Biomolecules (Carbohydrates)

Carbohydrates

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Carbohydrates are a major class of organic compounds that serve as a primary source of energy for living organisms. They are composed of carbon, hydrogen, and oxygen, often with the general formula $C_x(H_2O)_y$.

Classification Of Carbohydrates

Carbohydrates are classified based on the number of saccharide units they contain:

1. Monosaccharides: Simple sugars that cannot be hydrolyzed into simpler carbohydrates. They are the basic building blocks of carbohydrates.
2. Disaccharides: Carbohydrates that yield two monosaccharide units upon hydrolysis (e.g., sucrose, lactose, maltose).
3. Polysaccharides: Carbohydrates that yield a large number of monosaccharide units upon hydrolysis (e.g., starch, cellulose, glycogen).

Alternative Classification: Carbohydrates can also be classified based on their functional groups:

• Aldoses: Monosaccharides containing an aldehyde group.
• Ketoses: Monosaccharides containing a ketone group.

Monosaccharides

Definition: Simple sugars that cannot be broken down into simpler carbohydrates by hydrolysis.

General Formula: $(CH_2O)_n$, where $n$ usually ranges from 3 to 7.

• Trioses: $n=3$ (e.g., glyceraldehyde, dihydroxyacetone)
• Tetroses: $n=4$ (e.g., erythrose, threose)
• Pentoses: $n=5$ (e.g., ribose, deoxyribose - components of nucleic acids)
• Hexoses: $n=6$ (e.g., glucose, fructose, galactose)
• Heptoses: $n=7$ (e.g., sedoheptulose)

Glucose ($C_6H_{12}O_6$): The most common monosaccharide, an aldohexose. It is the primary source of energy for most living organisms.

Fructose ($C_6H_{12}O_6$): A ketohexose, known as fruit sugar, which is sweeter than glucose.

Structure: Monosaccharides exist in both open-chain and cyclic forms. The cyclic forms (pyranose and furanose rings) are generally more stable in solution.

Disaccharides

Definition: Carbohydrates formed by the linking of two monosaccharide units through a glycosidic bond, with the elimination of a water molecule.

Examples:

• Sucrose (Table Sugar): Formed from glucose and fructose.
• Lactose (Milk Sugar): Formed from glucose and galactose.
• Maltose (Malt Sugar): Formed from two glucose units.

Properties:

• Disaccharides like sucrose and lactose are crystalline solids.
• They are soluble in water.
• Reducing sugars (like maltose, lactose) can reduce Tollens' reagent, while non-reducing sugars (like sucrose) cannot.

Polysaccharides

Definition: Carbohydrates formed by the linking of a large number of monosaccharide units through glycosidic bonds.

Examples:

• Starch: A storage polysaccharide in plants, composed of glucose units. It consists of two components:
• Amylose: Linear chain of $\alpha$-glucose units linked by $\alpha(1 \rightarrow 4)$ glycosidic bonds.
• Amylopectin: Branched chain polymer of $\alpha$-glucose units with $\alpha(1 \rightarrow 4)$ linkages and $\alpha(1 \rightarrow 6)$ linkages at branch points.
• Cellulose: A structural polysaccharide in plants, composed of $\beta$-glucose units linked by $\beta(1 \rightarrow 4)$ glycosidic bonds. It provides rigidity to cell walls. Humans cannot digest cellulose due to the different type of glycosidic linkage.
• Glycogen: Storage polysaccharide in animals, structurally similar to amylopectin but more highly branched. Found primarily in liver and muscles.
• Other Polysaccharides: Chitin (structural polysaccharide in arthropod exoskeletons and fungi), important gums and mucilages.

Properties: Polysaccharides are generally amorphous, insoluble or sparingly soluble in water, and have a high molecular weight.

Importance Of Carbohydrates

1. Energy Source: Carbohydrates are the primary source of energy for most organisms. Glucose is metabolized through cellular respiration to produce ATP.

2. Structural Components: They form structural components in organisms.

• Cellulose provides structural support to plant cell walls.
• Chitin forms the exoskeletons of insects and crustaceans and the cell walls of fungi.

3. Food Storage: Polysaccharides like starch (in plants) and glycogen (in animals) serve as stored forms of energy.

4. Biological Recognition: Carbohydrate units on cell surfaces play roles in cell-cell recognition and communication.

5. Nucleic Acid Structure: Ribose and deoxyribose (pentoses) are essential components of RNA and DNA.

6. Other Roles: Found in antibiotics, vitamins, and in the synthesis of fats and proteins.