Lipids

Question 1

What are lipids?

Answer 1

A group of chemicals that dissolve in organic solvents but not water

Question 2

What is an oil?

Answer 2

A lipid that is liquid a room temperature

Question 3

What is a fat?

Answer 3

A lipid that is solid at room temperature

Question 4

What are the function of lipids?

Answer 4

Insulation and maintenance of body temperature
Metabolic a physiological processes such as the maintenance and integrity of the cell wall
Physical protection to internal organs
Energy storage - the only form of prolonged storage in the body

Question 5

What proportion of energy intake do dietary lipids provide in the Western countries?

Answer 5

30-40%

Question 6

What is triacylglycerol?

Answer 6

Makes up 95% of dietary lipids

Its formed from a molecule of glycerol with 3 fatty acids attached

Question 7

Describe the structure of a fatty acids

Answer 7

They consist of an even numbered chain of carbon atoms with hydrogens attached
A methyl group (CH3) at one end - the omega end
A carboxylic group (COOH) at the other end - the delta end

Question 8

How does the structure of saturated vs unsaturated fatty acids differ?

Answer 8

Their carbon bonds are fully saturated with hydrogen atoms
When 2 hydrogen atoms are missing a double bond forms and monounsaturated (single double bond) or polyunsaturated (2 or more double bond) fatty acids result

Question 9

How do you determine which ‘family’ and unsaturated fatty acid belongs to?

Answer 9

The position of the first double bond relative to the methyl end of the chain

Question 10

What are the important families of unsaturated fatty acids?

Answer 10

Polyunsaturated fatty acids in which the first double bond is 3 carbons from the methyl end are called n-3 (omega 3)
n-6 (omega 6) and n-9 are also important families.

Question 11

Name some common saturated fatty acids and the foods they are present in

Answer 11

Acetic acid - vinegar
Butyric - dairy
Caproic - dairy
Lauric - coconut
Myristic - dairy and coconut
Palmitic and stearic - most plant and animal fats
Arachidic - peanuts

Question 12

Name some common monounsaturated fatty acids and where they are found

Answer 12

Oleic - all animal and plants
Eicosinoic - rapeseed and animal fats
Gadoleic - fish oils

Question 13

Name some common polyunsaturated fatty acids and the foods they are found it

Answer 13

Linoleic - plant oils - sesame and safflower
alpha-linolenic- plant oils - walnuts and linseeds
Arachidonic - small amount in animal tissues
Eicosapentaenoic acid (EPA) - fish and fish oils
Docosapentaenoic acid (DPA) - fish, fish oil and animal brain
Docosahexaenoic acid (DHA) - as above

Question 14

What is a phospholipid?

Answer 14

Comprise a very small amount of dietary lipds
They are a diglyceride in which the third position is occupied by phosphoric acid to which one of chalice, serine, inositol or enthanolamine is attached
These 4 phospholipids make up most of the phospholipids in the body
Phosphaditylcholine is the most abundant in nature and is otherwise known as lecithin.

Question 15

What are sterols?

Answer 15

Lipids that are made form carbon, hydrogen and oxygen arranged in a series of 4 rings with varying side chains
Cholesterol is the principle sterol of animal tissue

Question 16

What is the basic role of cholesterol?

Answer 16

Structural component of membrane and lipoproteins

Precursor of bile acids, vitamin D and steroid hormones

Question 17

Describe the hydrogenation of fat?

Answer 17

Addition of a hydrogen atom at the double bond of unsaturated fatty acids to increase the degree of saturation and therefore increase the melting point of the fat.
Partial hydrogenation also changes the formation of some of the double bonds from their cis to the trans form
Transfatty acids are associated with an increased risk of coronary heart disease

Question 18

Which fatty acids are essential?

Answer 18

omega 3 and omega 6 fatty acids. Humans are unable to insert a double bond before C7.

Question 19

What is a short chain fatty acid?

Answer 19

Have alipathic tails less than 6 carbons long

Generally made by bacterial fermentation of fibre e.g. butyric

Question 20

What are medium chain fatty acids?

