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Lipids  Lipids include a diverse group of compounds that are united by a common feature.  Lipids



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English for Biology A Teacher Resource Manual

Lipids 
Lipids include a diverse group of compounds that are united by a common feature. 
Lipids 
are 
hydrophobic (“water- fearing”), or insoluble in water, because they are nonpolar molecules. They are 
hydrocarbons that only contain nonpolar carbon-carbon or carbon-hydrogen bonds, which explains why. In a 
cell, lipids serve numerous purposes. Cells store energy as lipids called fats for long-term use. Lipids also shield 
plants and animals from their surroundings (Figure 12). Because they repel water, for instance, they aid in 
keeping aquatic mammals and birds dry.
Lipids are a crucial component of the plasma membrane and the 
building blocks of numerous hormones. Steroids, phospholipids, oils, and waxes are examples of 
lipids. 


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Figure 12.
Hydrophobic lipids in the fur of aquatic mammals (credit: Ken Bosma) 

fat 
molecule, such as a triglyceride, consists of two main components—glycerol and fatty acids. 
Three carbon atoms, five hydrogen atoms, and three hydroxyl (-OH) groups make up the organic compound 
glycerol. The term "fatty acid" refers to a long chain of hydrocarbons to which an acidic carboxyl group is 
attached. Fatty acids can have anywhere between 4 and 36 carbons; those with 12 to 18 carbons are the most 
prevalent. In a fat molecule, a fatty acid is attached to each of the three oxygen atoms in the –OH groups of the 
glycerol molecule with a covalent bond Figure 13.
Figure 13.
Steroids, phospholipids, and fats like triglycerides, which are composed of fatty 
acids and glycerol, are examples of lipids. 
The majority of oils, which are unsaturated fats that are liquid at room temperature, are. A 
monounsaturated fat is one in which there is only one double bond in the molecule; a polyunsaturated fat is one 
in which there are multiple double bonds (e.g.,, canola oil). At room temperature, saturated fats are solid due 
to their tendency to pack tightly. Examples of saturated fats include the stearic and palmitic acid-containing 
animal fats found in meat and the butyric acid-containing fat found in butter. Adipocytes, specialized cells 


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found in mammals, are where globules of fat are stored for later use. Oil or fat is stored in seeds of plants and 
used as a source of energy during the development of the embryo. 
Phospholipids
are the major constituent of the plasma membrane. They are made up of fatty acid 
chains joined to a glycerol or other similar backbone, just like fats. The glycerol backbone's third carbon is 
bonded to a phosphate group, so there are only two fatty acids attached as opposed to three. An alcohol is added 
to the phosphate group to change it. 

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