Biology 12E. McIntyre

Biological Macromolecules:

LipidsSee Chap 1.2 in your text: Biology 12

• All Lipids are hydrophobic. Consist of carbon, hydrogen and oxygen

• Contain fewer polar O-H bonds and more non-polar C-H bonds than carbohydrates.

• includes fats and oils, waxes, phospholipids, steroids, and some other related compounds.

• Primarily energy sources and structural compounds and signaling molecules.

• Large number of bonded hydrogens - therefore release a larger amount of energy than other organic compounds. Fats yield 9kcal/gm, carbohydrates 4kcal/gm

General Characteristics

Types of Lipids

• Lipids with fatty acids

Triglycerides (Fats and oils)

Phospholipids

Waxes

• Lipids without fatty acids

Steroids

Triglycerides

Triglycerides (fats and oils)• Composed of three fatty acids joined to a

glycerol molecule.

• Glycerol is a three carbon molecule with hydroxyl groups on each carbon.

• Fatty acids - long hydrocarbon chains with terminal carboxyl (COOH) group.

Hydroxyl groups

Triglycerides

• Triglycerides are synthesized via condensation reactions.

• Glycerol & fatty acids are joined with ester bonds (a condensation reaction involving hydroxyl and carboxyl groups)

…Types of Lipids

• The main distinction between fats and oils is whether they’re solid or liquid at room temperature

• Fats– mostly from animal sources– triglycerides containing saturated fatty acids (e.g. butter is

solid at room temperature)• Oils

– mostly from plant sources– triglycerides with unsaturated fatty acids (e.g. corn oil is

liquid at room temperature)

Fatty Acids

Structure of Fatty Acids• all fatty acids have long

hydrocarbon chains with terminal carboxyl (COOH) group

• The “tail” of a fatty acid is a long hydrocarbon chain, making it hydrophobic.

• Typically 12-18 carbon atoms (even number)

• Saturated fatty acids have no double bonds between their carbon atoms

• Unsaturated fatty acids have double bonds in the carbon chain

Saturated and Unsaturated Fatty Acids

Saturated Fatty acids• The terms saturated, mono-unsaturated, and poly-unsaturated refer to the

number of hydrogen attached to the hydrocarbon tails of the fatty acids as compared to the number of double bonds between carbon atoms in the tail.

• Saturated fats have all single bonds between the carbons in their fatty acid tails, thus all the carbons are also bonded to the maximum number of hydrogen possible…maximum possible amount of hydrogen saturated fats.

• The hydrocarbon chains in these fatty acids are, thus, fairly straight and can pack closely together. There are strong attractions between the fatty acids in triglycerides with saturated fatty acids. These fats are solid at room temperature.

Saturated and Unsaturated Fatty Acids

Unsaturated Fatty acids• Oils have some double bonds between some of the carbons in the

hydrocarbon tail, causing bends or “kinks” in the shape of the molecules.• Because some of the carbons share double bonds, they’re not bonded to as

many hydrogen as they could if they weren’t double bonded to each other. These oils are called unsaturated fats.

• Because of the kinks in the hydrocarbon tails, unsaturated fats can’t pack as closely together, making them liquid at room temperature.

• unsaturated fats are “healthier”than the saturated ones.

• Hydrogenated vegetable oil (as in shortening and commercial peanut butters where a solid consistency is sought) started out as “good” unsaturated oil. However, this commercial product has had all the double bonds artificially broken and hydrogen artificially added (in a chemistry lab-type setting) to turn it into saturated fat that bears no resemblance to the original oil from which it came (so it will be solid at room temperature).

Saturated and Unsaturated Fatty Acids

• In unsaturated fatty acids, there are two ways the pieces of the hydrocarbon tail can be arranged around a C=C double bond.

• In cis bonds, the two hydrogens on either side of the double bond are either both “up” or both “down,” (both are on the same side of the molecule).

Cis and Trans Bonds

• In trans bonds, the two hydrogens are on opposite sides of the double bond, One is “up” and one is “down”.

• Naturally-occurring unsaturated vegetable oils have almost all cis bonds, but using oil for frying causes some of the cis bonds to convert to trans bonds. If oil is used only once like when you fry an egg, only a few of the bonds do this so it’s not too bad. However, if oil is constantly reused, like in fast food French fry machines, more and more of the cis bonds are changed to trans until significant numbers of fatty acids with trans bonds build up.

…Cis and Trans Bonds

• Fatty acids with trans bonds are carcinogenic, or cancer-causing. The levels of trans fatty acids in highly-processed, lipid-containing products such as margarine are quite high.

