First, What is Fat?
In very general terms, fat is a storage system. It allows animals to lock away energy for later use, much as plants lock energy away in the form of starch. Of course plants make some fat too, though it’s usually in liquid form — oil. Animals make more fat because it’s a denser storage medium, and that’s better if you happen to have to move around (chase prey or, conversely, run away from predators). You can store 4 calories (kcals for you Europeans) in a gram of starch, bur more than twice as many (9 calories) in a gram of fat.
Chemically speaking, fats and oils are triglycerides, sub-members of the extremely broad lipid family of molecules. Triglycerides, as the name implies, are made up of a trio of fatty acids, all connected to a “backbone” of glycerol. They are all more or less “E”-shaped, though since the type and configurations of the fatty acid molecules they contain are highly variable, different triglycerides can behave in very different ways. One factor that greatly influences the behavior of a triglyceride is its degree of saturation (for a discussion of that, go here).
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