15.1 Introduction to Carbohydrates

Carbohydrates are the most abundant class of organic compounds in nature. They are macromolecules primarily composed of carbon, hydrogen, and oxygen atoms. Structurally, each carbon atom is bonded to at least one oxygen atom, and all carbohydrates contain either an aldehyde or a keto group, along with at least one hydroxyl group.

The general formula for many carbohydrates is ${\mathrm{C}}_{\mathrm{x}}{\left({\mathrm{H}}_2\mathrm{O}\right)}_{\mathrm{y}}$, which led to the historical name "hydrates of carbon."

Plants synthesize carbohydrates through photosynthesis:

$6{\mathrm{H}}_2{\mathrm{O}}_{(\ell )}+6{\mathrm{CO}}_{2(\mathrm{g})}\stackrel{\begin{array}{c}\text{Sunlight}\\ \text{Chlorophyll}\end{array}}{\to }{\mathrm{C}}_6{\mathrm{H}}_{12}{\mathrm{O}}_{6(\mathrm{aq})}+6{\mathrm{O}}_{2(\mathrm{g})}$

Plants then convert the synthesized glucose into more complex forms like starch and cellulose.

Functions of Carbohydrates

Carbohydrates are essential for living organisms and serve several key functions:

• (a) Energy Provision: They are the primary source of energy for cellular activities.
• (b) Structural Support: They form structural components, such as cellulose in plant cell walls.
• (c) Growth and Development: They are vital for the growth and development of organisms.

The complex, branched structures of carbohydrates allow for efficient storage. These large molecules can be easily hydrolyzed (broken down by water) into glucose, which cells can readily use for energy.

15.1.2 Classification of Carbohydrates

Carbohydrates are classified into three main groups based on the number of sugar units they contain upon hydrolysis.

1. Monosaccharides
2. Oligosaccharides
3. Polysaccharides

1. Monosaccharides

Monosaccharides are the simplest carbohydrates, often called simple sugars. They cannot be hydrolyzed into smaller carbohydrate units.

• General Formula: $({\mathrm{CH}}_2\mathrm{O}{)}_{\mathrm{n}}$, where n is typically 3 to 6.

• Classification by Carbon Atoms:

  • Trioses (3 carbons)
  • Tetroses (4 carbons)
  • Pentoses (5 carbons)
  • Hexoses (6 carbons)

• Examples:

  • Glucose and fructose are the most common monosaccharides, both with the formula ${\mathrm{C}}_6{\mathrm{H}}_{12}{\mathrm{O}}_6$.
  • Glucose is a pentahydroxy aldehyde (an aldohexose).
  • Fructose is a pentahydroxy ketone (a ketohexose).

• Open-chain structures:

• Physical Properties:
  • White crystalline solids.
  • Soluble in water.
  • Sweet taste.
  • Cannot be hydrolyzed.
  • Generally reducing sugars due to the presence of a free aldehyde or ketone group.
• Dextro-rotatory Sugars: Monosaccharides like glucose, mannose, and galactose can rotate plane-polarized light to the right (clockwise) and are known as dextrose sugars.

2. Oligosaccharides

Oligosaccharides are carbohydrates that yield 2 to 10 monosaccharide molecules upon hydrolysis.

• Classification by Number of Units:
  • Disaccharides (yield 2 units)
  • Trisaccharides (yield 3 units)
  • Tetrasaccharides (yield 4 units)
• Physical Properties:
  • White crystalline solids.
  • Sweet taste.
  • Soluble in water.
• Hydrolysis Examples:
  • Sucrose (a disaccharide) hydrolyzes to glucose and fructose: ${\mathrm{C}}_{12}{\mathrm{H}}_{22}{\mathrm{O}}_{11}+{\mathrm{H}}_2\mathrm{O}\to {\mathrm{C}}_6{\mathrm{H}}_{12}{\mathrm{O}}_6\text{ (glucose)}+{\mathrm{C}}_6{\mathrm{H}}_{12}{\mathrm{O}}_6\text{ (fructose)}$
  • Lactose (a disaccharide) hydrolyzes to glucose and galactose: ${\mathrm{C}}_{12}{\mathrm{H}}_{22}{\mathrm{O}}_{11}+{\mathrm{H}}_2\mathrm{O}\to {\mathrm{C}}_6{\mathrm{H}}_{12}{\mathrm{O}}_6\text{ (glucose)}+{\mathrm{C}}_6{\mathrm{H}}_{12}{\mathrm{O}}_6\text{ (galactose)}$
  • Maltose (a disaccharide) hydrolyzes to two glucose units: ${\mathrm{C}}_{12}{\mathrm{H}}_{22}{\mathrm{O}}_{11}+{\mathrm{H}}_2\mathrm{O}\to 2{\mathrm{C}}_6{\mathrm{H}}_{12}{\mathrm{O}}_6\text{ (glucose)}$

3. Polysaccharides

Polysaccharides are complex carbohydrates that yield a large number (hundreds to thousands) of monosaccharides upon hydrolysis.

• Examples: Starch, cellulose, and glycogen.
• Physical Properties:
  • Amorphous (non-crystalline) solids.
  • Tasteless.
  • Insoluble in water.
  • Generally non-reducing sugars, as the aldehyde/ketone groups are tied up in glycosidic bonds.

Table 15.1: Summary of Carbohydrate Properties

15.1.3 Carbohydrates and Health Conditions

• Cardiovascular Disease Risk: Diets high in simple sugars, particularly fructose, can increase triglyceride levels, raising the risk of heart disease.
• Blood Sugar Control & Diabetes: Careful management of carbohydrate intake is essential for controlling blood sugar levels and managing diabetes.
• Weight Gain & Obesity: Excessive intake of simple carbohydrates and high-calorie foods can lead to weight gain.
• Lowering Cholesterol: Fiber, a type of polysaccharide found in whole grains, helps reduce LDL ("bad") cholesterol.
• Digestive Health: Fiber promotes healthy gut bacteria and prevents constipation.

15.1.4 Nutritional Importance of Carbohydrates

Carbohydrates have five primary roles in the body:

1. Energy Production: Glucose from carbohydrates is the main energy source for cells. 1 gram of glucose provides about 15.6 kJ of energy.
2. Energy Storage: Excess glucose is stored as glycogen in the liver and muscles, or converted to fat.
3. Building Macromolecules: Glucose is converted into ribose and deoxyribose, essential components of Nucleic Acids→, RNA, DNA, and ATP.
4. Protein Sparing: When carbohydrate intake is sufficient, the body does not need to break down Proteins→ for energy, allowing it to be used for tissue building and repair.
5. Fat Metabolism Support: Carbohydrates are necessary for the complete breakdown of fats.

15.1.5 Glycogen - A Store House

Glycogen is a highly branched polysaccharide of glucose that serves as the primary form of energy storage in animals. It is mainly stored in the liver and muscles. When energy is needed, glycogen is quickly broken down into glucose, which then enters cellular respiration to produce ATP (adenosine triphosphate), the cell's energy currency.

15.1.6 Complex Carbohydrates for Joint Lubrication

Joints like elbows and knees require lubrication for smooth movement. This is provided by complex carbohydrates, including:

• Glucosamine
• Glycosaminoglycans (GAGs)
• Proteoglycans

Glycosaminoglycans (GAGs) are long, unbranched polysaccharides that are a major component of joint cartilage, providing essential lubrication and shock absorption.