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I guess the deeper question is: what makes it possible for proteins like those part of HDL and LDL to move through the bloodstream?

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  • $\begingroup$ Ah, got it, thanks! With deeper question I meant: How do the proteins in LDL and HDL know that they have to go to the liver? How does their "GPS" work? $\endgroup$ Jun 2, 2019 at 8:27
  • $\begingroup$ Maybe change the question - It think "Is there so called free cholesterol that travels through the blood?" should be some valid question, too, and sounds the same. $\endgroup$ Nov 7, 2021 at 14:21

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It is not LDL that does the Job. The target cells "know" which lipoproteins (LDL,HDL,VLDL...) they should take up. They express the specific Membrane receptor that binds the Protein part of e.g. LDL and therefore take up specifically LDL rather than e.g. HDL from the blood. Example: cells that express receptors for apolipoprotein B-100 (one of the Protein parts of LDL) specifically "filter" LDL from the blood. The "filtering" occurs via endocytosis. Hope that helps.

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  • $\begingroup$ Thanks! So I guess LDL and HDL just moves through the whole circulatory system until it randomly hits the target cells. $\endgroup$ Jun 7, 2019 at 4:04
  • $\begingroup$ In respect of "randomly hits..." the target might be at the other ending: how does LDL/HDL find the cholesterol. It does, and this refers to cholesterol being soluble in blood (maybe not soluble enough not to take the taxi..., some idea of mine). From Wikipedia: "ABCA1 mediates the efflux of cholesterol and phospholipids to lipid-poor apolipoproteins (apoA1 and apoE) (reverse cholesterol transport), which then form nascent high-density lipoproteins (HDL)." I do believe cholesterol not even esterfied is being shedded from cell for "random" uptake. $\endgroup$ Nov 7, 2021 at 18:48

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