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    Hi, I'm studying for my exam and this question came up the practice paper. If i could get a detailed answer it would be great, thanks.

    Is this gcse biology?
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    1. The cell membrane is composed of a phospholipid bilayer, so that substances that are lipophilic are more easily let through.
    2. Because of the fact that the membrane restricts passage of charged molecules [which tend to be polar], there is again a selectivity. Whether a molecule will be charged or uncharged at a particular pH depends on its PkA, and can be determined using the Hendelsen-Hasselbach equation.
    3. There are pores in the membrane that have a finite size, so only molecules with a mass number below a certain figure can pass through.
    4. The membrane has some active transport mechanisms [based on receptors which can be glycolipid/glycoprotein in nature] such as Na+, K+ - ATPase (sodium potassium pump) that fuels the exchange of 3 Na+ ions for 2 K+ ions. There are similar active transport mechanisms for absorption of digested food in the intestinal endothelial cells (for A*: e.g. for absorption of basic amino acids (those with extra NH2 group) e.g. histidine - a defect in this mechanism causes Hartnup disease).
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