Every cell is surrounded by a cell surface membrane, and eukaryotic cells also have membranes around organelles. Under the electron microscope, all these membranes look the same: two dark lines separated by a lighter band, with a total thickness of about 7 nm. Because this three-layered appearance is found in every membrane, it was called the unit membrane.The dark lines are the hydrophilic phosphate heads of phospholipids, and the light band is the hydrophobic fatty acid tails. So the unit membrane is a phospholipid bilayer: the heads face the watery cytoplasm on one side and the watery tissue fluid on the other, and the tails face inwards.
• fluid, because the phospholipids and many proteins can move sideways within their layer
• mosaic, because proteins are scattered through the bilayer like tiles in a mosaicIntrinsic proteins span the bilayer and act as channels and carriers; extrinsic proteins sit on one surface. Glycoproteins and glycolipids carry carbohydrate chains on the outer surface, used in cell recognition. Cholesterol sits between phospholipids and regulates fluidity.The hydrophobic core lets small non-polar molecules such as O2 and CO2 diffuse through, but blocks ions and large polar molecules, which need proteins to cross.Worked example: a membrane 7 nm thick in an image magnified 100 000 times appears 7 × 10−9 m × 100 000 = 7 × 10−4 m = 0.7 mm across.
• fluid, because the phospholipids and many proteins can move sideways within their layer
• mosaic, because proteins are scattered through the bilayer like tiles in a mosaicIntrinsic proteins span the bilayer and act as channels and carriers; extrinsic proteins sit on one surface. Glycoproteins and glycolipids carry carbohydrate chains on the outer surface, used in cell recognition. Cholesterol sits between phospholipids and regulates fluidity.The hydrophobic core lets small non-polar molecules such as O2 and CO2 diffuse through, but blocks ions and large polar molecules, which need proteins to cross.Worked example: a membrane 7 nm thick in an image magnified 100 000 times appears 7 × 10−9 m × 100 000 = 7 × 10−4 m = 0.7 mm across.