Lipids 1

Question 1

Name the classes of lipids.

Answer 1

Fatty acids
Triacylglycerols
Phospholipids
Glycolipids
Steroid

Question 2

Describe essential fatty acids.

Answer 2

Essential fatty acids need to be ingested (from plants) as the human body cannot introduce double bonds beyond carbon 9.
e.g. linoleic acids, alpha-linoleic acid.

Question 3

Describe the importance of omega-3 fatty acids.

Answer 3

They are derived from linoleic acids as essential fatty acids.
They lower plasma cholesterol, which prevents atherosclerosis.
They lower TAG, prevent obesity and reduce inflammation.

Question 4

Describe “good fats” from our diet.

Answer 4

Good fats are high in polysaturated fatty acids.
e.g. veggie, olive and sunflower oil.

Question 5

Describe “bad fats” from our diet.

Answer 5

Bad fats are high in saturated fatty acids.
e.g. stearic (beef).

Question 6

Describe “really bad fats” from our diet.

Answer 6

High in trans fatty acids, and are a result from hydrogenation of vegetable oil.
e.g. margarine (man-made).

Question 7

What are the biological functions of lipids?

Answer 7

Stored form of energy
Signalling molecules
Enzyme cofactors
Vitamins
Structural elements in membranes

Question 8

Describe triacylglycerols.

Answer 8

TAGs are esters of fatty acids and glycerol/ neutral uncharged lipids.
They are water insoluble
They are the major lipid component of adipose tissue and exist here as lipid droplets.
They act as a dietary fuel and insulation (keep us war, and insulate electrical signals).

Question 9

Describe phospholipids.

Answer 9

Phospholipids are amphipathic molecules.
They consist of glycerol + 2 fatty acids + an inorganic phosphate group.
The charged phosphate head is hydrophilic.
The tails are hydrophobic (lipophilic).
They are critical for simultaneous aqueous and non-aqueous interfaces; membranes, lipid droplets, local signalling molecules.

Question 10

Describe steroids as lipids.

Answer 10

Steroids are a major class of lipids, they have a ring structure.
There are three main classes:
Cholesterol, Steroid hormones and Bile salts.

Question 11

Give the functions of bile salts and steroid hormones as steroids.

Answer 11

Bile salts - Sodium salts of steroids used for emulsification
Steroid hormones - Serve as messengers in the body.

Question 12

Describe the main functions of cholesterol in the body.

Answer 12

Precursor of other substances: bile salts (in liver), steroid hormones, vitamin D, lipoproteins, membranes.
They can also modify proteins.

Question 13

Describe the synthesis of cholesterol.

Answer 13

Cholesterol is made mainly in the liver, and begins with Acetyl CoA (which comes from glycolysis, amino acids or fatty acids).
Acetyl CoA is converted to HMG-CoA.
HMG-CoA is converted to Mevalonate by the enzyme HMG-CoA reductase. (this is the rate determining step and is targeted by statins).
Mevalonte is converted to isoprene units.
Isoprene units are converted to Squalene.
Squalene is converted to Cholesterol.

Question 14

Describe the role of statins in the regulation of cholesterol biosynthesis.

Answer 14

Statins inhibit HMG-reductase.
They lower the LDL levels
= reduce the risk of developing cardiovascular disease.

Question 15

Describe the structure of cholesterol.

Answer 15

Cholesterol has a central sterol nucleus of 4 hydrocarbon rings.
It has a hydrocarbon tail and a hydroxyl group.
This structure makes it hydrophobic meaning it cannot flow through the blood stream alone.

Question 16

Describe the structure of eicosanoids.

Answer 16

Eicosanoids are a lipid class derived from 20 carbon unsaturated fatty acids (eicosanoid acids) and are synthesised throughout the body.

Question 17

Describe the function of eicosanoids as precursors.

Answer 17

They are precursors to:
Prostaglandins
Thromboxanes
Leukotrienes.

They are metabolised quickly due to their short half life, and are produced and act locally.

Question 18

What do eicosanoids regulate?

Answer 18

Lipid inflammatory response
Pain and fever (prostaglandins)
Blood pressure regulation (prostacyclin - inhibits platelet activation and dilates blood vessels)
Blood clotting induction/platelet homeostasis (thromboxanes)
Many reproductive functions (prostaglandins)
SMC constrictions and bronchoconstriction.
Mucus production in stomach (prostaglandins)

Question 19

Describe the importance of eicosanoids in medicines.

