MCAT Content / Lipids / Lipids Description Structure

Description; Structure

Topic: Lipids

Lipids include a diverse group of compounds that are largely nonpolar and hydrophobic in nature.

Lipids are various organic compounds that are hydrophobic (water-fearing). Examples of lipids are fats, waxes, oils, and hormones. One place they can be found is in the membrane structure of cells.

The lipid molecule is composed of a glycerol molecule with three fatty acids attached.

The fatty acid chain of lipids can come in two forms; saturated and unsaturated. Saturated fatty acid chains contain only single bonds between the carbons. Unsaturated fatty acids contain a double bond between two carbons in the chain. Unsaturated fatty acids contain either cis or trans double bonds. Cis fatty acids cannot pack close together tightly due to the ‘kink’ in the fatty acid chain.

Steroids, a type of lipid, are composed of four fused carbon rings. Many steroids contain an -OH functional group attached at a site. Cholesterol found below is the most common steroid.

Terpenes are hydrocarbons that consist of isoprene units. Isoprene units contain five carbon atoms attached to eight hydrogen atoms (C5H8). Terpenoids are oxygenated derivatives from terpenes and their isoprene units consist of five-carbon units attached to an oxygen-containing group.

Terpene

Terpenoid

Practice Questions

Khan Academy

MCAT Official Prep (AAMC)

Biology Question Pack, Vol. 1 Question 17

Practice Exam 1 B/B Section Question 13

Key Points

• Lipids are hydrophobic, non-polar, and made mostly of hydrocarbon chains

• Lipids store energy, provide insulation, are a component of cell membranes, and are the building blocks of hormones

• Saturated fatty acids contain no double bonds

• Unsaturated fatty acids contain a double bond and can be cis or trans


Key Terms

Lipid: Any of a class of organic compounds that are fatty acids or their derivatives and are insoluble in water but soluble in organic solvents.

Nonpolar: A molecule with no separation of charge, so no positive or negative poles are formed. The electrical charges of nonpolar molecules are evenly distributed across the molecule

Hydrophobic: The physical property of a molecule that is seemingly repelled from a mass of water;  tend to be nonpolar and, thus, prefer other neutral molecules and nonpolar solvents.

Glycerol: An alcohol with three carbons, five hydrogens, and three hydroxyl (OH) groups.

Fatty acids: Long chain of hydrocarbons with a carboxyl group attached and may have 4-36 carbons.

Saturated fat: A type of fat in which the fatty acid chains have all or predominantly single bonds.

Unsaturated fat: A fat or fatty acid in which there is at least one double bond within the fatty acid chain.

Steroids, a type of lipid, are composed of four fused carbon rings.

Terpenes: Hydrocarbons that consist of isoprene units.

Terpenoids: Oxygenated derivatives from terpenes and their isoprene units consist of five-carbon units attached to an oxygen-containing group.



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