A negatively charged molecule is filtered with more difficulty’ compared to a positive one because –
Correct Answer: Presence of negatively charged sialoproteins on the filtering membrane
Description: Ans. is 'a' i.e., Presence of negatively charged sialoproteins on the filtering membrane o Sialoproteins in the glomerular capillary wall are negatively charged, and studies with anionically charged and cationically charged dextrans indicate that the negative charges repel negatively charged substances in blood, with the result that filtration of anionic substances 4 nm in diameter is less than half that of neutral substances of the same size.o This probably explains why albumin, with an effective molecular diameter of approximately 7 nm, normally has a glomerular concentration only 0.2% of its plasma concentration rather than the higher concentration that would be expected on the basis of diameter alone; circulating albumin is negatively charged,o Filtration of cationic substances is greater than that of neutral substances.Filtration surface (Filtration barrierQ or glomerular membrane)o The filtration surface is a three-layered structure : -Glomerular capillary endotheliumQThe basement membrane0 (Basal lamina)Visceral epithelium of Bowman's capsule with their foot process (PodocytesQ).o However, even the three layers put together do not provide an adequate structural basis for the degree of sieving achieved during filtration.o The fenestration in the endothelium are 50-100 nm in diameter, and the slits between the foot process of epithelial cells (podocytes) are 25 nm wide.o That leaves only the basement membrane to account for the selectivity and the filtration function of the glomerularmembrane.o Basement membrane contains hydrated channels approximately 6 nm wideQ.o A further functional barrier in basement membrane is added by the fact that basement membrane glycoproteins are strongly negatively chargedQ.o That is why filtration surface is almost impermeable to plasma proteins which are negatively charged, e.g.,albuminQ (size7 NmQ).o The passage of neutral or positively charged molecules of comparable diameter is not so difficult.Ultimately, molecules less than 4 nm is diameter are freely filtered while molecules larger than 8 nm are not filtered at all and the molecules between 4-8 nm diameter are filtered with difficulty which increases proportionately with size.
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