Carbohydrates

 

Carbohydrates

Introduction

Carbohydrates are organic molecules composed of carbon (C), hydrogen (H), and oxygen (O), usually in the ratio 1:2:1 (CH₂O). They are the primary source of energy for living organisms and play a vital role in metabolism and structural functions.

Classification of Carbohydrates

Carbohydrates are classified into three major types:

1. Monosaccharides (Simple Sugars)

These are the simplest form of carbohydrates, consisting of a single sugar unit that cannot be broken down further.

Examples of Monosaccharides:

1. Glucose (C₆H₁₂O₆): Primary energy source for cells.
2. Fructose: Found in fruits and honey.
3. Galactose: A component of lactose, found in milk.
4. Ribose & Deoxyribose: Found in nucleic acids (RNA and DNA, respectively).

Properties of Monosaccharides:

1. Soluble in water.
2. Sweet in taste.
3. Reducing sugars (can donate electrons in chemical reactions).

 

2. Disaccharides (Double Sugars)

Formed by the condensation reaction (dehydration synthesis) of two monosaccharides with the removal of a water molecule.

Examples of Disaccharides:

1. Sucrose (Glucose + Fructose): Common table sugar.
2. Lactose (Glucose + Galactose): Milk sugar.
3. Maltose (Glucose + Glucose): Found in germinating grains.

Properties of Disaccharides:

1. Soluble in water.
2. Some (like lactose and maltose) are reducing sugars, while sucrose is a non-reducing sugar.

3. Polysaccharides (Complex Carbohydrates)

Polysaccharides are long chains of monosaccharides linked by glycosidic bonds. They serve as energy storage molecules and structural components.

Examples of Polysaccharides:

Storage Polysaccharides

• Starch: The storage form of carbohydrates in plants (found in potatoes, rice, wheat).
• Glycogen: The storage form in animals, primarily in the liver and muscles.

Structural Polysaccharides

• Cellulose: A major component of plant cell walls, providing rigidity.
• Chitin: Found in the exoskeleton of arthropods (e.g., insects, crustaceans) and fungal cell walls.

Properties of Polysaccharides:

1. Insoluble in water.
2. Do not taste sweet.
3. Serve structural and storage functions.

Functions of Carbohydrates

1. Energy Source: Glucose is the primary fuel for cellular respiration, generating ATP.
2. Energy Storage: Plants store energy as starch, and animals store it as glycogen.
3. Structural Role: Cellulose provides rigidity to plant cells, while chitin strengthens exoskeletons.
4. Biosynthesis of Macromolecules: Ribose and deoxyribose are essential for nucleic acid synthesis (DNA & RNA).
5. Precursor for Other Biomolecules: Carbohydrates form glycoproteins and glycolipids, essential for cell signaling.

 

Metabolism of Carbohydrates

Carbohydrate metabolism primarily involves glycolysis, the Krebs cycle, and oxidative phosphorylation, where glucose is broken down to produce ATP.

Key Metabolic Pathways:

1. Glycolysis: Breakdown of glucose into pyruvate, producing ATP and NADH.
2. Gluconeogenesis: Formation of glucose from non-carbohydrate sources.
3. Glycogenesis: Conversion of glucose to glycogen for storage.
4. Glycogenolysis: Breakdown of glycogen to release glucose.

Sources of Carbohydrates

• Plant Sources: Fruits, vegetables, grains (rice, wheat, maize), and tubers (potatoes, sweet potatoes).
• Animal Sources: Dairy products (milk, cheese, yogurt contain lactose).

Importance of Carbohydrates in Health

1. Essential for Brain Function: The brain relies on glucose for energy.
2. Maintains Blood Sugar Levels: Carbohydrates regulate insulin production and glucose levels.
3. Supports Digestive Health: Dietary fiber (a type of carbohydrate) aids in digestion and prevents constipation.
4. Prevention of Chronic Diseases: Whole grains and fiber-rich carbohydrates reduce the risk of heart disease and diabetes.

Deficiency and Disorders Related to Carbohydrates

1. Hypoglycemia: Low blood sugar due to inadequate carbohydrate intake.
2. Diabetes Mellitus: A condition where blood sugar levels are too high due to insulin resistance or deficiency.
3. Lactose Intolerance: Inability to digest lactose due to a deficiency of lactase enzyme.
4. Obesity and Cardiovascular Diseases: Excess intake of refined carbohydrates can lead to weight gain and heart problems.

 

References

1. Nelson, D. L., & Cox, M. M. (2017). Lehninger Principles of Biochemistry. W. H. Freeman.
2. Berg, J. M., Tymoczko, J. L., & Stryer, L. (2015). Biochemistry. W. H. Freeman.
3. Voet, D., Voet, J. G., & Pratt, C. W. (2016). Fundamentals of Biochemistry: Life at the Molecular Level. Wiley.
4. Guyton, A. C., & Hall, J. E. (2021). Textbook of Medical Physiology.