Sugars

Carbohydrates fulfil important functions in our lives. Firstly it serves as an energy storage, as a fuel and as metabolic intermediates. Secondly it serves in the structure of DNA and RNA. And lastly many sugars are coupled to proteins and fats, and fulfil a roll in recognition processes.
The carbohydrates form a large group of molecules. Carbohydrates are also known as saccharin’s or sugars. The name carbohydrate originates from the observation that these substances lose water (hudoor = water) when heat is applied and after that carbon is left. The general formula for carbohydrates is: Cn(H2O)n.
Carbohydrates are made in large quantities inside plants during the photosynthesis (assimilation). In this process carbohydrates are formed from water and carbon dioxide:
n H2O + n CO2 Cn(H2O)n + n O2
In this reaction, sun-energy is taken up and is preserved in chemical substances. Carbohydrates are energy storage molecules. This energy can be released by the opposite process: burning (dissimilation). All animal life uses the burning of carbohydrates to obtain energy:
(CH2O)n + n O2 n H2O + n CO2 + energy
Carbohydrates are classified based on of molecular size in three groups. The three groups are:

Monosaccharides (simple sugars)
(among others Glucose, Fructose, Ribose and Galactose)
Disaccharides (molecules composed of two monosaccharides)
(among others Saccharose, Maltose and Lactose)
Polysaccharides (macromolecules consisting of long chains composed of saccharides)
(among others Cellulose, Starch and Glycogen)




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Monosaccharides


Monosaccharides are the simplest carbohydrates. They can be divided in aldoses and ketoses. The formula for monosaccharides is (CH2O)n. The smallest monosaccharides are, with three carbon atoms (n = 3), glyceraldehyde and dihidroxyacetone. Glyceraldehyde is called an aldose because it contains one aldehyde group. Dihydroxyacetone is called a ketose because it has one keto group.



Glyceraldehyde has an asymmetrical carbon atom, because of that, two different forms of the molecule are possible. These two forms are indicated by D- and L-Glyceraldehyde. The letter D and L refer to the asymmetrical carbon atom which is located the farthest away from the aldehyde or keto group (here one after the lowest atom). There are for aldose with three carbon atoms (trioses) two different forms possible. For aldose with four carbon atoms (tetroses), there are four different forms possible, because there are two asymmetrical carbon atoms. For aldose with five C-atoms (pentoses) there are eight different and with six C-atoms (hexoses) there are sixteen different forms possible.
Below the different aldoses until six carbon atoms are represented. The aldehyde group is represented in green. These sugars have the D configuration and are indicated in red. For each D-aldose there is also an L-form.



In the group of the ketoses also much different forms are distinguishable. Ketoses also have D and L form, except dihydroxyacetone because it has no asymmetrical carbon atom. There are less different ketoses than aldoses because they have an asymmetrical carbon atom less.
Below the different ketoses until six carbon atoms are represented. They are called ketoses because they contain a keto group (represented in green). The group causing the D-configuration of the ketose is represented in red. For each D-ketose there is also an L is form.



The most important and most known monosaccharide are: glucose, fructose, galactose and ribose.

Glucose and fructose in a solution are not prevent in the closed extended form proposed until now, but a ring like form.
In glucose, the aldehyde group at the C-1 atom reacts with the hydroxyl group at C-5 to form a ring. This ring form is also called a pyranose because it resembles pyran.



By forming this ring an asymmetrical carbon atom on carbon atom 1 is formed. Because of this, two different forms of the sugar are possible. These two forms are called alpha or beta-D-Glucopyranose.
Ketose also form a ring structure. The keto group at C-2 reacts with the hydroxyl group at C-5 and form a ring structure. This five carbon atom ring is called a furanose because it resembles furan.



List of the different kinds of sugars


Glucose


Glucose is also called dextrose. Glucose is a sugar that consists of six carbon atoms. These carbon atoms lie in a ring. This gives an asymmetrical molecule of five carbon atoms in a ring and 1 carbon atom outside of the ring.


Glucose is generally present in living organisms. The human blood contains approximately 0.8 g/l (5 mmol/l) of glucose. In case of an illness, the glucose level may have changed. Glucose is also a many measured mark in hospital laboratories.
Glucose is taken up in the small intestine and is transported to the liver via the hepatic portal vein. The liver regulates the glucose concentrations in the blood. If there is too much glucose in the blood, glycogen is formed. And when there is too little glucose in the blood, this glycogen is broken down again into glucose. This is all regulated in the glycogen metabolism.
Glucose is used as a fuel, glucose is energy. The use of glucose as energy happens in the metabolic processes: Glycolysis and Citric acid cycle.

List of the different kinds of sugars


Fructose


Fructose, is found in high concentrations in fruits and is a constituent of honey. Fructose tastes less sweet than glucose. Fructose is a sugar that exists of six carbon atoms. These carbon atoms lie in one ring. This gives a symmetric molecule of four carbon atoms in one ring and two carbon atoms at both sides of the ring.


Fructose is not taken up as well as glucose by the small intestine.

List of the different kinds of sugars


Galactose


Galactose is also a sugar with six carbon atoms with a somewhat different structure as glucose.

Galactose is taken up better by the small intestine than fructose. Galactose arises in the small intestine from lactose (milk sugar) from milk. Lactose is a disaccharide that consists of glucose and galactose. This lactose is broken down by lactase, produced by the intestine glands.

List of the different kinds of sugars


Ribose


Ribose is a sugar that exists of five carbon atoms. It resembles fructose, only in ribose a side chain has been taken off.


