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Compare and contrast the digestion and absorption of carbohydrates and lipids.

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Compare and contrast the digestion and absorption of carbohydrates and lipids Digestion is the process that breaks down food to small molecules that can be absorbed by the body. Digestion involves the mixing of food, its movement through the digestive tract, and chemical breakdown of large molecules of food. The digestive or gastrointestinal tract comprises the mouth, oesophagus, stomach, small intestine and the large intestine. Digestion begins in the mouth, when we chew and swallow, and is then completed in the small intestine, where it is absorbed. Cells present in the inner layer of the stomach and the small intestine as well as the salivary glands, pancreas, and gall bladder secrete enzymes and other substances that help in the breakdown of food. The absorbed materials cross the mucosa into the blood, and are carried off in the bloodstream to other parts of the body for storage or further chemical change. This process varies with different types of nutrients. Digestion involves three fundamental processes: Secretion: secretion of digestive enzymes, mucus, ions into the lumen, and hormones into blood. Absorption: transport of water, ions and nutrients from the lumen, across the epithelium and into blood. Mobility: Contractions of smooth muscle in the wall of the tube that emulsify, mix and propel its contents. Food is used for two purposes, firstly, for fuel, to provide energy for cellular metabolism and secondly, as a source of building blocks, monomers for cellular biosynthesis. ...read more.


Acetyl-CoA is oxidised to form carbon dioxide, in the citric acid cycle. The citric acid cycle releases energy, this energy is stored in the form of NADH and FADH2, this is later converted to form ATP by a process known as oxidative phosphorylation. Water is also produced, by the citric acid cycle; electrons that are generated by the oxidation of acetyl-CoA and from NADH and FADH2 are transferred to oxygen, to produce water. In anaerobic conditions, pyruvate is converted to various products; this process is known as anaerobic fermentation or anaerobic glycolysis. This process yields much less energy per glucose molecule in comparison to aerobic oxidation. In humans anaerobic glycolysis produces lactate. Other sugars, such as fructose, galactose and mannose are broken down identically to glucose breakdown. When glycogen reservoirs are depleted, due to fasting, in order to maintain glucose levels, the liver synthesises glucose from non-carbohydrate precursors, this process is know as gluconeogenesis. Non-carbohydrate precursors that can be converted to glucose include lactate and pyruvate, from glycolysis, intermediates from the citric acid cycle and amino acids, except for leucine and lysine. All non-carbohydrate precursors must be converted to oxaloacetate, the starting material for gluconeogenesis. Fatty acids cannot serve as non-carbohydrate glucose precursors in humans and animals, because fatty acid breakdown leads to acetyl-CoA and there is no pathway in humans to convert acetyl-CoA to oxaloacetate. Several other biosynthetic pathways use intermediates from the citric acid cycle as starting materials, such as the biosynthesis of fatty acids and cholesterol starts with acetyl-CoA that is obtained by the breakdown of citrate, amino acid biosynthesis utilises either ?-ketoglutarate and oxaloacetate to synthesize glutamate and aspartate. ...read more.


Lipoprotein lipase cleaves the triglycerides to provide free fatty acid and glycerol, which are absorbed by the cells and used for energy generation or storage. The remainder of the chylomicrons, also called the chylomicron remnants, contain cholesterol; these remnants are taken up and processed by the liver. The liver secretes triglycerides and cholesterol into the blood, to peripheral tissues by packing them into transporters known as very-low-density lipoproteins. Again, the triglycerides are removed, from the VLDL particles in the capillaries of the peripheral tissues and converted to low-density proteins (LDLs), this transports cholesterol to various organs, include blood vessels. High-density lipoproteins (HDL) have a different role: they remove cholesterol molecules from the blood serum and either transfers them to VLDL and LDL or ship them back to the liver. The liver is the only organ that is capable of disposing of cholesterol by converting it to bile acids. While lipoproteins serve as useful and necessary transporters for lipid components, they are also involved in the formation of artherosclerosis and cardiovascular disease. Glycogen has several advantages as a short-term energy reservoir compared to fat. Muscles can mobilise the energy stored in the glucose units of glycogen much faster than they can mobilise the energy stored in fat. Glucose, in contrast to fatty acids, can be metabolised anaerobically and so provide a very fast pathway of energy generation. Additionally, humans and animals cannot convert fatty acids to glucose. Therefore, fat metabolism alone cannot adequately maintain blood glucose levels. Cynthia Kanagasundaram 1 ...read more.

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