Lipids are mostly hydrophobic, which means they naturally repel or do not mix with water. Some lipids are amphiphilic, meaning they contain both water-loving and water-repelling parts. This unique chemical property comes from long hydrocarbon chains, which are chains of carbon and hydrogen atoms.
Triglycerides are the main molecules that animals use to store energy for long periods. They consist of a glycerol head attached to three long fatty acid tails. Saturated fatty acids have only single bonds between carbon atoms and form solid fats at room temperature.
Unsaturated fatty acids contain one or more double bonds in their carbon chains. These double bonds create physical bends in the chains, which makes them liquid oils. Phospholipids have two hydrophobic tails and a hydrophilic, or water-loving, head.
In water, they automatically arrange themselves into a double layer called a lipid bilayer. This bilayer forms the basic protective barrier of all cell membranes. Steroids are a different class of lipids with a structure made of four fused carbon rings.
A common student misconception is that all lipids are bad for your health. In reality, your body needs lipids to build cells and make essential hormones.