What are lipids?
Lipids are a group of organic molecules that include fats and oils. Like carbohydrates they are made of carbon, hydrogen and oxygen, but they contain far less oxygen. Lipids do not dissolve in water; they dissolve in organic solvents such as alcohol. Fats are solid at room temperature, while oils are liquid.
Structure
A common lipid is a triglyceride, made of one molecule of glycerol joined to three fatty acids by condensation, releasing water and forming ester bonds. Fatty acids may be:
- Saturated: no double bonds between carbons; usually found in animal fats and solid at room temperature.
- Unsaturated: one or more double bonds; usually found in plant oils and liquid at room temperature.
A special lipid, the phospholipid, has one fatty acid replaced by a phosphate group; it forms the bilayer of the plasma membrane.
Key idea
Triglyceride = 1 glycerol + 3 fatty acids joined by ester bonds. Lipids store about twice the energy per gram of carbohydrates.
Functions
Lipids form a long-term energy store, insulate the body against heat loss, protect organs as padding, and are the main part of cell membranes. They also dissolve and carry the fat-soluble vitamins A, D, E and K around the body. Because they release so much energy, fats and oils are an important, though small, part of a balanced diet. The emulsion test detects lipids: the sample is shaken with ethanol and then added to water, and a milky-white emulsion shows a lipid is present. Sudan III stain, which turns the fat layer red, can also be used.
Example
A drop of cooking oil shaken with ethanol and then added to water forms a cloudy white emulsion, a positive lipid test.
Remember
Glycerol + 3 fatty acids = triglyceride. Emulsion test = milky white for lipid.