Starch
n., plural: starch, starches
[stɑɹtʃ]
Definition: a polysaccharide carbohydrate made up of glucose
Table of Contents
Starch Definition
Starch is a polysaccharide (C6H10O5)n consisting of a large number of glucose monomers joined together by glycosidic bonds. It occurs especially in seeds, bulbs, and tubers. It belongs to a group of carbohydrates, which are organic compounds made up of carbon, hydrogen, and oxygen, usually in the ratio of 1:2:1. Carbohydrates are one of the major classes of biomolecules.
As a nutrient, carbohydrates can be classified into two major groups: simple carbohydrates and complex carbohydrates. Simple carbohydrates, sometimes referred to as simply sugar, consist of one or two saccharide residues. They are readily digested and serve as a rapid source of energy. Complex carbohydrates (such as cellulose, starch, chitin, and glycogen) are those that need more time to be digested and metabolized. They often are high in fiber and unlike simple carbohydrates, they are less likely to cause spikes in blood sugar levels. Glycogen, in particular, is stored in the liver for quick access to energy as it is burnt before fat.
Etymology
- Old English stearc (“stark, strong, rough”)
Chemical Formula
- (C6H10O5)n
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History and Terminology
Starch has long been known and used as early as 100,000 years ago. It is believed to be used in food preparations, such as in making bread and in porridges. This hypothesis is based upon the stone tools unearthed from old caves. The tools were likely used to scrape and grind starch grains from wild sorghum. This observation led scientists to presume that the inclusion of starch in the prehistoric diet of early humans in the African savannahs and woodlands improved diet quality. The processing of grains into a staple marked the shift of the prehistoric diet and is believed to be a crucial step in human evolution. (Ref.1) The word starch may come from the Old English stearc (“stark, strong, rough”), which in turn might have a Germanic origin, i.e. starchī, meaning “strong”.
Characteristics
Starch is a complex polysaccharide made up of a large number of glucose units joined together by glycosidic bonds. It is white, tasteless, and odorless powder. It has a variable molar mass. It is insoluble in alcohol and in cold water. Its chemical formula is (C6H10O5)n. Two types of molecules comprise pure starch: amylose and amylopectin. Both amylose and amylopectin are polysaccharides comprised of glucose residues. They differ in structure: amylose is a linear chain of glucose molecules connected by α-(1,4) glycosidic bonds whereas amylopectin is a branched-chain of glucose molecules linked linearly with α-(1,4) glycosidic bonds and α-(1,6) bonds at intervals of 24 to 30 glucose subunits. Since starch is a polysaccharide consisting essentially of D-glucose, it, therefore, belongs to a group of α-glucans.
Amylopectin is more soluble in water and easier to digest than amylose. Its solubility is due to the many endpoints, which can form hydrogen bonds with water. In general, starch contains 75 -80% amylopectin and 20-25% amylose by weight.
Dehydration Synthesis
The chemical process of joining monosaccharide units is referred to as dehydration synthesis since it results in the release of water as a byproduct. Starch is produced by dehydration synthesis. Plants store glucose that is not in use as starch. First, glucose is phosphorylated into glucose-1-phosphate. Starch granules are stored inside the amyloplasts located inside the cells of various plant organs. Starch granules may be found in fruits, seeds, tubers, and rhizomes. Daisies, sunflowers, and Jerusalem artichokes are examples of plants that store inulin (which is a fructan) instead of starch.
Degradation
In plants, starch degradation occurs naturally at night. The enzyme glucan water dikinase phosphorylates the starch, particularly at C-6 of one of the glucose residues. Then, another enzyme (phosphoglucan water dikinase) phosphorylates the glucose residue at C-3. After phosphorylation, degrading enzymes can now act on starch to liberate simple sugars. For instance, beta-amylase liberates two glucose residues as maltose. Another degrading enzyme is the disproportionating enzyme-1 that at the end of the degradation process liberates glucose. Starch degradation gives rise to chiefly maltose and smaller amounts of glucose. These simple sugars will then be moved out of the plastid into the cytosol via transporters: maltose transporter for maltose and plastidic glucose translocater for glucose. They may be used later as a substrate for the biosynthesis of sucrose, which is essential in the mitochondrial oxidative pentose pathway that generates ATP at night. (Ref.2)
Hydrolysis
Hydrolysis is the process of converting a polysaccharide, such as starch, into simple sugar components. The process of converting polysaccharides into monosaccharides, in particular, is called saccharification. In humans, complex carbohydrates such as starch are digested through a series of enzymatic reactions. These enzymes are salivary amylase, pancreatic amylase, and maltase. Salivary amylase acts on the starch and breaks it down to maltose. When the partially-digested carbohydrates reach the small intestine, the pancreas secretes pancreatic juices that include the pancreatic amylase. This enzyme acts on the partially-digested carbohydrates by breaking them down into simple sugars. The brush border of the small intestine releases digestive enzymes such as isomaltase, maltase, sucrase, and lactase. Isomaltase digests polysaccharides at the alpha 1-6 linkages, and convert alpha-limit dextrin to maltose. Maltase breaks down maltose (a disaccharide) into two glucose units. Sucrase and lactase digest sucrose and lactose into monosaccharide constituents, respectively. The epithelial cells (enterocytes) at the brush border of the small intestine absorb monosaccharides and then release them into the capillaries. The simple sugars are then transported to the cells of other tissues, especially to the liver, from the bloodstream. Glucose in the blood may be utilized by the body to produce ATP. Otherwise, it is transported to the liver, together with the galactose and fructose (which are largely converted into glucose), for storage as glycogen.
