Chapter 2: Carbohydrates – Overview

2.1 Classification of Carbohydrates

• Carbohydrates are building blocks of the biosphere: simple sugars (monosaccharides), fatty acids, amino acids, and mononucleotides assemble into biopolymers (polysaccharides, lipids, proteins, nucleic acids).

• This chapter focuses on: simple sugars, storage carbohydrates, and structural polysaccharides.

• Stereochemistry of carbohydrates is reviewed; multiple structural representations are discussed.

• Physical/chemical properties of simple sugars described; structures of storage and structural polysaccharides provided.

• Roles of complex carbohydrates: structural (cell walls) and energy reservoirs (storage).

• Examples of structural carbohydrates: cellulose (major plant cell wall component) and peptidoglycans (bacterial cell walls).

• Storage polysaccharides: starch and glycogen, polymers produced/consumed per energy needs.

• Carbohydrates can be defined as polyhydroxy aldehydes/ketones or substances yielding such compounds on hydrolysis.

• Common empirical formula: $CH<em>2O$, with general form $ext{(CH}</em>2 ext{O)}_n$ where n ≥ 3.

• Limitations: some carbohydrates contain N, P, or S; simple sugar deoxyribose has formula $C<em>5H</em>{10}O<em>4$ (not $C</em>5H<em>{10}O</em>5$).

2.1 Classification of Carbohydrates

• Carbohydrates are building blocks of the biosphere: simple sugars (monosaccharides), fatty acids, amino acids, and mononucleotides assemble into biopolymers (polysaccharides, lipids, proteins, nucleic acids).

  • Monosaccharides: The simplest form of carbohydrates, common examples include glucose, fructose, and galactose. They are categorized based on the number of carbon atoms (e.g., hexoses like glucose and fructose having six carbons) and the type of carbonyl group (aldehyde group for aldoses, ketone group for ketoses).

  • Disaccharides: Formed by the glycosidic linkage of two monosaccharides, such as sucrose (composed of glucose and fructose) or lactose (composed of glucose and galactose).

  • Oligosaccharides: Typically contain 3 to 10 monosaccharide units linked together. They often play roles in cell recognition and binding.

• This chapter focuses on: simple sugars, storage carbohydrates, and structural polysaccharides.

• Stereochemistry of carbohydrates: A critical aspect involving the precise arrangement of atoms in 3D space. This includes concepts like D- and L-isomers, which are defined by the configuration of the chiral center furthest from the carbonyl group. In aqueous solutions, monosaccharides readily interconvert between open-chain and cyclic hemiacetal/hemiketal forms, leading to $\alpha$ and $\beta$ anomers at the anomeric carbon.

• Physical/chemical properties of simple sugars: Due to the presence of numerous hydroxyl groups, simple sugars are highly soluble in water. They are generally sweet-tasting and serve as immediate energy sources in biological systems. Their ability to exist in both linear and cyclic forms is fundamental to their diverse reactivity and biological roles. Many monosaccharides can undergo oxidation (forming aldonic or aldaric acids) and reduction (forming alditols), or form phosphate esters, which are essential intermediates in metabolic pathways.

• Structures of storage and structural polysaccharides will be provided.

• Roles of complex carbohydrates:

  • Structural carbohydrates (e.g., cell walls): Provide rigidity and support. Examples include cellulose (a major component of plant cell walls) and peptidoglycans (found in bacterial cell walls). The type of glycosidic linkage profoundly impacts their function; for instance, cellulose consists of glucose units linked by $\beta$-1,4 glycosidic bonds, which form strong, linear fibers. Chitin, found in the exoskeletons of insects and crustaceans, is another significant structural polysaccharide, composed of N-acetylglucosamine units.

  • Energy reservoirs (storage polysaccharides): Serve as long-term energy stores, like starch in plants and glycogen in animals. These polymers are characterized by predominantly $\alpha$-glycosidic bonds, which are easily hydrolyzed by enzymes to release glucose for energy. Starch is a mixture of amylose (linear) and amylopectin (branched), while glycogen is more highly branched than amylopectin, allowing for rapid glucose mobilization.

• Carbohydrates can be defined as polyhydroxy aldehydes/ketones or substances yielding such compounds on hydrolysis.

• Common empirical formula: $CH<em>2O$, with general form $(CH</em>2O)_n$ where n $\geq$ 3.

  • Limitations of the empirical formula: While $CH<em>2O$ is a general representation, some carbohydrates deviate due to chemical modifications. For example, some may contain nitrogen (e.g., amino sugars like glucosamine and N-acetylglucosamine, which are components of chitin and peptidoglycans), phosphorus (in phosphorylated sugars like glucose-6-phosphate, essential metabolic intermediates), or sulfur (in glycosaminoglycans). Deoxyribose, a simple sugar found in DNA, has the formula $C</em>5H<em>{10}O</em>4$, notably lacking one oxygen atom compared to the ideal pentose formula ($C<em>5H</em>{10}O_5$).

2.1 Classification of Carbohydrates

• Carbohydrates are building blocks of the biosphere: simple sugars (monosaccharides), fatty acids, amino acids, and mononucleotides assemble into biopolymers (polysaccharides, lipids, proteins, nucleic acids).

