Identification of Unknown Carbohydrates | Lab Report

One of the main types of nutrients is the carbohydrates. Carbohydrates are the most vital foundation of energy for your body. Our digestive system has a capacity to change carbohydrates into glucose or most commonly known as blood sugar. Our body gets energy used by our cells, tissues and organs from this sugar. Carbohydrates also stores additional sugar in our liver and muscles.
Carbohydrates may be simple or complex depending on its chemical structure. Simple carbohydrates are also known as simple sugars. They are commonly established in refined sugar such as white sugar. Complex carbohydrates of starches includes grain products like bread, crackers, pasta and rice.
A. Identification of Unknown Carbohydrate Samples
Approximately 1.00 ml of the known carbohydrate samples and the two unknown samples were transferred on separate labelled test tubes. About 1.00 ml of Molisch reagent then 1.00 ml of concentrated H2SO4 was added to each sample. The test was observed for any change and was recorded. With the use of new batch of samples each time, the remaining tests were conducted:
(a) Iodine test – 1.00 ml of iodine reagent was added to each sample.
(b) Benedict’s test – 1.00 ml of Benedict reagent was added to each sample then heated using water bath.
(c) Barfoed’s test – 1.00 ml of Barfoed’s reagent was added to each sample then heated using water bath.
(d) Seliwanoff’s test – 1.00 ml of Seliwanoff reagent was added to each sample then heated using water bath.
(e) 2,4-DNP test – 1.00 ml of 2,4-DNP was added to each sample then heated using water bath.
The identity of the unknown samples was determined by comparing it to the known carbohydrate samples.
B. Hydrolysis of Starch
Exactly 50.0 ml of 5% starch solution was transferred in a 100-ml beaker. Precisely 5.00 ml of concentrated sulfuric acid or hydrochloric acid was added. The sample was covered with aluminium foil and was heated using water bath. Two 1.00 ml volume of the sample were transferred in a test tube. Exactly 1.00 ml of iodine reagent was added to one tube and 1.00 ml of Benedict’s reagent was added to the other. The reaction was observed. The sample was heated continuously. Two 1.00 ml volume of the sample was transferred between every 5 minute interval and tested with iodine and Benedict’s reagent as above until formation of blue-black complex in iodine stops and formation of brick red colour in Benedict’s reagent ensues.
In Molisch test, the result turned out to be positive or slow reaction. It is because of the formation of the reaction with alpha-naphthol in the occurrence of sulfuric acid. In this test, all type of carbohydrates will give a positive result.
Benedict’s solution is a deep-blue alkaline solution used in testing the existence of the aldehyde functional group, -CHO. Benedict;s reagent consists of blue copper (II) ions which are condensed to copper (I).These ions will then be precipitated as red copper (I) oxide which is not soluble in water. In Benedict’s test, monosaccharides and disaccharides except for sucrose give a positive result. It is when the result is a brick red precipitate.
In Barfoed’s test, the copper ion in solution oxidizes reducing monosaccharides. This is for the formation of a carboxylic acid and red precipitate of copper (I) oxide in 3 minutes.
In Seliwanoff’s test, the reagent dehydrates ketohexoses to form 5-hydroxymethylfurfural which will further react with resorcinol, that is present in the reagent, to produce a red product in 2 minutes.
In Iodine test, all polysaccharides such as glycogen and starch give positive result. The sample turns to blue-black color.

Chemistry of Pectin Carbohydrates and its Application in the Food System


 -Chemistry of Carbohydrates and its Application in Food System



Pectin is a carbohydrate polymer, with a relatively high molecular weight and is found naturally in most plants and particularly in fruits (Flutto, 2003). Fruits such as berries, citrus and apples contain naturally occurring pectin. Pectin comes from the Greek word pektos which means firm and hard, reflecting its ability to form gels and stabilize products (Flutto, 2003). For centuries pectin has been known for its gelling properties, but it was not until the beginning of the twentieth century that it started to be used commercially (Flutto, 2003). 


Pectin is classified as a complex carbohydrate that is relatively soluble in water (Kent, 2016). It is a polysaccharide material and found in fiber within the cell walls and lamella of most plants (King et. al, 2013). Pectin also contributes to the cell structure of plants.


