A steroid is a biologically active organic compound with four rings arranged in a specific molecular configuration. Steroids have two principal biological functions: as important components of cell membranes which alter membrane fluidity; and as signaling molecules. Hundreds of steroids are found in plants, animals and fungi. All steroids are manufactured in cells from the sterols lanosterol (opisthokonts) or cycloartenol (plants). Lanosterol and cycloartenol are derived from the cyclization of the triterpene squalene.The steroid core structure is typically composed of seventeen carbon atoms, bonded in four "fused" rings: three six-member cyclohexane rings (rings A, B and C in the first illustration) and one five-member cyclopentane ring (the D ring). Steroids vary by the functional groups attached to this four-ring core and by the oxidation state of the rings. Sterols are forms of steroids with a hydroxy group at position three and a skeleton derived from cholestane.: 1785f Steroids can also be more radically modified, such as by changes to the ring structure, for example, cutting one of the rings. Cutting Ring B produces secosteroids one of which is vitamin D3.
Examples include the lipid cholesterol, the sex hormones estradiol and testosterone,: 10–19 and the anti-inflammatory drug dexamethasone.Intensified is an album by Desmond Dekker & the Aces released in 1970.September is the ninth month of the year in the Julian and Gregorian calendars, the third of four months to have a length of 30 days, and the fourth of five months to have a length of fewer than 31 days. September in the Northern Hemisphere and March in the Southern Hemisphere are seasonal equivalent.
In the Northern hemisphere, the beginning of the meteorological autumn is on 1 September. In the Southern hemisphere, the beginning of the meteorological spring is on 1 September.
September marks the beginning of the ecclesiastical year in the Eastern Orthodox Church. It is the start of the academic year in many countries of the northern hemisphere, in which children go back to school after the summer break, sometimes on the first day of the month.
September (from Latin septem, "seven") was originally the seventh of ten months in the oldest known Roman calendar, the calendar of Romulus c. 750 BC, with March (Latin Martius) the first month of the year until perhaps as late as 451 BC. After the calendar reform that added January and February to the beginning of the year, September became the ninth month but retained its name. It had 29 days until the Julian reform, which added a day.