Lipids

Question 1

Explain the structure of fatty acids.

Answer 1

• They are carboxylic acids that contain 4-28 carbons (carboxyl group)
• Short chain (less than 6), medium-chain (6-12), long chains (13-21) and very long chain (over 22)
• Most naturally occurring fatty acids contain straight chain (no branching), even number of carbons

Question 2

What does it mean for a hydrocarbon chain to be saturated and unsaturated?

Answer 2

• Saturated: only carbon-carbon single bonds
• Unsaturated: one or more carbon-carbon double binds
• Monounsaturated (one double bond), polyunsaturated (many double bonds)

Question 3

Why are fatty acids not soluble in water?

Answer 3

• The long non-polar hydrocarbon chain results in a molecule that has great non-polar characteristics
• The carboxyl group is highly polar and can form hydrogen bonds, yet the non-polar chain has greater characteristics

Question 4

What type of bonding exists between fatty acids?

Answer 4

• Intermolecular forces, depends on the length of the chain and whether it is saturated or unsaturated
• Longer chains have stronger London dispersion forces, high melting points
• Saturated fatty acids are very straight, allow better contact between fatty acid chains, increases strength of London dispersion forces
• Unsaturated fatty acids have a kink (bend), weaker London dispersion forces and lower melting point

Question 5

What are triglycerides?

Answer 5

• Fatty acids that are bonded together with a glycerol through a condensation reaction
• 1 glycerol bonds to 3 fatty acids since it contains three hydroxyl groups (can form 3 water molecules)
• Reaction between fatty acids (carboxylic acids) and an alcohol result in an ester, ester linkage
• Reverse reaction is hydrolysis (addition of H2O)

Question 6

Explain the melting points of triglycerides.

Answer 6

• Triglycerides a.k.a fats/oils
• Difference between them is their state at room temperature
• Fats are solid at r.t.p (usually from animal sources), oils are liquid at r.t.p, usually from plants

Question 7

What makes triglycerides non-linear? Explain the melting points of saturated and unsaturated.

Answer 7

• Carbon-carbon double bonds cause kinks, chain is not linear
• Weaker London dispersion forces between triglycerides, lower melting point, likely to be liquid at r.t.p
• Saturated, more linear, more surface area between triglycerides, increases strength of forces, higher melting point

Question 8

What is the iodine number?

Answer 8

• Quantitative measurement of unsaturated for fats
• Mass of iodine that is absorbed by 100g of a chemical substance
• The more sites of unsaturated result in a greater iodine number (addition reaction with halogens)
• Iodine is slowly added to a sample of a lipid until no further colour change is observed, yellow-brown remains (it turns colourless before)
• When it becomes yellow-brown, there are no more double bonds present
• Method used to indicate number of double bonds present

Question 9

Which lipids have more unsaturation?

Answer 9

• Vegetable oils have more unsaturation than fats from animal sources
• Vegetable oils iodine number: 80-200
• Animal fats iodine number: 40-70, higher melting point, high degree of saturation, low iodine number

Question 10

How do you calculate the iodine number?

Answer 10

• Expressed as a percentage of mass of the iodine reacted by the mass of lipid reacted

Question 11

What are phospholipids?

Answer 11

• Other form of lipid
• Derived from triglycerides and perform function for the cell by forming a protective layer around the cell (cell membrane)
• The structure is similar to triglycerides, except that the glycerol is bonded to 2 fatty acids and one phosphate group (PO4 3-)
• Phosphate group contains ionic and non-polar function groups

Question 12

What characteristics do phospholipids have?

Answer 12

• Phosphate group results in water solubility, has ion-dipole attraction between negative phosphate group and polar water molecules
• Hydrophilic heads (water soluble) and hydrophobic tails (water insoluble)
• The phosphate heads form ion-dipole attraction with water molecules and the fatty acid tails avoid water
• Form phospholipid bilayer in aqueous environments

Question 13

What role do phospholipids play in solubility, the cell membrane and compartmentalisation?

Answer 13

• Phospholipid bilayer can form around hydrophobic (non-polar) vitamins, allows them to be dissolved and transported in bloodstream
• In the cell membrane consists of a bilayer and proteins and cholesterol, forms protective layer
• The cell contains inside and outside water, the tails form the inside
• Cell membrane is selectively permeable, some substances can cross, some cannot
• Phospholipid bilayer forms around organelles, allows compartmentalisation within the cell

Question 14

What type of reactions do phospholipids undergo?

Answer 14

• Condensation reactions involving glycerol
• Hydrolysis in acidic or basic media, releasing fatty acid chains
• Hydrolysis involving enzymes (catabolic)

Question 15

What are steroids? Explain the structure.

Answer 15

• Chemical substances naturally produced in the body, act as structural components in cell membranes or as hormones
• Contains steroidal backbone which is made of four fused rings: three cyclohexane rings and one cylcopentane (17 carbons)
• The backbone is non-polar, yet the presence of polar functional groups help increase the polarity and water solubility
• Most steroids are water insoluble, carrier proteins used that bind to the steroid to better increase its solubility

Question 16

What is cholesterol?

