Sugar – Part 2: The Different Avatars of Sugar


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Myself . By T.V. Antony Raj
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Image source: nourition.com
Image source: nourition.com

Sugar is the universal name for a variety of carbohydrates or saccharides that have a sweet taste.

The word ‘sugar’ immediately brings to our mind the white crystals we add to tea and coffee to make it sweet.

However, scientifically, the term ‘sugar’ refers to various types of substances derived from different sources: simple sugars known as monosaccharides, and compound sugars: disaccharides, oligosaccharides and polysaccharides.

Any word that ends with “-ose” would most probably denote a sugar.

The range of sweetness we experience when eating foods is determined by the different proportions of sugars found in them.

Many chemically-different substances that are non-carbohydrates may also have a sweet taste, but are not classified as sugars. Some of these are used as low-calorie food substitutes for sugar and are categorized as artificial sweeteners.

Saccharides

Saccharides (Greek sacchar: sugar)  are one of the most important biomolecules. They are also known as carbohydrates and control the energy in cells, provide structural integrity, and provide a role in the immune system, development and fertilization in all living things.

Natural saccharides are generally simple carbohydrates called monosaccharides having the general formula (CH2O)n  where n is three or more.

Plants use carbohydrates to store energy and to provide supporting structures. Animals and humans consume plants to get their share of carbohydrates as a source of carbon atoms for synthesis of other compounds.

Carbohydrates supply energy for working muscles. They provide the fuel for the central nervous system, enable fat metabolism, and prevent protein from being used as energy.

Monosaccharides

Monosaccharides (Greek monos: single, sacchar: sugar) or simple sugars are the most basic units of carbohydrates with the general formula C6H12O. Examples of Monosaccharides include Glucose (dextrose), fructose (levulose) and galactose. They have one sugar unit with six carbon atoms and five hydroxyl groups (−OH).  They are the building blocks of disaccharides and polysaccharides (such as cellulose and starch). 

Monosaccharides normally found in food (Source: socialphy.com)
Source: socialphy.com

Each carbon atom that supports a hydroxyl group (except for the first and last) is chiral (a molecule that has a non-superposable mirror image), giving rise to a number of isomeric dextro– and laevo-rotatory forms all with the same chemical formula. For instance, galactose and glucose are both aldohexoses, but have different physical structures and chemical properties.

Monosaccharides form an aqueous solution when dissolved in water.

Glucose also known as D-glucose, dextrose, corn sugar, grape sugar and blood sugar is a simple dietary monosaccharide found in plants. It is one of the three dietary monosaccharides, along with fructose and galactose, that are absorbed directly into the bloodstream during digestion.

Glucose

The name “glucose” is derived from the Greek word γλευχος, meaning “sweet wine, must”. The suffix “-ose” denotes a sugar.

In a biological sense, glucose is found everywhere. It occurs naturally in fruits and plant juices. It is the primary product of photosynthesis. Most ingested carbohydrates are converted into glucose during digestion and it is the form of sugar that is transported around the bodies of animals in the bloodstream. It is used as an energy source by most organisms, from bacteria to humans.

Use of glucose may be by either aerobic respiration, anaerobic respiration, or fermentation. Glucose is the human body’s key source of energy, through aerobic respiration, providing about 3.75 kilo calories (16 kilojoules) of food energy per gram. Aerobic respiration requires oxygen.

Simplified reaction:

C6H12O6 (s) + 6 O2 (g) → 6 CO2 (g) + 6 H2O (l) + heat
ΔG = −2880 kJ per mol of C6H12O6

The negative ΔG indicates that the reaction can occur spontaneously.

Glucose can be manufactured from starch by the addition of enzymes or in the presence of acids. Glucose syrup is a liquid form of glucose that is widely used in the manufacture of foodstuffs.

Fructose or fruit sugar, is a simple dietary monosaccharide found in honey, fruits that grow on trees and vines, flowers, berries, and most root vegetables. It is the sweetest of the sugars.

Fructose

Fructose, a 6-carbon polyhydroxyketone is an isomer of glucose – both have the same molecular formula (C6H12O6but they differ structurally. It is often bonded to glucose to form the disaccharide sucrose.

Along with glucose and galactose, fructose is absorbed directly into the bloodstream during digestion.

Commercially, fructose is processed from sugarcane, sugar beets, and maize.

Galactose (Greek galakt: milk), a monosaccharide sugar, is a constituent of the disaccharide lactose along with the glucose. It does not occur in the free state. It is less sweet than glucose.

