1. BIOLOGICAL MOLECULES
BZ 002
Prof. Silvia Díaz

2. L8. Lipids Objective: Describe the structure of fatty acid molecules.
By the end of this lecture students should be able to:
Describe the structure of fatty acid molecules.
Explain the differences between saturated and unsaturated fatty acids.
Illustrate the formation of a triglyceride from glycerol and three fatty acid molecules.

3. L8. Lipids
Contents
1. Types of lipids.
2. Fatty acids.
3. Triglycerides.

4. - Types of lipids

5. Lipids
Lipids are a diverse and heterogeneous group of organic molecules whose only common property is their inability to dissolve in water.
Unlike other macromolecules like polysaccharides, proteins and nucleic acids, lipids are not formed by the step-wise polymerization reactions.
They are regarded as macromolecules because of their large molecular weights.

6. Types of Lipids The most important types of lipids are:
Triglycerides (fats and oils),
phospholipids,
glycolipids
and steroids
Others include waxes, hormones, vitamins and terpenes.
Vitamins A, D, K, and E are all lipids as are a variety of hormones and such light absorbing plant pigments as the chlorophylls and carotenoids.

7. The most abundant lipids are the fats, compounds that are stored by animals and by many plants as an energy reserve.
Electron micrograph of a thin section of a fat storage cell or adipocyte. L, the single large fat droplet; N,
nucleus; M, mitochondria; En, endothelium of a capillary containing an erythrocyte (E); CT, connective tissue ground substance which contains collagen fibers (Co) and fibroblasts (F). The basement membranes (BM) surrounding the endothelium and the fat cell are also marked.

8. Types of Lipids
These structures represent four major classes of biologically important lipids: (a) phospholipids, (b) triacylglycerols (triglycerides), (c) terpenes, and (d) steroids.

9. 2. Fatty acids

10. Small organic molecules as building blocks
of the cell
Large units
of the cell
MONOSACCHARIDES
POLYSACCARIDES
FATTY ACIDS
FAT, LIPIDS, MEMBRANES
The three most abundant biological macromolecules—
Proteins
nucleic acids
and polysaccharides—are all polymers
composed of multiple covalently linked identical or nearly identical small molecules
AMINO ACIDS
PROTEINS
NUCLEOTIDES
NUCLEIC ACIDS

11. Biological macromolecules
The three most abundant biological macromolecules—
Polysaccharides
Proteins
and nucleic acids
are all polymers
and are composed of multiple covalently linked identical or nearly identical small molecules, or monomers.
The three most abundant biological macromolecules—
Proteins
nucleic acids
and polysaccharides—are all polymers
composed of multiple covalently linked identical or nearly identical small molecules

12. In contrast, fatty acids are molecules that form macromolecules but not polymers: LIPIDS
A fatty acid molecule has two distinct parts:
a single carboxylic (-COOH) group at one end
and a long hydrocarbon tail.
The long hydrocarbon tail with –C-C- and –C-H bonds is non-polar and hydrophobic, giving lipids their water insoluble property.

13. The length of the hydrocarbon tail varies in different fatty acids
But the number of carbons is always even and usually between 2 and 22.
However, 16 and 18-carbon fatty acids are the most common in nature.
On the other hand, the carboxylic group is highly polar and hydrophilic.
Fatty acid molecules are therefore amphipathic since they have both polar and non-polar components.

14. Saturated fatty acids
do not contain double bonds in their hydrocarbon tails. Therefore, they have the maximum number of hydrogen atoms.
The molecules are relatively straight and can pack together rather nicely in a crystal lattice.

15. Saturated fatty acids
This property gives saturated fatty acids and fats derived from them relatively high melting points and most are solids or semi-solids at room temperature.

16. Saturated fatty acids
An example is the 18-carbon stearic acid which has a melting point of 70˚C.
Because they are highly reduced, saturated fatty acids give a great deal of energy upon oxidation and are therefore efficient forms of energy store.
Stearic acid is the major component of animal triacylglycerols.

17. Unsaturated fatty acids
have one or more double bonds in their hydrocarbon chains (but not usually more than three).
The double bond usually occurs between carbon atoms 9 and 10 as in oleic acid.