Ribose is one of the most important building blocks of large molecules. Ribose is part of the molecules: AMP, ADP, ATP, cyclical AMP and RNA. Also in DNA ribose is built in, but in the case DNA a variant called desoxiribose. Ribose can be produced by the body in the metabolic process
the pentose cycle, in the form of ribose-5-phosphate.

List of the different kinds of sugars


Disaccharides


When two cyclic monosaccharides (acetal bonding) are coupled by means of a glucoside bonding a disaccharides arises. A glycoside bonding arises together with a split off of water.
Disaccharides are commonly present in nature and form important components of foods. Disaccharides are also important in the formation of polysaccharides and monosaccharides.
The most important disaccharides are: Saccharose, Maltose and Lactose.

List of the different kinds of sugars


Saccharose


Saccharose is also known as: sugar, table sugar, sucrose. The organic chemical name is Alpha-D-Glucopyranosyl-(1->2)-Beta-D-fructofuranoside. This disaccharide is composed of the monosaccharides glucose and fructose.


During the digestion, saccharose is broken down by the enzyme invertase in glucose and fructose. Invertase is also called sucrase. This enzyme is produced in the small intestine where the breakdown of sucrose occurs.
The known sugar from the daily life consists of saccharose, just as powdered sugar and sugar syrup.

List of the different kinds of sugars


Maltose


Maltose is also called malt sugar. This disaccharide is composed of two of the monosaccharides glucose. These glucose units are connected with an alpha 1,4 bond. Maltose can be broken down in our digestive system. The enzyme maltase can break down maltose in individual glucose units. The enzyme maltase is produced in the small intestine. Maltase hydrolysis the alpha-(1-4)-bond in maltose in which glucose is released. Glucose is absorbed in the small intestine.


The word alpha, in the name of the alpha 1,4 bond between the two glucose units, indicate that the two units are in a straight area. The numbers 1,4 indicate that the bond is present between the carbon atoms 1 and 4.

List of the different kinds of sugars


Lactose


Lactose is also known as milk sugar and is present in milk. The organic chemical name is Beta-D-Galactopyranosyl-(1->4)-Alpha-D-Glucopyranose. Lactose is composed of the monosaccharides galactose and glucose. The two molecules are connected by a Beta-1,4-bond.



The word Beta , in the name of the bond between the two monosaccharides units, indicates that the two units are not in a straight area, they are slant on each other. The numbers 1.4 mean that the bonds is present between the carbon atom 1 of galactose and carbon atom 4 of glucose.

Lactose intolerance

Lactose is broken down in the digestion to galactose and glucose by the enzyme lactase. Most children can break down lactose in their digestion. In contrast to children, there is a large group of adults in the world that can not produce the enzyme lactase. These people are intolerant to milk. After drinking milk, lactose accumulates in the lumen of the intestine because there is no mechanism to take up this disaccharide. The symptoms of lactose intolerance are nausea, cramp, pain and diarrhoea.
Lactose intolerance is a genetic variation that mostly presents itself in adolescent or in adult life. Lactose intolerance is prevalent in 3% of Danish people, and in 97 % of Thai people. There is lactase treated milk available for lactose intolerant people. The ability of people to break down lactose also in maturity appears to have been developed since people started to domesticate cow cattle. This was some thousand years ago.

List of the different kinds of sugars



Polysaccharides


Polymers are biomolecules composed of units in which we can recognize the monosaccharides. Some polysaccharides can be hydrolysed by enzymes in our digestion. In this process polysaccharides are split up in monosaccharides.
The most important polysaccharides are: Cellulose, Starch and Glycogen.

List of the different kinds of sugars


Cellulose


Cellulose is an important "building material" in plants. Cellulose is harvested from vegetable material such as straw and cotton plants.
Natural textile fibre such as linen (flax), cotton and viscose (artificial silk) consist of cellulose. Just as paper, cardboard, cotton wadding and cellophane.
Cellulose can not be broken down in our digestive system. Humans do not have an enzyme that is able to break down the beta bonds in cellulose. Cellulose and other indigestible compounds form the dietary fiber, which are important in our digestive system.



List of the different kinds of sugars


Starch


Starch is an energy storage molecule for plants. There are two forms of starch. The not subdivided form is called Amylose. In Amylose the glucose units are coupled by Alpha-1,4-bonding. The second form is Amylopectine. In this form, there are branchings in the form of alpha-1,6-bonds that occur once every thirty units. This form resembles glycogen, but with less alpha-1,6 branchings.
Starch can be broken down in our digestive system. Starch is broken down in individual glucose units by the action of the enzyme amylase. Amylase is present in our saliva and is produced by the pancreas. Amylase hydrolysis the alpha-(1,4)-bonds. Glucose is absorbed in the small intestine.

Glycogen


Glycogen serves as an energy storage in humans. It consists of glucose units that are coupled by Alpha-1,4-bonds, with branchings as alpha-1,6-bonds. The 1,6 bonds occur every ten units.



For an extensive discussion about glycogen and the glycogen metabolism, see the glycogen metabolism.

List of the different kinds of sugars



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Literature

[1]. Stryer, Lubert;- Biochemistry - fourth edition; New York: W. H. Freeman and Company
(1995). ISBN 0-7167-2009-4
[2]. Van of the Lake, A.G.A.;- Organic Chemistry for the MLO - first edition; Wooden:
Bohn Stafleu Van Loghum b. v. (1995). ISBN 90-313-1736-5



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