• Cis fats exist naturally and, because the hydrogen atoms are crowded on one side of the molecule, they bend, allowing other chemicals and enzymes to bind to them.

• Trans fats do not exist naturally, with a very few exceptions. Because the structure is uncrowded, they do not bend and so other molecules and enzymes find it more difficult to bind to them.

…Cis and Trans Bonds

• In fact, it is the very fact that they are straight that allows trans fats to solidify at room temperature. Natural, cis fats are curved and so can't pack into a crystal formation at normal temperatures. Trans fats, on the other hand, are straight and CAN pack into a crystal formation, which allows them to solidify at room temperature.

Stop & Think

How would the melting point of stearic acid compare to the melting points of oleic acid and linoleic acid? Assign the melting points of –17°C, 13°C, and 69°C to the correct fatty acid. Explain.

– stearic acid (18 C) saturated

– oleic acid (18 C) one double bond

– linoleic acid (18 C) two double bonds

Solution to ‘Stop & Think’

Stearic acid is saturated and would have a higher melting point than the unsaturated fatty acids. Because linoleic has two double bonds, it would have a lower mp than oleic acid, which has one double bond.– stearic acid mp 69°C

– oleic acid mp 13°C

– linoleic acid mp -17°C

Stop & Think

• Identify each fatty acid as a– trans unsaturated fatty acid– cis unsaturated fatty– saturated fatty acid

Oleic acid Elaidic acid Stearic acid

Solution to ‘Stop & Think’

Oleic acid Elaidic acid Stearic acidOleic acid is a cis unsaturated fatty acid that comprises 55–80% of olive oil.

Elaidic acid is a trans unsaturated fatty acid often found in partially hydrogenated vegetable oils.

Stearic acid is a saturated fatty acid found in animal fats and as the intended product in hydrogenation.

Stop & Think

• Which two fatty acids are isomers?

Solution to ‘Stop & Think’

• Which two fatty acids are isomers?

Phospholipids - two fatty acids attached to phosphate group

• phosphate heads are hydrophilic (water soluble) but tails are hydrophobic (water insoluble).

• spontaneously form miscelles in water.

• very important because they form biological membranes…

…Types of Lipids

Membrane Structure– Phospholipid bilayer

• Hydrophillic heads in contact with aqueous environment

• Hydrophobic tails reside in interior of bilayer

• Cholesterol is a structural component

– Proteins• Integral

• Peripheral

• Lipid anchored

– Dynamic structure

Stop & Think

• Sketch the arrangement of miscelles in a oily environment

Solution to Stop & Think

• Compact hydrophobic molecules containing four fused hydrocarbon rings and several different functional groups

Examples:

• Cholesterol-precursor

to sex hormones and

vitamin D

• Sex hormones

Steroids (Sterols)

Cholesterol• High concentrations of

cholesterol in the bloodstream and a diet rich in saturated fats have been linked to the development of atherosclerosis, a condition in which fatty deposits, called plaque, form on the inner lining of blood vessels, blocking the flow of blood to tissues. Deprived of oxygen and nutrients, the cells of the tissues die.

…Steroids (Sterols)

…Cholesterol• Vitamin D is formed by the action of UV light in

sunlight on cholesterol molecules that have “risen” to near the surface of the skin.

• Many medical journals state that people not shower immediately after being in the sun, but wait at least ½ hour for the new Vitamin D to be absorbed deeper into the skin.

• Our cell membranes contain a lot of cholesterol (in between the phospholipids).

…Steroids (Sterols)

…Cholesterol

• Cholesterol is not the bad guy. It’s how it is packaged.

• LDL is the ‘bad’ cholesterol

• HDL is the ‘good cholesterol.

…Steroids (Sterols)

Stop & Think• Examine the cholesterol and phospholipid

molecules. Predict how the cholesterol molecules are oriented among the phospholipids of a membrane.

• Cholesterol is an amphipathic molecule, meaning, like phospholipids, it contains a hydrophilic and a hydrophobic portion. Cholesterol's hydroxyl (OH)

Solution to Stop & Think

group aligns with the phosphate heads of the phospholipids. The remaining portion of it tucks into the fatty acid portion of the membrane.

Soaps• The “tail” of a fatty acid is a long hydrocarbon chain, making it hydrophobic. The “head” of the molecule is a carboxyl group which is hydrophilic. Fatty acids are the main component of soap, where their tails are soluble in oily dirt and their heads

are soluble in water to emulsify and wash away the oily dirt. However, when the head end is attached to glycerol to form a fat, that whole molecule is hydrophobic.