Impermeable

Actually I just have one last question: Why are cell membranes permeable to non-polar molecules and relatively impermeable to polar molecules?

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... the endodermis, there is the 'apoplast block' - the cellulose cell walls of the cells of the endodermis have a substance called suberin which is impermeable and prevents the movement of water. The suberin makes up what is called the Casparian strip. At this point, all the water has to move into ...

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... osmolarity of the interstitial fluids in the renal medulla (so it gets concentrated and stays that way). In the ascending loop the membrane is impermeable so water cannot get in, but there is an active reabsorption of ions like Na+, K+ and Cl- meaning that the solution gets steadily more dilute ...

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... in the medulla of the kidney for the collecting ducts later. On the ascending loop very little water is allowed back in because the walls are impermeable, so the high concentration of solutes in the Loop is maintained despite the increasing concentration of water in the interstitial space ...

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... mitochondria, it passes easily into the matrix. There, mitochondrial esterases cleave off the lipid group (AM), yielding BCECF which is membrane-impermeable and consequently trapped in the matrix. The effect of pH on the emission intensity of BCECF is shown in the figure below. (i) Vesicles containing ...

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