Cell Membrane
Phospholipid Bilayer Formed by Surface Tension
Phospholipids, the building blocks of the cell membranes, are both hydrophilic and hydrophobic. The hydrophobic pair of fatty acids are embedded in the middle of the membrane and the hydrophilic phosphate projects outward forming hydrogen bonds with a tight, organized layer of water on each face of the membrane.
Phospholipids are Glycerol with Two Fatty Acids and a Phosphate
Glycerol is a small, three-carbon sugar. In phospholipids, two fatty acids have been attached (through ester linkages to two hydroxyl groups) on two carbons and the third carbon is attached to a phosphate group. Phospholipids from cells of different tissues have various molecules, e.g. choline, also attached to the phosphates on the surface of the membrane.
Phospholipids Form a Single Layer
An early experiment to determine the structure of biological membranes isolated the membranes from red blood cells and then poured the lipids on the surface of water in a trough. The lipids formed a layer with a surface area exactly twice what was needed to cover the number of red blood cells used in the original extraction. The phospholipids formed a single layer on the water with the phosphate in the water and the fatty acids pairs toward the air. The water was stretched tight, showing surface tension, as each phosphate hydrogen bonded into the surface layer of organized water.
Cell Membrane Is a Bilayer
As the red blood cell experiment showed, the cell membrane is made of two layers of phospholipids, i.e. a lipid bilayer. In the bilayer, the hydrophobic fatty acid tails of each layer are held together by weak van der Waals forces in the center of the membrane and the phosphates hydrogen bond with the water layers on the cytoplasmic and outer surfaces.
Membrane Strength Comes from Surface Tension
The van der Waals forces holding the fatty acid tails together are not strong enough to hold a membrane together if the face layers of organized water are not attached by hydrogen bonds to the phosphates. Some snake venoms rupture red blood cells and cause hemolysis, because they contain an enzyme that removes phosphates from phospholipids, i.e. a phospholipase. The action of the snake venom phospholipase on red blood cell phospholipids removes the phosphates leaving a hydroxyl group. The hydroxyl group is small and makes only a few hydrogen bonds. The minimal attachment to the water layer is not enough to prevent the lipid layer from twisting and forming small aggregates of lipids, micelles.
Cell Membrane Is a Fluid Mosaic of Proteins Embedded in a Phospholipid Bilayer
The phospholipid bilayer produces the properties of the cell membrane, by providing a strong boundary that prohibits charged molecules, with shells of hydrogen bonded water molecules, such as sodium and potassium ions, and large molecules, such as proteins, e.g. insulin or enzymes, from entering or leaving cells. Proteins are embedded and move around like floating rafts in the fluid lipid bilayer. The membrane proteins can bind and respond to the presence of hormones, such as insulin, or transport molecules, such as glucose, into cells. It is the surface tension of the water exposed to the lipid bilayer on both faces that holds the membrane and the cell together.
Cell Membrane Structure
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