18. Unsaturated fatty acids
Those with more than one double bond are said to be polyunsaturated.

19. Unsaturated fatty acids
The double bonds in unsaturated fatty acids can be of a cis or trans configuration, which significantly affects the molecule's molecular configuration.
Naturally-occurring unsaturated vegetable oils have almost all cis- bonds.

20. Unsaturated fatty acids
But using oil for frying causes some of the cis- bonds to convert to trans bonds.
Cis-double bonds cause the fatty acid chain to bend, an effect that is more pronounced the more double bonds there are in a chain.
These bends or kinks prevent unsaturated fatty acid molecules from packing closely in a crystal lattice as saturated fatty acids.

21. Unsaturated fatty acids
This gives them low melting points and most are liquids at room temperature.
Oleic acid, an 18 -carbon fatty acid just like stearic acid but has a much lower melting point of 13˚C (compared to 70˚C of stearic acid) and is a liquid at room temperature.
This in turn plays an important role in the structure and function of cell membranes.

22. 3. Fat and oils. Triglycerides

23. Fats and oils are also called triglycerides or triacylglycerols
They are the most common lipids in nature and are the main constituents of vegetable oils and animal fats.
Triglycerides have lower densities than water (they float on water), and at room temperature may be solid or liquid.

24. Triglycerides are formed by combining glycerol with three molecules of fatty acid
Each fatty acid has a carboxyl (-COOH) group, very hydrophilic and reactive.
In triglycerides, the hydroxyl groups of the glycerol join the carboxyl groups of the fatty acid to form ester bonds.
The esterification of each of the three fatty acid components of a triglyceride to glycerol is coupled with a loss of a water molecule and is therefore a condensation or dehydration reaction.

25. Glycerol is a small molecule
Like sugars, glycerol has many hydroxyl groups and tastes sweet, as its name suggests.
However, it is not a sugar because it cannot adopt an oxygen-containing ring structure.
Rather, because compounds containing hydroxyl groups are called alcohols, glycerol is a polyalcohol.

26. Cells can join fatty acids and glycerol to make glycerides.
The bond is formed by removing the elements of water between the carboxyl group of the fatty acid and a hydroxyl group of glycerol.
Any bond of this type, between a carboxyl group and a hydroxyl group, is called an
ester bond.
Glyceride examples
Triglycerides
Phosphoglyderides
Trioleoylglycerol

27. Simple triglycerides
The double bonds create kinks in the molecules, interfering with their ability to pack together in a solid mass.
Saturated fats tend to be solid at room temperature and come mostly from animal sources.
Oils on the other hand have unsaturated fatty acid components which are kinked and can therefore not pack closely with the result that they are liquid at room temperature.
It is the absence or presence of these double bonds that accounts for the difference between hard (saturated) and soft (polyunsaturated) margarine.

28. Simple and mixed triglycerides
Triglycerides containing the same kind of fatty acids in all the three ester bonds with glycerol are called simple triglycerides and are named after the fatty acid they contain, for example triestearate.
However, such simple triglycerides, rarely occur in nature.
All the triglycerides obtained from natural fats and oils contain two or three different fatty acid components and are thus termed mixed triglycerides.

29. Final questions: Identify and name the following chemical structures:B) C)

30. Summary of key points:
Lipids can not be defined precisely because of their diverse chemical properties.
They are a diverse and heterogeneous group of organic molecules whose only common property is their inability to dissolve in water, a property that explains their varied biological functions.
Lipids are not formed by the step-wise polymerization reactions. They are however regarded as macromolecules because of their large molecular weights.

31. Summary of key points:
Triglycerides include fats an oils, which have fatty acids as building blocks.
Fatty acids are amphipathic molecules and their hydrophobic hydrocarbon tail can be saturated or unsaturated.
Fatty acids are esterified to glycerol to form triglycerides.

32. Self assessment
1. Which of the following statements are correct. Explain your answers.
Both fatty acids and polysaccharides can be important energy stores in the cell.
Fatty acids are said to be “amphipathic.”
The hydrophobic tails of phospholipid molecules are repelled from water.

33. Reading suggestions Biological Molecules Module, Unit 4. pp. 49-54
Alberts et al., Essential Cell Biology, 3rd Ed Chapter 2, pp Panel 4-2.

34. Lipids. Phospholipids and glycolipids
Next lecture
Lipids. Phospholipids and glycolipids

35. Practical 3
Test for lipids