Carbohydrates

Table of contents

Definition

Carbohydrates are biomolecules consisting of carbon (C), hydrogen (H), and oxygen (O), typically following the empirical formula (CH₂O)n, where “n” represents the number of carbon atoms. Carbohydrates are essential for energy storage, structural support, and many metabolic processes in living organisms.

Properties of Carbohydrates

Chemical Composition:
- Made up of carbon (C), hydrogen (H), and oxygen (O) atoms, generally in the ratio of 1:2:1.
Solubility:
- Simple carbohydrates (monosaccharides and some disaccharides) are generally water-soluble because they have hydroxyl (-OH) groups, which form hydrogen bonds with water molecules.
- Complex carbohydrates (polysaccharides) vary in solubility; some, like cellulose, are insoluble due to their large size and structure.
Sweetness:
- Simple sugars like glucose and fructose taste sweet, while complex carbohydrates like starch and cellulose lack sweetness.
Reducing and Non-reducing Properties:
- Reducing sugars (e.g., glucose and lactose) have free aldehyde or ketone groups to participate in redox reactions.
- Non-reducing sugars (e.g., sucrose) lack free aldehyde or ketone groups and do not act as reducing agents.
Polymerization:
- Carbohydrates can polymerize to form complex molecules. For instance, glucose molecules can link to form starch, glycogen, or cellulose, each with distinct properties and functions.
Structural Variability:
- Carbohydrates exhibit structural diversity, allowing them to form linear, branched, or highly complex chains, contributing to their function versatility.

Functions of Carbohydrates

Primary Energy Source:
- Carbohydrates are the main source of energy for living organisms. During digestion, carbohydrates are broken down into glucose, which cells use to produce ATP (adenosine triphosphate) through cellular respiration.
- Example: Glucose is quickly absorbed and provides immediate energy, which is particularly crucial for brain function and physical activities.
Energy Storage:
- Polysaccharides like starch (in plants) and glycogen (in animals) are stored energy reserves.
- Example: Glycogen in the liver and muscles can be rapidly broken down into glucose to maintain blood sugar levels during fasting or exercise.
Structural Support:
- Some carbohydrates provide structural integrity to cells and tissues.
- Example: Cellulose, a polysaccharide found in plant cell walls, gives rigidity and support to plants. Chitin, another polysaccharide, forms the exoskeleton of insects and crustaceans.
Cell Recognition and Signaling:
- Carbohydrates attached to proteins and lipids on cell surfaces (glycoproteins and glycolipids) play critical roles in cell recognition, signaling, and immune response.
- Example: Blood group antigens on red blood cells are carbohydrate structures determining blood type and compatibility.
Protection and Lubrication:
- Some carbohydrates serve protective roles in the body or function as lubricants.
- Example: Mucus, which contains glycoproteins, protects internal body surfaces (such as the respiratory and digestive tracts) and lubricates tissues.
Fiber and Digestive Health:
- Indigestible carbohydrates, such as dietary fiber (mainly cellulose), help maintain digestive health by adding bulk to stool, promoting regular bowel movements, and preventing constipation.
- Example: Fiber-rich foods like whole grains, fruits, and vegetables contribute to gut health and reduce the risk of various digestive disorders.
Osmotic Balance:
- Carbohydrates play a role in osmotic balance in cells, helping to regulate water movement in and out of cells.
- Example: In plants, the carbohydrate sucrose is involved in osmoregulation, ensuring proper cell water uptake.

Classification of Carbohydrates

Carbohydrates are classified into three major types based on the number of sugar units they contain:

Monosaccharides – Simple sugars, single-unit carbohydrates.
Disaccharides – Composed of two monosaccharide units.
Polysaccharides – Complex carbohydrates made of many sugar units.

Monosaccharides

Monosaccharides are the simplest and most basic form of carbohydrates. They are single sugar molecules that cannot be hydrolyzed (broken down) into simpler sugar units. They are the building blocks for all other types of carbohydrates.

Structure and Types of Monosaccharides

Monosaccharides can be classified based on:

The number of carbon atoms:
- Trioses: 3 carbon atoms (e.g., glyceraldehyde).
- Pentoses: 5 carbon atoms (e.g., ribose, found in RNA).
- Hexoses: 6 carbon atoms (e.g., glucose, fructose).
The type of functional group:
- Aldoses: Contain an aldehyde group (-CHO). Example: Glucose.
- Ketoses: Contain a ketone group (-CO). Example: Fructose.

