Cell Organelles & Water Relations

• Biochemical Identity of Sialic Acid: Sialic acid is the generic designation for a family of nine-carbon acidic amino sugars derived from neuraminic acid, with the predominant biological form being N-Acetylneuraminic acid (Neu5Ac).
• Biological Role: Neu5Ac occupies terminal positions on glycoprotein and glycolipid oligosaccharide chains on animal cell surfaces, providing negative surface charge and mediating cellular recognition, adhesion, and pathogen binding.
• Distinction from Other Amino Sugars: $N$-Acetylglucosamine (GlcNAc) and $N$-Acetylgalactosamine (GalNAc) are hexosamines, while $N$-Acetylmuramic acid (NAM) is a bacterial cell wall peptidoglycan component.

Correct option: (3) N-Acetyineuraminic acid

• Statement (I) is False: Prostaglandins are eicosanoids synthesized from 20-carbon polyunsaturated fatty acids (primarily arachidonic acid, $20:4\Delta^{5,8,11,14}$), and therefore contain 20 carbon atoms arranged into a cyclopentane ring and two side chains, not 15 carbons.
• Statement (II) is True: Prostaglandin $\text{PGE}$ and $\text{PGF}$ were first isolated and purified from human seminal fluid and sheep prostate/vesicular glands by Sune Bergström and Ulf von Euler.
• Conclusion: Statement (I) is false but Statement (II) is true.

Correct option: (4) Statement (I) is false but Statement ({I) is true.

• Assertion (A) is True: Ethylene reduces auxin biosynthesis and significantly impairs basipetal polar auxin transport (PAT) in stems and petioles by downregulating PIN efflux carriers, thereby inducing cell separation in the abscission zone.
• Reason (R) is False: Ethylene signaling functions by de-repressing master transcription factors (EIN3/EIL1), which specifically activate ethylene-responsive genes and transcriptional cascades (e.g., triggering de novo synthesis of cell-wall hydrolyzing enzymes like cellulase and polygalacturonase), rather than causing general mRNA and enzyme inhibition.
• Conclusion: (A) is true but (R) is false.

Correct option: (3) (A) is true but (R) is false.

• Antagonistic Action of ABA on Gibberellin: In germinating cereal seeds (barley, wheat), gibberellins (GA) induce the de novo transcription of hydrolytic enzymes (such as $\alpha$-amylase) in the aleurone layer by promoting GAMYB expression.
• Suppression Mechanism: Abscisic acid (ABA) directly suppresses this GA-mediated induction by activating protein kinases (such as PKABA1) and transcriptional repressors that silence the $\alpha$-amylase promoter, thereby maintaining seed dormancy.

Correct option: (3) Abscisic acid