Lipids

Classification as Lipids

• All organic molecules that are nonpolar are called lipids: it is a ragtag group.
• Because they are nonpolar, they are hydrophobic (“water fearing”)
  • They are insoluble in water (and other polar solvents).
  • They are soluble in nonpolar solvents like ether, benzene, etc.
  • Remember the key idea: “like dissolves like.”
• Unlike carbohydrates, proteins, and nucleic acids, they do not form polymers.

Functions of Lipids

What Is a Fat?

• A fat is made of two things:
  • One glycerol molecule, which is a three-carbon alcohol.
  • Three fatty acids, which are long, linear carbon chains with a carboxylic acid at the end.
• Fats form by dehydration synthesis (illustrated below):
  • The alcohol functional group of a glycerol reacts with the carboxylic acid functional group of a fatty acid.
  • This forms a bond between the two molecules, while also producing water.
• Fats are also known as triglycerides.

Figure 5.10, page 65, Campbell's Biology, 5th Edition

Saturated vs. Unsaturated Fatty Acids

Saturated

• “Saturated” with hydrogen atoms
• Only single bonds
• Fats that are solid at room temperature

Figure 5.11(a), page 66, Campbell's Biology, 5th Edition

Unsaturated

• Not “saturated” with hydrogen atoms
• Double bonds
• Fats that are liquid at room temperature—oils

Figure 5.11(b), page 66, Campbell's Biology, 5th Edition

Structural Lipids

Phospholipids

• Identical to triglycerides except that the third carbon in the glycerol is attached to a phosphate group instead of a fatty acid.
• Phosphate end is polar.
• Therefore, they have a hydrophobic end and a hydrophilic end.

Figure 5.12(a) and (b), page 67, Campbell's Biology, 5th Edition

Phospholipid Symbol

This is the way you will see phospholipids represented in diagrams.

Figure 5.12(c), page 67, Campbell's Biology, 5th Edition

Phospholipid Bilayer

• Hydrophilic on the outer parts of the lipid bilayer which encounter water (where the spherical phosphate "heads" are located)
• Hydrophobic in the middle of the lipid bilayer where there is no water (where the long fatty acid "tails" meet)
• This is the structure of the cell membrane

Figure 5.13(b), page 67, Campbell's Biology, 5th Edition

Cell Membrane

Figure 8.14, page 141, Campbell's Biology, 5th Edition

Steroids

• Chemical messengers
• Based on the cholesterol molecule, shown here.

Figure 5.14, page 68, Campbell's Biology, 5th Edition

Sex Hormones

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Ducks and Sex Hormones

• The subtle variations in the molecular architecture of these two lipid molecules influence the development of the anatomical and physiological differences between male and female vertebrates.
• It is these two molecules, and their slight differences in location of functional groups, that determine the plumage of the female wood duck (left) and the male wood duck (right).

Figure 4.8, page 53, Campbell's Biology, 5th Edition

Cis vs. Trans Fatty Acids

The arrangement of hydrogens in a double bonded fatty acid.

• Cis: hydrogens are on the same side of the molecule.
• Trans: hydrogens are on the opposite sides of the molecule.

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Dangers of Trans Fatty Acids

• Trans Fatty Acids 101
• Trans Fatty Acids, Butter and Margarine
• FDA Fact Sheet
• Trans Fatty Acids and Coronary Heart Disease