charge selectivity.
The glomerular capillary wall (endothelium, basement membrane, podocyte slit diaphragm) carries fixed negative charges, largely due to heparan sulfate proteoglycans and the sialoprotein coat (e.g. podocalyxin).

Step 2: Apply electrostatics.
Because the barrier is negatively charged, it repels negatively charged (anionic) molecules and attracts/permits positively charged (cationic) molecules. Therefore, for the same molecular size:
\[ \text{Filterability:}\quad \text{Cation} > \text{Neutral} > \text{Anion} \]

Step 3: Map to the curves.
Curve A (highest filterability) = cation; curve B = neutral; curve C (lowest, labelled anions) = anion. So the most filterable molecule based on charge is A, the cation.

Step 4: Why others are wrong.
• B (neutral) is filtered less than the cation.
• C (anion) is filtered least because of charge repulsion - this is the basis of selective albuminuria when the negative charge barrier is lost (e.g. minimal change disease).
• They are not filtered equally - charge clearly discriminates.

Key fact: The negatively charged glomerular barrier favours filtration of cations over neutral molecules over anions; albumin (anionic) is normally restricted partly by charge.