highest percentage of triacylglycerol by weight is Chylomicrons

041. The human plasma lipoprotein containing the highest percentage of triacylglycerol by weight is:

1. VLDL

2. Chylomicrons

3. HDL

4. LDL

Answer

2. Chylomicrons

Reference:

Harper 26^th Edition Page 206 Table 25.1

Lippincott 3^rd Edition Page 227

Chaterjee 6^th Edition Page 338 Table 21.2

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Discussion

Chylomicrons are large lipoprotein particles (having a diameter of 75 to 1,200nm) that are created by the absorptive cells of the small intestine. Chylomicrons transport lipids to adipose tissue where they are broken down by lipoprotein lipase.

Explanation

1. VLDL has 90 to 93 percent of lipids, mostly Triacyl Glycerol

2. Chylomicrons have 98 to 99 percent of lipids, mostly Triacyl Glycerol

3. HDL₁ has just 68 % of lipids

4. LDL has 79 percent of lipids, mostly cholesterol

Comments

The chylomicrons are released by exocytosis from enterocytes into lacteals, lymphatic vessels originating in the villi of the small intestine, and are then secreted into the bloodstream, at the place where the lymphatic system falls into the blood stream, at the thoracic duct's connection with the left subclavian vein.

Tips

The following table has been adapted from Harper

Lipoprotein		Source	Diameter (nm)	Density (g/ml)	Composition		Main Lipid Components	Apolipoproteins
					Protein (%)	Lipids(%)
Chylomicrons		Intestine	90-1000	<0.95	1-2	98-99	Tricacylglycerol	A-I, A-II, A-IV, B-48, C-I, C-II, C-III, E
Chylomicron remnant		Chylomicrons	45-150	<1.006	6-8	92-94	Tricacylglycerol, Phospholipids, Cholesterol	B-48, E
VLDL		Liver (intestine)	30-90	0.95 – 1.006	7-10	90-93	Tricacylglycerol	B-100, C-I, C-II, C-III
IDL		VLDL	25-35	1.006 – 1.019	11	89	Tricacylglycerol, Cholesterol	B-100, E
LDL		VLDL	20-25	1.019 – 1.063	21	79	Cholesterol	B-100
HDL	HDL1	Liver, Intestine, VLDL, Chylomicrons	20-25	1.019 – 1.063	32	68	Phospholipids, Cholesterol	A-I, A-II, A-IV, C-I, C-II, C-III, D, E
	HDL2		10-20	1.063-1.125	33	67
	HDL3		5-10	1.125-1.210	57	43
	Preb HDL		<5	>1.210				A-I
Albumin/free faty acids				>1.281	99	1	Free fatty acids
HDL : High Density lipoproteins IDL : Intermediate Density lipoproteins LDL : Low Density lipoproteins VLDL : Very Low Density lipoproteins

fuel oxidation serves not to produce ATP but to generate heat is Brown adipose tissue

040. The specialized mammalian tissue/organ in which fuel oxidation serves not to produce ATP but to generate heat is:

1. Adrenal gland

2. Skeletal muscle

3. Brown adipose tissue

4. Heart

Answer

3. Brown adipose tissue

Reference:

Harper 26^th Edition Page 217

Chaterjee 6^th Edition Page 342

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Thermogenesis is the process by which the body generates heat, or energy, by increasing the metabolic rate above normal. This rise in metabolic rate is referred to as the thermogenic effect, thermogenic response, or specific dynamic action (SDA). Thermogenesis is activated by a few different mechanisms, including supplements, nutrition, exercise, and exposure to cold.

Explanation

Thermogenesis is the process of heat production. Non-shivering thermogenesis usually occurs in brown adipose tissue (brown fat) that is present in newborn and hibernating mammals and in human infants.

Comments

Features of Brown adipose tissue (BAT)

1. a well developed blood supply and

2. a high content of mitochondria and

3. a high content of cytochromes

4. low activity of ATP Synthase.

5. rich in carnitine

6. has the enzyme glycerokinae

Thus the energy obtained by oxidation is not “captured” by ATP, but is dissipated as heat.

Tips

The brown colour of the fat is due to the high content of cytochrome

The amino acid which serves as a carrier of ammonia from skeletal muscle to liver is Alanine

038. The amino acid which serves as a carrier of ammonia from skeletal muscle to liver is:

1. Alanine

2. Methionine

3. Arginine

4. Glutamine

Answer

1. Alanine

Reference:

Lippincott 3^rd Edition Page 248

Chaterjee 6^th Edition Page 416

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Two mechanisms are available in humans for the transport of ammonia from the peripheral tissues to the liver for its ultimate conversion to urea.

