Practice question

Plant membranes replace cholesterol phytosterols where stigmasterol and sitosterol are prominent structurally related functionally overlapping but distinct regulation. Both derive cycloartenol via CYP51 SMT1 SMT2 sterol methyltransferases, with sitosterol bearing twenty four alpha ethyl group dominating sixty percent free sterols stigmasterol bearing additional trans double bond C22 C23 introduced CYP710A comprising ten to thirty percent campesterol ten percent remainder. In most crops soybean Arabidopsis tomato sitosterol quantitative major but stigmasterol characteristic marker plasma membranes detergent resistant rafts showing inducible increase pathogen challenge avirulent bacteria senescence mediated PLD phospholipase D signaling and ROS tolerance membrane remodelling. Cholesterol minor less than five percent, ergosterol dominates fungi mycobacteria. Phytosterols regulate membrane order permeability ion channels H ATPase activity auxin transport PIN localization brassinosteroid precursor campesterol growth regulation. Ratio sitosterol stigmasterol modulates fluidity freezing tolerance via CBF pathway COR genes. Understanding phytosterol diversity illustrates evolutionary adaptation where cholesterol function fulfilled ethyl sterols explaining dietary plant sterol interference human cholesterol absorption via Niemann Pick C1 Like one transporter NPC1L1 competitor heart health and plant physiology cold adaptation topics in exams.