B4 Lipids

B.4.1 Compare the composition of the three types of lipids found in the human body
Triglycerides (fats and oils), phospholipid (lecithin) and steriods (cholesterol)

Triglycerides (fats and oils)
- glycerol and 3 fatty acids
- Glycerol has 3 carbon atoms, each having an -OH group
- Fatty acids are long chains with carboxylic acid terminal group. (R-COOH)

Source: IB Chemistry textbook

Phospholipids
- similar to triglyceride
- glycerol and 2 fatty acids
- third -OH has a phosphate group
- different phospholipids vary fatty acids and phosphate groups attached
- have polar/ hydrophilic heads (phosphate group) and 2 non-polar tails (the hydrocarbon chains)
- hence, spontaneous form is a phospholipid bilayer, which creates the basis of membrane structures
- maximum interaction with polar head and water, creating a non-polar/ hydrophobic interior
Most common phospholipid: Lecithin 

Phospholipid bilayer
Source: IB Chemistry Textbook

The structure of phospholipids
Source: IB Chemistry Textbook

Steriods
- 4 fused rings
Most important: Cholesterol
- used in synthesis of many other steroids e.g. sex hormones

Structure of cholesterol
Source: IB Chemistry Textbook

B.4.2 Outline the differences between HDL and LDL cholesterol and outline its importance
Cholesterol is insoluble in blood
HDL
- high density lipoprotein
- good cholesterol
- tends to carry away cholesterol from the arteries thus slowing the build up
- high levels of HDL seems to protect against heart attack
- poly-unsaturated fats e.g. in fish, nuts and corn oil seem beneficial in lowering LDL

LDL
- low density lipoprotein
- bad cholesterol
- main source: saturated fat/oil and trans fat
- high levels of LDL are associated with increased deposition in the walls of arteries

Omega-3-polyunsaturated fatty acid
- in flaxseeds and fish oil
- help reduce risk of heart disease and optimum neurological development
- cannot be made by the body and must be included in diet, hence called "essential fatty acids"


B.4.3 Describe the difference in structure between saturated and unsaturated fatty acids.
Most naturally occurring fats contain a mixture of saturated, mono-saturated and poly-saturated fatty acids and are classified according to the predominant type of unsaturation present.

Saturated 
- all C-C single bonds
- tetrahedral 109.5 bond angles between atoms
- so molecules pack closely together, significant VDW forces.
- hence created saturated triglycerides with relatively high mpt that are solid at room temp.
- called 'fats' e.g. butter and lard

A saturated triglyceride

 Source: IB Chemistry Textbook

Unsaturated
- one or more C=C double bond
- 120 bond angles with kinks in the chain (made by the double bond) making it more difficult for them to pack closely
- unsaturated triglycerides with weaker IMF, lower mpt
- liquids at room temp.
- called 'oils', mostly in plants and fish e.g. corn oil and cod liver oil

An unsaturated triglyceride

Source: IB Chemistry Textbook

**Strong correlation between saturated fat rich diets and elevated levels of LDL cholesterol, associated increase in the cause heart disease.**


B.4.4 Compare the structures of the two essential fatty acids linoleic (omega-6-fatty acid) and linolenic (omega-3-fatty acid) and state their importance.
Body is unable to produce either fatty acid
Both are "essential fatty acids"
- from plants and fish e.g. shellfish, leafy vegetables, canola oil, flaxseed oil
- aids metabolic processes e.g. synthesis of lipids: prostaglandins (which helps lower blood pressure)

'Omega-3' 'Omega-6' represents the position of the first double bond in the molecule relative to terminal -CH3 group. Referred to as 'Omega' (the last letter in Greek alphabet) to represent its distance from -COOH group (the 1st carbon). 'Omega-3' essentially means third carbon counting backwards.

