Why must membrane lipids be amphipathic?

Answer 1

Membranes are one of endless examples in Biology of how structure equals function. If membranes require their structure be formed of specifically amphipathic lipids, this must be directly related to overall membrane function.

The primary lipid components of membranes are phospholipids. Phospholipids consist of two hydrophobic ("water fearing") fatty acid tails attached to a hydrophillic ("water loving") phosphate head. Because their fatty acid tails are poorly soluble in water, phospholipids spontaneously form bilayers in aqueous solutions, with the hydrophobic tails buried in the interior of the membrane and the polar head groups exposed on both sides, in contact with the aqueous solution. This amphipathic structure of phospholipid bilayers forms a stable barrier between two aqueous compartments and represent the basic structure of all biological membranes.

This structure directly relates to membrane function of forming a selectively permeable barrier between cells or between organelles within cells. The membrane is selectively permeable because only lipid soluble molecules and small uncharged molecules (such as water, carbon dioxide, molecular oxygen, ethanol, etc) may pass freely. Large polar molecules and ions require special transport proteins to cross the membrane and thus allow the cell to monitor what and how much of these molecules it is letting in or out at a given time.

In summary, the amphipathic structure of phospholipids allow membranes to form a stable selectively permeable lipid barrier between aqueous compartments within/ between cells.