Starch is an organic structure (carbohydrate) composed of two distinct polymers, amylose and amylopectin that are all made up of repeated glucose molecules. It is used as a reserve of energy, providing plants with glucose molecules (and consequently energy) when photosynthesis can not occur (at night or in winter). In humans it's a source of glucose necessary for energy production. Starch is also used in papermaking, glue and laundry.
Starch is packed into semicrystalline granules (containing crystalline and non-crystalline section) called starch granules. These granules are either contained in plant leaves or stored for long term usage in the plant's seeds/roots/fruit. In the leaves, the starch granules are smaller and are located inside chloroplasts. This starch is termed transitory starch and is accessed during the night to provide the plant with energy. The granules contained in the plant's other organs hold starch referred to as storage starch which is reserved for long-term usage. These granules are stored inside special double-envelopped organelles called amyloplasts. Potato tubers contain this type of starch and are used as the potato plant's "battery" when the shoot of the plant has died and thus can not provide the plant with any energy (glucose) via photosynthesis.
The structure of starch granules has been debated and it's not yet clear. Nevertheless, scientists have identified some components. As the two polymers that make up starch are just repeated glucose molecules, starch consists only of glucose. Amylose is polymerized into a coiled chain of glucose molecules (no branching), while amylopectin shows a linear but branched structure. The granules consist of 10-30% amylose (percentage varies depending on source) and 70-90% amylopectin. The branched chains of amylopectin interact together and form double helices while the linear part of amylopectin that is not surrounded by its branches resides together with amylose chains. These amylose chains form the amorphous (non-crystalline) part of the granules while the packed double helices form the crystalline one.
As starch is just a chain of glucose, , molecules (and is broken down by the enzyme amylase found in our saliva) it can provide an organism with its main source of energy through aerobic or anaerobic respitation. This process uses glycose with (aerobic) or without oxygen (anaerobic) to create ATP, the energy "currency" of organisms.
When we boil rice for example, we observe the grains swelling and becoming much more soft. This manifests itself in the process of starch gelatinization.
At high temperatures, the intermolecular bonds of the starch molecules (for example the double helices formed by amylopectin) are broken down (new water-starch hydrogen bonds are formed) and thus the structure of the starch granules is altered. First the amorphous regions are disrupted and then the granule's whole structure gets effected. The granules lose their integrity and burst. Amylose (and a smaller amount of amylopectin) molecules leave the granule and contribute to the increased viscosity of the liquid.
When the temperature drops, recrystallization occurs. This is referred to as starch retrogradation and is responsible for bread staling.
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