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.