Carbohydrate Structure and Metabolism, an Overview, Animation.
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(USMLE topics) Structure of monosaccharides, disaccharides and polysaccharides. Digestion of carbs. Glucose metabolic pathways. This video also answers common dietary questions such as: what is the difference between simple and complex carbohydrates? what is fiber? why we need fibers? why high-fructose corn syrup is bad for your health?…
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Voice by: Ashley Fleming
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Carbohydrates are biomolecules that consist of carbon, hydrogen and oxygen. Carbohydrates play crucial roles in living organisms.
Carbohydrates are made of base units called monosaccharides. Monosaccharides consist of a carbon chain with a hydroxyl group attached to all carbons except one, which is double-bonded to an oxygen. This carbonyl group can be in any position along the chain, forming either a ketone or an aldehyde.
Monosaccharides exist in open-chain form and closed-ring form. The ring forms can connect to each other to create dimers, oligomers and polymers, producing disaccharides, oligosaccharides and polysaccharides. Examples of disaccharides: sucrose, maltose, and lactose. Common polysaccharides include glycogen, starch and cellulose, all of which are polymers of glucose. Their differences arise from the bonds between monomers. Glycogen and starch: monomers are bonded by alpha-linkages. Some monomers can make more than one connection, producing branches. Starch in food can be digested by breaking alpha bonds, with the enzyme amylase.
Cellulose, the major structural component of plants, consists of unbranched chains of glucose bonded by beta-linkages, for which humans lack the enzyme to digest. Cellulose and other non-digestible carbohydrates in food do not supply energy, but are an important part of human diet, known as dietary fibers. Fibers help slow digestion, add bulk to stool to prevent constipation, reduce food intake, and may help lower risk of heart diseases.
Digestion of starch starts with amylase in the saliva and continues in the small intestine. Sucrose and lactose are hydrolyzed by intestinal enzymes sucrase and lactase. Simple sugars are then transported in the bloodstream to tissues.
Foods rich in simple sugars deliver glucose to the blood quickly, and can be helpful in case of hypoglycemia, but regular diets of simple sugars produce high spikes of glucose and may promote insulin insensitivity and diabetes. Complex carbohydrates take longer to digest and release simple sugars. Eating complex carbohydrates helps dampen the spikes of blood glucose and reduce diabetes risk.
Glucose is central to cellular energy production. Cells break down glucose when energy reserves are low. Glucose that is not immediately used is stored as glycogen in liver and muscles. Glycogen is converted back to glucose when glucose is in short supply.
Energy production from glucose starts with glycolysis, which breaks glucose into 2 molecules of pyruvate. Glycolysis involves multiple reactions and is tightly regulated by feedback mechanism.
In the absence of oxygen, such as in the muscles during exercise, pyruvate is converted into lactate. This anaerobic pathway produces no additional energy, but it regenerates NAD+ required for glycolysis to continue.
When oxygen is present – cellular (aerobic) respiration – pyruvate is degraded to form acetyl-CoA. Significant amounts of energy can be extracted from oxidation of acetyl-CoA to carbon dioxide, by the citric acid cycle and the following electron transport system. When present in excess, acetyl-CoA is converted into fatty acids. Reversely, fatty acids can breakdown to generate acetyl-CoA during glucose starvation.
When blood sugar level is low and glycogen is depleted, new glucose can be synthesized from lactate, pyruvate, and some amino-acids, in gluconeogenesis.
Fructose feeds into the pathway at the level of 3-carbon intermediate, and thus bypasses several regulatory steps. Fructose entrance to glycolysis is therefore unregulated, unlike glucose. This means production of acetyl‐CoA from fructose, and its subsequent conversion to fats, can occur unchecked, without regulation by insulin.
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Chapters
0:00 Introduction
0:55 Uses of carbohydrates
1:44 Health benefits
2:17 Nutrition
3:02 Risks
A carbohydrate is a biomolecule consisting of carbon (C), hydrogen (H) and oxygen (O) atoms, usually with a hydrogen–oxygen atom ratio of 2:1 (as in water) and thus with the empirical formula Cm(H2O)n (where m may or may not be different from n). However, not all carbohydrates conform to this precise stoichiometric definition (e.g., uronic acids, deoxy-sugars such as fucose), nor are all chemicals that do conform to this definition automatically classified as carbohydrates (e.g. formaldehyde and acetic acid).
The term is most common in biochemistry, where it is a synonym of saccharide, a group that includes sugars, starch, and cellulose. The saccharides are divided into four chemical groups: monosaccharides, disaccharides, oligosaccharides, and polysaccharides. Monosaccharides and disaccharides, the smallest (lower molecular weight) carbohydrates, are commonly referred to as sugars. The word saccharide comes from the Ancient Greek word σάκχαρον (sákkharon), meaning “sugar”. While the scientific nomenclature of carbohydrates is complex, the names of the monosaccharides and disaccharides very often end in the suffix -ose, which was originally taken from glucose, from the Ancient Greek word γλεῦκος (gleûkos), meaning “wine, must”, and is used for almost all sugars, e.g. fructose (fruit sugar), sucrose (cane or beet sugar), ribose, lactose (milk sugar), etc.
Carbohydrates perform numerous roles in living organisms. Polysaccharides serve as an energy store (e.g. starch and glycogen) and as structural components (e.g. cellulose in plants and chitin in arthropods). The 5-carbon monosaccharide ribose is an important component of coenzymes (e.g. ATP, FAD and NAD) and the backbone of the genetic molecule known as RNA. The related deoxyribose is a component of DNA. Saccharides and their derivatives include many other important biomolecules that play key roles in the immune system, fertilization, preventing pathogenesis, blood clotting, and development.
Carbohydrates are central to nutrition and are found in a wide variety of natural and processed foods. Starch is a polysaccharide. It is abundant in cereals (wheat, maize, rice), potatoes, and processed food based on cereal flour, such as bread, pizza or pasta. Sugars appear in human diet mainly as table sugar (sucrose, extracted from sugarcane or sugar beets), lactose (abundant in milk), glucose and fructose, both of which occur naturally in honey, many fruits, and some vegetables. Table sugar, milk, or honey are often added to drinks and many prepared foods such as jam, biscuits and cakes.
Cellulose, a polysaccharide found in the cell walls of all plants, is one of the main components of insoluble dietary fiber. Although it is not digestible by humans, cellulose and insoluble dietary fiber generally help maintain a healthy digestive system by facilitating bowel movements. Other polysaccharides contained in dietary fiber include resistant starch and inulin, which feed some bacteria in the microbiota of the large intestine, and are metabolized by these bacteria to yield short-chain fatty acids.
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