Structure/Chemistry of Pectin


Pectin is known as a pectic polysaccharide that is rich in galacturonic acid (GalA). Different types of plants produce a number of different polymers of pectin and have different functional properties. The different polymers of pectin vary depending on the molecular weight, amount of natural sugar and chemical configuration. Pectin contains a homogalacturonic acid backbone that is made up of a galacturonic acid chain (Figure 1) that is partly esterified with methyl esters and linked by -1,4 glyosidic bonds (Thibault et al., 1993; Zhan et al., 1998). The distribution of esters are critical as it affects the charge density of the polymer. When the molecules interact with other molecules such as calcium, protein or other pectins it will reduce the repulsion between molecules. Acetyl groups are also important in the classification of specific pectin types.

Figure 1



Figure 1: pectin structure consisting of long sequence of anhydrous galacturonic acid and esterified methanol

The amount of galacturonic acid (GA) present in the entire molecule is known as %GA. To be qualified as a food additive it must have a minimum of 65% GA (Flutto, 2003).  The amount of galacturonic units that are esterified are known as degree of esterification (DE), for high-ester pectins (HE) should be great than 50% and for low-ester pectins (LE) should be less than 50%. The total units amidated in the entire molecules is known as degree of amidation (DA) and is regulated to a maximum of 25%.  

There are two methods of producing pectin, precipitation and amidation. Precipitation is normally used for HE and low-ester conventional pectin and amidation is used to prepare low ester amidated pectin. Using the amidation process pectin mixture is amidated with ammonia and this forms galacturonamide units in the chain and is an important aspect in commercial pectin.     

Another component of the backbone are neutral sugars. Areas in the backbone where neutral sugars are present are known as hairy regions and areas without are called smooth regions (Figure 2). The sugars form short segment side chains. Examples are d-galactose, L-rhamnose, L-arabinose and D-xylose. Some neutral sugars are a part of the side chains, whereas some are incorporated into the backbone and some such as L-rhamnose which causes kinks in the chain. The D-xylose is found in apple peels and can be attached in to the backbone. 


Figure 2



Figure 2: Pectin: primary and secondary structure

Although there are a lot of important components of the backbone, the position of each of these components are also of significance.  For example, in apple pectin the distribution is found to be random but in citrus it is block wise. These distributions can affect the charge density and how molecules will repel each other. The structure of pectin is very resistant to heat, even at a lower pH (~3.5) the backbone is only slightly depolymerized at a higher temperature. In order to improve the heat-stability, the water-activity of the system must be lowered. By adding sugar this can lower the water-activity, this is why sugar is added in jam to improve the gel-forming capabilities. Pectins are known for forming gels with sugar and acid (May, 1990)   


Based on the degree of polymerization, number and location of methyl-ester groups these will affect the solubilization rate of pectin. Pectin is very soluble in water but is insoluble in most organic solvents ( Flutto, 2003). The solvent used to dissolute pectin is also important as pH, temperature and ionic strength can affect the rate of dissolution.


Production of Pectin


Pectin is most commonly found in the form of pectic or protopectin substance and is very important within in the cell wall structure. In this structure these substances are not soluble in water, therefore they act as a hydrating mechanism and cement for the cellular network.

Although there is not a complete comprehension of pectic substance, it is known that it is a convoluted structure that pectin attaches to other parts of the cell wall through covalent, hydrogen and/or ionic bonds.

The commercial production of pectin is a complex process where different fruits, most commonly apple pomace and citrus peels are mixed with water and hot dilute mineral acid (pH~2) and an extraction process is performed (Figure 3).  Pectin is separated from the peels and this allows it to be in a soluble form (Silva Team, 2017). Using a filtration system, the liquid is drained off from the peels that are suspended in the liquid and the protopectin is extracted by a hydrolysis in aqueous solution (SilvaTeam, 2017).  The concentrated liquid is either mixed with alcohol (usually isopropanol) this is known as precipitation or mixed with ammonia called amidation. In order to create high-ester and low ester conventional pectin the precipitation method is used. For low-ester amidated pectins the amidation process is used. The gelatinous mass is then pressed and washed to remove the alcohol or ammonia and is dried and ground up. The pectin then goes through a standardization process to ensure consistency within products.



