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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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    $\begingroup$ As regards the first part of your question (that contained in the header), the fundamental problem is that cholesterol is practically insoluble in water, and HDL and LDL may be thought of as (one of) Nature's methods for solving the 'oil and water don't mix' problem. I don't fully understand what you mean by your second question (the 'deeper question'). $\endgroup$ – user1136 Jun 1 '19 at 8:47
  • $\begingroup$ 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? $\endgroup$ – Juan Alonso Jun 2 '19 at 8:27
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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. $\endgroup$ – Juan Alonso Jun 7 '19 at 4:04

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