Carbohydrates

Introduction to Carbohydrates

Carbohydrates are a major class of biomolecules that serve as a primary source of energy in living organisms. They are composed of carbon (C), hydrogen (H), and oxygen (O), typically with the empirical formula CxHyOz. Carbohydrates are also known as saccharides, meaning "sugars." Their study is essential for understanding metabolic processes, cellular structure, and energy storage.

• Photosynthesis: Plants convert CO2 and H2O into glucose and O2 using sunlight.

• Respiration: In animals, glucose is oxidized to CO2 and H2O, releasing energy.

Classification of Carbohydrates

Carbohydrates are classified based on the number of sugar units present:

• Monosaccharides: The simplest carbohydrates, consisting of a single sugar unit.

• Disaccharides: Composed of two monosaccharide units joined by a glycosidic bond.

• Polysaccharides: Large polymers containing many monosaccharide units.

Monosaccharides

Structure and Classification

Monosaccharides contain several hydroxyl (-OH) groups attached to a carbon chain (3–8 carbons). They are further classified by:

• Aldose: Contains an aldehyde group.

• Ketose: Contains a ketone group.

The number of carbon atoms is indicated by prefixes: triose (3C), tetrose (4C), pentose (5C), hexose (6C).

Fischer Projections

Fischer projections are two-dimensional representations of carbohydrate structures. The most oxidized group is placed at the top, with horizontal lines representing bonds coming out of the plane and vertical lines going into the plane.

Chirality and Stereochemistry

A chiral carbon is a carbon atom bonded to four different groups. Most monosaccharides have one or more chiral centers, leading to stereoisomerism.

D and L Enantiomers

Monosaccharides exist as two enantiomers:

• D-isomer: The –OH group on the chiral carbon farthest from the carbonyl is on the right.

• L-isomer: The –OH group on the chiral carbon farthest from the carbonyl is on the left.

The D-isomer is more common in nature and is used in biological systems.

Important Monosaccharides

• D-Glucose: An aldohexose (C6H12O6), primary blood sugar, found in fruits and honey.

• D-Fructose: A ketohexose (C6H12O6), the sweetest sugar, found in fruit juices and honey.

• D-Galactose: An aldohexose, component of lactose, important in brain and nerve tissues.

Blood Glucose Levels

Blood glucose is tightly regulated. Normal fasting blood glucose is <110 mg/dL; diabetes is diagnosed if fasting glucose is ≥126 mg/dL on two occasions. The oral glucose tolerance test measures glucose response after ingestion of a glucose solution.

Ring Structures of Monosaccharides

Haworth Structures: Pyranose and Furanose Rings

Monosaccharides can cyclize to form ring structures. A six-membered ring is called a pyranose, and a five-membered ring is a furanose. The ring forms when the carbonyl carbon reacts with a hydroxyl group on the same molecule, creating a hemiacetal or hemiketal.

• Groups on the left in the Fischer projection are oriented up in the Haworth structure; groups on the right are oriented down.

α and β Anomers

The newly formed –OH group at the anomeric carbon (C1 in pyranose, C2 in furanose) can be oriented either down (α-anomer) or up (β-anomer).

• α-anomer: –OH is down

• β-anomer: –OH is up

Mutarotation

Mutarotation is the interconversion between α- and β-anomers in aqueous solution, passing through the open-chain form. The β-anomer is usually more stable and predominant.

Chemical Properties of Monosaccharides

Oxidation: Reducing Sugars

Monosaccharides with a free carbonyl group (aldoses and some ketoses) can be oxidized to carboxylic acids and are called reducing sugars. Benedict’s test detects reducing sugars by forming a red/orange precipitate.

Oxidation of Ketoses

Although ketones are generally resistant to oxidation, ketoses like fructose can isomerize to aldoses under basic conditions and then be oxidized, thus acting as reducing sugars.

Reduction: Formation of Sugar Alcohols

Reduction of the carbonyl group in monosaccharides produces sugar alcohols (alditols), such as sorbitol from glucose. These are used as sweeteners in sugar-free products.

Disaccharides

Structure and Formation

Disaccharides are formed by a dehydration reaction between two monosaccharides, creating a glycosidic bond. Common disaccharides include maltose, lactose, and sucrose.

• Maltose: Two glucose units, α-(1→4) glycosidic bond.

• Lactose: Galactose and glucose, β-(1→4) glycosidic bond.

• Sucrose: Glucose and fructose, (1→2) glycosidic bond; not a reducing sugar.

Polysaccharides

Structure and Types

Polysaccharides are large polymers of monosaccharides. The four most important are amylose, amylopectin, cellulose, and glycogen, all composed of D-glucose but differing in glycosidic linkages and branching.

• Amylose: Unbranched, α-(1→4) glycosidic bonds.

• Amylopectin: Branched, α-(1→4) and α-(1→6) glycosidic bonds.

• Glycogen: Highly branched, similar to amylopectin but more frequent branching.

• Cellulose: Unbranched, β-(1→4) glycosidic bonds; not digestible by humans.

Biological Importance

• Starch: Main storage form of glucose in plants (amylose and amylopectin).

• Glycogen: Main storage form of glucose in animals (liver and muscle).

• Cellulose: Structural component of plant cell walls; indigestible by humans.