L 17 - Carbohydrates PDF

17 Chemical Blocks of Life: CHO: ILOs By the end of this lecture, students will be able to 1. Describe how carbohydrates help us function and survive 2. Describe the building blocks of carbohydrates 3. Discuss carbohydrates’ role in health and disease Carbohydrates are generall...

17 Chemical Blocks of Life: CHO: ILOs By the end of this lecture, students will be able to 1. Describe how carbohydrates help us function and survive 2. Describe the building blocks of carbohydrates 3. Discuss carbohydrates’ role in health and disease Carbohydrates are generally hydrates of carbon containing hydrogen and oxygen in a ratio of 2:1, with general molecular formula (CH2O)n, where n > 3. ❖ Classification There are three major classes of carbohydrates - Monosaccharides (mono = one) - Oligosaccharides (oligo= few) 2-10 monosaccharide units. - Polysaccharides (Poly = many) >10 monosaccharide units. Monosaccharides: (Simple sugars) - Depending on the number of carbon atoms, the monosaccharides are named trioses (C3),tetroses (C4), pentoses (C5), hexoses (C6), heptoses (C7).The most abundant monosaccharides in nature are the 6- carbon sugars like glucose and fructose. - They either contain an aldehyde group and are called aldoses, or a keto group and are called ketoses. - The carbon atom that is double bonded to an oxygen atom forms a carbonyl group. Physical properties of Monosaccharides: - Asymmetric carbon atom is a carbon atom attached to 4 different groups or atoms. It gives the ability to show isomerism - Isomers are compounds that have the same chemical formula but have different structures as fructose, glucose and galactose that have the same chemical formula, C6H12O6. Oligosaccharides: The most abundant are disaccharides as: - Maltose: o It is the Malt sugar. Maltose is the major degradative product of Starch. It is hydrolyzed to two molecules of glucose - Lactose: o Lactose is composed of glucose and galactose subunits. It is found exclusively in milk of mammals (Milk sugar). Page 1 of 3 o It is the least sweet sugar. Although glucose can be obtained from several food types, Lactose is the only source of galactose which plays an integral role in various neural and immunological processes including the ABO blood types. o Moreover it was found that Lactose plays a role in absorption of calcium, copper and zinc especially in infancy. o Some patients suffer from “Lactose Intolerance”, they are unable to fully digest the lactose in milk. As a result, they have diarrhea, gas and bloating after eating or drinking dairy products. There are many Lactose free dairy products available now in the market for those patients. - Sucrose (Cane sugar): o Sucrose is composed of glucose and fructose subunits. It is obtained from cane sugar. It is also present in various fruits. o The fructose component gives it a very sweet taste. Therefore, sucrose derivatives are sometimes used by diabetics and obese patients as an alternative to glucose giving them sweet taste with less concentration aiming to lower their insulin and blood glucose levels. However, it was found that high levels of fructose increase hunger and sugar craves. Polysaccharides: Most of the carbohydrates found in nature occur in the form of high molecular polymers called polysaccharides. Important polysaccharides include: - Glycogen: Are the branched form of polysaccharides from animal sources and the main storage form in humans. - Starch: Are the plant storage form of CHO and the most abundant CHO in diet. Found in potatoes, rice and pasta. It consists of 2 main molecules Amylose and Amylopectin. Both consist of glucose molecules linked together while Amylopectin is the branched component in starch. - Dextrins: These are highly branched. Since they do not easily go out of vascular compartment they are used for intravenous infusion as plasma volume expander in the treatment of hypovolumic shock. - Unbranched cellulose: (Structural form of CHO in plants). Humans cannot digest cellulose. However, it is consumed in the diet as fiber. - Fiber helps the digestive system to keep the food moving through the gut and to avoid constipation. Examples of food containing cellulose include green leafy vegetables and some fruits as unpeeled apples. - Each of these polysaccharides is a polymer of glucose. ❖ Carbohydrate Derivatives: These are sugar molecules that have been modified with substituents other than hydroxyl groups. Most sugar derivatives occur naturally and have important biological functions. Examples include: - Deoxy-monsaccharides: In which The –OH is replaced –H as Deoxyribose sugar the DNA sugar - Amino Sugar: -OH is replaced by –NH2, the sugar is then designated as –osamaine as glucosamine in proteoglycans (see in Structure & Chemistry of C.T. Ground Substance) - Sugar Acids: In which the –CH2OH is oxidized to –COOH as Glucuronic acid which is synthesized in the liver and plays a major role in detoxification and elimination of toxic compounds. Page 2 of 3 - Immature babies have low levels of glucuronic acid this makes them more prone to jaundice due to accumulation of bilirubin without being detoxified by glucuronic acid in severe cases “kernicterus” this may lead to affection of the brain leading to mental retardation and vision problems. - Patients with liver failure are very prone to drug toxicity due to decrease in glucuronic acid synthesis and inability to detoxify the drug. - Sugar Alcohols: In which COOH is reduced to CH2OH. Examples include mannitol used as diuretic to help the body produce more urine in case of edema And sorbitol used to relieve constipation and as an artificial sweetner for patients following low caloric diets as it contains nearly 50% less calories than normal sugars and have the ability to highly stimulate the taste buds giving the same taste as sugar. Sorbitol is normally present in some fruits as Cherries giving them a sweet taste with low calories. However excess use of sorbitol leads to bloating and digestion problems. ❖ What is the importance of Carbohydrates? Carbohydrates have a wide range of functions, including providing a significant fraction of the dietary calories for most organisms, supplying and acting as a storage form of energy in the body, CHO also serve as cell membrane components that mediate some forms of intercellular communication. Although a common source of energy, where 1 gram of dietary CHO provides 4 Cal. However, carbohydrates are not essential nutrients, because humans are able to obtain 100% of their daily energy requirement from proteins and fats. However, they are more readily converted than proteins or fats into the key energy monosaccharide: glucose. When digested, all carbohydrates are broken down to glucose, which is then transported in the blood to cells for energy production. This helps CHO to induce early but short term satiety when compared to proteins and fats. ❖ Do Carbohydrates play a role in disease? Carbohydrates may directly influence human diseases by affecting physiological and metabolic processes, acting as risk factors for many diseases. For Example: Abnormal metabolism (usage by the body) of glucose is related to many serious diseases with Diabetes Mellitus coming on top also certain types of cancers might be related. Inborn errors in CHO metabolism can cause wide range of abnormalities including mental retardation and hepatomegaly (Increase in the liver size). Some GIT (digestive system) diseases and disturbances are related to abnormal digestion or breakdown of CHO as vomiting, bloating and diarrhea. Ingestion of too much CHO (a very common dietary habit nowadays) is directly related to obesity and its dangerous health consequences as heart diseases. Page 3 of 3