Resistant starch
Resistant starch is a form of starch that resists digestion in the small intestine of humans. It is also dietary fiber. It is metabolized instead in the large intestine by the colonic microbiota. The microbes in the colon ferment it and produce metabolic byproducts such as gases and short-chain fatty acids. The short-chain fatty acids, in particular, are absorbed and provide health benefits to the human body. Fermentation of resistant starch also helps promote the growth of beneficial bacteria.
Plant starch vs. Animal starch
Animal starch is not a starch per se. It refers to the constituent of the animal’s glycogen owing to the similarity in the structure and composition of amylopectin. While plants store excess glucose in the form of starch, the animals also do so in the form of glycogen. Glycogen is a branched polymer of glucose that is mainly produced in liver and muscle cells, and functions as secondary long-term energy storage in animal cells. Similar to starch, glycogen is a complex carbohydrate that primarily serves as a storage carbohydrate. The difference between the amylopectin in plants and the amylopectin in animals is that the latter has more extensive branching at every 8 to 12 glucose units.
Biological Importance
All plant seeds and tubers contain starch which is predominantly present as amylose and amylopectin. Plants use starch as a way to store excess glucose, and thus also use starch as food via the mitochondrial oxidative phosphorylation during at night or when photosynthesis is unlikely. Plants store excess starch in amyloplasts, which are leucoplasts that function primarily in storing starch granules through the polymerization of glucose and in converting these reserves back into simpler sugars (e.g. maltose and glucose), especially when light is not available. Chloroplasts, pigmented organelles involved primarily in photosynthesis, are also capable of storing starch.
Animals do not store excess glucose as starch; they store them as glycogen. However, certain animals feed on starch-laden food.
Dietary starch is present in many staple foods, such as maize, rice, wheat, potatoes, cassava, barley, rye, taro, yams, etc. It is also present in various food products such as cereals, noodles, pancakes, bread, pasta, etc. Starch provides about 4.2 kilocalories per gram. In humans, starch may serve as a major source of glucose. Glucose is essential as it is involved in general metabolism, e.g. glycolysis (for energy synthesis), glycogenesis (for glycogen synthesis), pentose phosphate pathway (for pentoses and NADPH syntheses for use in nucleic acid synthesis and lipid synthesis, respectively).
Starch has many commercial uses, such as in papermaking, as a food, in the production of commercial grape sugar, for stiffening linen in laundries, in making a paste, in the printing industry, in hydrogen production, etc.
Health risk
Too much starch in the diet is associated with dental caries, obesity, and diabetes mellitus. Starch (especially cooked and contained in processed foods) can cause spikes in blood glucose levels after a meal. Thus, starch consumption is advised to be in moderation. Individuals with celiac disease and congenital sucrase-isomaltase deficiency may need to avoid starchy foods. (Ref.3)
Try to answer the quiz below to check what you have learned so far about starch.
Further Reading
References
- Porridge was eaten 100,000 years ago. (2009, December 18). Retrieved from telegraph.co.uk/news/uknews/6834609/Porridge-was-eaten-100000-years-ago.html Link
- Wikipedia Contributors. (2019, February 25). Starch. Retrieved from en.wikipedia.org/wiki/Starch#Energy-store-of-plants Link
- Starch: Foods, Digestion, Glycemic Index. (2016, June 4). Retrieved from nutrientsreview.com/carbs/polysaccharides-starch.html Link
Reviewed by: Todd Smith, PhD
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