Answer to Question #15303 in Biochemistry for nida sadaqat
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.
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.
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