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I'm attending an introductory high school course to cell biology. Based on my understanding, lipids – the building blocks of membranes – are formed in the smooth ER. Are all/most/any membranes comprised of lipids that are formed in the smooth ER? If not, where do the lipids of membranes come from?

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It sounds like we're mainly interested in phospholipids here, as they're the main component of cell membranes. Since your question didn't specify, we'll talk about Bacteria/Archaea and Eukarya separately

Bacteria/Archaea

Here, the answer is fairly intuitive. Bacteria and Archaea don't have endoplasmic reticulum, so they can't synthesize phospholipids in it. In this case, it seems like they create fatty acids directly from enzymes in the midst of the cytosol.

Eukarya

Here, the answer is a bit more complex. Fortunately, there are some good reviews about the topic that answer the question. Here, they talk about the entire phospholipid biosynthesis pathway and only mention the ER as an organelle, although it's worth noting that there are several sub-locations within the ER where specific groups are added. However, they fail to distinguish between rough and smooth. In another source, they mention not only the ER but also the mitochondria as having some ability to synthesize phospholipids. That's pretty cool, in my opinion.

TL;DR:

Bacteria and Archaea certainly don't rely on the smooth ER for lipid synthesis. In eukaryotes, there are several areas where lipids can be made but the smooth ER does seem to be the main source.

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