Figure 3




Figure 3: process of producing pectin



Functionality of Pectin

Pectin has a variety of uses in the food and pharmaceutical industries. It appears as a white to light brown powder. The role of pectin in plants is to ensure the plant walls of adjacent cells stay joined together. Protopectin is a precursor substance found in immature fruits, as fruits start to ripen the protopectin is converted to pectin and increases its water-soluble capacity ( Britannica, 2018). Pectin then aids in maintaining firmness and shape of fruits. As the pectin begins to break down to simple sugars and completely water-soluble the fruit overripens, loses shape and firmness (Britannica, 2018). As the use of pectin is still a newer technology and methods are still developing, it is likely that with more knowledge pectin will contribute new and better functional properties in the near future.

Table 1



High-Ester Pectin

pH and Ionic Strength

-above 4.5 beta-elimination can occur which causes depolymerization of the galacturonic side chain and the esterified carboxyl will be cleaved

Pectin chains carries a negative charge and often repel each other

Depending on charge density it will affect the amount of repulsion

The higher the pH and charge density results in a lower degree of esterification and stronger repulsion  and more difficulty forming gels

Under these conditions hydrogen bonding is impossible between ionized pectin chains

–A lower pH is necessary for high-ester pectins to gel

The low pH (~3.6) lowers repulsions between molecules enough that the distance between chains is adequate to allow hydrogen bonding to occur

Under a certain pH (critical level) the gel strength is reduced as the gelling occurs to quickly and an unorganized polymer network and precipitation wil occur

Gelling Mechanism

Gel with sugar and acid by forming cross-linking polymers in junction zones (Flutto, 2003)

–to stabilize the molecular network the water activity of the system must be reduced

–sugar is added to achieve sufficient hydrophobic interactions

Although more common in low-ester pectins, calcium can alter gelling properties

Hydrogen bonding and hydrophobic attractions between methyl-ester groups play a significant role

Calcium bridges are also form if there are free acids from esters positioned in blocks

–The distribution of esters on the backbone that are marked in a block distribution will contribute to calcium gelling and will greatly increase gelling temperature (Flutto, 2003)

Pectin Concentration

With higher concentration of high-ester pectin will increase the number of junction zones and side chains with elastic activity, which in turn will increase over-all gel strength

An increase in molecular weight will also have same effect (Flutto, 2003)

Degree of Esterification

The galacturonic acid degree of esterification affects the charge density and how many sites are available for hydrophobic interaction (Flutto, 2003)

The higher degree of esterification the less charged and can form gels at a higher pH and temperature

The degree of esterification will determine the optimal pH for digestion (Flutto, 2003)

Acetylation and Branching

As the side of the acetyl groups decreases pectin chains are not close enough to each other to interact

Neutral sugars present can result in steric hinderance and reduce the molecular interaction between molecules and makes it difficult to form gels

Water Activity


Reducing water activity allows hydrophobic interaction to occur easier and this increases gelling rates and strength of gel

By using sugar, the water activity is reduced, it allows less space between molecules so interaction occur easier

Cooling and Storage

gelling will occur under ideal conditions, where intermolecular interactions are formed, and the molecular movement has ceased to allow closer interactions

as cooling rate is increase so is gelation rate

When cooled gel is stored the texture turns into a stronger final gel

Slowly the network will re-organize and there will be an enlargement of existing junction zones and creation of new junction zones between pectin molecules

Should be cooled slowly to avoid difficulty in forming hydrophobic interaction and hydrogen bonding


Table 1:  Functionality of high-ester pectin’s









Table 2



Low-Ester Pectins 

pH and Ionic Strength

 Used in food systems high in sugar and low pH due to their specific properties

Low sugar content the pH is decreased, and pectin molecules are neutralized with protons

-reduces amount of junction zones interacting with calcium

At a low pH there is more calcium requirements and creates a looser gel texture

At a lower pH natural calcium will be increased and reduces need for extra added calcium

Gelling Mechanism

When conditions are not met with high-ester pectins, low-ester pectins are used

Properties are dependent on type of pectin used (conventional or amidated)

Occur through ionic linkages through calcium bridges of two pectins carboxylic groups which form hydrogen bonding (Flutto, 2003)

-occurs upon cooling system

-pectin chains bridges by ions (usually calcium, could also be magnesium or potassium) which integrate into their coordination shells two polyanion oxygen atoms from one pectin molecule and three from another chain (Flutto, 2003)

Calcium is ideal for bridging in complex carbohydrates as it ionic radius is big enough (0.1 nm) to interact with multiple oxygen atoms

