What Are Carbohydrates?

Carbohydrates are the most abundant organic molecules on Earth. They are essential for all living organisms, from bacteria to humans, serving as the primary source of energy. The name carbohydrate literally means "hydrates of carbon," as these molecules are composed of carbon (C), hydrogen (H), and oxygen (O) atoms, with the general formula C_x(H₂O)_y. While many carbohydrates fit this formula, some do not, and some non-carbohydrates do.

1.0Classification of Carbohydrates

On the basis of number of saccharide units obtained upon hydrolysis, Carbohydrates are classified as:

Monosaccharides 

• They are the simplest sugars that cannot be further hydrolysed.
• In their generalised formula, x is mainly equal to y, i.e., the. number of Carbon and Oxygen atoms is the same.
• All monosaccharides occur in d and l forms, except Dihydroxyacetone, because a chiral carbon is absent in Dihydroxyacetone.
• Chiral Carbon:- A Carbon atom that is attached to four different types of atoms or groups of atoms.
• The structure of saccharides is either a ring or a straight chain.
• A six-membered ring is known as a pyranose and a five-membered ring is furanose. Pyranose and furanose were given by "Haworth."

Classification of monosaccharides based on the number of carbons

Oligosaccharides

• Oligosaccharides are those carbohydrates that, on hydrolysis, yield 2 to 10 monosaccharide units (monomers).
• In oligosaccharides, monosaccharides are linked together by glycosidic bonds.
• The aldehyde or ketone group of one monosaccharide reacts with the alcoholic group of another monosaccharide to form a glycosidic bond.
• One molecule of H₂O is eliminated during glycosidic bond formation (dehydration synthesis).
• The direction of the glycosidic bond is mostly 1'-4".
• When another monosaccharide unit is fructose, then the direction of linkage is 1'-2". (Non-reducing sugars). For e.g. Sucrose

Disaccharides 

• Composed of two monosaccharide units. e.g. Maltose, Sucrose, Lactose, Trehalose.
• All disaccharides are water soluble and sweet in taste, so they are known as sugar.
• Sucrose: A common table sugar made of glucose + fructose.

• Lactose: The sugar found in milk, made of glucose + galactose.
• Maltose: A sugar found in germinating seeds and fermented foods, made of glucose + glucose.

• All monosaccharides are reducing in nature.
• Some disaccharides/oligosaccharides are reducing in nature while others are non-reducing in nature. (Sucrose and Trehalose)
• All polysaccharides are non-reducing in nature.

Summary Table for Disaccharides

Polysaccharides/Glycans

• Polysaccharides are composed of large number of monosaccharide units.
• Polysaccharides are insoluble in water and do not taste sweet.
• All polysaccharide are non-reducing but in a polysaccharide chain, the right end is called the reducing end and the left end is called the non-reducing end.
• According to function, they are classified as nutritive and structural.

Homopolysaccharides Composed of same monomers. Biologically important homopolysaccharides are as follows:

Cellulose:– Unbranched, linear polymer of -D-glucose units (6000 to 10,000). It has  1'-4" linkage.

• Cellulose is the main component of the plant cell wall. In wood, cellulose is 50% and in cotton, it is 90%.
• It is the most abundant organic compound of the plant kingdom.
• It is also used to form Rayon fibre (Artificial silk).
• Paper made from plant pulp is cellulose.

• Starch – It is the main stored food in plants. Starch is a polymer of -D-glucose units. Starch gives a blue colour with iodine solution. Starch consists of two types of chains.
• Glycogen: The main energy storage polysaccharide in animals and fungi. It's stored primarily in the liver and muscles. Glycogen is a highly branched polymer of glucose.
• Cellulose: A major structural component of plant cell walls. It is the most abundant organic polymer on Earth. Humans can't digest cellulose due to the specific type of glycosidic bonds (beta-1,4), so it acts as dietary fiber.
• Chitin: A structural polysaccharide found in the exoskeletons of arthropods (like insects and crustaceans) and the cell walls of fungi.

