Monosaccharides and Disaccharides

• The stoichiometric formula (CH2O)n represents carbohydrates, where n indicates the number of carbons, the ratio of carbon to hydrogen to oxygen is 1:2:1.
• "Carbohydrate" comes from "carbo" (carbon) and "hydrate" (water).
• Carbohydrates are classified into monosaccharides, disaccharides, and polysaccharides.

Monosaccharides

• Monosaccharides are simple sugars, the most common of which is glucose.
• Monosaccharides typically contain three to seven carbons and their names usually end in "-ose".
• An aldose contains an aldehyde group (R-CHO), while ketose contains a ketone group (RC(=O)R').
• Sugars are classified as trioses (3 carbons), pentoses (5 carbons), or hexoses (6 carbons), depending on the number of carbons.
• Glucose's chemical formula is C6H12O6 and is a source of energy.
• During respiration, energy is released from glucose to make ATP.
• Plants synthesize glucose using carbon dioxide and water, which meets the plant's energy requirements.
• Galactose (in lactose) and fructose (in sucrose) are other common monosaccharides.
• Glucose, galactose, and fructose have the same chemical formula (C6H12O6) but differ structurally and chemically and are isomers.
• Glucose, galactose, and fructose are isomeric monosaccharides (hexoses) and have similar chemical formulas but have slightly different structures.
• Glucose and galactose are aldoses, while fructose is ketose.
• Monosaccharides exist as linear chains or ring-shaped molecules; in aqueous solutions, they are usually in ring forms
• In ring form, glucose possesses varying hydroxyl group arrangements (OH) around the anomeric carbon.
• The hydroxyl group is in the alpha (α) position if it is below carbon number 1, and in the beta (β) position if it is above the plane.
• Five- and six-carbon monosaccharides in equilibrium exist between linear and ring forms.
• Fructose and ribose can also form rings, but they form five-membered rings while glucose forms a six-membered ring.

Disaccharides

• Disaccharides form when two monosaccharides undergo a dehydration reaction (or condensation reaction/dehydration synthesis).
• In the process of the monosaccharides joining together, a hydroxyl group and hydrogen release a water molecule to form a covalent bond.
• The covalent bond that forms between carbohydrate molecules is called a glycosidic bond.
• Glycosidic bonds can be alpha or beta.
• An alpha bond is formed when the OH group on the carbon-1 of the first glucose is below the ring plane, while 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 comprises the monomers glucose and galactose and is naturally found in milk.
• Maltose, or malt sugar, forms from the dehydration reaction between two glucose molecules.
• Sucrose, or table sugar, is comprised of glucose and fructose monomers.

Polysaccharides

• Polysaccharides consist of a chain of monosaccharides linked by glycosidic bonds.
• Polysaccharide chains may be branched or unbranched and may contain different monosaccharides.
• The molecular weight of a polysaccharide can reach 100,000 daltons or more, depending on the number of monomers that are joined.
• Starch, glycogen, cellulose, and chitin are primary examples of polysaccharides.
• Amylose and amylopectin (both glucose polymers) comprise the starch stored by plants.
• Glucose starch comprises monomers joined by α 1-4 or α 1-6 glycosidic bonds, where the numbers refer to the carbon number of the residues forming the bond.
• Amylose contains unbranched glucose monomer chains (α 1-4 linkages), while amylopectin is a branched polysaccharide (α 1-6 linkages at branch points).
• Glycogen is the storage form of glucose in humans and other vertebrates and contains glucose monomers.
• Glycogen has a similar structure to amylopectin, it is more branched.
• Whenever blood glucose levels decrease, glycogen breaks down to release glucose in a process scientists call glycogenolysis.
• Cellulose is a natural biopolymer, it comprises a plant's cell wall.
• Glucose monomers form cellulose connected by β 1-4 glycosidic bonds.
• Every other glucose monomer in cellulose flips over, creating tightly packed, extended chains that contribute to the rigidity and tensile strength of plant cells.
• Human digestive enzymes are unable to break down the β 1-4 linkage.
• Herbivores can digest plant material rich in cellulose with the help of specialized flora and cellulase-secreting bacteria and protists in their digestive systems.
• Arthropods have an exoskeleton comprised of chitin, a nitrogen-containing polysaccharide.
• Chitin is made of repeating N-acetyl-β-d-glucosamine units (a modified sugar) and is found in fungal cell walls.

Benefits of Carbohydrates

• Carbohydrates are an important part of the human diet and are not necessarily bad for you.
• Artifacts from ancient civilizations show an ancestral presence of wheat, rice, and corn.
• As part of a well-balanced diet, carbohydrates should supplement proteins, vitamins, and fats.
• Per gram, carbohydrates provide 4.3 Kcal (compared to 9 Kcal/g for fats).
• Carbohydrates contain soluble and insoluble elements (fiber).
• Fiber promotes regular bowel movement, regulates blood glucose consumption rate, and lowers cholesterol by preventing its entry into the bloodstream.
• Fiber-rich diets protect against colon cancer, promotes fullness with whole grains and vegetables.
• Glucose, derived from carbohydrates, is an immediate source of energy that breaks down during cellular respiration.
• Without carbohydrates, the availability of "instant energy" decreases.

Explore the relationship between glucose, galactose, and fructose, including ring formation and isomeric forms. Learn about glycosidic bonds, disaccharide formation, and the classification of monosaccharides. Understand the structural differences between aldoses and ketoses.