Carbohydrates

Carbohydrates are commonly associated with starches in foods like bread and pasta. Biologically, they are built from sugar, which serves as their fundamental building block. Beyond providing energy, carbohydrates also offer structural support, as seen in cellulose in plants and chitin in insect exoskeletons and fungi.

Sugars are scientifically called saccharides. Carbohydrates are categorized by the number of sugar molecules they contain: monosaccharides (one sugar unit, e.g., glucose), disaccharides (two sugar units, e.g., sucrose/table sugar, lactose/milk sugar), oligosaccharides (three to ten sugar units), and polysaccharides (many sugar units, e.g., glycogen, starch).

All carbohydrates share an empirical formula with a 1:2:1 ratio of carbon, hydrogen, and oxygen (C:H2O), hence the name 'carbohydrate'. Glucose, a six-carbon monosaccharide, is fundamental for life, used in cellular respiration and produced by plants. Other key monosaccharides include fructose (a five-sided sugar found in fruit, sweeter than glucose) and galactose (less sweet than glucose). These are easily absorbed into the bloodstream.

Disaccharides consist of two sugar molecules bonded together. Sucrose is glucose linked to fructose, while lactose is glucose linked to galactose. Enzymes like sucrase and lactase are needed to break these down into monosaccharides for absorption. Lactose intolerance results from a lack of lactase. Oligosaccharides (3-10 sugar units) are important in glycoproteins and glycolipids on cell membranes, aiding cell identification and attachment. Carrots are a good source of oligosaccharides, though cooking is needed to release them.

Polysaccharides are large macromolecules composed of hundreds to thousands of sugar units. Starch, found in potatoes, is how plants store energy as hundreds of glucose molecules. Glycogen is the animal equivalent, storing thousands of glucose molecules in the liver for energy. Cellulose provides structural support in plants, with hydrogen bonds making it incredibly durable. Humans cannot digest cellulose without the help of microorganisms, unlike animals such as cows.

Carbohydrate polymers are broken down through hydrolysis, which involves adding water to cleave bonds, forming smaller sugar units. Conversely, dehydration reactions remove water to form covalent bonds, building larger carbohydrate molecules from smaller ones. These processes are crucial for energy acquisition and storage.

Humans are evolutionarily programmed to like sugar as it signals the presence of nutrient-rich fruits. However, modern diets often include excessive amounts of added sugars, such as high-fructose corn syrup, which is linked to increased rates of heart disease and diabetes. While sugar is essential for energy, too much can be detrimental to health.