Unit 4
Lipids and Membranes

Overview

Lipids are a far more heterogenous class of biomolecules than are proteins, carbohydrates, or nucleic acids, but lipids do have some common features; for example, they are virtually insoluble in water, and generally have one or more unbranched hydrocarbon chains, 10–20 carbons in length. Lipids also have a polar or ionic functional group attached to one end of a hydrocarbon chain. This structure allows lipids to form organized aggregates with the carbon-hydrogen chains stacked together (excluding water) and the polar head groups interacting with water molecules. These lipid aggregates (membranes) can efficiently separate the aqueous compartments of the body.

Chemically, lipids differ from other biomolecules in that they are composed of reduced hydrocarbons. This property means that per unit mass, more chemical energy is available from lipids than from, say, carbohydrates (in other words, French fries have more calories—energy—than plain potatoes).

In Unit 4, we discuss the structure and functions of lipids. The unit is divided into five lessons:

  1. Lipid Classification and Structure
  2. Micelles, Bilayers, and Liposomes
  3. Biological Membranes
  4. Lipoproteins
  5. Transport through Membranes

Objectives

After completing this unit, you should be able to

  1. identify the different classes of lipids.
  2. describe the properties of lipid aggregates.
  3. describe the structure of biological membranes.
  4. define “lipoprotein,” and explain the structure and functions of lipoproteins.
  5. discuss the dynamic nature of membranes, and describe some membrane transport mechanisms.
Top

Glossary

bilayers two lipid layers stacked so that the hydrocarbon chains face each other, and the head groups face out and interact with water
cis-C=C double bond the hydrogens on each of the two carbon atoms are on the same side of the C=C plane (as opposed to trans-C=C bonds in which the hydrogen atoms are on opposite sides)
essential fatty acids polyunsaturated fatty acids; synthesized by plants but not by mammals
fatty acid unbranched hydrocarbon chain with a carboxylic acid functional group at one end
lipoprotein protein with covalently attached lipids
liposome lipid bilayers forming small vesicles in aqueous solution
micelle aggregation of lipids in aqueous solution
phospholipids (or glycerophospholipids) major components of cell membrane; two fatty acids and phosphoric acid esterified to glycerol
(CH2OH—CHOH—CH2OH)
polyunsaturated (fatty acid) fatty acid containing more than one C=C double bond
saturated (fatty acid) fatty acid containing no double bonds
sphingolipids complex lipids with sphingosine, a C18 amino alcohol, as backbone; found predominantly in nerve and brain tissue
transition temperature temperature at which a cooling lipid bilayer begins to undergo a phase change to become gel-like
triacylglycerol three fatty acids esterified to glycerol
Top