Examples of Monosaccharides:

Glucose: The most important hexose sugar in human metabolism. Glucose is the primary energy source for most living organisms and is crucial for cellular respiration.
- Sources: Found in fruits, honey, and as part of many carbohydrates.
Fructose: Known as the sweetest naturally occurring sugar, fructose is commonly found in fruits and honey.
- Sources: Fruits, honey, root vegetables.
Galactose: Less sweet than glucose and fructose, galactose is part of lactose, the sugar found in milk.
- Sources: Milk and dairy products (as part of lactose).

Functions of Monosaccharides:

Immediate energy source: Monosaccharides like glucose are easily absorbed into the bloodstream and provide quick energy for cellular functions.
Building blocks for more complex carbohydrates: Monosaccharides are the foundational units synthesizing disaccharides and polysaccharides.
Role in cellular communication: Some monosaccharides are involved in cell recognition and signaling, forming part of glycoproteins and glycolipids in cell membranes.

Disaccharides

Disaccharides comprise two monosaccharide units linked by a glycosidic bond (a covalent bond formed through dehydration). They are a common form of carbohydrates found in many foods.

Examples of Disaccharides:

Sucrose (Glucose + Fructose):
- Common name: Table sugar.
- Sources: Found naturally in plants like sugarcane and sugar beet.
- Function: Major dietary source of energy. When consumed, sucrose is broken down into glucose and fructose for absorption.
Lactose (Glucose + Galactose):
- Common name: Milk sugar.
- Sources: Found in milk and dairy products.
- Function: Provides energy for infants and mammals. It requires the enzyme lactase for digestion, and lactose intolerance occurs when lactase production is insufficient.
Maltose (Glucose + Glucose):
- Common name: Malt sugar.
- Sources: Formed during the breakdown of starch, found in germinating grains and malted products like beer and cereals.
- Function: An intermediate in the digestion of starch, maltose is broken down into glucose molecules for energy.

Functions of Disaccharides:

Energy source: Like monosaccharides, disaccharides are broken down into their component sugars during digestion, providing energy.
Transportable form: In plants, disaccharides like sucrose are transported through the phloem to various parts of the plant as energy storage.
Serve as sweeteners: Disaccharides like sucrose are used as sweeteners in food products due to their taste and energy content.

Polysaccharides

Polysaccharides are large, complex carbohydrates of long chains of monosaccharide units, often numbering in the hundreds or thousands. Glycosidic bonds link these sugar chains. Polysaccharides are important in energy storage, structural integrity, and biological processes.

Polysaccharides are subclassified into two groups.

Homopolysaccharides (Homoglycans): When a polysaccharide comprises several units of the same monosaccharide unit only, it is called homopolysaccharide.

a. Starch

b. Glycogen

c. Cellulose

Heteropolysaccharides (Heteroglycans): They contain two or more different types of monosaccharide units or their derivatives.

For example, Glycosaminoglycans

Types of Polysaccharides:

Storage polysaccharides: Serve as energy reserves.
- Starch: The primary storage form of carbohydrates in plants. Starch is composed of two types of molecules:
  - Amylose: A linear chain of glucose molecules.
  - Amylopectin: A branched chain of glucose molecules.
  - Sources: Potatoes, grains (rice, wheat), corn, and legumes.
  - Function: Starch is broken down into glucose for energy during digestion.
- Glycogen: The storage form of glucose in animals, mainly stored in the liver and muscles. Glycogen is highly branched and can be rapidly mobilized when the body needs glucose for energy.
  - Sources: Found in animal tissues (muscle, liver), though not consumed significantly in the diet.
  - Function: Glycogen is broken down into glucose when blood sugar levels are low or during physical exertion.
Structural polysaccharides: Provide structural support.
- Cellulose: A major component of plant cell walls. Cellulose is a long, unbranched chain of glucose molecules linked by β-1,4-glycosidic bonds, making it indigestible for humans.
  - Sources: Found in all plant-based foods such as fruits, vegetables, and grains.
  - Function: Provides structural strength to plants, contributing to the rigidity and stability of cell walls. In humans, cellulose is a dietary fiber, aiding digestion and promoting intestinal health.
- Chitin: Found in the exoskeletons of arthropods (e.g., insects, crabs) and the cell walls of fungi. Chitin is similar to cellulose but contains nitrogen.
  - Sources: Crustaceans, insects, fungi.
  - Function: Provides structural strength and protection in animals and fungi.

Functions of Polysaccharides:

Energy storage: Starch in plants and glycogen in animals serve as long-term energy reserves. They can be broken down into glucose when energy is needed.
Structural role: Cellulose in plant cell walls and chitin in exoskeletons and fungi provide structural integrity and protection.
Regulation of digestion: Indigestible polysaccharides, such as dietary fibers, are crucial in maintaining healthy digestion and regulating blood sugar levels.