Lipids
(Canadian Campbell 2nd ed -5.3)

Lipids are a varied group of molecules most of which are insoluble in water. Like carbohydrates, carbon, hydrogen, and oxygen are the principal elements of lipids although the oxygen content is much reduced. Fats, phospholipids, steroids, carotenoids, and waxes are lipids. Fats are the most abundant group of lipids in the biological world. Fats are composed of three fatty acid molecules bonded to a molecule of the alcohol glycerol. Fats have at least twice the energy storing capacity per unit weight as carbohydrates do. Cholesterol is an important steroid that is a part of some hormones. Steroids differ from most other lipids by virtue of the structure of steroids consisting of carbon rings instead of chains.

One of the biological important characteristics of fats, and lipids in general, is their insolubility in water. Lipids are made of long chains of hydrocarbons with relatively little oxygen (see figure to right). As a result of this, they tend to be non-polar, meaning they do not dissolve in polar solvents such as water. Today we are going to demonstrate this by dissolving a fat (vegetable oil) into water and ether. The fat we will use, like that shown in the adjacent figure, is a typical fat with three fatty acid chains. Another group of important lipids are the phospholipids. Phospholipids differ from fats in only having two fatty acid chains and a polar head rather than a glycerol head. The insolubility in water (hydrophobic) of the fatty acid tails and the solubility of the polar head (hydrophilic) is important in the functioning of the phospholipid membrane of cells.

Exercise 4 - Solubility of Fats

Click Here to view a tube containing 5 ml of vegetable oil and 5 ml of water.
Blue dye was added to the water for increased visibility.

Consider the following questions. For answers to the questions consult your lab manual, textbook and lecture notes. If you are still unsure consult with one of the instructors.

• Do the oil and water mix?
• What can you conclude about the polarity of the oil if you know that water is polar?
• The fat in the tube is a plant triglycerol (vegetable oil). Why is it a liquid at room temperature?
• Would you expect an animal triglycerol to be solid or liquid at room temperature? Why?

Click Here to view the same tube with 5 ml of soap solution added to the mix.

Consider the following questions. For answers to the questions consult your lab manual, textbook and lecture notes. If you are still unsure consult with one of the instructors.

• Did the oil and water mix when you added the soap?
• What did the soap do to the fat?
• Can you think about process and locations were a compounds like the soap would be important to an animal?

Click Here to view a tube containing 10 ml of vegetable oil and 10 ml of ether.

Consider the following questions. For answers to the questions consult your lab manual, textbook and lecture notes. If you are still unsure consult with one of the instructors.

• Did the oil and ether mix?
• Now that you know that oil is non polar what can you conclude about the polarity of the ether?
• Fats do not dissolve in water but phospholipids used to form membranes can disperse in water to form micelles. Why?

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