1. Phospholipids have a glycerol backbone, which is the hydrophilic (water-soluble) head, and two fatty acid tails, which are hydrophobic (water-insoluble). The hydrophobic tails face each other and form a bilayer.
2. Lipid-soluble substances (e.g., 0>, C02, steroid hormones) cross cell membranes because they can dissolve in the hydrophobic lipid bilayer.
3. Water-soluble substances (e.g., Na\ CI", glucose, HoO) cannot dissolve in the lipid of the membrane, but may cross through water-filled channels, or pores, or may be transported by carriers.
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