kosh_ki
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We know that only lipid soluble molecules (e.g. oxygen, carbon dioxide) can pass through the phospholipid bilayer without any help using simple diffusion, and that water soluble molecules (e.g. glucose) use facilitated diffusion via protein channels.

We also know this is explained by how the phosphate head is hydrophilic, and the fatty acid tails are hydrophobic.

1. What makes the lipid soluble molecules even able to cross the hydrophilic phosphate head region?

2. Why can protein channels cut across both hydrophilic and hydrophobic sections of the membrane then?

Thanks so much
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macpatgh-Sheldon
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Hi,

1. Lipid solubility or lipophilicity is one factor that aids diffusion through the phospholipid bilayer, but it is not the only one.

--- A small molecule of lipid as against larger ones [like palmitic acid or stearic acid], pass through more easily [as can small ions [which are not lipophilic, so the converse applies] to a certain extent, although most have special mechanisms that permit active transport]

--- Uncharged particles pass through the membrane more easily than charged ones as uncharged molecules tend to be more polar.

Certain drugs e.g. topical eyedrops are formulated to possess both a lipophilic and a hydrophilic part, so that they can cross both the lipophilic corneal epithelium [and endothelium], and the hydrophilic corneal stroma, respectively [the cornea consists of five layers: from the outside going in: epithelium, Bowman's layer [basement membrane of the epithelium], stroma, Descemet's membrane, and endothelium].

2. The protein channels are part of the structure of the membrane [here integral proteins] so although they are somewhat flexible [fluid mosaic], they do not need to "cut across".

Hope this helps & be safe!
M.
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