In contrast, the nonpolar portion is composed of a tetracyclic triterpenoid and an isooctyl hydrocarbon chain.This hydrophobic part of a sterol extends inwards and interacts with phospholipid fatty acid tails; more specifically, the tetracyclic structure and the side chain align adjacently to the fatty acid chain. In the bilayer, cholesterol and phospholipids interact to enhance mechanical coherence of the membrane by reducing fluidity and increasing rigidity. This has the effect of suppressing passive permeability, which improves the ability of a cell to control the movement of various molecules, especially polar ones, across the membrane and into the cell. There are also cholesterolrich regionstermed lipid raftsin the phospholipid bilayer.These lipid rafts have physical features that are quite distinct from those of the surrounding membrane landscape and are often associated with integral membrane proteins. However, these sites tend to be enriched in sphingolipids. Currently, our understanding of lipid raft distribution, structure, and function in cell membranes is relatively basic, especially in insects.A relatively small amount of sterol is required for metabolic purposes, in particular for producing molting hormone.The details of insect molting have been reviewed thoroughly elsewhere, so we provide a quick overview with an emphasis on variation in molting hormone structure as a function of the sterol precursors being used.In most insects, hydroxyecdysone, and cholesterol is the required precursor.Some insects, including plantfeeding heteropterans, leafcutting ants, and honeybees, use the ecdysteroid makisterone as their molting hormone. These insects have lost the ability to dealkylate phytosterols.Instead, they directly convert campesterol into makisterone A or sitosterol into makisterone C. Cholesterol is the most common sterol found in insects.Hydroxyecdysone is the major steroid hormone in most insects; cholesterol is its precursor.Some insects use another group of steroid hormones, makisterone A or C; they differ from E in that they contain an buy Tacalcitol additional methyl or ethyl group, respectively, at C, and campesterol are common phytosterols.They differ structurally from cholesterol by a methyl or ethyl group; alkylcholesterol is often seen in evolutionarily derived plants. Ergosterol at C; it also has two additional double bonds at C, in contrast to cholesterol.This inability to produce sterols likely exists because insects lack the enzymecoding gene that converts farnesyl pyrophosphate to squalene. The fact that insects have lost the ability to generate sterols de novo is puzzling.Perhaps there is an evolutionary advantage to sterolauxotrophy given that insects have innate oxygensupply limitations as a function of their blindended tracheal respiratory system cholesterol synthesis is extremely oxygen consuming and metabolically expensive.An inadequate oxygen supply could place a significant constraint on the synthesis of sterols, and this could reduce andor delay insect growth. Since the last comprehensive review on insect sterol utilizationpublished years ago several technological advances, including advanced gas and liquid chromatography, gene chips, highthroughput sequencing and screening, and cellular and molecular immunology and biochemistry, have been applied to better understand insect sterol nutrition and physiology.Our goal in this review is to provide an overview of the most recent advances concerning insect sterol nutrition, ranging from metabolism to homeostasis, physiological ecology, and the potential of exploiting insect sterol metabolic constraints to manage insect herbivore pests; in addition, we aim to more deeply explore sterol biology, including its implications for humans.