Carbohydrate
The molecule of D-galactose bonds with a molecule of D-glucose to form lactose, a disaccharide found in animal milk. This specific linkage is known as beta-1-4 glycosidic linkage. For the simplest carbohydrates, the carbon-to-hydrogen-to-oxygen atomic ratio remains constant at 1:2:1. Scientists often represent these compounds using the empirical formula CH2O. Monosaccharides are the simplest carbohydrates because they cannot be hydrolyzed into smaller carbohydrate units. These molecules usually follow the general formula Cm(H2O)n. Disaccharides like sucrose appear frequently in nature alongside polysaccharides such as starch and cellulose. Saccharides function as polyhydroxy aldehydes or ketones containing acetal type linkages. Many carbohydrates feature OH groups appended to or replaced by N-acetyl groups. Sulfate modifications create substances like glycosaminoglycans. Carboxylic acid and deoxy modifications produce unique sugars like fucose and sialic acid.
History of sugar cane cultivation began in New Guinea where it was first grown. Mass cultivation techniques developed later in India for isolating crystalline sugar. Cane sugar reached Europe around the 13th Century before expanding to the New World. Constantin Kirchhoff discovered that grape sugar forms when starch is boiled with acid in 1811. The starch industry started operations the following year. Henri Braconnot found that sugar forms through sulfuric acid action on cellulose in 1819. William Prout gave this group of substances the name saccharine after chemical analyses. German chemist Carl Schmidt proposed the term carbohydrate in 1844. French physiologist Claude Bernard discovered glycogen, a form of carbohydrate storage in animal livers, in 1856. Emil Fischer received the 1902 Nobel Prize in Chemistry for work on sugars and purines. Otto Meyerhof won the 1922 Nobel Prize in Physiology or Medicine for discovering glucose metabolism. Hans von Euler-Chelpin and Arthur Harden shared the 1929 Nobel Prize in Chemistry for research on sugar fermentation. Bernardo Houssay and the Coris received the 1947 Nobel Prize in Physiology or Medicine for discoveries regarding pituitary glands and glycogen conversion. Luis Leloir earned the 1970 Nobel Prize in Chemistry for finding sugar nucleotides. Raymond Dwek coined the term glycobiology in 1988.
Plants synthesize carbohydrates from carbon dioxide and water using photosynthesis to store energy internally. This stored energy often appears as starch or lipids within plant components. Animals and fungi consume these plant components to use as fuel for cellular respiration. Oxidation of one gram of carbohydrate yields approximately 16 kilojoules of energy. The oxidation of one gram of lipids produces about 38 kilojoules instead. The human body stores between 300 and 500 grams of carbohydrates depending on total body weight. Skeletal muscle contributes to a large portion of this storage capacity. Energy obtained from metabolism usually gets stored temporarily within cells as ATP. Organisms capable of anaerobic and aerobic respiration metabolize glucose and oxygen together. Carbon dioxide and water emerge as byproducts during this process. Glucose serves as a nearly universal and accessible source of energy for most organisms. Many organisms can metabolize other monosaccharides but prioritize glucose usage first. In Escherichia coli, the lac operon expresses enzymes for lactose digestion when present. If both lactose and glucose exist, the system represses lactose digestion to use glucose first.
Carbohydrate consumed in food yields 3.87 kilocalories of energy per gram for simple sugars. Complex carbohydrates provide 3.57 to 4.12 kilocalories per gram in most other foods. Relatively high levels of carbohydrate appear in processed foods or refined plant products like sweets. Table sugar, honey, soft drinks, breads, crackers, jams, and fruit products contain significant amounts. Refined carbohydrates such as white bread or rice are readily digestible with high glycemic indices. High-GI foods cause an abrupt increase in blood glucose concentration that declines rapidly after meals. Low-GI foods produce lower blood glucose concentrations returning gradually following consumption. The Institute of Medicine recommends American and Canadian adults get between 45 and 65 percent of dietary energy from whole-grain carbohydrates. Food and Agriculture Organization and World Health Organization jointly recommend national guidelines set a goal of 55, 75 percent total energy from carbohydrates. They suggest only 10 percent directly from sugars. A 2017 Cochrane Systematic Review concluded insufficient evidence supports claims that whole grain diets affect cardiovascular disease. Carbohydrates form one of the main components of insoluble dietary fiber found in nature.
Many techniques analyze glycans including NMR spectroscopy despite spectral overlap challenges. High-resolution mass spectrometry and high-performance liquid chromatography apply to glycan cleavage processes. Glycolipids can be analyzed directly without separating lipid components. N-glycans from glycoproteins undergo routine analysis via reversed phase HPLC after fluorescent tagging. Labels like 2-aminobenzamide, anthranilic acid, and 2-aminopyridin appear frequently in recent years. O-glycans usually analyze without any tags attached. Fractionated glycans from HPLC instruments further analyze by MALDI-TOF-MS for structure information. Multiple reaction monitoring performs on triple quadrupole instruments detecting predetermined precursor ions. Carbohydrate synthesis generates natural and unnatural carbohydrate structures as a sub-field of organic chemistry. Selective formation of glycosidic linkages remains very important for chemical manipulation. Usage of protecting groups extends extensively throughout these reactions. Amadori rearrangement and Koenigs, Knorr reaction represent main organic reactions involving carbohydrates. Ferrier rearrangement and Lobry de Bruyn, Van Ekenstein transformation also feature prominently. Cyanohydrin reaction and Wohl degradation contribute to the broader field of carbohydrate chemistry.
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Common questions
What is the chemical formula for simple carbohydrates?
Simple carbohydrates follow the empirical formula CH2O with a constant carbon-to-hydrogen-to-oxygen atomic ratio of 1:2:1. These molecules usually adhere to the general formula Cm(H2O)n as monosaccharides cannot be hydrolyzed into smaller carbohydrate units.
When did Carl Schmidt propose the term carbohydrate?
German chemist Carl Schmidt proposed the term carbohydrate in 1844 after William Prout named the group saccharine following earlier chemical analyses. This terminology emerged decades before Henri Braconnot discovered sugar formation through sulfuric acid action on cellulose in 1819.
How much energy does one gram of carbohydrate yield during oxidation?
The oxidation of one gram of carbohydrate yields approximately 16 kilojoules of energy while producing carbon dioxide and water as byproducts. Carbohydrate consumed in food provides 3.87 kilocalories of energy per gram for simple sugars and between 3.57 to 4.12 kilocalories per gram for complex carbohydrates.
Who won the Nobel Prize in Chemistry for work on sugars and purines?
Emil Fischer received the 1902 Nobel Prize in Chemistry for his research on sugars and purines. Other notable laureates include Otto Meyerhof who won the 1922 Nobel Prize in Physiology or Medicine for discovering glucose metabolism and Luis Leloir who earned the 1970 Nobel Prize in Chemistry for finding sugar nucleotides.
What percentage of dietary energy should adults consume from whole-grain carbohydrates?
The Institute of Medicine recommends American and Canadian adults get between 45 and 65 percent of dietary energy from whole-grain carbohydrates. The Food and Agriculture Organization and World Health Organization jointly recommend national guidelines set a goal of 55 to 75 percent total energy from carbohydrates with only 10 percent directly from sugars.