Answer 20

They have between 6-12 carbons
e.g. caproic acid, caprylic acid and lauric acid
They get absorbed into the portal system and don’t require carnitine to be transported in to eh mitochondria - meaning they are easily converted into energy and less likely to be stored as fat.

Question 21

What are eicosinoids and what are they made from?

Answer 21

20 carbon molecules used locally for cell signalling. 3 main groups, PG1, 2 and 3.
Linoleic acid acts as the precursor form PG1, arachodonic for PG2 and ALA for PG3.

Question 22

What is omega 6 and its metabolites?

Answer 22

Linoleic acid, GLA and DGLA

Question 23

What is omega 3 and it’s metabolites?

Answer 23

Alpha linolenic acid, EPA and DHA.

Question 24

Describe the ALA pathway

Answer 24

Delta-6-desaturase converts ALA to EPA, which then gets converted to PG-3 or DHA. PG-3 is anti-inflammatory

Question 25

Describe the omega 6 pathway

Answer 25

Linoleic acid gets converted to GLA via delta-6-desaturase. GLA then gets converted to DGLA. DGLA can get converted to PG-1, which is anti-inflammatory. It can also get converted to arachidonic acid by delta-5-desaturase, and then onto PG-2, which is inflammatory. If the diet is high is omega 3 then the delta-5-desaturase will be used up and GLA is more likely to turn into PG1.

Question 26

What is a simple lipid?

Answer 26

Fatty acids esterified with alcohols (e.g. glycerol)

Question 27

How are lipids named?

Answer 27

After the number of carbons, the number of double bonds and the position of the first double bond e.g. 18:2n-6 = 18 carbons, 2 double bonds, 1st double bond is at the 6th carbon from the methyl end

Question 28

What are the 3 phases of fat digestion?

Answer 28

Gastric, duodenal and ileal

Question 29

Describe the gastric phase of fat digestion

Answer 29

lingual lipase may begin fat digestion in the mouth. It then may continue into the stomach where is acts as a gastric lipase and degrades upto 10% of the ingested fat. The entry of the resulting FAs and monoacylglycerols acts to stimulate production of cholecystokinin and inhibits gut motility. Chyme is produced in the gut, with the fat sitting on top of an aqueous layer - which delays fat entry into the duodenum. The pyloric sphincter controls the rate of emptying - it opens twice per minute allowing about 3 mls through at a time.

Question 30

Describe the duodenal phase of fat digestion

Answer 30

The entry of chyme containing minor lipolytic products into the duodenum stimulates - the release of CCK which inhibits gut motility - secretion of bile acids from the gallbladder - the release of pancreatic juice containing lipases

Question 31

Describe lipolysis in the duodenum

Answer 31

Hydrolysis of bonds between a FA and the glycerol backbone of the TAG and phospholipids and cholesterol in the cholesterol esters. The reaction is catalysed by pancreatic enzymes including lipase and phospholipase A2. FA from position 1 is removed first, and then from position 3, generating 2 monoacylglycerol (2MAG)

Question 32

How are fats rendered soluble in the small intestine?

Answer 32

They are solubilised by association with bile salts and phospholipids to form micelles

Question 33

Why do bile salts render fats soluble in water?

Answer 33

Bile salts and phospholipids are amphipathic molecules - they are capable of interfacing with both hydrophobic and hydrophilic environments

Question 34

What are the primary bile salts?

Answer 34

Cholic and chenodeoxycholic acids | Produced in the liver from cholesterol under the action of the rate-limiting enzyme 7-alpha-hydroxylase

Question 35

How are short and medium-chained fatty acids absorbed?

Answer 35

Directly into the portal system and then transported bound to albumin, where they are rapidly oxidised.

Question 36

Describe the ileal phase of fat digestion

Answer 36

Involves the transit of dietary fats from the micelles into the enterocyte Once in the cell there is rapid conversion of MAGs top TAGs under the influence of enzyme acyl-coA-cholesterol acyltransferase (ACAT) - helping to maintain a concentration gradient into the enterocyte.

Question 37

What is the role of chylomicrons

Answer 37

Once in the enterocyte the TAG and CEs get taken up into chylomicrons. Chylomicrons leave the basement membrane by exocytosis and cross into the lymphatic vessels