Comparing structures: (from markscheme)
- both have 18 carbon atoms;
- both have -COOH;
- linoleic acid has 2 double carbon bonds AND linolenic acid has 3 double carbon bonds/ linoleic acid has less double bonds;
- both have first double carbon bond on C9/ first double bond of linoleic is after the 6th C atom and first of linolenic is after the 3rd C atom counting from -CH3 terminal group;

Source: American Heart Association

B.4.5 Define the term iodine number and calculate the number of C=C double bonds in an unsaturated fat/oil using addition reactions
Iodine number - number of grams of iodine which will react with 100g of fat
1 mole of iodine will react with 1 mole of double bonds in fat. Higher no. double bonds per molecule = large iodine number
Used to determine the degree of unsaturation in fat
- addition reaction, breaks C=C (alkene) and adds I

- reacting known amount of fat with known amount of iodine
- after reaction completes, excess iodine is calculated by titration with Na2S2O3 (Sodium Thiosulphate), and reacted iodine is found.

- Find Mr > Find moles > Find mole ratio of iodine to fat/oil > iodine number


B.4.6 Describe the condensation of glycerol and three fatty acid molecules to make a triglyceride
Esterification is a condensation reaction. Condensation means the reaction has a side product, but not necessarily water.

Ester linkages between the glycerol and fatty acids are created during the reaction.
Each fatty acid chain is different. (R1, R2, R3) They differ in these two ways:
Hydrocarbon chain length
- most abundant fatty acids have even number of carbon atoms between 14-22 Cs.

Number and position of C-C bonds in chain
Fatty acids with:
no double bonds = saturated
just 1 double bond = mono-saturated
several double bonds = poly-saturated

Their natures affects their melting points and other important properties.

Source: IB Chemistry Textbook

B.4.7 Describe the enzyme-catalysed hydrolysis of triglycerides during digestion
Fats and oils cannot be transported in blood
- broken into fatty acids and glycerol in the gut (digestion)
- hydrolysis reaction (water used), under control of lipase enzyme

Lipase
- secreted in different parts of the gut
- act sequentially to digest lipids
- usually slowest molecules to digest, may take hours before they are soluble and are absorbed into the blood
- enzyme is made of protein, hence sensitive to pH change; control pH in different area of the guts is one way the body controls lipid digestion.


B.4.8 Explain the higher energy value of fats compared to carbohydrates
Lipids
- stored energy, released when broken down (respiration, oxidation produces carbon dioxide and water)
- less oxidised than carbs, hence undergoes more oxidation and releases more energy per unit mass when used as respiratory substrate.

**energy from a gram of lipid = x2 energy from a gram of carbohydrate

- but because of its insolubility, lipid energy is not as readily available as carbohydrates (so it's not good for immediate energy source e.g. for a marathon)
- but makes ideal storage molecules (e.g. a trip to the Arctic), called adipose tissue

Adipose tissue 
- in different parts of the body
- reservoirs of energy
- helps protect some body organs e.g. kidneys and layer of fat under the skin helps insulate the body


B.4.9 Describe the importance roles of lipids in the body and the negative effects that they can have on health.
Important roles include:
- energy storage
- insulation and protection of organs
- helps synthesis of steroid hormones
- structural component of cell membrane (phospholipids)
- helps determine selective transport of metabolites across cell boundaries
- in nerves the myelin sheath (special layer of phospholipids) gives electrical insulation to the nerves and speeds up nervous transmission.
- omega-3 poly-unsaturated fatty acids reduce the risk of heart disease
- poly-unsaturated fats may lower levels of LDL cholesterol
- cholesterol also important in plasma membrane structure
- lipids help absorb fat-soluble vitamins (A, D, E, K)

Negative effects include:
- increased risk of heart disease from elevated levels of LDL cholesterol and trans fatty acids; the major source of LDL cholesterol is saturated fats, in particular lauric (C12), myristic (C14) and palmitic (C16) acids
- obesity
- excess lipids, because of their low solubility causes them to deposit on the walls of the main blood vessels (restricts blood flow - atherosclerosis, associated with high blood pressure and leads to heart disease)