Carbohydrates are biomolecules that consist of carbon, hydrogen and oxygen atoms, usually in the ratio of 1:2:1. Carbohydrates play crucial roles in living organisms. Among other functions, they serve as major sources of energy, and structural components. 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. Some monosaccharides share the same molecular formula, but are different in structure due to different positions of atoms. These seemingly small structural details result in completely different sugars, with different properties and metabolism pathways. 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, respectively. Examples of disaccharides are 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 serve as energy storage in animals and plants, respectively. Their monomers are bonded by alpha-linkages. Some monomers can make more than one connection, producing branches. Starch in food can be digested by breaking these 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. During digestion, digestible carbohydrates are broken down into simple sugars. Digestion of starch starts with amylase in the saliva and continues in the small intestine by other enzymes. Sucrose and lactose are hydrolyzed by their respective intestinal enzymes. Simple sugars are then absorbed through the intestinal wall and transported in the bloodstream to tissues, for consumption or storage. 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, releasing a small amount of energy. 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+, which is required for glycolysis to continue. When oxygen is present, pyruvate is further 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 a process called gluconeogenesis, which is almost the reverse of glycolysis. Metabolism of other simple sugars converges with the glycolytic pathway at different points. For example, 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.