Many food items employ lipid components as emulsifiers, such as mono- and diglycerides and lecithin. Lipids, such as omega-3 fatty acids and conjugated linoleic acid, are also nutritionally beneficial. However, like other emulsifiers, they can have adverse effects on certain consumers, such as causing gastrointestinal problems for those who are sensitive. Lipids are the most abundant component of cells and tissues. They are used by organisms as an energy source and can also act as structural components of cell membranes. There are three main classes of lipids: phospholipids, triglycerides, and sterols. Each class has many different molecules that can be combined together to form more complex structures. For example, phosphatidylcholine is a common phospholipid that makes up part of the membrane of almost all animal cells; sphingomyelin is a common triglyceride that is found in cellular membranes; and cholesterol is a common sterol that forms part of the membrane of cells of the immune system and reproductive systems. Lipids are highly stable compounds that are difficult to decompose even under high heat conditions. This means that they are not as susceptible to degradation as other organic compounds and can be added to foods without leading to significant changes in flavor or texture.