Answer 19

Eicosanoids can induce lipid inflammatory effects so their regulation is important in drugs.

Question 20

Describe how synthesis of eicosanoids is regulated.

Answer 20

The synthesis of eicosanoids is regulated by cyclooxygenase (COX) 1 and 2, (for prostanoids).
The synthesis of Hetes (leukotrienes and lipoxins) is regulated by LOX.

Question 21

Describe the synthesis of prostanoids.

Answer 21

All eicosanoids synthesis starts with arachidonic acid being esterified in the membrane phospholipid.
An enzyme called phospholipase A2 cleaves the phospholipid to produce arachidonic acid.
Arachidonic acid is then converted by a two-step reaction pathway involving COX 1 and COX 2 enzymes.

Question 22

Describe the synthesis of hetes.

Answer 22

All eicosanoids synthesis starts with arachidonic acid esterified in membrane phospholipid.
An enzyme called phospholipase A2 cleaves the phospholipid to produce arachidonic acid.
Arachidonic acid is then converted by a linear pathway, initiated by members of the LOX family of enzymes.

Question 23

Describe the process of lipid digestion.

Answer 23

Lipids are digested by pancreatic enzymes (lipases) and is promoted by emulsification (dispersion) by bile salts and peristalsis (mixing).
This process occurs in the small intestine, where fat is converted to emulsified fats and are then broken down further into fatty acids and glycerol (by lipases).

Question 24

Describe the role of bile salts in lipid digestion.

Answer 24

Bile salts (2 major: taurocholic and glycholic) act as biological detergents to form emulsions an mixed micelles.
This saves lipids coalescing in an aqueous environment.

Question 25

Describe the digestion of triacylglycerol.

Answer 25

Triacylglycerol is digested by pancreatic lipase, which removes 2 fatty acids from Carbon #1 and #3.
This removal produces monoacylglycerol and 2 fatty acids.

Question 26

Describe the digestion of cholesterol esters.

Answer 26

Cholesterol esters are digested to cholesterol and free fatty acids.

Question 27

Describe the digestion of phospholipids.

Answer 27

Phospholipids are hydrolysed to fatty acids and lysophospholipid.

Question 28

Describe the process of the uptake of lipids into cells, after digestion.

Answer 28

Products of lipid digestion form mixed micelles with bile salts, this allow them to be transported by the hydrophobic part of the bile salt. Mixed micelles approach the brush border membranes of enterocytes and release lipid products which enter these cells by diffusion.
*(short and medium fatty acids do not require micelles for absorption)

Question 29

Describe what happens to lipids once absorbed by enterocytes.

Answer 29

Intestinal cells resynthesise triacylglycerol, phospholipids and cholesterol esters for export
Fatty acids are insoluble so are packaged with a solubilising protein into chylomicrons for export.
Chylomicrons (lipoproteins) are released by exocytosis into lymph and then blood.

Question 30

Define a lipoprotein.

Answer 30

A lipoprotein consists of lipid particles coated with phospholipids to allow process round body.

Question 31

Describe what happens to lipids such as TAG when transferring from blood to tissue.

Answer 31

The TAG containing chylomicron si hydrolysed to Fatty acids and glycerol by lipoprotein lipase (this is found primarily in the capillaries of skeletal muscle and adipose tissue).
The resulting free fatty acids are used for energy or are re-esterified to TAG for storage.

Question 32

What happens to chylomicrons once they are depleted of the lipid they were exporting.

Answer 32

Chylomicrons depleted of lipid (TAG) are called chylomicron remnants which go to the liver.
Glucose is used by the liver to produce glycerol - 3 phosphate (glycolysis, gluconeogenesis).

Question 33

Describe the transport of fatty acids through blood.

Answer 33

Fatty acids are transported through blood in complex with serum albumin (non-esterified).
Esterified fatty acids are carried in lipoproteins.

Question 34

Give examples of the classes of lipoproteins.

Answer 34

Classified by density: least to most dense-
Chylomicrons (TAG rich, transports from intestine to tissue)
VLDL (TAG rich, transports from liver to tissue)
LDL (Cholesterol rich, transports to extrahepatic tissue)
HDL (protein and cholesterol rich, from tissue to liver for elimination = good cholesterol)

Question 35

Define atherosclerosis.

Answer 35

Atherosclerosis is a gradual build up of plaque on the arteries.
This can be due to unhealthily high cholesterol levels that can come from too much LDL