C₆H₁₂O₆ — Glucose

Glucose is a simple sugar or monosaccharide with the molecular formula \(C_6H_{12}O_6\). It is one of the most important carbohydrates in biological systems, serving as the principal energy source for both plants and animals. Often referred to as dextrose or blood sugar, glucose is the main fuel used by the body’s cells to generate energy through cellular respiration.

In nature, glucose is found in fruits, honey, and plant sap. It is a product of photosynthesis and forms the structural foundation for many carbohydrates like starch, cellulose, and glycogen. The name ‘glucose’ is derived from the Greek word glykys, meaning sweet. In the human body, glucose concentration in blood is tightly regulated to maintain homeostasis and supply continuous energy to vital organs such as the brain and muscles.

Glucose is an aldohexose, meaning it contains six carbon atoms and an aldehyde group (\(-CHO\)). Its open-chain structure can be represented as:

However, in aqueous solution, glucose primarily exists in a cyclic form due to intramolecular reaction between the aldehyde group and a hydroxyl group, forming a hemiacetal. This results in two cyclic isomers—α-D-glucose and β-D-glucose—depending on the position of the hydroxyl group on the first carbon atom (anomeric carbon).

• α-D-glucose: The hydroxyl group on the anomeric carbon (C1) is positioned below the plane of the ring.
• β-D-glucose: The hydroxyl group on the anomeric carbon is positioned above the plane of the ring.

The cyclic form of glucose is more stable and accounts for over 99% of glucose molecules in solution. These forms interconvert in solution through a process called mutarotation.

Glucose is widely distributed in nature and serves as a fundamental energy source for almost all living organisms. It is formed during the photosynthesis process in plants:

Natural sources of glucose include fruits such as grapes, bananas, apples, and berries. Honey also contains a high proportion of glucose along with fructose. In the human body, glucose is produced by digestion of carbohydrates and through the gluconeogenesis pathway in the liver, which synthesizes glucose from non-carbohydrate sources like amino acids and glycerol.

1. From Starch Hydrolysis:

Industrially, glucose is prepared by hydrolyzing starch from corn, rice, or potatoes using acid or enzymatic catalysts:

This process is widely used to produce glucose syrup and dextrose monohydrate used in food and pharmaceuticals.

2. From Cellulose Hydrolysis:

Cellulose, found in plant cell walls, can also be hydrolyzed using concentrated acids to yield glucose:

3. Photosynthetic Production:

In green plants, glucose forms through the photosynthetic reaction involving carbon dioxide, water, and sunlight as described earlier. This natural synthesis sustains nearly all life on Earth by serving as a primary energy source.

Physical Properties:

• Appearance: White crystalline solid.
• Taste: Sweet, less sweet than sucrose.
• Solubility: Readily soluble in water due to hydrogen bonding.
• Optical Activity: Glucose is optically active and exists mainly as D-glucose in nature.
• Crystallization: Forms monohydrate crystals when crystallized from water.

Chemical Properties:

• Reducing Property: Glucose reduces Fehling’s and Tollen’s reagents due to its aldehyde group in the open-chain form.

\(RCHO + 2[Ag(NH_3)_2]^+ + 3OH^- \rightarrow RCOO^- + 2Ag + 4NH_3 + 2H_2O\)

• Fermentation: Glucose undergoes fermentation by yeast to produce ethanol and carbon dioxide.

\(C_6H_{12}O_6 \xrightarrow{yeast} 2C_2H_5OH + 2CO_2\)

• Oxidation: Mild oxidizing agents convert glucose into gluconic acid, while strong oxidizing agents yield saccharic acid.
• Esterification: Reacts with acetic anhydride to form glucose pentaacetate, confirming the presence of five hydroxyl groups.

Glucose is the chief source of energy in living cells. During cellular respiration, glucose undergoes oxidation to release energy in the form of ATP (adenosine triphosphate):

This energy fuels vital biological processes such as muscle contraction, nerve impulse transmission, and biosynthesis of macromolecules. The brain depends almost entirely on glucose for energy.

Glucose also acts as a precursor for the synthesis of glycogen (storage form in animals), starch (storage form in plants), and fatty acids. It is metabolized via three main pathways: glycolysis, Krebs cycle, and the pentose phosphate pathway.

Glucose plays a vital role in human nutrition. It maintains blood sugar levels within the normal range (70–110 mg/dL). Deficiency of glucose can lead to hypoglycemia, causing weakness, confusion, and fatigue. On the other hand, excessive glucose levels, known as hyperglycemia, are a hallmark of diabetes mellitus.

In medical treatments, glucose solutions (like 5% dextrose) are administered intravenously to provide quick energy to patients and restore fluid balance. It is also used as a sweetening and energy agent in sports drinks, oral rehydration salts, and nutritional supplements.

• Food Industry: Used as a sweetener and fermentation substrate in baking, brewing, and dairy products.
• Pharmaceuticals: Serves as an energy source in IV fluids and as a stabilizer in medicinal formulations.
• Biotechnology: Used as a carbon source for microbial fermentation processes.
• Textile and Leather Industries: Acts as a reducing agent in dyeing and tanning processes.
• Biofuel Production: A key raw material for producing ethanol and biogas through fermentation.