Galactose

Glactose, is a component of the antigens found on the surface of red blood cells that determine blood groups.

Disaccharides

Sucrose, maltose, and lactose are compound sugars or disaccharides, with the general formula  C12H22O11. They are formed by the combination of two monosaccharide molecules with the exclusion of a molecule of water.

Sucrose is the granulated sugar that we customarily use as additive in our food. It is a disaccharide with one molecule of glucose covalently linked to one molecule of fructose.

Sucrose

Animated sucrose molecule model
Model of a sucrose molecule (Author: RedAndr)

Sucrose is found in the stems of sugar cane and roots of sugar beet. It also occurs naturally alongside fructose and glucose in other plants, in particular fruits and some roots such as carrots.

After eating, during digestion, a number of enzymes known as sucrase split sucrose into its constituent parts, glucose and fructose.

Maltose also known as maltobiose or malt sugar, is a disaccharide formed during the germination of certain grains, the most notable one being barley, which is converted into malt, the source of the sugar’s name. It is less sweet than sucrose, glucose, or fructose.

Maltose

A molecule of maltose is formed by the combination of two molecules of glucose.

Maltose is formed in the body during the digestion of starch by the enzyme amylase and is itself broken down during digestion by the enzyme maltase

Lactose is the naturally occurring disaccharide derived from galactose and glucose found in milk. A molecule of lactose.is formed by the combination of a molecule of galactose with a molecule of glucose.

Lactose

A molecule of galactose is formed by the combination of a molecule of glucose with a molecule of lactose.

After consuming milk, during digestion, lactose is broken down into its constituent parts by the enzyme lactase. Children have this enzyme in them. In some adults the enzyme lactase does not form as they grow up and are unable to digest lactose.

Oligosaccharides

Oligosaccharides (Greek oligos: a few, sacchar: sugar) are polymeric carbohydrate molecules containing a small number, typically three to nine, monosaccharide units. They are commonly found on the plasma membrane of animal cells where they play a role in cell–cell recognition.

Fructo-oligosaccharides (FOS), also sometimes called oligofructose or oligofructan, are oligosaccharide fructans. They consist of short chains of fructose molecules.

FOS occur naturally and are found in many vegetables.

FOS exhibit sweetness levels between 30 and 50 percent of sugar in commercially prepared syrups and are used as an alternative sweetener. Due to consumer demand for healthier and calorie-reduced foods, FOS emerged commercially in the 1980s.

The range of sweetness we experience when eating foods is determined by the different proportions of sugars found in them.

Galactooligosaccharides (GOS) occur naturally, and consist of short chains of galactose molecules. These compounds can be only partially digested by humans.

Mannan oligosaccharides (MOS) are widely used in animal feed to improve gastrointestinal health, energy levels and performance. They are normally obtained from the yeast cell walls of Saccharomyces cerevisiae.

Polysaccharides

Polysaccharides are polymeric carbohydrate molecules composed of long chains of monosaccharide units bound together by glycosidic bonds. Typically, polysaccharides contain more than ten monosaccharide units.

Cellulose, starch, glycogen, xanthan gum in plants, etc., are polysaccharides.

3D structure of cellulose, a beta-glucan polysaccharide. (Autho - Ben Mills)
3D structure of cellulose, a beta-glucan polysaccharide. (Autho – Ben Mills)

Polysaccharides, have a general formula of Cx(H2O)y where x is usually a large number between 200 and 2500. Considering that the repeating units in the polymer backbone are often six-carbon monosaccharides, and the general formula can also be represented as (C6H10O5)n where 40≤n≤3000.

Definitions of how large a carbohydrate must be to fall into the categories polysaccharides or oligosaccharides vary according to personal opinions of scientists.

Polysaccharides are an important class of biological polymers. Their function in living organisms is usually either structure or storage-related. Starch (a polymer of glucose) is used as a storage polysaccharide in plants, being found in the form of both amylose and the branched amylopectin. In animals, the structurally similar glucose polymer is the more densely branched glycogen, sometimes called ‘animal starch’. Glycogen’s properties allow it to be metabolized more quickly, which suits the active lives of moving animals.

The range of sweetness we experience when eating foods is determined by the different proportions of sugars found in them.

 

← Previous – Sugar – Part 1: History of Canesugar

Next → Sugar – Part 3: Oh Sweet Poison, Thy Name is Sugar!

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