Fat metabolism, Biology


Animals  digest  and utilize three main  types  of fatty compounds or lipids  neutral or true  fats ( triglycerides ),  phospholipids  and sterols. Endproducts  of fat  digestion  are fatty  acids. Glycerol, free cholesterol,  phosphate radicals  and

Monoglycerides, The absorptive  cells of intestinal  mucosa  resynthesize  fats, phospholipids  and sterols from  most of these digestion  products  after  absorbing  these  from  the intestine . Except  for  some  water soluble  lipids  and fatty  acids,  which  are absorbed into  the blood ,all  these fatty products of absorptive cells  aggregate in    minute droplets,  called  chylomicrons.  Some  protein molecules  become   adsorbed at  surfaces  of chylomicrons. Then  the latter  are absorbed into  lacteals of villi. From  the lacteals ,  these  are transported up the  thoracic  duct and emptied into venous  blood  at juncture of  jugular and subclavian veins. However  all chylomicrons  are  removed  from the blood  within  an hour or two  by adipose tissues  and liver, because  normally, facts  circulate in  the blood  for transportation  to different  tissues as smaller globules called  lipoproteins, containing mixtures  of triglycerides,  phospholipids ,  cholesterol and proteins,  Lipoproteins are formed almost  entirely in the liver. Their  normal concentration  in blood   is about 700mg  per 100ml  of blood i,e, 0.7%.

Cellular  metabolism  of  lipids   in the body  has five aspects  fat  storage,  breakdown  for energy  synthesis  of phospholipids  and sterols,  synthesis  of fats  from  carbohydrates  and synthesis   of fats from  proteins.

1.      Fat Storage : After a fat rich   meal lipid concentration  in blood may rise upto 2%  due to chylomicrons, The  plasma may become  turbid  for this reason.  However  within  an hour  or after  absorption  most chylomicrons are removed  from blood    by fat cells  of adipose tissues  for  storage. Adipose tissues are  virtually the  fat depots  of body   since this  storage serves  to provide  energy  elsewhere in the body  it is a  food reseve . A subsidiary functional  of adipose tissues  is to  provide  heat  insulation  and protection. Remaining  chylomicrons are removed  from  the blood  by liver  cells  which  store  some  fat and utilize  the remainder for  forming  lipoproteins  for circulation  and for  various other  purposes.

2.      Breakdown for energy: Glucose is though, the  main fuel   substance  almost  all body  cells  except  nerve  cells and perhaps  some other tissue  cells normally  derive about 30%  to 40%  of their  energy requirements  from  fats. It must be  noted that about 30% to 50%  of carbohydrates,  digested from  food are also  converted  to  fats, stored and later  utilized  for energy   production  in between  the meals.

All  adipose tissues are highly  active sites of fat metabolism in the body.    Their  old fat is continuously hydrolysed into fatty  acids and glycerol, while  new fat is continuously synthesized to replace the  lost fat, Thus  the fat of these tissues is reneqed every two to three weeks. The  breakdown products fatty acids and glycerol  are both released into the blood but, for  synthesis of fats adipose tissues  take only  fatty acids from the blood  and synthesize the required glycerol by  themselves, The fatty  acids  circulate in bold in combination with plasma albumins and are called free fatty  acids FFA , In starvation and diabetes, fat hydrolysis and blood concentration of FFA  may  increase manifold.  

Most body   cells obtain  some glycerol  and fatty acids  from  blood  for energy production,  The glycerol is immediately changed into  glycerol 3  phosphate which  enters into embden  Meyerhof pathway  for  oxidation   The  fatty acids are degraded and oxidized for  energy only  in the mitochondria . In this process a fatty  acids  molecule is  sequentially broken down  into 2  acetyl units ,each  linked  to  coenzyme   A , forming  a molecules  of acetyl coenzyme  A. Acetyl  CoA  molecules,  then enter into  krebs cycle  for oxidations.

By  weight each  gram  of fat  yields  about 9.3  kcal. Of plysioligically  available  bioenergy i.,e.  More  than double of that  yielded by glucose . 

