Lipid Bilayer
- A lipid bilayer is a two-layer membrane structure formed when phospholipid tails face inward and phospholipid heads face watery fluid.
- Phospholipids have a glycerol backbone, a water-attracting hydrophilic head, and two water-avoiding hydrophobic fatty-acid tails.
- Hydrophilic means water-attracting, so hydrophilic parts of a phospholipid face intracellular fluid or extracellular fluid.
- Intracellular fluid is the watery fluid inside a cell, and extracellular fluid is the watery fluid outside a cell.
- Hydrophobic means water-avoiding, so hydrophobic fatty-acid tails point toward one another inside the membrane.
- The hydrophobic lipid bilayer forms a barrier that favors lipid-soluble molecules and resists many water-soluble molecules.
- Lipid-soluble substances can dissolve in membrane lipid, which allows them to cross cell membranes more easily than charged or strongly water-soluble substances.
- Oxygen (O₂), carbon dioxide (CO₂), and steroid hormones cross cell membranes readily because they are sufficiently lipid soluble.
- Water-soluble substances usually need channels, pores, or carriers because they do not dissolve well in the hydrophobic membrane interior.
- Sodium ion (Na⁺), chloride ion (Cl⁻), glucose, and water (H₂O) cross membranes through water-filled channels, pores, or transport carriers rather than by dissolving directly in membrane lipid.
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