Carbohydrates are fundamental biological molecules defined chemically by carbon, hydrogen, and oxygen atoms, often in a ratio of (CH₂O)n. They serve primarily as a source of energy for living organisms and are a major component of the human diet. This family includes simple sugars, like those in fruits, and complex starches found in grains and vegetables.
The Basic Building Block
The fundamental repeating unit, or monomer, of a carbohydrate is called a monosaccharide, which translates literally to “single sugar.” These simple sugars are the smallest form of carbohydrate and cannot be broken down further by hydrolysis. A monosaccharide typically has an empirical formula where ‘n’ ranges from three to seven. For example, common monosaccharides like glucose have the chemical formula C₆H₁₂O₆. The presence of multiple hydroxyl groups makes these molecules highly soluble in water, and in solution, they usually exist in a ring-shaped form.
Key Types of Simple Sugars
Three monosaccharides are particularly important in human nutrition: glucose, fructose, and galactose. These are classified as hexoses because they each contain six carbon atoms and share the molecular formula C₆H₁₂O₆. They are structural isomers, meaning their atoms are arranged differently, which alters their biochemical behavior.
Glucose, commonly known as blood sugar, functions as the primary fuel source for nearly all cells in the body. Fructose, or fruit sugar, is the sweetest monosaccharide and is found naturally in honey and fruits. Galactose is usually a component of lactose, the disaccharide found in milk, rather than a free sugar. The liver can interconvert these three sugars to ensure glucose is available for energy needs.
From Monomer to Complex Structure
Monosaccharides link together to form larger carbohydrate structures through dehydration synthesis, also known as a condensation reaction. This process involves the removal of a water molecule as two sugar units join. A new covalent bond, called a glycosidic bond, links the two sugar molecules via an oxygen atom. When two monosaccharides join, they form a disaccharide, such as sucrose (table sugar), which is made from one glucose and one fructose molecule. This process can repeat, linking hundreds or thousands of monomers to create polymers called polysaccharides, including starch, glycogen, and cellulose.
Primary Roles in Biology
The most recognized role of carbohydrates is providing energy. Glucose is broken down through cellular respiration to produce adenosine triphosphate (ATP), the energy currency of the cell. Certain cells, such as red blood cells and brain cells, rely almost exclusively on glucose for fuel.
Carbohydrates also serve as energy storage and structural support. In animals, excess glucose is stored as the highly branched polysaccharide glycogen, primarily in the liver and muscle tissues, which is quickly broken down when energy is required. Plants store energy as starch and use the unbranched polysaccharide cellulose to form rigid cell walls, providing structural stability.