Lipids and biological membranes
Readings: p.54-55, 70-71, 363-372
By the end of this topic, you should be able to:
• Describe and distinguish between the different classes of lipids
• Describe the biological roles of the different classes of lipids
• Explain the properties of lipid bilayers, and why certain lipids form bilayers
All cells are enclosed by a membrane composed of lipids, proteins and carbohydrates.
Eucaryotic cells also have internal membranes that divide the cell into different
compartments (Fig 11-3, p.364). Biological membranes have several important functions,
including the following.
1. They separate the contents of a cell or organelle from the surrounding
2. They control import and export of molecules (e.g., nutrients, waste, ions) into and
out of the cell or organelle, using proteins that span the membrane.
3. They contain sensors or receptors that allow the cell to respond to external stimuli
including communications from other cells.
4. They are involved in cell movement.
Biological membranes are based on certain types of lipid molecules. Lipids are biological
molecules that have little or no solubility in water, but are soluble in organic solvents. In
addition to being structural components of biological membranes, lipids also function as
energy storage molecules (see Topic 16), enzyme cofactors, signalling molecules, and
pigments. Structurally, lipids may be divided into several different classes, some of which
are described below (see Panel 2-4, p.70).
Fatty acids: Hydrocarbon chains ending in a carboxylic acid group (Movie 2.2). Fatty
acids usually contain an even number of carbon atoms, and range in length from 4 to 36
carbons. If the hydrocarbon chain has no double bonds, it is called saturated. If it has one
double bond, it is called monounsaturated. If it has two or more double bonds, it is called
polyunsaturated. The first C-C double bond in unsaturated fatty acids is usually between
C9 and C10, counting from the COOH end; it is almost always cis, forming a kink in the
chain. Any additional double bonds are usually at every third carbon, as in the 18-carbon
fatty acid shown on the next page.
The structure of a fatty acid can be indicated by a short form in which the letter C is
followed by the number of carbon atoms and double bonds in the fatty acid, separated by
a colon. For example, the saturated fatty acid with 16 carbons would be represented
C16:0. The 18-carbon unsaturated fatty acid shown below is most properly abbreviated 9, 12
C18:2 cis,cis-Δ, although sometimes the cis isomer or the positions of the double
bonds are assumed, such that the abbreviation could be simply C18:2. In another
nomenclature system, the double-bond position is counted from the methyl end of the
molecule, and the number of the double-bonded carbon closest to the methyl end is given
in brackets. In this system, the 18-carbon fatty acid shown below would be represented as
C18:2(n-6). In older nomenclature, the terminal carbon is called the ω (omega) carbon,
and sometimes you will see the abbreviation in the form C18:2(ω-6).
Triacylglycerols: Glycerol is a three-carbon molecule with hydroxyl groups at each
carbon (see figure). Triacylglycerols (also called triglycerides) are obtained by attaching a
fatty acid to each hydroxyl via an ester linkage (see Panel 2-4, p.70). Most triacylglycerol
molecules contain two or three different types of fatty acids. Triacylglycerol is used to
store fatty acids as energy reservoirs in adipocytes.
Glycerophospholipids: These are like triacylglycerols, except that the fatty acid on one
end of glycerol is replaced with phosphate (Fig. 11-10, p.367). The phosphate group is
often conjugated to a polar alcohol like ethanolamine (to make
phosphatidylethanolamine, Fig 11-10B, p.367), serine (to make phosphatidylserine, Fig
11-7, p.366) or choline (to make phosphatidylcholine, Fig 11-6, p.366).
OH CHH CH CH O C3
CH CH CH2 OHH OH OH
CH NH 3 OH CH3
CH 2HH CH3
Sphingolipids: These are based on sphingosine (see figure). If a fatty acid is attached to
the nitrogen of sphingosine via an amide linkage, the molecule is called a ceramide. The
terminal hydroxyl group can be modified with phosphoethanolamine or phosphocholine
to make sphingomyelins (found in the myelin sheath of nerve cell