Cis double bonds bend the molecule at the double bond, trans double bonds do not. Trans double bonds are not considered healthy in a diet. The number of double bonds in a molecule decreases the melting point of the fat. This has significance in the behavior of a fatty membrane under different temperature conditions. Cell membranes are composed of a mixture of phospholipids, one of which is lecithin.
Neutral fats are the main component of stored fat in fat cells.
Neutral fats and phospholipids are both amphipathic. For this reason, phospholipids especially are perfect for forming membranes. When water is present, they do not form large blobs easily as the phosphate-glycerol end seeks water and it is most readily found at the interface of water and lipid. Think about why membranes form naturally from phospholipids that are arranged in a double layer with the phosphate-glycerol ends facing toward water. These are called the linolenic and linoleic acid series respectively.