Phospholipid Bilayer

The basic structure of every membrane is a phospholipid bilayer. A phospholipid molecule is made from glycerol, two fatty acids and a phosphate group.The phosphate-containing head is charged, so it is hydrophilic (attracted to water). The two fatty acid tails are non-polar, so they are hydrophobic (repelled by water).In water, phospholipids arrange themselves automatically into a double layer. The hydrophilic heads face outwards, towards the watery cytoplasm on one side and the watery tissue fluid on the other. The hydrophobic tails point inwards, towards each other, away from water. This forms a membrane with a hydrophobic core.The hydrophobic core is a barrier to substances that are water-soluble:
• Small, non-polar molecules such as oxygen and carbon dioxide, and lipid-soluble molecules such as steroid hormones, dissolve in the core and diffuse straight through.
• Ions (such as Na⁺ and Cl⁻) and large polar molecules (such as glucose) cannot pass through the core. They need the help of membrane proteins.
• Water is polar but very small, so a little can pass through the bilayer; most moves through protein channels (aquaporins).The bilayer is not rigid. Individual phospholipids move sideways within their layer, which makes the membrane flexible and allows it to form vesicles and fuse with other membranes. Membranes with more unsaturated fatty acid tails (which have kinks) are more fluid.