Carbohydrates and Monosaccharides

• Carbohydrates are a class of naturally occurring compounds and derivatives formed from them
• Carbohydrates contain carbon, hydrogen, and oxygen atoms
• Many carbohydrates have the empirical formula (CH2O)n, where n is an integer
• Carbohydrates were originally regarded as "hydrates of carbon," hence the name

Monosaccharides

• Monosaccharides, also known as simple sugars, are the simplest form of carbohydrates
• They cannot be hydrolyzed into smaller carbohydrates
• Monosaccharides are the monomers from which more complex carbohydrates are constructed
• They are polyhydroxy aldehydes or ketones, meaning they contain multiple hydroxyl (-OH) groups and either an aldehyde (CHO) or ketone (C=O) group
• Monosaccharides are classified based on the number of carbon atoms they contain
  • Trioses: 3 carbons
  • Tetroses: 4 carbons
  • Pentoses: 5 carbons
  • Hexoses: 6 carbons
  • Heptoses: 7 carbons
• The suffix "-ose" indicates that a molecule is a carbohydrate
• Monosaccharides can exist as stereoisomers
• Stereoisomers have the same chemical formula but different spatial arrangements
• Enantiomers are stereoisomers that are mirror images of each other (D- and L- forms)
• Most sugars in living organisms are D-isomers
• Epimers are stereoisomers that differ at one chiral carbon
• Glucose, fructose, and galactose are common hexoses
• Glucose is the most important energy source for most organisms
• Fructose is found in fruits and is the sweetest monosaccharide
• Galactose is a component of lactose (milk sugar)
• Ribose and deoxyribose are pentoses found in RNA and DNA, respectively
• Monosaccharides can form ring structures in solution
• The aldehyde or ketone group reacts with a hydroxyl group on the same molecule to form a cyclic hemiacetal or hemiketal
• The cyclic forms exist as α and β anomers, which differ in the configuration at the anomeric carbon (the carbon derived from the carbonyl carbon of the open-chain form
• Monosaccharides can be modified with phosphate groups, amino groups, or other substituents
• These modifications can alter their properties and functions
• For example, glucose-6-phosphate is an important intermediate in metabolism
• Monosaccharides are important sources of energy
• They are also building blocks for larger carbohydrates
• Monosaccharides participate in various metabolic pathways

Disaccharides

• Disaccharides are carbohydrates composed of two monosaccharides joined by a glycosidic bond
• A glycosidic bond is a covalent bond formed between the hydroxyl group of one monosaccharide and the hydroxyl group of another
• The formation of a glycosidic bond involves the removal of a water molecule (dehydration)
• Common disaccharides include:
  • Sucrose (table sugar): glucose + fructose
  • Lactose (milk sugar): galactose + glucose
  • Maltose (malt sugar): glucose + glucose
• Sucrose is produced by plants and is used as a major transport form of sugar
• Lactose is found in milk and is an important source of energy for infants
• Maltose is produced during the breakdown of starch
• The glycosidic bond can be α or β, depending on the configuration of the anomeric carbon involved in the bond
• For example, sucrose has an α-1,2-glycosidic bond, lactose has a β-1,4-glycosidic bond, and maltose has an α-1,4-glycosidic bond
• Disaccharides are digested into monosaccharides before they can be absorbed by the body
• Enzymes called disaccharidases catalyze the hydrolysis of glycosidic bonds
• Lactose intolerance is caused by a deficiency of the enzyme lactase, which breaks down lactose
• Disaccharides provide a readily available source of energy
• They also contribute to the taste and texture of foods

Polysaccharides

• Polysaccharides are complex carbohydrates composed of many monosaccharides linked together by glycosidic bonds
• They are also known as glycans
• Polysaccharides can be linear or branched
• They serve various functions, including energy storage and structural support
• Common polysaccharides include:
  • Starch: a storage polysaccharide in plants, composed of glucose monomers
  • Glycogen: a storage polysaccharide in animals, composed of glucose monomers
  • Cellulose: a structural polysaccharide in plant cell walls, composed of glucose monomers
  • Chitin: a structural polysaccharide in the exoskeletons of arthropods and cell walls of fungi, composed of N-acetylglucosamine monomers
• Starch consists of two types of glucose polymers: amylose and amylopectin
  • Amylose is a linear polymer of glucose linked by α-1,4-glycosidic bonds
  • Amylopectin is a branched polymer with α-1,4-glycosidic bonds in the main chain and α-1,6-glycosidic bonds at the branch points
• Glycogen is similar to amylopectin but is more highly branched
• Cellulose is a linear polymer of glucose linked by β-1,4-glycosidic bonds
• The β-linkages make cellulose more resistant to hydrolysis
• Chitin is similar to cellulose but contains N-acetylglucosamine instead of glucose
• Polysaccharides are broken down into monosaccharides by enzymes called glycosidases or amylases
• Starch and glycogen are important energy reserves
• Cellulose and chitin provide structural support to plants and animals
• Polysaccharides also play roles in cell signaling and recognition
• Some polysaccharides are modified with sulfate groups or other substituents
• These modifications can affect their properties and functions
• Polysaccharides have diverse applications in food, medicine, and industry

Carbohydrates are natural compounds containing carbon, hydrogen, and oxygen, often with the formula (CH2O)n. Monosaccharides, or simple sugars, are the most basic carbohydrates and cannot be hydrolyzed. They are classified by the number of carbon atoms they contain, such as trioses, tetroses, and pentoses.