Iodine, a chemical element (I), is the heaviest stable halogen and is widely used as a versatile reagent and diagnostic tool in various scientific fields. Its unique properties allow it to interact with specific substances, making it invaluable for identifying or measuring compounds.
Detecting Starch
One recognized application of iodine in biology is detecting starch. When an iodine solution, such as Lugol’s iodine or an iodine-potassium iodide solution, encounters starch, it produces a distinctive blue-black or deep purple coloration, indicating starch presence.
The reaction involves iodine molecules interacting with amylose, a component of starch. Amylose has a helical structure, trapping iodine molecules within its coil. This entrapment forms a unique complex that absorbs light differently, resulting in a deep blue color. While amylose is primarily responsible for this intense blue, amylopectin, the branched component of starch, typically produces a reddish-brown or purple solution due to its different structure.
The iodine-starch test is widely applied in laboratory experiments and food analysis. It commonly tests for starch in foods like potatoes, bread, or cereals. In biological studies, it detects starch in plant tissues, indicating energy storage or photosynthetic activity. This simple yet effective test also helps identify adulteration or monitor starch breakdown during hydrolysis.
Thyroid Function Assessment
Iodine, particularly its radioactive forms, is used in medical diagnostics to evaluate thyroid gland function. Radioactive isotopes like Iodine-123 (I-123) and Iodine-131 (I-131) are employed, distinct from the elemental iodine used in the starch test. The thyroid gland uniquely absorbs and utilizes iodine from the bloodstream to produce hormones, making these isotopes highly effective diagnostic tools.
The Radioactive Iodine Uptake (RAIU) test measures radioactive iodine absorbed by the thyroid gland over time. Patients swallow a small dose of the radioisotope, and a gamma probe measures accumulated radioactivity at intervals, commonly at 6 and 24 hours. The uptake percentage provides insight into thyroid activity. High uptake indicates an overactive thyroid (hyperthyroidism), often seen in Graves’ disease, while low uptake suggests an underactive thyroid or thyroiditis.
I-123 is generally preferred for diagnostic imaging due to its shorter half-life and pure gamma emission, resulting in lower patient radiation exposure and clearer images. I-131, also used diagnostically, emits both gamma and beta radiation with a longer half-life, making it more suitable for therapeutic purposes like destroying thyroid cells in hyperthyroidism or thyroid cancer. These tests diagnose various thyroid conditions by assessing how efficiently the gland processes iodine.
Microbial and Cellular Staining
Iodine also serves a valuable purpose in biological staining techniques, particularly within microbiology, where it functions as a mordant. A mordant helps chemically bind a dye to a cellular structure, intensifying the stain or making it more permanent. This enhances the visibility of cells and their components under a microscope.
The most prominent example of iodine as a mordant is in the Gram stain procedure, a fundamental bacterial classification technique. After cells are stained with crystal violet, iodine forms a large crystal violet-iodine complex within them. This complex becomes trapped in the thick peptidoglycan cell walls of Gram-positive bacteria, causing them to retain the purple color even after decolorization. Gram-negative bacteria, with thinner peptidoglycan, do not retain the complex as effectively and are decolorized, allowing counterstaining pink or red.
Beyond the Gram stain, iodine visualizes specific cellular components. It stains glycogen granules in animal and bacterial cells, and starch in plant cells, by reacting with these polysaccharides to produce a visible color. The primary objective in these staining applications is to enhance contrast, enabling clearer identification and differentiation of microorganisms and internal cellular structures for microscopic examination.