Composition And Metabolism Of Lipids
Question 1. Classification of lipids.
Answer.
Definition Of Lipids
Lipids are organic substances, relatively insoluble in water, soluble in organic solvents (alcohol, ether), related to fatty acids and utilized by living cells
Classification Of Lipids
Lipids are classified into the following groups
Simple Lipids
These are esters of fatty acids with alcohol.
They are of two types:
- Fats and oils (triacylglycerol)
These are esters of fatty acids with glycerol.
The difference between fat and oil is only physical.
Thus oil is a liquid while fat is a solid at room temperature
- Waxes
These are esters of fatty acids with alcohol other than glycerol.
Cetyl alcohol is most commonly found in waxes
Complex Or Compound Lipids
These are esters of fatty acids with alcohols containing additional groups such as phosphate, nitrogenous base, carbohydrate and protein.
They are further divided as
Phospholipids
These are lipids containing fatty acid, alcohol, phosphoric acid and nitrogenous base.
They are of the following types:
- Glycero-phospholipids –
They contain glycerol as the alcohol e.g.- Phosphatidyl choline (lecithin)
- Phosphatidyl ethanolamine (cephalin)
- Sphingo-phospholipid
They contain sphingol as the alcohol. e.g. Sphingomyelin - Phospho – inositide
They contain inositol as the alcohol. e.g. Phosphatidyl inositol
Glycolipids
These are lipids containing fatty acid, sphingol, nitrogenous base and carbohydrate e.g.
- Cerebroside
- Ganglioside
Lipoprotein
They are a combination of lipids and proteins e.g.
Chylomicron
- VLDL
- LDL
- HDL
Question 2. Functions of lipids.
Answer.
Functions Of Lipids
- Lipids are constituents of membrane structure
- Lipids regulate membrane permeability
- Lipids are a source of fat soluble vitamins (A, D, E and K)
- Lipids take part in regulation of cell metabolism
- Lipids are an important source of energy (9.5 C/gm)
- Lipids can be stored in the body in unlimited amounts
- Lipids exert an insulating effect in the body
- Lipids around internal organs like kidney may provide padding and protect the organ
- Lipids are essential for proper functioning of the nervous system
- Essential fatty acids (PUFA’s) are required to be taken in the diet for normal health and growth
Question 3.Digestion and absorption of lipids.
Answer.
Digestion of lipids:
Digestion In The Mouth
- The enzyme lingual lipase is secreted from the dorsal surface of the tongue (Ebners gland).
- It acts at a pH of 4 to 4.5.
- It breaks down TG to fatty acid and glycerol
Digestion In The Stomach
- The enzyme gastric lipase acts at a pH of 7 to 8.
- It requires calcium ion for its activity.
- Activity of gastric lipase is seen when the intestinal contents are regurgitated into the stomach.
- Fats delay the rate of emptying of the stomach and thus have high satiety value.
Digestion In The Small Intestine
- Pancreatic juice enters the small intestine through the pancreatic duct and bile enters the small intestine through the bile duct.
- Secretion of pancreatic juice is stimulated by the hormones secretin and Cholecystokinin – Pancreozymin (CCK-PZ)
- Bile salts help in emulsification of fats (breakdown of fats into smaller units).
- Pancreatic lipase acts at a pH of 6 and breaks down triglycerides into fatty acids and glycerol.
- The enzyme cholesterol esterase breaks down cholesterol esters and phospholipase breaks down phospholipids.
Absorption Of Lipids:
- The resynthesized triglycerides in the intestinal epithelial cells cannot pass to lymphatics or to the portal blood as they are insoluble in water.
- Triglycerides get covered with a layer of phospholipid, cholesterol and apoprotein to from the lipoprotein chylomicron.
- Chylomicrons are soluble; they pass out through the intestinal epithelial cell and enter the blood and lymphatics.
Question 4. Classify lipoproteins with their functions.
Answer.
Lipoproteins Definition
- Lipoproteins are molecular complexes that consist of lipids and proteins.
- They are conjugated proteins and they function as transport vehicles for lipids in the blood.
Structure Of Lipoproteins
- A lipoprotein consists of a neutral lipid core (triglyceride and cholesterol ester), surrounded by a shell of phospholipid, apoprotein and cholesterol.
- The phospholipid and cholesterol are exposed on the surface of the lipoprotein so that the lipoprotein is soluble in aqueous solution
Classification Of Lipoproteins
Lipoproteins can be classified by two methods:
- Ultracentrifugation
- Electrophoresis
Classification By Ultracentrifugation
By ultracentrifugation lipoproteins are classified as:
- Chylomicrons – density lowest – floats
- VLDL
- LDL
- HDL – density highest, sinks to the bottom (settles below)
Functions Of Lipoproteins
Chylomicrons
- Chylomicrons are synthesised in the intestinal mucosal cells
- They transport the exogenous triglycerides (triglycerides taken in the diet) from the intestine to the liver.
VLDL
- VLDL are synthesised in the hepatic cells
- VLDL transport the endogenous triglyceride (triglyceride synthesised in the body) from the liver to the peripheral tissues
IDL
- VLDL is acted upon by the enzyme lipoprotein lipase which removes the triglycerides, to from IDL (intermediate density lipoprotein).
