IB DP Biology: SL复习笔记2.2.2 Fatty Acids

• Triglycerides are a form of lipid, made up of one molecule of glycerol with three fatty acids attached to it
• These fatty acids have long hydrocarbon ‘tails’
• Fatty acids occur in two forms:
  • Saturated fatty acids
  • Unsaturated fatty acids
    • Unsaturated fatty acids can be monounsaturated or polyunsaturated
• The difference between these fatty acid types is found in their hydrocarbon tails

Saturated fatty acids

• In saturated fatty acids, the bonds between the carbon atoms in the hydrocarbon tail are all single bonds
• The fatty acid is said to be ‘saturated’ with hydrogen
  • This means that each carbon atom in the hydrocarbon tail (except for the final carbon atom) is bonded to two hydrogen atoms
• Saturated fatty acids can be synthesised industrially by hydrogenation (reaction with hydrogen gas) of unsaturated fatty acids
• All the carbon-to-carbon bonds are single bonds in saturated fatty acids

An example of a saturated fatty acid

Unsaturated fatty acids

• In unsaturated fatty acids, the bonds between the carbon atoms in the hydrocarbon tail are not all single bonds
  • There is at least one carbon-carbon double bond; a fatty acid with one C=C double bond is known as monounsaturated fatty acid
  • In some unsaturated fatty acids, there are many carbon-carbon double bonds; these are known as polyunsaturated fatty acids
  • These double bonds can cause the hydrocarbon tail of unsaturated fatty acids to kink (bend slightly), meaning they are not as straight as saturated fatty acids
• The fatty acid is said to be ‘unsaturated’ because the hydrocarbon tail does not contain the maximum number of hydrogen atoms possible
  • This is because each carbon atom in a carbon-carbon double bond can only bond to one hydrogen atom (instead of two)

An example of a monounsaturated fatty acid

An example of a polyunsaturated fatty acid

The different isomers of unsaturated fatty acids

• A single C-C bond in a hydrocarbon chain is able to rotate along its axis so that rotation of one part of the molecule in relation to others is possible
• The rotation may cause conformational changes in molecules but they all remain identical
• However, when there is a C=C double bond in a hydrocarbon chain, no rotation is possible
• This causes isomers of fatty acids to be formed, each with different properties
• The isomers are labelled cis and trans isomers

Cis-isomers

• At a C=C double bond, the attached hydrogen atoms are on the same side of the hydrocarbon chain as each other
  • This causes a kink in the fatty acid chain which means the fatty acid chains are less tightly packed together, lowering their melting point (less kinetic energy is needed to break them apart)
  • Triglycerides that contain cis-unsaturated fatty acids are liquid (oils) at room temperature

The Structural Differences Between a Saturated and a Cis-Unsaturated Fatty Acid

Trans-isomers

• At a C=C double bond, the attached hydrogen atoms are on the opposite sides of the hydrocarbon chain to each other
  • The lack of bend in the hydrocarbon chain allows them to pack more closely together meaning they have a higher melting point (this property makes them more attractive to food manufacturers)
  • Triglycerides that contain trans-saturated fatty acids are solid are room temperature

The Structure of a Trans-Unsaturated Fatty Acid

The use of trans-fatty acids in the food industry

• Trans-fatty acids occur in small quantities in natural products such as dairy and red meat
• Artificial trans-fats (which contain trans-fatty acids) are made industrially by the hydrogenation of liquid vegetable oils
• Trans-fats are favoured by food manufacturers for commercial reasons
  • They are more solid at room temperature
  • They create more stable emulsions in shortening agents
  • Food products with trans-fats appear (in their retail packaging) drier and less 'greasy' to consumers
  • Alleged taste benefits (though this is subjective)
  • They can be reused more times eg. in large-scale deep-fat fryers
  • Many countries have legislated to restrict the use of trans-fats in the foodservice industry
• Many foods that contains trans-fats (often labelled as 'partially hydrogenated vegetable oils') are sold as processed products in supermarkets
  • Biscuits/cookies
  • Cakes
  • Doughnuts
  • Pie crusts
  • Crisps
  • Pizza bases
  • Certain kinds of margarine and spreads

Trans-fatty acids have associated health risks

• The two types of fat that lead to health problems are namely saturated fats and trans-fats
  • Doctors recommend limiting your intake of these types of fats
• Trans-fats alter the balance of various types of cholesterol
  • They increase low-density lipoprotein (LDL) levels in circulation (so-called 'bad' cholesterol)
  • They decrease high-density lipoprotein (HDL) levels in circulation (so-called 'good' cholesterol)
• LDLs are known to increase the risk of coronary heart disease, blood clotting and strokes
• Doctors recommend that the bulk of fats intake should come from monounsaturated fats, which reduce LDL levels
  • Omega-3 fats and oils are a well-publicised source of monounsaturated fats; these are found in fish, pulses and certain nuts

Excess consumption of trans-fatty acids and saturated fats can lead to the buildup of cholesterol and the blockage of coronary arteries, causing Coronary Heart Disease (CHD)

Evaluating claims

• Evidence for the claims surrounding the health risks associated with trans-fats often comes from 'cohort studies'
  • Eg. Greenland Eskimos, whose diet is rich in oily fish and meat, have a very low incidence of heart disease
• Other epidemiological studies can establish correlations between diet and incidence of disease
• Whilst it is rare to find a direct causal link between fat intake and heart disease, the many claims about fats suggest strongly that trans-fats have an overall detrimental effect on health when consumed in high quantities
• Other conditions linked to trans-fats include
  • Allergy
  • Breast cancer
  • Colonic cancer
  • Cardiovascular diseases
  • Premature birth
  • Preeclampsia (a condition associated with pregnancy)
  • Disorders of the nervous system
  • Vision defects in infants
  • Diabetes
  • Obesity