Carbohydrates Notes

Carbohydrates

• Biological Importance:
  • Energy
  • Structural
  • Cell markers (ID)
  • Communication
• Structure:
  • (CH2O)n or 1:2:1 ratio of Carbon, Hydrogen, and Oxygen atoms.
  • A single chain of Carbon atoms with hydroxyl groups. Can contain a carbonyl group.
  • Isomers exist due to different arrangements of atoms.
• Monomers:
  • Monosaccharides: glucose, fructose, galactose
  • Distinguished by the number of Carbon atoms and the placement of the carbonyl group.
  • Small monomers are chains, larger ones are rings.

Monosaccharides

• Simple sugars, consist of one ring.
• General formula: CnH{2n}O_n (1:2:1 ratio).
• Classified by the number of Carbon atoms they contain.
• Main source of fuel for cellular respiration.
• Carbon skeleton serves as raw material for the production of other macromolecules.
• Functional Groups: Hydroxyl (-OH).

Examples of Monosaccharides

• Glucose (blood sugar): used by cells for energy.
• Fructose: found in fruits, sweeter than glucose.
• Galactose: found in milk.

Two Forms of Glucose

• Alpha ($\alpha$) and Beta ($\beta$) glucose.
• Differ in the spatial arrangement of the hydroxyl group attached to carbon number 1.

Disaccharides

• Two monosaccharides bonded together by a glycosidic linkage.
• Formed by a condensation (dehydration synthesis) reaction where a molecule of water is removed.

Counting Carbons

• Carbons in monosaccharides are numbered for reference in glycosidic linkage formation.

Position of Hydroxyl Group

• The position of the hydroxyl group on the first Carbon atom determines whether it is alpha or beta glucose.

Monosaccharides

• $\beta$ (Beta) Glucose diagram.

Alpha and Beta Glucose

• Diagrams illustrating the structural difference between alpha ($\alpha$) glucose and beta ($\beta$) glucose.

Monosaccharides

• Galactose diagram.

Monosaccharides

• Fructose diagram.

Disaccharides

• Composed of two monosaccharides.
• Covalent bond is called a glycosidic linkage, forms between specific hydroxyl groups.
  • Lactose = Galactose + Glucose
  • Maltose = Glucose + Glucose
  • Sucrose = Fructose + Glucose

Disaccharides Example

• Two glucose molecules are shown

Disaccharides

• Formation of Maltose by joining two glucose molecules with an alpha 1,4 glycosidic linkage and releasing a water molecule (H_2O).

Joining Carbohydrate Monomers

• The process of forming a glycosidic bond between two monosaccharides with the release of water (H_2O).

Breaking Carbohydrate Polymers

• The process of breaking the glycosidic bond requires the addition of water.
  • This process occurs through hydrolysis.

Condensation Reaction

• $\alpha$-1,4 glycosidic linkage
  *Note: the glycosidic linkage is made between 2 hydroxyl functional groups!

Forming Disaccharides by Condensation (Dehydration) Synthesis

• (a) glucose + glucose -> maltose + H_2O
• (b) glucose + fructose -> sucrose + H_2O

Forming Disaccharides

• monosaccharide + monosaccharide -> disaccharide + H_2O
• $\alpha$(1$\rightarrow$4) glycosidic linkage
• $\beta$(1-4) linkage
• ($\alpha$1 - $\beta$2) linkage

Carbohydrates – Polysaccharides

• Polymer
• Energy Storage
• Cellulose
• Starch: Amylose and Amylopectin
• Glycogen
• Linkages:
  • Glycosidic: C-O-C
  • Starches and glycogen (Storage): $\alpha$1-4. Enzymes allow us to break down starch and glycogen.
  • Cellulose (structural support): $\beta$1-4
• Reactions:
  • Build: Condensation (dehydration) synthesis
  • Break: Hydrolysis
• All composed of glucose

Some Common Disaccharides

• Sucrose (Glucose + Fructose)
• Lactose (Galactose + Glucose)
• Maltose (Glucose + Glucose)
• Cellobiose (Glucose + Glucose)
• Isomaltose (Glucose + Glucose)

Polysaccharides

• Several hundred to thousands of monosaccharides bonded together.
• Examples: Glycogen, Starch, cellulose, chitin
• Function: energy storage or structural molecules.

Amylose Structure

• Amylose is an unbranched component of starch, formed from 1-4 linkage of $\alpha$-glucose molecules.
• The chain of 1-4 linkages is formed; it is quite long and may wind up into a helix (a spiral shape).

Two Forms of Starch

• Amylose ($\alpha$ 1-4 glycosidic bond)
• Amylopectin

Storage Polysaccharides:

• Plants: Starch
• Amylose (1,4 glycosidic linkage) is a type of simple starch that is unbranched (20-30% starch, hard to digest – insoluble in water).
• Amylopectin (1,4 & some 1,6 glycosidic linkage) is a complex starch that is branched (70% starch, soluble, easy to degrade – many enzyme attachment points).

Glycogen

• carbohydrate storage in animals; stored in the liver
• $\alpha$ 1-4 glycosidic bond

Storage Polysaccharides

• Animals: Glycogen
• Chains of glucose molecules.
• Highly branched (1,4 glycosidic linkage & lots of 1,6 glycosidic linkage).
• Glycogen is hydrolyzed when blood sugar drops.
• Depleted in 1 day unless storage is replenished by consumption of food.

Cellulose

• Provides structure to plant cell walls.
• Every other glucose is upside-down compared to starch.
• Cannot be digested by humans.
• Consists of glucose polymers bonded by 1-4 beta glycosidic linkages.

Structural Polysaccharides

• Cellulose in plants is the Major component of cell walls.

Structural Polysaccharides

• Polymers of beta glucose
• Cellulose chains

Cellulose

• Cellulose – the most abundant organic compound on earth, but…
• Cellulose CANNOT be digested due to its $\beta$ bonds!
• Animals lack the enzyme to catalyze the reaction.
• However, humans have found applications for the strong cellulose fibrils: manufacturing paper, lumber products, and cotton.

Structural Polysaccharides

• Chitin
  • Exoskeleton of insects and crustaceans
  • Cell wall of fungi

Chitin

• Biomedical devices
• Drug delivery
• Catalysts
• Water purification
• Cosmetics
• Antimicrobials

N-Acetylglucosamine

Carbohydrates and Their Function

• Cellulose: Cellulose is a structural polysaccharide found in the cell walls of plants; its glucose subunits are joined in a way that cannot be broken down readily. Cleavage of the links between the glucose subunits in cellulose requires an enzyme most organisms lack. Some animals, such as cows, are able to digest cellulose by means of bacteria and protists they harbor in their digestive tract, which provide the necessary enzymes.
• Chitin: Chitin is a type of structural polysaccharide found in the external skeletons of many invertebrates, including insects and crustaceans, and in the cell walls of fungi. Chitin is a modified form of cellulose with a nitrogen group added to the glucose units. When cross-linked by proteins, it forms a tough, resistant surface material.

Cellulose vs Chitin

Diagrams of Cellulose and Chitin