Answer 16

• Type of steroid, adds rigidity to phospholipid bilayer
• Has four fused rings, very non-polar (hydrophobic)
• Hydroxyl functional group is attracted to charged phosphate head, rest of cholesterol is non-polar

Question 17

What are steroids used for?

Answer 17

• Medical conditions can result in lower levels of steroids, prescribed to have normal levels
• Also used for non-medical reasons, enhanced growth or athletic performance
• Increases levels of cholesterol in bloodstream due to steroids, increase risk of CHD and stroke

Question 18

How are steroids used for enhanced athletic performance?

Answer 18

• Testosterone used with other drugs, growth and development of male characteristics
• Increased levels of testosterone can improve strength and endurance
• Can lead to hair loss and decreased fertility

Question 19

Why must fats be consumed through the diet?

Answer 19

• Allows body to obtain energy and recycle nutrients into lipids
• Form structural components within the body
• Used to produce steroids for growth and development or build new cell membranes to replace old ones
• Over-consumption can lead to health effects

Question 20

Why are lipids good sources of energy?

Answer 20

• Lipids can be used as an energy source for organisms reacting in a combustion reaction
• Converts three fatty acids from triglycerides to CO2 and H2O, releases energy
• Lipids are more reduced than carbohydrates, they have more hydrogen and less O2 and produce more energy when oxidised, more bonds made, exothermic (mexo)
• Fatty acids require more oxygen to oxidise to make CO2 (fatty acids have O2 oly from carboxyl)
• Lipids yield more energy per gram when oxidised, compared to carbohydrates, lipids almost double the amount as carbs

Question 21

What negative effect can triglycerides (saturated) have on health?

Answer 21

• Excess energy obtained from fats, stored for later use, too much can interfere with metabolic pathways
• Saturated fatty acids have a linear structure, greater surface area for London dispersion forces
• Hence saturated fats are solid at room temperature, excess intake can increase cholesterol levels in blood

Question 22

What type of unsaturated fatty acids exist and what effect do they have on health?

Answer 22

• Naturally occur in cis form, synthetic produced as trans fats
• Trans fats are more linear than cis fats, hence trans fats resemble saturated fats in structure and function
• High consumption of trans fats result in higher blood cholesterol levels, increased risk for heart disease

Question 23

What are lipoproteins and what are the two types?

Answer 23

• Lipoproteins carry cholesterol through aqueous bloodstream since cholesterol is non-polar
• Lipoproteins make it more soluble
• High density and low density

Question 24

What do high density lipoproteins (HDL) do?

Answer 24

• Travel through the bloodstream, absorb excess cholesterol and transport it to the liver for removal
• HDL is ‘good’ cholesterol, composed of more protein and less triglycerides

Question 25

What do low density lipoproteins (LDL) do?

Answer 25

• Travel through the bloodstream, deposit cholesterol in the walls of blood vessels
• ‘bad’ cholesterol, composed of less protein and more triglycerides
• Can cause buildup known as plaque
• Plaque can form blockage in blood flow, result in heart attack or stroke

Question 26

What is rancidity?

Answer 26

• If fats are not consumed right away, they can react with oxygen becoming rancid (spoil)
• Distinct rancid odours and flavours
• Hydrolytic rancidity and Oxidative rancidity

Question 27

What is hdrolytic rancidity?

Answer 27

• Process of hydrolysis, turns triglycerides back into glycerol and three fatty acids
• One of the fatty acids produces butanoic acid
• Unpleasant odour
• Certain enzymes, increased temperature and presence of water can increase the rate of rancidity
• Use of refrigerator reduces rate of oxidation
• Diary products become rancid quickly

Question 27

What is hdrolytic rancidity?

Answer 27

• Process of hydrolysis, turns triglycerides back into glycerol and three fatty acids
• One of the fatty acids produces butanoic acid
• Unpleasant odour
• Certain enzymes, increased temperature and presence of water can increase the rate of rancidity
• Use of refrigerator reduces rate of oxidation
• Diary products become rancid quickly

Question 28

What is oxidative rancidity?

Answer 28

• When oxygen reacts with carbon-carbon double bonds of unsaturated fats, catalysed by light
• Variety of compounds produced, ketones and aldehydes, unpleasant aroma
• Oxidative rancidity occurs more frequent when substance is exposed to light and greater exposure to oxygen
• E.g. fish (unsaturated fats)
• Unsaturated more susceptible to oxidative rancidity

Question 29

What roles do lipids have in the body?

Answer 29

• Energy storage, release more energy when oxidised
• Insulation and protection of organs, stored in adipose tissue
• Steroid hormones, e.g. progesterone, testosterone, anabolic steroids
• Cell membranes

Question 30

What is hydrogenation and when does it usually occur?

Answer 30

• Unsaturated fats are hydrogenated to saturated fats with a higher melting point by adding hydrogen under pressure in the presence of a heated nickel catalyst
• Happens in cis-unsaturated fatty acids
• Yet partial hydrogenation may occur, turned into trans-isomers, promote formation of LDL cholesterol, increase risk of heart disease