01The phospholipid bilayer
Each phospholipid has a hydrophilic phosphate head and two hydrophobic fatty-acid tails. In water they self-assemble into a bilayer: heads face the water on both sides, tails hide in the middle.
This structure is why the membrane is selectively permeable — the oily core blocks ions and large polar molecules.
02The fluid mosaic model
The membrane is fluid (lipids and many proteins drift sideways) and a mosaic (many different proteins embedded in it).
Integral proteins span the bilayer; their middle sections are hydrophobic, their ends hydrophilic. Peripheral proteins sit loosely on the surface.
Cholesterol in animal membranes acts as a fluidity buffer: it keeps membranes from getting too fluid when warm and too stiff when cold.
Glycoproteins and glycolipids carry sugar chains on the outside surface and act as ID tags for cell recognition (like blood types).
03Notebook box
Unsaturated tails (kinks) → more fluid. Saturated tails (straight) → packed, less fluid. Cold-climate organisms have more unsaturated fats in their membranes.
Worked example: A membrane protein has a region of nonpolar amino acids flanked by polar ones. Where is it? → Spanning the bilayer; the nonpolar part sits in the hydrophobic core.