The Biological Role of Starch
Starch is a fundamental molecule in the biological world, primarily serving as the energy storage mechanism for plants. During photosynthesis, green plants convert carbon dioxide and water into glucose using energy from sunlight. While some of this glucose is used immediately for energy, any excess is converted into starch and stored for later use. This ensures that the plant has a ready supply of energy to draw upon during periods without sunlight, such as at night, or during seasonal dormancy. Starch provides a compact and osmotically inactive way for plants to store large amounts of glucose, unlike free glucose which would take up more space and cause osmotic issues. It is this vital role that makes starch so prevalent in plant-based foods that form the basis of many human diets.
Where is starch stored in plants?
Starch is stored in different parts of a plant, depending on the species and its function. This storage can be categorized into two main types based on location:
- Transitory Starch: This is starch that is produced and stored temporarily in the chloroplasts of green leaves during the day, where photosynthesis occurs. It is broken down at night to provide the plant with a continuous energy supply.
- Reserve Starch: This refers to starch stored for long-term use in specialized storage organs. This is the type of starch we most commonly consume in our diets.
Common storage locations include:
- Tubers: Potatoes are a classic example, storing large quantities of starch in their underground tubers.
- Seeds and Grains: The endosperm of grains like wheat, rice, and corn is packed with starch to provide energy for the developing seedling.
- Roots: Root vegetables such as cassava and sweet potatoes are major sources of starch.
- Fruits: Fruits like unripe bananas contain starch, which converts to sugar as they ripen.
The chemical structure of starch: Amylose and Amylopectin
Starch is not a single molecule but rather a mix of two types of polymers made from glucose units: amylose and amylopectin.
- Amylose: A linear, unbranched chain of glucose molecules linked by alpha-1,4 glycosidic bonds. It forms a helical structure that is responsible for the characteristic deep blue-black color reaction with iodine.
- Amylopectin: A highly branched chain of glucose molecules with both alpha-1,4 and alpha-1,6 glycosidic bonds. It comprises the majority of starch (75-80%) and gives a less intense orange-yellow color with iodine.
Detecting the presence of starch with iodine
The most common and reliable method for detecting starch is the iodine test. This simple chemical test takes advantage of the unique interaction between iodine and the amylose component of starch.
How the iodine test works
- Preparation: A sample of the substance to be tested (e.g., a slice of potato or a leaf) is placed on a tile or in a test tube. For leaves, chlorophyll must first be removed by boiling the leaf in alcohol.
- Application: A few drops of iodine solution (containing potassium iodide) are added to the sample.
- Observation: If starch is present, the brownish-yellow iodine solution turns a distinct blue-black or deep purple color almost immediately. If no starch is present, the color of the solution remains unchanged.
The distinctive blue-black color is due to the polyiodide ions in the solution becoming trapped within the helical structure of the amylose molecule, forming a colored complex. This test is specific to starch and does not produce a positive result for other sugars like glucose or sucrose.
Comparing common starch sources
Different plants produce starch granules with varying characteristics, including size, shape, and amylose-to-amylopectin ratio. These properties influence how the starch behaves in food and industrial applications.
| Feature | Potato Starch | Rice Starch |
|---|---|---|
| Granule Size | Larger granules (up to 100µm) | Relatively smaller granules (about 2µm) |
| Granule Shape | Spherical, oval, or irregular | Polygonal and angular |
| Amylose Content | Relatively low (~20%) | Higher (~25%) |
| Gelatinization Temp | Lower gelatinization temperature | Higher gelatinization temperature |
| Texture in Food | Excellent thickening agent, forms a clear gel | Tends to form a thicker, more opaque paste |
| Birefringence | Strong, distinct "Maltese cross" under polarized light | Less pronounced than potato starch |
Conclusion
In conclusion, starch is primarily found as a crucial energy reserve in the cells of green plants, stored in granular form within various plant organs such as seeds, tubers, and roots. Its presence is readily and reliably detected using the iodine test, which produces a characteristic blue-black coloration due to the reaction with amylose, a component of the starch granule. From the microscopic granules within a plant cell to the staple foods on our plates, starch is ubiquitous, serving as a fundamental energy source for plant life and a major nutritional component of the human diet. Understanding how is starch found provides insight into plant biology and the composition of the foods we consume daily.
