Carbohydrates: Building Blocks of Life

Study Notes

Carbohydrates: Building Blocks of Life

Carbohydrates are essential biomolecules with diverse functions: energy sources, energy storage, structural components, and signaling molecules.
Energy source: Glucose is the primary fuel for cellular metabolism.
Energy storage: Carbohydrates like glycogen (in animals) and starch (in plants) store energy for later use.
Structural roles: Cellulose provides structure to plant cell walls, and chitin forms the exoskeleton of insects and crustaceans.
Informational roles: Carbohydrates are involved in cell-cell communication and immune responses, often in the form of glycoproteins and proteoglycans.

Carbohydrate Classes

Monosaccharides: Single sugar molecules, containing either an aldehyde group (aldoses) or a ketone group (ketoses). Most common example: D-glucose. Monosaccharides with 4 or more carbons typically form cyclic structures in solution.
Disaccharides: Composed of two monosaccharide units linked by a glycosidic bond. Example: Sucrose (table sugar), which consists of glucose and fructose. Most common mono- and disaccharides have names ending in “-ose."
Oligosaccharides: Chains of a few monosaccharide units (usually 3-10) linked together. In cells, they are often covalently attached to proteins or lipids, forming glycoproteins or glycolipids.
Polysaccharides: Long chains (greater than 20 units) of monosaccharides, serving roles in energy storage and structural support. Examples: Starch, glycogen, cellulose.

Monosaccharides – Aldoses

Aldoses are monosaccharides containing an aldehyde group (-CHO) at the end of the molecule.
Carbon backbone: Classified based on the number of carbon atoms:
- 3C (triose): Glyceraldehyde
- 4C (tetrose): Erythrose
- 5C (pentose): Ribose
- 6C (hexose): Glucose
Aldoses are more common in nature than ketoses.

Monosaccharides – Ketoses

Ketoses are monosaccharides containing a ketone group (C=O) typically at the second carbon.
- 3C (triose): Dihydroxyacetone
- 6C (hexose): Fructose
D-fructose is the most common ketose in nature.

Stereochemistry of Monosaccharides

Chirality: All monosaccharides, except dihydroxyacetone, are chiral molecules, meaning they have at least one carbon atom bonded to four different groups.
Chiral Centers: The number of stereoisomers a molecule can have is determined by the number of chiral centers (n), calculated by the formula 2^n.
Enantiomers: Mirror images that cannot be superimposed, such as D-glucose and L-glucose. The D- or L- configuration is determined by the configuration of the chiral center furthest from the carbonyl group.
Epimers: Monosaccharides that differ in configuration at only one carbon atom (e.g., D-glucose and D-galactose differ at C4).

Cyclization of Monosaccharides

Monosaccharides with 5 or more carbons typically form cyclic structures in aqueous solutions.
Hemiacetals: Form when an aldehyde reacts with an alcohol group, leading to ring closure.
Hemiketals: Form when a ketone reacts with an alcohol group.
Example:
- D-glucose forms a six-membered ring structure known as pyranose.
- D-fructose forms a five-membered ring called furanose.
The carbon atom that becomes the new chiral center during cyclization is called the anomeric carbon.

Hexose Derivatives

Phosphate derivatives: For example, glucose-6-phosphate is a phosphorylated form of glucose and a central intermediate in glycolysis.
Amino sugars: An amine group (-NH₂) replaces a hydroxyl group (-OH), forming compounds like glucosamine and galactosamine. These derivatives are important in structural polymers (e.g., bacterial cell walls).

Glycosidic Bond Formation

A glycosidic bond is the linkage between two monosaccharide units.
The bond forms between the anomeric carbon of one sugar and the hydroxyl group of another.
Acetal: The anomeric carbon involved in a glycosidic linkage (no free reducing end).
Hemiacetal: The anomeric carbon not involved in the glycosidic linkage, leaving a free reducing end.
Reducing sugars: Sugars that contain a free anomeric carbon capable of acting as a reducing agent (e.g., glucose, lactose).
Non-reducing sugars: Sugars with both anomeric carbons engaged in glycosidic bonds, like sucrose.

Common Disaccharides

Sucrose: Composed of glucose and fructose, linked by an α(1→2) glycosidic bond. It is a non-reducing sugar.
Lactose: Composed of galactose and glucose, linked by a β(1→4) glycosidic bond. It is a reducing sugar.
Maltose: Composed of two glucose molecules, linked by an α(1→4) glycosidic bond. It is a reducing sugar.
Trehalose: Composed of two glucose molecules linked by an α(1→1) glycosidic bond, it is a non-reducing sugar.

Polysaccharides

Homopolysaccharides: Made of only one type of monosaccharide (e.g., starch, glycogen, cellulose).
Heteropolysaccharides: Contain more than one type of monosaccharide (e.g., agarose).
Starch: The primary storage carbohydrate in plants, composed of amylose (unbranched) and amylopectin (branched).
Glycogen: The primary storage carbohydrate in animals, highly branched and more compact than starch.
Cellulose: A structural polysaccharide in plants, forming fibers that provide strength and support. Humans cannot digest cellulose due to the lack of the enzyme cellulase.

Description

This quiz explores the essential roles of carbohydrates in biological systems, including their functions as energy sources and structural components. Test your knowledge on different classes of carbohydrates such as monosaccharides and disaccharides. Dive into the significance of carbohydrates in cellular metabolism and communication.