Application of polarizing microscope in observing starch
Polarization microscopy is widely used in fields such as minerals and chemistry, as well as in biology and botany. For example, in botany, such as identifying fibers, chromosomes, spindles, starch granules, cell walls, and whether crystals are present in cytoplasm and tissue. The application in medicine is to use a polarizing microscope to examine crystals in joint fluid. We all know that starch can be determined by staining with iodine solution, but using a polarizing microscope, it is not necessary to stain to determine whether it is starch. This is the starch granules of potatoes. Simply cut a small piece of potatoes, apply it on a glass slide, and drop a drop of water to make a water embedded glass slide for observation. At low magnification, there is no staining but ordinary particles.
Generally, starch is white or almost white, insoluble in organic solvents such as ether, ethanol, acetone, and also insoluble in cold water. Starch exists in granular form in endosperm cells, and the shape and size of starch from different sources vary. Microscopic observation can distinguish different types of starch or determine the type of unknown sample. The shape of starch particles can be roughly divided into three types: circular, elliptical, and polygonal. Generally, plants with high water content and low protein content have larger starch particles that are mostly circular or elliptical in shape, such as potato starch; On the contrary, the particles are smaller and polygonal, such as rice starch. Under a 400-600 magnification microscope, it can be observed that some starch surfaces have ring patterns, similar to the annual rings of trees, while potato starch ring patterns are extremely obvious.
However, as long as the polarizer is rotated, the world will be different. A cross will appear on the starch granules, which has a special name called Maltese Cross. The intersection of the crosses is located at the navel of the starch granules. This Malta Cross has a history. Zoom in a bit more and you can see the circular pattern on the starch granules, with the center point being the location of the navel.
