Certain lipids form an essential part of the membranes that enclose and protect every living cell. Similar membranes surround all bodies within the cell, so that each cell body can do its job without unwanted interference from other cell bodies. A, D, E, and K, which do not dissolve in water. Kinds of lipids. Lipids are classified as simple lipids or complex lipids, according to their structure.
Simple lipids contain only carbon, hydrogen, and oxygen.
They consist of an alcohol in combination with certain organic acids containing a variable number of carbon atoms. Waxes, another common group of simple lipids, contain an alcohol molecule that is larger than the glycerol molecule. Complex lipids have a more complicated structure than simple lipids. Phospholipids are found in all bacteria, and in the cells of all plants and animals. They are most plentiful in sperm, eggs, embryos, and brain cells.