Monosaccharides: Simple Sugars

• Carbohydrates have a stoichiometric formula of (CH2O)n, where n is the number of carbon atoms
• The ratio of carbon to hydrogen to oxygen is 1:2:1 in carbohydrate molecules
• Scientists classify carbohydrates into monosaccharides, disaccharides, and polysaccharides

Monosaccharides

• Monosaccharides are simple sugars; glucose is most common
• They contain three to seven carbon atoms
• Monosaccharide names end with the suffix -ose
• Sugars with an aldehyde group (R-CHO) are aldoses
• Sugars with a ketone group (RC(=O)R') are ketoses
• Trioses contain three carbons
• Pentoses contain five carbons
• Hexoses contain six carbons
• Glucose has the chemical formula C6H12O6
• Glucose provides a source of energy
• During cellular respiration, energy is released from glucose to make ATP
• Plants use carbon dioxide and water to synthesize glucose, meeting their energy needs
• Humans and animals catabolize starch to get glucose
• Galactose and fructose are other common monosaccharides
• Glucose, galactose, and fructose share the chemical formula (C6H12O6)
• Isomers structurally and chemically differ in the arrangement of functional groups around the asymmetric carbon
• Glucose, galactose, and fructose are isomeric monosaccharides (hexoses)
• Glucose and galactose are aldoses
• Fructose is a ketose
• Monosaccharides exist as linear chains or ring-shaped molecules, but exist in ring forms in aqueous solutions
• In ring form, glucose has two hydroxyl group arrangements (OH) around the anomeric carbon: alpha (α) position, and beta (β) position

Disaccharides

• Disaccharides form when two monosaccharides undergo dehydration reaction (condensation reaction/dehydration synthesis)
• During this process, one monosaccharide's hydroxyl group combines with another monosaccharide's hydrogen
• This then releases a water molecule and forms a covalent bond between carbohydrate molecules
• The covalent bond between carbohydrates is called a glycosidic bond
• Glycosidic bonds (or glycosidic linkages) can be alpha or beta type
• 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 consists of the monomers glucose and galactose; and is naturally found in milk
• Maltose (or malt sugar) forms from two glucose molecules
• Sucrose (or table sugar) consists of glucose and fructose monomers

Polysaccharides

• A long chain of monosaccharides linked by glycosidic bonds is a polysaccharide
• Polysaccharides can be branched or unbranched and contain different types of monosaccharides
• The molecular weight may be 100,000 daltons or more
• Starch, glycogen, cellulose, and chitin are primary examples of polysaccharides
• Plants store sugars in the form of starch
• Amylose and amylopectin (both glucose polymers) comprise these sugars
• Plants synthesize and store excess glucose as starch in different plant parts, including roots and seeds
• Starch in seeds provides food for the embryo and serves as humans/animals' food source
• Amylase in saliva breaks down starch into smaller molecules (maltose and glucose)
• Monomers in glucose starch are joined by α 1-4 or α 1-6 glycosidic bonds
• The numbers 1-4 and 1-6 refer to the carbon number of the two residues joined to form the bond
• Unbranched glucose monomer chains (α 1-4 linkages) form the starch amylose
• Amylopectin is a branched polysaccharide (α 1-6 linkages at branch points)
• Glycogen is the storage form of glucose in humans/other vertebrates, comprised of monomers of glucose
• Glycogen is the animal equivalent of starch, stored in liver/muscle cells
• When glucose levels drop, glycogen breaks down to release glucose in glycogenolysis
• Cellulose makes up a plant's cell wall, providing structural support
• Wood and paper are mostly cellulosic
• Glucose monomers comprise cellulose linked by β 1-4 glycosidic bonds
• Glucose monomers form hydrogen bonds, holding chains firmly together side-by-side, forming strong microfibrils
• Human digestive enzymes cannot break down the β 1-4 linkage
• Herbivores (cows, koalas, and buffalos) can digest plant material rich in cellulose
• Species of bacteria/protists in the rumen secrete the cellulase enzyme
• Cellulases break down cellulose into glucose monomers for energy
• Termites can also break down cellulose because of organisms in their bodies
• Carbohydrates serve various functions in different animals
• Exoskeletons in Arthropods (insects, crustaceans, etc.) are made of chitin (nitrogen-containing polysaccharide)
• Chitin has repeating N-acetyl-β-d-glucosamine units and is found in fungal cell walls

Benefits of Carbohydrates

• Carbohydrates are an important part of the human diet
• A gram of carbohydrate contains 4.3 Kcal; fat contain 9 Kcal/g
• Fiber, the insoluble part of carbohydrates, promotes bowel movement, regulates blood glucose, and removes excess cholesterol
• Fiber attaches to cholesterol and prevents cholesterol particles from entering the bloodstream
• Fiber-rich diets may reduce in colon cancer
• Meals with whole grains/vegetables gives a feeling of fullness
• Glucose breaks down during cellular respiration to make ATP, providing an immediate source of energy

Monosaccharides are simple sugars classified by the number of carbon atoms they contain (3-7). Glucose, fructose, and galactose are common examples. They provide a source of energy for cells through processes like cellular respiration and photosynthesis.