Lipids are molecules that are soluble in nonpolar compounds, like alcohol, but not soluble in water. Lipids are an important part of cell membranes and play a vital role in storing energy.
Fatty acids are one variety of biologically important lipid. Fatty acids are composed of a carboxylic acid head with a hydrogen and carbon tail; the head is polar and hydrophilic while the tail is nonpolar and hydrophobic. This means that fatty acids tend to orient themselves with their heads facing water and tails facing away from water. The metabolism of fatty acids creates large amounts of ATP, the energy molecule of cells, making them an important part of the metabolism of living things.
Similarly to fatty acids, phospholipids have hydrophilic heads and hydrophobic tails. Phospholipids are essential in the formation of cell membranes. Two rows of phospholipids align such that the hydrophilic heads of one row face the cytoplasm in the cell interior while the heads of the second row face the fluid surrounding the cell; the hydrophobic tails of both rows face each other. This configuration separates the cell's fluid from the fluid around it. Lipid bilayers surround not only a cell itself, but also the cell's nucleus and many of its organelles. This structure is vital in regulating the transfer of ions and other molecules into and out of cells.