Carbohydrates include sugars and polysaccharides; sugars are made up of one, monosaccharide, or two, disaccharide, carbohydrate units. Polysaccharides comprise long, and also branched chains of sugar units. Starch is the storage polysaccharide of plants, where as in animals it is glycogen. These polysaccharides, also disaccharides are broken down to their monomers, monosaccharides, during digestion and then are consequently absorbed by the body. There are also non-digestible carbohydrates this is dietary fibre. Fibre cannot be broken down to monosaccharides by the enzymes of the digestive tract, and so they are not absorbed by the body and therefore do not contribute to the energy and nutrient requirements of the body. However, fibre is important in maintaining the health and functioning of the digestive tract.
Most of the carbohydrates ingested in the body, is converted to energy. Glucose, from our diet in the form of sugar or carbohydrates, is the preferred fuel molecule for the brain. The liver monitors and maintains a constant level of glucose in the blood, and so that the brain is supplied with sufficient energy at all times. When there is excess glucose in the body, it is polymerised and stored in the form of the polysaccharide glycogen from glucose-6-phosphate, in the liver and muscle cells. Glycogen structure is similar to that of amylopectin, but glycogen is more highly branched, there are branch points at every 8 to 12 glucose residues. When the glucose level in the blood drops significantly, may be due to high energy demands in muscle cells, to protect the brain from a potential fuel shortage, glycogen is broken down to form glucose, in the liver; this glucose enters the blood circulation to restore glucose concentrations. Glycogen’s glucose units are mobilized by their sequential removal from the chain’s ends. The highly branched structure allows the rapid degradation of glycogen through the simultaneous release of the glucose units at the end of every branch.
Carbohydrates serve not only as fuel molecules but they and their breakdown products are also important precursors for biosynthetic pathways. The breakdown of glucose in the body, glucose oxidation, generates an important source of energy, in humans. A process called glycolysis breaks down, glucose (a six carbon atom molecule), in a complex series of reactions, where glucose molecules are split into two identical molecules with three carbon atoms called pyruvate.
Pyruvate is converted to acteyl-CoA, in aerobic conditions, and this then enters the citric acid cycle. 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 α-ketoglutarate and oxaloacetate to synthesize glutamate and aspartate.
Lipids (fats and oils) are a diverse group of substances made up mostly of nonpolar groups. As a result of their nonpolar character, lipids are very hydrophobic, this characteristic is very important in cells because lipids tend to associate into nonpolar groups and barriers, as in cell membranes. Besides having important roles in membranes, lipids are stored, mainly in adipose cells, and used in cells as an energy source, for when food supply is low. Lipids link covalently with carbohydrates to form glycolipids and with proteins to form lipoproteins. Triglycerides and phospholipids are made up of fatty acids; the breakdown of these compounds is a major source of energy for the body. Triglycerides, also called fats, accounts for roughly 90% of the long-term energy storage in humans. Phospholipids, cholesterol as well as other minor components of lipids such as glycolipids and sphingolipids are the constituents of cell membranes. Lipids also serve as precursors for hormones; steroid hormones are synthesized from cholesterol. Cholesterol is also the precursor of bile acids, biological detergents that aid in the digestion of lipids. Products such as meat, milk, dairy products and eggs are in general rich in fat. The major lipid component of dietary fat is triyglyceride, and a smaller portion consisting of fatty acids, phospholipids, glycolipids, and steroids: cholesterol.
Lipids have to be broken down in the digestive system, like carbohydrates, to be absorbed by the body. Very little lipid digestion occurs until the lipid globules in chyme arrive in the duodenum. The enzymes, lipases and phospholipases in the stomach and the small intestine, produce fatty acids, monoglycerides and choleseterol, by breaking down lipids, mostly triglycerides, phospholipids and cholesterol.
The presence of lipids in the duodenum, serves as a stimulus for the secretion of bile. The bile salt micelles are secreted into the duodenum, where they break up the fat droplets into tiny emulsification droplets of triglycerides, this process is called emulsification. By emulsifying the fat, the droplets are smaller and greater in number, this will help digestion, therefore a greater surface area, than when the fat first entered the duodenum. Pancreatic lipase (enzyme) coats the emulsification droplets, and then anchors the lipase enzyme to them. Lipase removes two of the three fatty acids from each triglyceride molecule, to release free fatty acids and monoglycerides. Phospholipids such as lecithin are digested by Phospholipase A, to release fatty acids and lysolecithin. Free fatty acids, monoglycerides and lysolecithin, which are more polar, than the undigested lipids, quickly become associated with micelles of bile salts, lecithin, and cholesterol to form ‘mixed micelles’. These micelles are then absorbed at the brush border of the intestine.
The free fatty acids, monoglycerides and lysolecithin, are used to resynthesis, triglycerides and phospholipids within the epithelial cells. This process is different when compared to the absorption of amino acids and monsaccharides, which pass through the epithelial cells without being altered. Once the triglycerides, phopholipids and cholesterol are resynthesised back, they are combined with protein, lipoprotein, inside the epithelial cells to form small particles called chylomicrons. These chylomicrons act as transporter molecules for lipid components in the fluids of the body. The chylomicron particles are secreted into the venous blood by way of the thoracic duct via the central lacteals, the lymphatic system, of the intestinal villi. Where as the amino acids and monosaccharides enter the hepatic portal vein.
While the chylomicrons are travelling through the blood vessels, their triglyceride content is removed by the enzyme lipoprotein lipase, which is attached to the endothelium of blood vessels. 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.
