Biology question about cholesterol

[nayryecal]

Hi!

I read in the destroyer that cholesterol is the precursor to steroid hormones such as estrogen, progesterone, cortisone, cortisol...etc, thus they are lipid soluble and able to pass through the plasma membrane. In the CliffsAP biology I read that proteins that transport cholesterol in blood (low-density lipoproteins, or LDLs) target specific cells by receptor-mediated endocytosis and require a secondary messenger to get across the cell membrane.

So why cant cholesterol in the blood easily pass through the cell like the cholesterol that is the precursor to steroid hormones? What is the difference?

Thanks

 

[ChrisM07]

My reasoning would be that cholesterol is hydrophobic and would not be able to travel around in the aqueous portion of the blood. By having a protein carrier, it allows the cholesterol to become soluble. Now, the carrier itself is a protein and is not able to pass through the lipid bilayer so it therefore needs to utilize the receptor mediated endocytosis.

Similarly, these steroid hormones should travel via the same method. Once near the membrane, they'll have to dissociate from the carrier in order to pass through the membrane. Even when in the cell though, these hormones and cholesterol based molecules need a transporter to get them into the nucleus because they are again not soluble in the aqueous environment of the cytosol. Once in the nucleus, the hormone and carrier complex is called a hormone response element (HRE) that can bind to DNA and cause the transcription of mRNA for protein formation.

 

[denttal]

So cholesterol itself is lipid soluble. However, this makes cholesterol not soluble in H20/blood so cholesterol travels in the blood surrounded by carrier proteins like LDL and HDL. The inside layer of LDL is hydrophobic so thats where the cholesterol is. The outside layer of LDL is hydrophilic. Therefore, when the LDL (with the cholesterol inside) reaches the plasma membrane, it cannot pass through. This is where the LDL receptor proteins are needed to bring the LDL into the cell via endocytosis. Once inside the cell, lysosomes will lyse the LDL releasing the cholesterol into the cell.

The steroid hormones, however, are only derived from cholesterol. They are not cholesterol. These hormones are lipid soluble so can pass directly through the plasma membrane. The steroid hormones also need to be carried through the blood by hydrophilic protein carriers. However, for steroid hormones, the protein carriers simply release the steroid hormone when it reaches the cell membrane (so no receptor proteins are needed).