- IDL loses some more triglyceride to form LDL
- LDL is formed by the degradation of VLDL
- LDL is very rich in cholesterol and cholesterol ester and poor in triglyceride
- LDL transports cholesterol from the liver to the peripheral tissues
- This is known as forward transport of cholesterol
- Excessive deposition of cholesterol by LDL in the blood vessels can cause atherosclerosis; hence LDL is called BAD cholesterol
HDL
- HDL is synthesised in the liver
- HDL transports cholesterol from the peripheral tissues to the liver, where the cholesterol is degraded
- This movement of cholesterol by HDL is called reverse transport of cholesterol
- Since HDL helps in reducing blood cholesterol level, it is called GOOD cholesterol
Question 5. Functions Of Cholesterol.
Answer.
Functions Of Cholesterol.
Cholesterol has four functions:
- Synthesis of bile acids
- Hormone synthesis
- Vitamin D synthesis
- Coprostanol synthesis
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Question 6. Add a note on atherosclerosis.
Answer.
Atherosclerosis Definition
Atherosclerosis is a complex disease characterised by thickening or hardening of the arteries due to accumulation of cholesterol in the inner arterial wall
Mechanism Of Atherosclerosis
- Increased levels of cholesterol for prolonged periods will cause deposition of cholesterol in the subintimal region of the arteries
- Coronary vessels and cerebral vessels are predominantly affected by this atherosclerotic process
- LDL deposits the cholesterol in the macrophages, which become overloaded with cholesterol esters and are called “foam cells” and this leads to the formation of atherosclerotic plaques
- The formation of atherosclerotic plaque leads to narrowing of the vessel wall
- The blood flow through the narrow vessel lumen is more turbulent and there is tendency for clot formation
- Finally a clot is formed, which occludes one of the major vessels
- Thrombosis leads to ischaemia of the tissue supplied, due to decreased oxygen flow
- Finally infarction or ischaemic death of the tissue occurs
Disorders That May Cause Atherosclerosis
Certain diseases are associated with atherosclerosis, such as:
- Diabetes mellitus
- Hyper – lipoproteinemia
- Nephrotic syndrome
- Hypothyroidism
Causes Of Atherosclerosis
Factors which may cause atherosclerosis are:
- Obesity
- High consumption of saturated fat
- Excessive smoking
- Lack of physical exercise
- Stress
Question 7. Describe beta oxidation of palmitic acid. Add a note on its energetics.
Answer.
Beta Oxidation Definition
- In beta oxidation the fatty acids are oxidised by the removal of 2 carbon atoms at a time.
- The carbon atom in the beta position is attacked first; hence it is called beta oxidation
Tissues In Which Beta Oxidation Is Carried Out
- Beta oxidation takes place in the liver, heart, kidney, testes and adipose tissue.
- In cardiac muscle, 80 % of the energy is derived from beta oxidation
Site Of Beta Oxidation In The Cell
Beta oxidation takes place in the mitochondria
Activation Of Fatty Acids
- Fatty acids are activated to acyl CoA by the enzyme thiokinase.
- Two ATP’s are required for the activation of fatty acids.
Role Of Carnitine
- Acyl CoA is formed in the cytosol whereas beta oxidation takes place in the mitochondrial matrix.
- Acyl CoA cannot penetrate the mitochondrial membrane on its own.
- Acyl CoA can penetrate the mitochondrial membrane only in combination with carnitine.
- Carnitine is present in high concentration in the liver and muscles.
- The normal blood level of carnitine is 7 to 14 micrograms %.
- Carnitine is synthesised in the liver and kidneys from lysine and methionine.
- Carnitine acts as a carrier molecule or ferry boat and transports acyl CoA across the mitochondrial membrane.
- The enzymes required are carnitine palmitoyl transferase 1 and 2.
Role Of Carnitine
- Acyl CoA is formed in the cytosol whereas beta oxidation takes place in the mitochondrial matrix.
- Acyl CoA cannot penetrate the mitochondrial membrane on its own.
- Acyl CoA can penetrate the mitochondrial membrane only in combination with carnitine.
- Carnitine is present in high concentration in the liver and muscles.
- The normal blood level of carnitine is 7 to 14 micrograms %.
- Carnitine is synthesised in the liver and kidneys from lysine and methionine.
- Carnitine acts as a carrier molecule or ferry boat and transports acyl CoA across the mitochondrial membrane.
- The enzymes required are carnitine palmitoyl transferase 1 and 2.
Reactions Of Beta Oxidation
- The end products of beta oxidation are acyl CoA and acetyl CoA.
- The acyl CoA enters beta oxidation cycle again and the acetyl CoA enters the Krebs cycle where it oxidised to carbon dioxide and water.
Beta Oxidation
Palmitic Acid Is C15H31COOH.
- Hence for complete beta oxidation it has to undergo 7 cycles producing 7 acetyl CoA.
- One acetyl CoA is produced extra in the last cycle.
- Hence the total acetyl CoA produced by beta oxidation of palmitic acid is 8.
Energetics Of Beta Oxidation
- 4 ATP’s are generated in each cycle of beta oxidation – 2.5 from NADH and 1.5 from FADH2.
- Hence 7 cycles will generate 28 ATP’s.
- 8 acetyl CoA when oxidised in the Krebs cycle will generate 80 ATP’s.
- Hence 108 ATP’s (28 + 80) are generated.
- 2 ATP’s are required for activation of fatty acids.
- Hence net generation of ATP’s is 106.
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