Lipids 1

Question 1

What are lipids?

Answer 1

They are heterogeneous organic molecules that are insoluble in water but soluble in organic solvents. They can be found as phospholipids in the membrane, droplets of liquid in adipose tissue and lipoproteins in blood.

Question 2

What are some of the biological functions of lipids?

Answer 2

Stored form of energy. 
Membrane molecules. 
Enzyme cofactors. 
Hormones (steroid)
Vitamins A, B, E, K
Signalling molecules (prostoglandins)

Question 3

Describe the five lipid classes

Answer 3

Fatty acids are the simplest acids. Triacylglycerol is the main type of stored fat.
Phospholipids are used in the membrane.
Glycolipids are lipids with a carbohydrate group attached and these are used in nerve myelination and as blood group antigens.
Steroids have a ring structure.

Question 4

What are essential fatty acids?

Answer 4

These are ones that the body cannot produce itself as it can’t make fatty acids with double bonds from the 9th carbon so must ingest them.
Linoleic and linoleic acid are essential fatty acids and arachidonic acid can be synthesised from linoleic acid.

Question 5

What are omega 3 fatty acids?

Answer 5

These are derived from linoleic acid. They lower plasma cholesterol, prevent atherosclerosis, lower TAG, prevent obesity and reduce inflammation.

Question 6

Describe the some causes and effects of fatty acid deprivation

Answer 6

Infants that are dependant on milk, extreme vegans and people with chronic gut diseases can all be deficient in essential fatty acids.
It can result in growth retardation, reproductive failure, skin lesions, kidney/liver problems, depression, ADHD and slight neurological or visual problems.

Question 7

What is triacylglycerol?

Answer 7

It is an ester of fatty acids and glycerol. It is a neutral, uncharged lipid and is water insoluble. They provide dietary fuel and insulation.

Question 8

How are lipids digested?

Answer 8

The small intestine is the main site of digestion. Pancreatic enzymes called lipases digest lipids and this is promoted by emulsification by bile salts and peristalsis.

Question 9

What are bile salts?

Answer 9

They act as biological detergents to form emulsions and mixed micelles which stops lipids sticking together. They are derivatives of cholesterol (cholic acid +taurine/glycine).

Question 10

What are triacylglycerols digested into?

Answer 10

They are broken down by pancreatic lipase to form monoacylglycerol and two fatty acids. It can be further broken down to remove the last fatty acid.

Question 11

What are cholesterol esters digested into?

Answer 11

They are broken down into cholesterol and a free fatty acid.

Question 12

What are phospholipids digested into?

Answer 12

They are hydrolysed to a lysophospholipid and a free fatty acid.

Question 13

How do digested lipids get absorbed into cells?

Answer 13

Short and medium fatty acid chains are absorbed straight into the epithelia. Long chain fatty acids form mixed micelles with bile salts. The mixed micelles approach the brush border of enterocytes and release the lipid product which is diffused into the cell.

Question 14

What is steatorrhea?

Answer 14

Steatorrhea is excess fat in faeces causing them to float, have an oily appearance and smell foul. This can be because of lipid malabsorption due to defects in bile secretion, pancreatic function or intestinal cell uptake. The gallbladder secretes bile so removal of this inhibits digestion and absorption of fats.

Question 15

What happens to the absorbed fatty acids from digestion?

Answer 15

Intestinal cells resynthesise triacylglycerols, phospholipids and cholesterol esters for export. These are insoluble so are packaged in chylomicrons (combined with solubising protein apoB-48). Chylomicrons are exocytosed into the lymph then the blood.

Question 16

What happens to the blood chylomicrons when they reach tissue?

Answer 16

Triacylglycerols in chylomicrons are hydrolysed to fatty acids and glycerol by lipoprotein lipase. Lipoprotein lipase is mainly found in capillaries of skeletal muscle and adipose tissue. The remaining free fatty acid is used for energy or is reesterified to triacylglycerols for storage. The chylomicrons depleted of triacylglycerol are called chylomicron remnants and these go to the liver. Glycerols is used by liver to produce glycerol-3-phosphate.

Question 17

How is triacylglycerol stored?

Answer 17

In adipose cells triacylglycerol is stored as droplets of depot fat. This is the most efficient form of fuel as they are highly reduced and nearly anhydrous (contains no water). Then fatty acids are released from this when energy supplies are low.

Question 18

How are fatty acids released from triacylglycerol in adipose tissue and how do they then travel in the blood?

Answer 18

Hormone sensitive lipase (HSL) is activated by phosphorylation in response to epinephrine. This HSL releases fatty acids from stored triacylglycerol. High plasma glucose and insulin promote the dephosphorylation of this.
Free fatty acids form a complex with serum albinum (plasma protein) to travel in the blood but over 90% of fatty acids are esterified and these are carried in lipoproteins.

Question 19

Name the four classes of lipoprotein and describe their function

Answer 19

Chylomicrons are triacylglycerol rich and carry these from the intestine to the tissues and it is the least dense lipoprotein.
VLDL is triacylglycerol rich and carries it from the liver to the tissue.
LDL is cholesterol rich and carries this to the extrahepatic tissue.
HDL is protein and cholesterol rich and it transports cholesterol from tissue to the liver for elimination as bile salts and HDL is the most dense lipoprotein.