(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 highfructose 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 doublebonded to an oxygen. This carbonyl group can be in any position along the chain, forming either a ketone or an aldehyde.
Monosaccharides exist in openchain form and closedring 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 alphalinkages. 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 betalinkages, for which humans lack the enzyme to digest. Cellulose and other nondigestible 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 acetylCoA. Significant amounts of energy can be extracted from oxidation of acetylCoA to carbon dioxide, by the citric acid cycle and the following electron transport system. When present in excess, acetylCoA is converted into fatty acids. Reversely, fatty acids can breakdown to generate acetylCoA during glucose starvation.
When blood sugar level is low and glycogen is depleted, new glucose can be synthesized from lactate, pyruvate, and some aminoacids, in gluconeogenesis.
Fructose feeds into the pathway at the level of 3carbon 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.