  • Monosaccharides: The simplest form of carbohydrates, common examples include glucose, fructose, and galactose. They are categorized based on the number of carbon atoms (e.g., trioses with 3 carbons like glyceraldehyde, pentoses with 5 carbons like ribose, and hexoses with 6 carbons like glucose and fructose). They are also classified by the type of carbonyl group: those with an aldehyde group are called aldoses (e.g., glucose, galactose), while those with a ketone group are called ketoses (e.g., fructose). Their open-chain forms exist in equilibrium with cyclic hemiacetal (for aldoses) or hemiketal (for ketoses) forms in aqueous solutions, a process known as mutarotation.

  • Disaccharides: Formed by the glycosidic linkage of two monosaccharides through a dehydration reaction. Common examples include sucrose ($\alpha$-glucose and $\beta$-fructose, joined by an $\alpha$,$\beta$-1,2-glycosidic bond), lactose ($\beta$-galactose and $\alpha$- or $\beta$-glucose, with a $\beta$-1,4-glycosidic bond), and maltose (two glucose units linked by an $\alpha$-1,4-glycosidic bond).

  • Oligosaccharides: Typically contain 3 to 10 monosaccharide units linked together. They often play crucial roles in cell recognition and binding, forming parts of glycoproteins and glycolipids on cell surfaces, which are vital for cell-cell communication and immune responses.

• This chapter focuses on: simple sugars, storage carbohydrates, and structural polysaccharides.

• Stereochemistry of carbohydrates: A critical aspect involving the precise arrangement of atoms in 3D space. Key concepts include:

  • Chiral Carbons: Carbons bonded to four different groups, giving rise to stereoisomers.

  • D- and L-Isomers: Defined by the configuration of the chiral center furthest from the carbonyl group. In biological systems, D-isomers are predominantly found.

  • Fischer Projections: A 2D representation showing the spatial arrangement of atoms, with horizontal lines representing bonds coming out of the plane and vertical lines representing bonds going into the plane.

  • Cyclic Forms and Anomers: In aqueous solutions, monosaccharides readily interconvert between open-chain and cyclic hemiacetal/hemiketal forms. The formation of a new chiral center at the former carbonyl carbon (C1 for aldoses, C2 for ketoses) creates anomers (e.g., $\alpha$ and $\beta$ anomers), which differ in the configuration of the hydroxyl group at this anomeric carbon. This interconversion is called mutarotation and leads to an equilibrium mixture.

• Physical/chemical properties of simple sugars: Due to the presence of numerous hydroxyl groups, simple sugars are highly soluble in water, forming hydrogen bonds with water molecules. They are generally sweet-tasting and serve as immediate energy sources in biological systems. Their ability to exist in both linear and cyclic forms is fundamental to their diverse reactivity and biological roles. Many monosaccharides can undergo various reactions:

  • Reduction: Carbonyl group can be reduced to a hydroxyl group, forming alditols (e.g., glucose forms sorbitol).

  • Oxidation: Aldehyde group can be oxidized, forming aldonic acids (e.g., gluconic acid from glucose) or aldaric acids (oxidation at both ends).

  • Esterification: Hydroxyl groups can react with acids (e.g., phosphoric acid) to form phosphate esters (e.g., glucose-6-phosphate), essential metabolic intermediates.

  • Glycoside Formation: The anomeric carbon's hydroxyl group can react with an alcohol to form a glycosidic bond, creating a glycoside, which is stable and non-reducing.

• Structures of storage and structural polysaccharides will be provided.

• Roles of complex carbohydrates:

  • Structural carbohydrates (e.g., cell walls): Provide rigidity and support. Examples include cellulose (a major component of plant cell walls) and peptidoglycans (found in bacterial cell walls). The type of glycosidic linkage profoundly impacts their function; for instance, cellulose consists of glucose units linked by $\beta$-1,4 glycosidic bonds, which form long, strong, linear fibers that can aggregate into microfibrils, providing high tensile strength. Chitin, found in the exoskeletons of insects and crustaceans, as well as fungal cell walls, is another significant structural polysaccharide, composed of N-acetylglucosamine units linked by $\beta$-1,4 glycosidic bonds, giving it similar strength and insolubility to cellulose.

  • Energy reservoirs (storage polysaccharides): Serve as long-term energy stores, like starch in plants and glycogen in animals. These polymers are characterized by predominantly $\alpha$-glycosidic bonds, which are easily hydrolyzed by enzymes (e.g., amylases) to release glucose for energy. Starch is a mixture of two glucose polymers: amylose (linear, unbranched, $\alpha$-1,4 linkages) and amylopectin (branched, with $\alpha$-1,4 and $\alpha$-1,6 linkages at branch points). Glycogen is the animal equivalent of starch and is even more highly branched than amylopectin, allowing for rapid mobilization of glucose from multiple ends when energy is needed.

• Carbohydrates can be defined as polyhydroxy aldehydes/ketones or substances yielding such compounds on hydrolysis.

• Common empirical formula: $CH<em>2O$, with general form $(CH</em>2O)_n$ where n $\geq$ 3.

  • Limitations of the empirical formula: While $CH<em>2O$ is a general representation, some carbohydrates deviate due to chemical modifications. For example, some may contain nitrogen (e.g., amino sugars like glucosamine and N-acetylglucosamine, which are components of chitin and peptidoglycans), phosphorus (in phosphorylated sugars like glucose-6-phosphate, essential metabolic intermediates in glycolysis), or sulfur (in glycosaminoglycans like chondroitin sulfate, which are important components of connective tissue). Deoxyribose, a simple sugar found in DNA, has the formula $C</em>5H<em>{10}O</em>4$, notably lacking one oxygen atom compared to the ideal pentose formula ($C<em>5H</em>{10}O_5$) because a hydroxyl group at