The plasma membrane consists of an amphipathic phospholipid bilayer with a hydrophobic internal hydrocarbon core.
Small non-polar molecules ($O_2, CO_2, N_2$) and small uncharged molecules (ethanol) diffuse freely across the bilayer.
Water is a polar molecule; although trace amounts cross by simple diffusion, water is not freely permeable across pure lipid bilayers and requires specialized transmembrane protein channels (aquaporins) for rapid facilitated osmotic transport.
Correct Option: Option 2 (freely permeable to water).
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Q.2
Fish Physiology and Biochemistry- ICAR JRF/SRF 2024 (Q.75)
MCQ (Single Correct)+4−1
Which prime ion transporter influences the absorption of both Na+ and Cl- by both Na#/Cl- and Na+/K+/2Cl-cotransporters?
In branchial and intestinal epithelial cells, the basolateral $Na^+/K^+$-ATPase (sodium-potassium pump) couples ATP hydrolysis to pump $3\ Na^+$ ions out of the cell in exchange for $2\ K^+$ ions pumped inward.
This active transport maintains a low intracellular $Na^+$ concentration and a negative intracellular membrane potential, generating a powerful inward electrochemical $Na^+$ gradient.
This gradient provides the essential driving force for secondary active cotransporters:
$Na^+/Cl^-$ cotransporter (NCC) in freshwater ion absorption.
$Na^+/K^+/2Cl^-$ cotransporter (NKCC1 / NKCC2) in seawater ion secretion and intestinal fluid absorption.
nalysis:
(Na+/K+-ATPase): Correct.
Correct option: (4) Na+/K+-ATPase
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