Fat Digestion
The body is able to digest most types of foods simply by secreting the right kind of digestive enzyme. We have protease to digest proteins, amylase to digest starch, and so on. Most digestive enzymes are able to work effectively in the chemical environment inside our digestive tract. This is because food and body fluids are mostly water, and most molecules can dissolve in water and undergo chemical reactions there.However, fat digestion presents a special problem. Fats and other lipids do not dissolve in water; instead, they tend to congeal together into large masses. The same thing happens in salad dressing, when the oil and vinegar (mostly water) are allowed to separate. This separation of lipid and water reduces the effectiveness of fat-digesting enzymes, which are known as lipase. Without extra assistance, the lipases would not have access to most of the fat molecules, and fats would pass through the digestive tract without being digested.
The problem is solved by using bile, a substance that is produced by the liver and stored (and released) by the gall bladder. Bile consists of molecules that have a dual nature. Half of the molecule is attracted to water, and the other half is attracted to fats. The bile molecules therefore place themselves in between the fat and water. In this way the fat droplets remain suspended in water rather than merging together. This process is also called emulsification, and is similar to the way that detergents remove grease from dirty dishes. In the digestive tract, emulsification allows lipase to gain access to the fat molecules and thus facilitates digestion.