Heteropolysaccharides:– Composed of different monosaccharide units.

• Hyaluronic acid – Found in the extracellular matrix of connective tissue in the form of a lubricating agent. It also occurs in the animal cell coat as a binding material (Animal cement). Hyaluronic acid is made up of D-glucuronic acid and N–acetyl–D–glucosamine arranged in alternate order.

It accounts for the toughness and flexibility of cartilage and tendons.

• Chondroitin – Chondroitin occurs in connective tissue.
• Heparin – an anticoagulant used to prevent blood clotting.
• Pectins – Pectin is found in the cell wall, where it binds cellulose fibrils in bundles. Salts of pectin, i.e. Ca and Mg-pectates, form the middle lamella in plants (Plant cement).
• Hemicellulose – Present in the cell wall.
• Agar is a mucopolysaccharide obtained from red algae. It is composed of galactose and 3,6-anhydro-L-glucopyranose. It is used as a culture medium in the laboratory.

2.0Examples of Carbohydrates in Food

Carbohydrates are found in a wide variety of foods. Here are some common examples:

• Simple Carbohydrate Sources:
• • Natural Sugars: Fruits (e.g., apples, bananas, berries), milk, and honey.
  • Added Sugars: Candy, soda, cookies, pastries, and many processed foods.
• Complex Carbohydrate Sources:
• • Grains: Bread, pasta, rice (especially whole-grain options), oatmeal, and cereals.
  • Starchy Vegetables: Potatoes, corn, peas, and sweet potatoes.
  • Legumes: Beans, lentils, and chickpeas.
  • Fiber-Rich Foods: Whole grains, fruits, vegetables, nuts, and seeds.

Functions of Carbohydrates

Carbohydrates are vital for life and perform a variety of crucial functions:

• Energy Source: The most important function. Glucose is broken down through cellular respiration to produce ATP, the energy currency of the cell.
• Energy Storage: Starch in plants and glycogen in animals serve as readily available energy reserves.
• Structural Components: Cellulose provides rigidity to plant cells, and chitin strengthens the exoskeletons of insects.
• Biochemical Precursors: Carbohydrates can be converted into other important biological molecules like fatty acids and amino acids..

3.0Carbohydrate Benefits

Carbohydrates are not just energy suppliers — they play multiple vital roles in maintaining overall biological balance and body function. Below are the key benefits of carbohydrates in living organisms:

1. Primary Energy Source

Carbohydrates are the body’s main source of energy. Glucose, derived from carbohydrate breakdown, fuels cells, tissues, and organs — especially the brain and muscles.

2. Energy Storage

In animals, excess glucose is stored as glycogen in the liver and muscles. Plants store carbohydrates as starch, ensuring a continuous supply of energy when needed.

3. Protein-Sparing Action

When carbohydrates are available, the body uses them for energy instead of breaking down proteins. This allows proteins to perform their primary functions — tissue repair and enzyme formation.

4. Fat Metabolism Regulation

Carbohydrates aid in the oxidation of fats. Without adequate carbohydrates, fats are incompletely oxidized, leading to the production of ketone bodies (a condition known as ketosis).

5. Structural Support

Complex carbohydrates like cellulose and chitin provide structural strength in plants and arthropods, contributing to their rigidity and protection.

6. Brain and Nervous System Function

The brain relies almost entirely on glucose for its energy. A steady carbohydrate supply ensures proper nerve impulse transmission and cognitive performance.

7. Digestive Health

Dietary fibers, which are complex carbohydrates, promote healthy digestion by adding bulk to stool, supporting gut bacteria, and preventing constipation.

8. Cellular Communication

Carbohydrates form part of cell membranes as glycoproteins and glycolipids, which help in cell recognition, signaling, and immune response.

9. Maintaining Blood Sugar Levels

Carbohydrates help regulate blood glucose levels, preventing hypoglycemia or excessive spikes that can affect metabolic stability