Sphaerosomes

A sphaerosome, as its name suggests, is spherical in shape and — like the microbodies covered just before it (6.6.11) — is bounded by a single unit membrane, not a double envelope. Its defining functional role is fat (lipid) storage: sphaerosomes are the site where a plant cell packages and holds its stored fat reserves, and the textbook's own example is the endosperm cells of oil-storing seeds (oilseeds), where large numbers of sphaerosomes accumulate to store the fat the germinating seedling will later mobilise as an energy and carbon source. Sphaerosomes are generally understood to bud off from the smooth endoplasmic reticulum, which is consistent with why they carry only a single membrane (an ER-derived vesicle) rather than the double membrane of a true plastid or mitochondrion — this also places them functionally alongside other single-membrane-bound organelles like the lysosome and the microbodies (peroxisomes, glyoxysomes) discussed in this same run of sections, all of whi …