Introduction to Carbohydrates

Carbohydrates Overview

• Carbohydrates, also called saccharides, are compounds made from carbon, hydrogen, and oxygen
• Monosaccharides and disaccharides are small molecules often called sugars
• Polysaccharides are very large carbohydrates like starch and cellulose
• Carbohydrates are a source of energy, e.g., glucose, and a storage of energy, e.g., starch in plants
• They serve as building blocks for polysaccharides, e.g., cellulose in plants and glycogen in humans
• Carbohydrates are components of other molecules, e.g., DNA, RNA, glycolipids, glycoproteins, ATP

Monosaccharides

• Monosaccharides are the simplest carbohydrates and building blocks for larger carbohydrates
• The general molecular formula for monosaccharides is (CH2O)n, where n can be 3, 5, or 6
• Molecules that have the same molecular formula but different structural formulae are called structural isomers
• Monosaccharides are classified by the number of carbon atoms:
• Trioses have 3-carbon atoms, e.g., glyceraldehyde
• Pentoses have 5-carbon atoms e.g., ribose and deoxyribose
• Hexoses have 6-carbon atoms, e.g., fructose, glucose & galactose
• Glyceraldehyde is a triose with the molecular formula C3H6O3
• Glyceraldehyde contains an aldehyde group and two hydroxyl groups
• Glyceraldehyde is a reducing sugar and gives a positive test with Benedict's reagent
• Aldehydes and ketones are organic compounds which incorporate a carbonyl functional group, C=O
• If at least one of the substituents of a carbonyl functional group is hydrogen, the compound is an aldehyde
• If neither substituent of a carbonyl functional group is hydrogen, the compound is a ketone
• A hydroxyl group is a functional group in organic chemistry that consists of one oxygen atom bonded to one hydrogen atom
• Pentoses and hexoses can exist in cyclic and non-cyclic forms
• Aldoses have an aldehyde group and are reducing sugars
• Ketoses have a ketone group and are non-reducing sugars
• Reducing sugars can donate electrons, while non-reducing sugars cannot

Specific Monosaccharides

• Glucose is the most important carbohydrate fuel in human cells
• It exists in cyclic forms a- glucose and -glucose
• Glucose reacts to form the disaccharide maltose and polysaccharides like starch and cellulose
• Alpha and beta glucose differ only in the orientation of the hydroxyl group (-OH) on the first carbon atom
• In alpha glucose, the hydroxyl group is positioned below the plane of the ring, and in beta glucose, it is positioned above the plane of the ring
• Galactose has a similar structure to glucose
• Galactose exists in α and β forms, and reacts with glucose to make lactose
• Fructose is a hexose classified as a ketose
• Fructose forms a five-atom ring compared to glucose's six-atom ring
• Fructose reacts with glucose to form sucrose
• Ribose and Deoxyribose are pentoses that form part of nucleotides in RNA (ribose) and DNA (deoxyribose)

Disaccharides

• A maltose molecule is formed from two α-glucose molecules in a condensation reaction
• A water molecule splits out and the two glucose units are linked by a glycosidic bond
• An α-lactose molecule is formed from one ß-galactose molecule and one α-glucose molecule in a condensation reaction
• A water molecule splits out and the two monosaccharide units are linked by a glycosidic bond
• A sucrose molecule reacts with a water molecule to form one α-glucose molecule and one α-fructose molecule through hydrolysis
• The glycosidic bond is broken in the reaction and the two monosaccharide units are formed.
• Sucrose (α-glucose + α-fructose) is a non-reducing sugar
• Maltose (α-glucose + α-glucose) is a reducing sugar
• Lactose (α-glucose + β-galactose) is a reducing sugar
• Disaccharides are soluble in water, but too big to pass through the cell membrane by diffusion
• Disaccharides break down during digestion in the small intestine to give smaller monosaccharides that pass into the blood
• Monosaccharides are used quickly by cells, and converted into disaccharides in the cell by condensation reactions for storage
• Further condensation reactions result in the formation of polysaccharides

Polysaccharides

• Polysaccharides are formed from successive condensation reactions of monosaccharides
• Condensation polymerisation refers to monosaccharides undergoing a series of condensation reactions to create polysaccharides
• Polysaccharide properties depend on length, branching, folding, and whether the chain is straight or coiled
• Starch is the energy storage in plants, and exists in both amylose (unbranched) and amylopectin (branched) forms
• Amylose forms a helical structure, amylopectin is insoluble in water
• Glycogen is similar to amylopectin but with shorter branches
• Glycogen acts as a carbohydrate energy store in human cells
• Cellulose is made of β-glucose molecules forming straight chains
• It constitutes plant cell walls, providing toughness due to hydrogen bonding between chains
• Cellulose is indigestible by humans but can be broken down by herbivores with cellulose enzyme