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Medicine LibreTexts

1.08: Fats

Fats or lipids are important in the plasma membrane around cells and form the insulating fat layer under the skin. They are also a highly concentrated source of energy, and when eaten in the diet provide more than twice as much energy per gram as either carbohydrates or proteins.

Like carbohydrates fats contain carbon, hydrogen and oxygen, but unlike them, there is no particular relationship between the number of hydrogen and oxygen atoms.

The fats and oils animals eat in their diets are called triglycerides or neutral fats. The building blocks of triglycerides are 3 fatty acids attached to a backbone of glycerol (glycerine). When fats are eaten the digestive enzymes break down the molecules into separate fatty acids and glycerol again.

Fatty acids are divided into two kinds: saturated and unsaturated fatty acids depending on how much hydrogen they contain.(Saturated fats contain more hydrogen than unsaturated fats). The fat found in animals bodies and in dairy products contains mainly saturated fatty acids and tends to be solid at room temperature. Fish and poultry fats and plant oils contain mostly unsaturated fatty acids and are more liquid at room temperature.

Phospholipids are lipids that contain a phosphate group. They are important in the plasma membrane of the cell.


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  • Ruth Lawson (Otago Polytechnic; Dunedin, New Zealand)