Ä The first found in most tissues, uses glutamine synthetase to combine ammonia with glutamate to form glutamine – a non toxic transport form of ammonia. This glutamine is transported in the blood to liver where it is cleaved by glutaminase to produce free ammonia and glutamate.

Ä The second transport mechanism, used primarily by muscle, involves transamination of pyruvate (the end product of aerobic glycolysis) to form alanine. Alanine is transported by the blood to the liver, where it is converted to pyruvate, again by transamination. In the liver, the pathway of gluconeogenesis can use the pyruvate to synthesize glucose, which can enter the blood and be used by muscle – a pathway called the Glucose-Alanine Cycle

Explanation

1. Alanine serves as a carrier of ammonia from skeletal muscle to liver

2. Methionine (Met, M. C5H11NO2S) is an essential nonpolar amino acid, and a lipotropic.

3. Arginine is a semiessential basic amino acid, required in urea formation and creatinine synthesis

4. Glutamine serves as a carrier of ammonia from peripheral tissues

Comments

Methionine and cysteine are the only sulfur-containing proteinogenic amino acids. The methionine derivative S-adenosyl methionine (SAM) serves as a methyl donor. Methionine plays a role in cysteine, carnitine and taurine synthesis by the transsulfuration pathway, lecithin production, the synthesis of phosphatidylcholine and other phospholipids. Improper conversion of methionine can lead to atherosclerosis. Methionine is a chelating agent.

Tips

Methionine is one of only two amino acids encoded by a single codon (AUG) in the standard genetic code (tryptophan, encoded by UGG, is the other).

amino acids are converted to succinyl – CoA, except Histidine

039. All of the following amino acids are converted to succinyl – CoA, except:

1. Methionine

2. Isoleucine

3. Valine

4. Histidine

Answer

4. Histidine

Reference:

Harper 26^th Edition Pages 249, 262

Lippincott 3^rd Edition Page 261 to 263

Chaterjee 6^th Edition Page 457

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Amino acids that forms Succinyl CoA are

1. Methionine through Propionyl CoA

2. Valine through L-Methyl Malonyl CoA

3. Isoleucine through Propionyl CoA

4. Threonine through Propionyl CoA (Threonine undergoes non oxidative deamination to become Propionyl CoA)

Propionyl CoA is converted to Succinyl CoA. Other sources of Propionyl CoA are

1. Beta oxidation of odd chain fatty acids

2. Oxidative decarboxylation of alpha keto glutarate

3. Activation of Succinic Acid

Explanation

Histidine metabolism yield

1. Urocanic acid

2. 4-imidazolone-5-propionate

3. Formiminoglutamic acid (Fliglu)

4. Glutamate

Comments

Histidine also forms

1. Ergothioneine

2. Carnosine

3. Anserine

Tips

Carnosine is a dipeptide of histidine with beta-alanine (b-alanyl histidine). It is present in skeletal muscle

The ligand-receptor complex dissociates in the endosome because Of the acidic pH of the vesicle

037. The ligand-receptor complex dissociates in the endosome because:

1. Of its large size

2. The vesicle looses its clathrin coat

3. Of the acidic pH of the vesicle

4. Of the basic pH of the vesicle

Answer

3. Of the acidic pH of the vesicle

Reference:

Lippincott 3^rd Edition Page 230

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Discussion and Explanation

The pH of the endosome falls due to the protein pumping activity of endosomal ATPase and this allows separation of the ligand from the receptor. Late endosomes are formed as the pH continues to drop due to the activity of endosomal ATPase

Comments and Tips

The diagram below show the Fate of an LDL particle and its receptor after endocytosis. The same pathway is followed by other ligands, such as insulin and other protein hormones, that are internalized by receptor-mediated endocytosis and degraded in the lysosome. After an LDL particle binds to an LDL receptor on the plasma membrane, the receptor-ligand complex is internalized in a clathrin-coated pit that pinches off to become a coated vesicle. The clathrin coat then depolymerizes to triskelions, resulting in an early endosome. This endosome fuses with a sorting vesicle, known as a late endosome, where the low pH (approximately 5) causes the LDL particles to dissociate from the LDL receptors. A receptor-rich region buds off to form a separate vesicle that recycles the LDL receptors back to the plasma membrane. A vesicle containing an LDL particle may fuse with another late endosome but ultimately fuses with a lysosome to form a larger lysosome. There, the apo-B protein of the LDL particle is degraded to amino acids and the cholesterol esters are hydrolyzed to fatty acids and cholesterol. Abundant imported cholesterol inhibits synthesis by the cell of both cholesterol and LDL receptor protein.