Ketosis: The  initial degradation   and splitting  of fatty acids  intoacetyl  CoA .is  most  active  in liver  cells. All  acetyl  CoA, thus  formed  cannot  be  utilized by liver  cells  themselves. Hence  surplus acetyl  CoA, molecules  condense  in pair to form  acetoacetic  acid (keto acid), acetone and beta  hydroxybutyric  acids collectively  called  ketone bodies. The  latter are released into  blood  and are  taken by  other cells  which  degrade these   back  to  acetyl CoA for  energy production .

In  starvation  diabetes and repeated fat rich  diet, when  glucose becomes deficient in blood ,or   cells are  unable to use  it, almost  all energy  must comes from  fat breakdown ,Hence  ketone  bodies are  formed  inliver  and released into the blood at  a rapid rate.Now  since cells  have a limited capacity  of  utilizing   ketone bodies due  to several  reasons ,the  concentration  of these  bodies  tremendously (up to 10 times  increases in blood .   leading  to extreme acidosis .This  condition  is called  ketosis  causes hard  breathing , depression  of nervous  system, poisoning , unconsciousness,  coma  and , ultimately  death, Being  a sweet smelling , volatile substance , this acetone can  be detected in the breath  of a person  suffering  from  ketosis.

3.      Synthesis of fats from carbohydrates: When  glucose level in blood considerably rises due  tjocarbohydrate rich diet, and  glucose  storage as glycogen  in liver  and muscle cells is  saturated, the  surplus  glucose is rapidly converted to fat   ( triglycerides) and stored in adipose tissues, This  conversion also occur  in the  liver and  it is necessary  to maintain  the normal  blood  sugar level. Glucose  in this is first  degraded to acetyl  CoA by  glycolysis .Long chain  fatty acids  are, then synthesized  by polymerization  of acetyl  groups of acetyl  CoA molecules  or active acid  derived by  molecules  of acetyl CaA in pairs.  The  glycerol portion    of fats in also  derived from  glucose  degradation.

4.      Synthesis  of fats  from proteins:  Endproucts of protein  digestion circulate  in blood   as amino  acids  which  are taken by cells  according  to their  requirement  almost  all amino  acids  can  be converted  to fat for storage when       due to repeated  protein rich  diet, The  amino  acids  concentration  in blood   increases, This  conversion also  occurs  in the liver,  The amino acids  for this  are first deaminated in liver cells  i.e.  their  carboxyl  and amino  group  are separted . The  carboxyl group  are degraded  to acetyl CoA from which  fatty   acids  are formed by polymerization  directly  or by   way of acetoacetic   acids  as in the  case of glucose. Conversion  of amino acids  into  fatty  acids  or keto acid  ( acetoacetic  acid ) is called  ketogenesis .

5.      Synthesis  of phospholipids  and cholesterol: Phospholipids  are fatty acids  conjugated with  phosphoric  acid,  and often  also with nitrogenous  bases . Cholesterol  belong  to a class  of  fatty  derivatives characterized by a  sterol ring  structure synthesized from acetyl CoA, large   amounts of  both  phospholipids  and  cholesterol  occur in plasma  membrane and membrane  organelles of the cells .These  compound  are known  to have  controlling  effects  upon  the permeability  of these  membrane  .Other  important  functions of phospholipids  are to participate  in synthesis thromboplastin  of blood  and myelin of nerve fibres  nad to donate phosphate   radical  inseveral metabolic reactions,  Similarly other   important  function  of cholesterol  are  to participate  in synthesis   of bile  salts, adrenocortical  hormones, sex hormones and  vitamin D, and to render the skin  resistant to water, acids  and  other  injurious  chemicals .

Both  phospholipids  and cholesterol  are digested from food  as well  as, synthesized  in all   body  cells. But more  than 90%  of these are synthesized in the  liver from  fats and fatty acids.

Posted Date: 10/4/2012 2:41:07 AM | Location : United States

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