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Module 1: Carbohydrates

Introduction to Carbohydrates

• Definition: Carbohydrates are organic compounds classified as hydrates of carbon, consisting of carbon (C), hydrogen (H), and oxygen (O) in a specific ratio (typically 1:2:1). They are technically identified as polyhydroxy aldehydes and ketones. The term "saccharide" originates from the Greek word "sakkron," meaning sugar, and encompasses all carbohydrate types.

• Molecular Formula: The general empirical formula for carbohydrates is represented as (C_x(H_2O)_y), indicating their nature as carbon and water hydrates.

Importance

• Carbohydrates fulfill 70-80% of human energy needs through glucose metabolism, which is vital for bodily functions, especially for brain activity as it is the brain's primary energy source.

• They represent over 90% of the dry matter in plants, serving as a building block for various organic matter and contributing significantly to biomass.

• Carbohydrates also play essential roles in cellular functions, metabolic processes, and maintaining homeostasis within the body.

Properties of Carbohydrates

• Energy Storage: Carbohydrates efficiently serve as energy reserves, storing fuels in the form of glycogen in animals and starch in plants. Their polymeric nature allows for the aggregation of glucose molecules for rapid energy release during metabolic demands.

• Structural Role:

  • Sugars: Simple sugars like ribose and deoxyribose form the structural framework of genetic materials, RNA and DNA, respectively, enabling the storage and transfer of genetic information.

  • Polysaccharides: Examples include cellulose, which provides structural integrity to cell walls in bacteria and plants, and chitin, which forms exoskeletons in arthropods.

• Cell Interactions: Carbohydrates often interact with proteins (glycoproteins) and lipids (glycolipids), playing vital roles in cell signaling, adhesion, and immunity.

• Chemical Nature: Carbohydrates are organic molecules characterized by the presence of aldehydes or ketones paired with multiple hydroxyl (–OH) groups, defining their reactivity and interactions.

Functions of Carbohydrates

• Energy Source: They provide a primary source of energy for living organisms, e.g., glucose being a central metabolite fueling cellular respiration.

• Energy Storage: Glycogen in animal tissues and starch in plants serves as energy stores, which can be mobilized in response to energy needs.

• Structural Components: Carbohydrates serve as critical scaffolding materials:

  • Glycosaminoglycans contribute to the extracellular matrix in animal tissues.

  • Cellulose in plant cells adds rigidity, while chitin offers protection and support in fungi and arthropods.

• Dietary Fiber: Non-digestible carbohydrates, particularly cellulose, contribute dietary fiber, which is essential for healthy digestion and regulation of blood sugar levels.

• Nucleic Acids: Ribose and deoxyribose play crucial roles as constituents of RNA and DNA, vital for protein synthesis and genetic information storage and transfer.

• Cellular Functions: Carbohydrates are involved in cellular lubrication (mucins), communication (receptors), immunity (antibody recognition), and detoxification processes (binding toxins).

Classification of Carbohydrates

1. Monosaccharides: The simplest form, consisting of single polyhydroxy aldehyde or ketone units (e.g., glucose, fructose). They are the building blocks for more complex carbohydrates.

2. Disaccharides: Formed by the linkage of two monosaccharides through a glycosidic bond (e.g., sucrose, formed from glucose and fructose).

3. Oligosaccharides: Consist of 3 to 10 monosaccharide units (e.g., raffinose), often found in plants and involved in cell recognition.

4. Polysaccharides: Composed of long chains of hundreds or thousands of monosaccharide units, either straight or branched (e.g., cellulose in plants, glycogen in animals, and starch in plants).

Stereochemistry and Configuration

• Molecular Configuration: Refers to the spatial arrangement of bonds in a molecule, particularly important in defining carbohydrate forms.

• Configuration in Monosaccharides: Determined by the orientation of the hydroxyl group on the highest-numbered asymmetric carbon, designated as D (right) or L (left) forms in Fischer projections.

• Stereoisomerism: Molecules that share the same chemical formula but differ in three-dimensional arrangements can exhibit different properties and functions.

• Chirality: A chiral carbon is an asymmetric carbon atom attached to four different atoms or groups, yielding two non-superimposable mirror images, increasing the complexity of carbohydrate diversity.

• Vant Hoff's Rule: The number of potential stereoisomers is determined by the number of asymmetric carbon atoms (n), resulting in 2^n possible stereoisomers. The penultimate carbon atom serves as the reference point for configuration comparison.

Chirality and Handedness

• Chiral molecules relate to each other as left and right hands are to a mirror image, emphasizing the significance of molecular orientation in biological functionality.

Chiral Carbons and Stereoisomers

• Identifying Chiral Carbons: Chiral centers arise where any carbon atom is bonded to four distinct groups. Increased chiral carbons lead to greater stereoisomeric possibilities.

• For n chiral carbons, the potential total of stereoisomers is calculated as 2^n.

Examples of Stereoisomers

• D- and L-Glyceraldehyde: These forms serve as reference points for establishing the configurations of aldoses and ketoses based on the configuration of the penultimate carbon.

• Optical Activity: The ability of a substance to rotate polarized light; substances can be dextrorotatory (+) or levorotatory (-). Racemic mixtures, comprising equal amounts of enantiomers, exhibit no net optical rotation due to canceling effects.

Anomerism and Ring Structures

• Cyclic Forms: Monosaccharides can frequently form stable rings through intramolecular hemiacetal or hemiketal formation; this process involves a reaction between an aldehyde or ketone group with a hydroxyl group. The configuration of the anomeric carbon determines whether the sugar is classified as alpha (α) or beta (β).

• Haworth Projection: A commonly used method to depict cyclic forms of carbohydrates, illustrating the anomeric center and the orientations of hydroxyl groups that define specific carbohydrate forms.

Summary of Biological Importance

• Monosaccharides and Their Functions: Various monosaccharides such as glucose and ribose are integral to metabolism, energy storage, and as foundational blocks for nucleic acids. Each monosaccharide's classification aligns with its carbon count and the placement of its functional groups (aldehyde vs. ketone).

• This module on carbohydrates offers a comprehensive overview of their biological roles, chemical properties, and significance in metabolic processes, vital for further exploration in biochemistry and molecular biology.