Starch granules: shape, A-type and B-type
Starch is a carbohydrate produced by plant photosynthesis. It serves as an energy storage within plants and is a primary source of energy for humans, especially in East Asian countries where rice is a staple food. Amylopectin and amylose cluster together to form granules whose sizes rang from sub-micrometers to several hundred micrometers. Starch granules exhibit a variety of shapes, including spherical, oval, disc-shaped, polyhedral, and elongated rods.
Starch granules are categorized into two types: Type A and Type B. Lens-shaped Type A starch granules are approximately 20 micrometers in length, while spherical Type B starch granules are less than 10 micrometers. Research indicates that the majority of cereal starches are Type A, whereas tuber starches are typically Type B. Additionally, there is a type C starch granule that is intermediate between types A and B, and pea is the common example. There are tiny pores on type A starch surface, while type B have none. These pores extend from surface to interior and serve as action sites for hydrolase. During digestion and germination, these pores facilitate the rapid hydrolysis of starch granules.
Despite their varying shapes and sizes, their internal structure is almost universally shared among plants: double helices, lamella, blocklets and growth rings.
Growth Rings
Under an electron microscope, all starch granules display alternating light and dark concentric rings attributed to starch production during plant photosynthesis. There is no clear boundary between the semi-crystalline and amorphous regions, and they transition gradually. The bright rings are amorphous regions about 100nm thick, which expand outward from the hilum evenly and alternately. The randomly distributed amylose is their primary component ,and some amylopectin extend from the semi-crystalline regions. The loose structure makes them easily destroyed by enzymes or dilute acids.
The dark rings are semi-crystalline regions ranging from 100 to 500nm in thickness. These rings are thinner on the starch periphery and thicker internally. Amylopectin side chains are wound into double helices and are regularly arranged into crystals. There is also a little of amylose distributed in the crystal gaps loosely, and they link to amylopectin without double helices. The following facts illustrate that amylopectin is the main component of the crystalline rings, and responsible for structure of starch granules. The birefringence is still existed in starch granules even the amylose is removed by dilute acid; the X-ray diffraction patterns of waxy starch granules, which contain only amylopectin, still indicate a crystalline structure; and the decrease in crystallinity in amylose-rich peas.
Blocklets
In the conventional clustered model, the lamella is the next level of the growth ring. However, researchers have observed protrusions on starch surface with the help of atomic force microscopy (AFM) and scanning electron microscopy (SEM). The protrusions are considered an intermediate structure named blocklets. Their size depends on plant species and location within the granule. Blocklets are 400-500 nm long in B-type starch (potato) and 25-100 nm long in A-type starch (wheat). Larger blocklets are distributed on the starch granule surface, while smaller blocklets are located inside the granule. Whatever the size, they are very similar in structure. The ellipsoid-like blocklet is composed of multiple amylopectins that play an important role in blocklet architecture. They are arranged in parallel with the reducing ends towards the hilum of starch granules. Amylose is also considered to be a supplementary material for blocklets or a link connecting other blocklets, thus enhancing the strength and flexibility of the starch granule.
Blocklets are found in both the amorphous and crystalline regions of the growth ring. They are continuously distributed throughout the starch. Normal blocklets are tightly arranged in the crystalline region, whereas defective blocklets make the amorphous region loose and chaotic. It is interesting that the blocklets size may be unrelated to starch granule size and the thickness of growth rings.
Crystalline and Amorphous Lamella
X-ray diffraction experiments have shown that the amylopectin-rich crystalline region also displays an alternating structure. The side chains of amylopectin are responsible for constructing the crystalline lamella. The branches near the branching points are no longer arranged parallel, so they form the loose amorphous lamella with amylose. Each repeated unit that is about 10 nm thick contains one amorphous lamella and one crystalline lamella. The crystalline lamella is about 4-6 nm thick, while the amorphous lamella is 3-6 nm thick. The size of the repeating units is conserved in all plants. About several dozen repeating units constitute the crystalline region of growth ring.
Amylose and Amylopectin
Amylopectin and amylose are the smallest structures in starch granules. Amylose is a long, unbranched chain of glucose molecules linked by 1,4-glycosidic bonds. Some amylose molecules may have a little of branches (less than 1% branching points). Their degree of polymerization ranges from several hundred to several thousand. Amylopectin is a highly branched chain of glucose molecules linked by 1,4-glycosidic bonds and 1,6-glycosidic bonds. Each branch averages 20-30 glucose residues. Amylopectin is a very large molecule with a degree of polymerization ranging from several thousand to several hundred thousand. Generally, about 70% of the starch in plants is amylopectin, which is the main component of the crystalline region. Amylose, on the other hand, is distributed in the amorphous regions. They are abundant around the periphery of the starch granule and near the hilum.