A triglyceride molecule

A triglyceride molecule.

Lipids are a broad group of biomolecules containing an assortment of structurally diverse types. What makes a biomolecule a lipid is its solubility in organic solvents, so lipids invariably have large portions of their structure with only carbons and hydrogens, though many important lipids are amphiphilic. An amphiphile is a chemical compound possessing both hydrophilic and hydrophobic moeities properties.

It helps in categorizing biological lipids to see that they fall into two categories based on biosynthesis, in that they originate entirely or in part from two distinct types of biochemical subunits or building-blocks: ketoacyl and isoprene groups. Although the biosynthesis of macromolecular precursors as a general topic is one step past the 1st semester biochem approach in the new MCAT, knowing a little bit about lipid biosynthesis is recommended because a little bit goes a long way in making lipids more coherent. Important classes of lipids include fatty acids, glycerolipids, glycerophospholipids, sphingolipids, polyketides, sterol lipids and prenol lipids.

WikiPremed Resources

Lipids Images
Image gallery for study with links to larger teaching JPEGs for classroom presentation

Question Drill for Lipids
Conceptual Vocabulary Self-Test

Basic Terms Crossword Puzzle

Basic Puzzle Solution

Learning Goals


Understand that the definition of the category of lipids is not strictly structural but relies on solubility.

Be able to draw the structure of a fatty acid, glycerol, and a triglyceride molecule. Understand the acyl substitution dehydration pathway of formation of a tryglyceride molecule from glycerol and three fatty acids.

Be able to explain why saturated fats have higher melting points than unsaturated fats.

Understand the structure of soaps and detergents and how they function.

Be able to distinguish the structure of phospholipids from triglycerides in general terms as well as distinguish a few different varieties of phosopholipids from each other including lecithin and sphingomyelin. Understand how their behavior underlies the properties of cell membranes.

In basic terms, understand why the terpenes usually have a number of carbon atoms that is a multiple of 5.

Be able to draw the generic structural formula for steroids. Be able to name several examples of steroids and describe their functions.

Recognize the eicosanoid lipids: the prostaglandins, thromboxanes, and leukotrienes.

Suggested Assignments

Review lipids terminology with practice using the question server. Complete the fundamental terms crossword puzzle. Here is the solution to the puzzle.

Read pp. 112-113 in ExamKrackers Chemistry. Perform practice items 65-72 on pp. 116-117 (practice items for carbohydrates, proteins, lipids & nucleic acids).

Read pp. 3-10 in ExamKrackers Biology I. Perform practice items 1-8 on pg. 11 (subject matter for lipids, carbohydrates, and nucleic acids).

Study the lipid web resources carefully.

Conceptual Vocabulary for Lipids