what is alternative forms of cholesterol formed in plants and bacteria ?
Cholesterol in plants:
More than 250 steroids have been described in plants. Of these, perhaps sitosterol, which differs from cholesterol by an ethyl substituent at position 24, is the most common. But plants also contain cholesterol both free and esterified. Cholesterol occurs as a component of plant membranes and as part of the surface lipids of leaves where it is sometimes the major sterol. The quantity of cholesterol is generally small when expressed as percent of total lipid. While cholesterol averages perhaps 50 mg/kg total lipid in plants, it can be as high as 5 g/kg (or more) in animals.
Cholesterol in bacteria:
The ability to synthesize sterols is rarely found in prokaryotes. Consequently, when bacteria or blue-green algae are grown in sterol-free media, sterols are not usually detected in their membranes. The lack of cholesterol in microbial membranes is thus frequently cited as a feature distinguishing them from eukaryotic cell membranes. The wall-less mycoplasmas resemble other prokaryotes in their inability to synthesize sterols. Yet, unlike the other prokaryotes, most of the mycoplasmas require exogenous cholesterol for growth and incorporate large quantities of it into their cell membrane.