Starch is a storage carbohydrate composed of glucose units joined into large molecules. Produced by plants and green algae, it accumulates as semicrystalline granules and supplies carbon and energy for growth and maintenance. Most starch contains two structurally distinct components, amylose and amylopectin. Its abundance in crop seeds, roots, and tubers makes it an important food constituent and a raw material for manufacturing. (pmc.ncbi.nlm.nih.gov)
Molecular structure
Starch is a polymer whose principal components differ in chain organization. Amylose consists mainly of long, relatively unbranched glucose chains, although some branching occurs. Amylopectin is a much larger, extensively branched molecule. Both contain α(1→4) glycosidic bonds along their chains and α(1→6) bonds at branch points. Amylopectin commonly accounts for about 75–90 percent of ordinary plant starches, but composition varies among species and cultivars. (pmc.ncbi.nlm.nih.gov)
Amylopectin chains form double helices that pack into ordered regions, interspersed with less ordered regions containing many branch points. This organization produces the semicrystalline structure of starch granules. Amylose occupies parts of the surrounding matrix. Granule architecture is therefore more complex than a simple mixture of two dissolved polymers, and differences in chain arrangement influence physical behavior. (pmc.ncbi.nlm.nih.gov)
Starch resembles glycogen in its linkage types, but glycogen has a different branching organization and generally does not form the same semicrystalline granules. It also differs from cellulose, whose β-linked glucose chains serve a structural rather than a storage function in plants. These differences illustrate how linkage geometry and branching can give glucose polymers distinct biological properties. (pmc.ncbi.nlm.nih.gov)
Formation and biological role
In green plants, starch synthesis occurs inside plastids: chloroplasts in photosynthetic tissues and amyloplasts in many storage tissues. Carbon assimilated through photosynthesis can enter starch synthesis through intermediates connected to the Calvin–Benson cycle. The activated glucose donor is ADP-glucose, which starch synthases use to extend growing chains. (pmc.ncbi.nlm.nih.gov)
Several enzymes act together during granule formation. Granule-bound starch synthase is chiefly responsible for amylose synthesis; other starch synthases extend amylopectin chains. Branching enzymes introduce α(1→6) linkages, while debranching enzymes remove selected branches, helping establish an organization compatible with crystalline packing. This is a coordinated process rather than a simple sequence of independent reactions. (pmc.ncbi.nlm.nih.gov)
Leaf starch often functions as a temporary reserve, accumulating during daylight and being consumed when photosynthesis stops. Experiments with Arabidopsis thaliana show that nighttime degradation adjusts to both the available starch and the anticipated duration of darkness. Storage starch in seeds and underground organs instead supports processes such as germination and subsequent growth. (pmc.ncbi.nlm.nih.gov)
Granules and responses to heating
Starches from different botanical sources vary in granule structure and processing behavior. Extracted starch is usually handled as a dry powder, but its useful properties emerge largely through interaction with water. Native granules remain suspended rather than forming a true molecular solution in cold water. Their organized internal structure changes substantially during heating. (fao.org)
With sufficient water and heat, granules undergo gelatinization: water uptake and swelling accompany disruption of ordered structures, and some polymer chains enter the surrounding liquid. Continued heating and mechanical treatment produce a paste whose viscosity depends on the starch source and processing conditions. Gelatinization occurs over a range rather than at one universal temperature; water availability and other ingredients affect the transition. (pmc.ncbi.nlm.nih.gov)
During cooling and storage, dispersed chains can reassociate into more ordered structures, a process called retrogradation. Amylose generally reassociates faster than amylopectin. Retrogradation changes gel firmness and can increase resistance to enzymatic breakdown. It is distinct from gelatinization: the former involves renewed molecular association, whereas the latter disrupts the native granule’s organization. (pmc.ncbi.nlm.nih.gov)
Digestion
Human digestion begins breaking down starch through salivary amylase and continues through pancreatic amylase in the small intestine. These enzymes cleave internal α(1→4) linkages, producing smaller carbohydrates that undergo further enzymatic conversion to absorbable sugars. Starch accessibility depends not only on molecular composition but also on food structure and processing. (pmc.ncbi.nlm.nih.gov)
Starch enclosed within intact plant tissues may be less accessible to digestive enzymes. Cooking commonly increases accessibility by gelatinizing granules, whereas subsequent retrogradation can make part of the starch less susceptible to digestion. Interactions with other food constituents also affect breakdown, so amylose content alone does not determine a food’s digestive behavior. (pubmed.ncbi.nlm.nih.gov)
Resistant starch is the fraction that escapes digestion in the small intestine. Resistance can result from physical enclosure, native granule organization, retrogradation, or chemical modification. Some resistant starch reaches the large intestine and undergoes microbial fermentation. These categories describe mechanisms of resistance, not identical properties across all starch-containing foods. (pmc.ncbi.nlm.nih.gov)
Extraction, applications, and identification
Commercial sources include maize, wheat, potatoes, and cassava. Extraction separates granules from other plant constituents through milling or rasping, washing, screening, and sedimentation or centrifugation, followed by drying. Unlike flour, purified starch contains relatively little of the original plant’s other components. (fao.org)
Starch supplies thickening and binding functions in foods and is used in adhesives, textiles, and papermaking. Controlled hydrolysis produces glucose and shorter-chain derivatives. Physical, enzymatic, or chemical treatments produce modified starches with properties suited to particular processing conditions; cassava-derived products, for example, are used in sweetener, alcohol, paper, and textile manufacture. (fao.org)
A familiar laboratory test adds iodine in the presence of iodide, producing a deep blue or blue-black complex associated chiefly with amylose helices. The color reflects iodine-containing species accommodated within the helical structure, rather than the formation of a simple covalent starch–iodine compound. (pmc.ncbi.nlm.nih.gov)
References
- Formation of starch in plant cellspmc.ncbi.nlm.nih.gov
- Starch Metabolism in Arabidopsispmc.ncbi.nlm.nih.gov
- Protein Phosphorylation in Amyloplasts Regulates Starch Branching Enzyme Activity and Protein–Protein Interactionspmc.ncbi.nlm.nih.gov
- Arabidopsis plants perform arithmetic division to prevent starvation at nightpmc.ncbi.nlm.nih.gov
- Cassava processing - Cassava flour and starchfao.org
- Cassava processing - Utilization of cassava productsfao.org
- Save and grow: Cassavafao.org
- Effects of Extrusion on Starch Molecular Degradation, Order–Disorder Structural Transition and Digestibility—A Reviewpmc.ncbi.nlm.nih.gov
- Mechanisms of the different effects of sucrose, glucose, fructose, and a glucose–fructose mixture on wheat starch gelatinization, pasting, and retrogradationpmc.ncbi.nlm.nih.gov
- Starch gelatinization, retrogradation, and enzyme susceptibility of retrograded starch: Effect of amylopectin internal molecular structurepubmed.ncbi.nlm.nih.gov
- Retrogradation of Maize Starch after High Hydrostatic Pressure Gelation: Effect of Amylose Content and Depressurization Ratepmc.ncbi.nlm.nih.gov
- A mechanistic approach to studies of the possible digestion of retrograded starch by α-amylase revealed using a log of slope (LOS) plotpmc.ncbi.nlm.nih.gov