Biochemistry Carbohydrates

• Carbohydrates are represented by the stoichiometric formula (CH2O)n, where n is the number of carbon molecules.
• The ratio of carbon to hydrogen to oxygen is 1:2:1 in carbohydrate molecules.
• Carbohydrates are classified into three subtypes: monosaccharides, disaccharides, and polysaccharides.

Monosaccharides

• Monosaccharides are simple sugars, the most common of which is glucose.
• The number of carbons in monosaccharides ranges from three to seven.
• Most monosaccharide names end with the suffix -ose.
• If the sugar has an aldehyde group (R-CHO), it is an aldose.
• If the sugar has a ketone group (RC(=O)R'), it is a ketose.
• Depending on the number of carbons, monosaccharides can be trioses (three carbons), pentoses (five carbons), and/or hexoses (six carbons).
• Scientists classify monosaccharides based on the position of their carbonyl group and the number of carbons in the backbone.
• Aldoses have a carbonyl group at the end of the carbon chain.
• Ketoses have a carbonyl group in the middle of the carbon chain.
• Trioses, pentoses, and hexoses have three-, five-, and six-carbon backbones, respectively.
• The chemical formula for glucose is C6H12O6.
• Glucose is an important source of energy.
• During cellular respiration, glucose releases energy, which helps make adenosine triphosphate (ATP).
• Plants synthesize glucose using carbon dioxide and water to provide energy requirements for the plant.
• Humans and other animals that feed on plants often obtain glucose from catabolized starch.
• Galactose (part of lactose, or milk sugar) and fructose (found in sucrose, in fruit) are other common monosaccharides.
• Glucose, galactose, and fructose all have the same chemical formula (C6H12O6) but differ structurally and chemically as isomers due to different arrangements of functional groups around the asymmetric carbon.
• Monosaccharides can exist as linear chains or ring-shaped molecules, usually found in ring forms in aqueous solutions.
• Glucose in a ring form can have two different hydroxyl group arrangements (OH) around the anomeric carbon (carbon 1).
• If the hydroxyl group is below carbon number 1, it is in the alpha (α) position.
• If the hydroxyl group is above the plane, it is in the beta (β) position.
• Five and six carbon monosaccharides exist in equilibrium between linear and ring forms.

Disaccharides

• Disaccharides form when two monosaccharides undergo a dehydration reaction (condensation reaction or dehydration synthesis).
• One monosaccharide's hydroxyl group combines with another monosaccharide's hydrogen, releasing a water molecule and forming a covalent glycosidic bond.
• Glycosidic bonds can be alpha or beta types.
• An alpha bond is formed when the OH group on the carbon-1 of the first glucose is below the ring plane.
• A beta bond is formed when the OH group on the carbon-1 is above the ring plane.
• Common disaccharides include lactose, maltose, and sucrose.
• Lactose is a disaccharide consisting of glucose and galactose monomers and is naturally found in milk.
• Maltose, or malt sugar, is a disaccharide formed by a dehydration reaction between two glucose molecules.
• Sucrose, or table sugar, consists of glucose and fructose monomers.
• Sucrose forms when a glucose monomer and a fructose monomer join in a dehydration reaction to form a glycosidic bond, releasing a water molecule.

Polysaccharides

• Polysaccharides are long chains of monosaccharides linked by glycosidic bonds.
• The chain may be branched or unbranched and may contain different types of monosaccharides.
• The molecular weight may be 100,000 daltons or more, depending on the joined monomers.
• Starch, glycogen, cellulose, and chitin are primary examples of polysaccharides.
• Plants store sugars in starch form, which comprises amylose and amylopectin (both glucose polymers).
• Enzymes break down starch into smaller molecules like maltose and glucose, which cells can absorb.
• Glucose starch comprises monomers joined by α 1-4 or α 1-6 glycosidic bonds.
• Unbranched glucose monomer chains (only α 1-4 linkages) form amylose, whereas amylopectin is a branched polysaccharide (α 1-6 linkages at the branch points).
• Glycogen is the storage form of glucose in humans and other vertebrates, comprised of monomers of glucose.
• Glycogen, the animal equivalent of starch, is a highly branched molecule stored in liver and muscle cells.
• When glucose levels decrease, glycogen breaks down to release glucose in a process called glycogenolysis.
• Cellulose is the most abundant natural biopolymer of plants, providing structural support as the main component of plant cell walls.
• Cellulose mostly comprises glucose monomers linked that β 1-4 glycosidic bonds.
• Every other glucose monomer in cellulose is flipped over, giving it rigidity and high tensile strength.
• Herbivores such as cows, koalas, and buffalos can digest plant material rich in cellulose with the help of specialized flora in their stomach which release cellulases.
• Arthropods (insects, crustaceans, and others) have exoskeletons made of chitin.
• Chitin is a nitrogen-containing polysaccharide made of repeating N-acetyl-β-d-glucosamine units.

Benefits of Carbohydrates

• Carbohydrates have been an important part of the human diet for thousands of years.
• A well-balanced diet should supplement carbohydrates with proteins, vitamins, and fats.
• A gram of carbohydrate provides 4.3 Kcal, and fat provides 9 Kcal/g.
• The insoluble part, fiber (mostly cellulose), promotes regular bowel movement by adding bulk and regulates the blood glucose consumption rate.
• Fiber helps remove excess cholesterol from the body by binding to cholesterol in the small intestine, preventing its absorption into the bloodstream.
• Fiber promotes regular bowel movement by adding bulk, and it regulates the blood.
• Fiber-rich diets help reduce the risk of colon cancer and provides an immediate source of energy.
• Glucose breaks down during cellular respiration to produce ATP (the cell's energy currency).

Monosaccharide classification, energy production, and the distinctions between aldoses and ketoses. Examines glucose synthesis in plants and catabolism in humans. It also touches on the molecular composition of carbohydrates.