Based on length of junction zones or number of galacturonic acid involved in bonds with calcium will affect how gel in formed

When at least 7 carboxyl groups from each chain are involved the bonds become stabilized (Flutto, 2003)

If junction zones become too long precipitation may form

Number/distribution of ester & amide groups 

 Was developed to achieve better gelling control by controlling functional properties of low-ester pectin

Amidation increases the gelling properties

–due to potential of hydrogen bonding with amide groups

Gels formed with amidation do not require as much calcium resulting in a much firmer gel and are more thermo-reversible

Degree of Esterification

Amount of calcium required depends, process parameters , rate of cooling and on degree on esterification

-increase in ionic strength, pH or decrease in esterification lowers amount of calcium required

Calcium bonds can only occur in esterification-free zones, so gel strength increases with decreasing esterification

The degree of esterification should be above 30% to control length of junction zones

Molecular weight 

 The length of the polymer affects how many junction zones needed to make a network

The higher the molecular weight will increase gelation rate, lower calcium requirements and create a more cohesive and elastic gel  reducing syneresis (Flutto, 2003)

Water Activity


 The more solid content the decrease in amount of calcium required

–also accelerates gelling, increases setting temperatures and overall gel strength (Flutto, 2003)

A higher degree of esterification should be used for a higher solid level

Ionic Strength 

 At higher ionic strength there is an increase in gel strength

As the polymers are neutralized by ions the chains become closer together and leads to organization of the network and a stronger gel


Table 2: Functionality of Low-ester Pectins


Figure 4: Eggbox model


Figure 4: Egg-box model: overview of low-ester pectin gel mechanism including calcium bridges and possible hydrogen bonding types

Application  in the Food Industry

Pectin has the ability when heated to form a thick gel-like solution and therefore is used as a thickening agent in cooking and baking. The most traditional use for pectin is in jams, jellies and preserves (IPPA, 2001). Higher quality jams are usually made with better quality fruit and requires less pectin and therefore less sugar (May, 1990). Fruits with high pectin and pH levels such as grapefruits and lemons are difficult to make a high-fruit content jam as they tend to create an over-strong gel and must be carefully controlled.

Table 3


Different types of commercial pectin


Rapid Set Pectin

Slow Set Pectin

High sugar products:


Some jams


bakery and biscuit jams

Stabilizing Pectin

Stabilizing acidic protein products: yogurts

Whey and soya beverages

Low Methyl Ester and Amidated Pectin

Lower sugar products:

Reduced sugar preserves

Dessert gels and toppings

Sauces and marinades

Table 3: Comparison of different types of commercial pectins and application in the food industry

Pectin has the unique ability to reduce low-density lipoprotein (LDL) and in turn, can lower cholesterol levels. Pectin also delays stomach emptying and helps to prevent swings in blood sugar (Flutto, 2003). Pectin also has a strong antibacterial effect on food spoilage microorganisms and therefore is a good method or food preservation techniques (Daoude et. al, 2013).


Pectin Interaction with Proteins


 Food proteins (e.g. casein) in acidic environments tend to form sediment and may dehydrate easily after heat treatment. Protein sources need an effective method to stabilize proteins in a low pH system. In ideal high-ester pectin concentrations it has been discovered to be a stabilizer in this environment. On the galacturonic backbone the presence of free carboxyl blocks of pectin allows protein stability through stearic repulsion (Flutto, 2003). The lower amount of carboxyl groups in high-ester pectins has been shown to be effective due to the weaker electrostatic interactions with protein and in turn can allow for static repulsion. This interaction will depend on where the carboxyl groups are located on the backbone and protein structure and distribution of ionizable groups on the surface (Flutto, 2003). Based on the overall system, pH, ionic strength and if sugars or fat are incorporated into the system. pH is the most important system factor as it affects ionization of protein and pectin and affects protein structure and interactions within a complex system. For an ideal interaction a pH of 3.6-4.5 in necessary. If the pH is too low the block structures will not properly bind to protein as they are not sufficiently ionized. If the pH is too high the protein-polysaccharide complex is not very strong and the protein-protein repulsions  become dominate and will not stabilize proteins.



Overall pectin is a great natural product with a variety of functional uses. Presently, pectin is used as a thickening and textural ingredient. With an increase in research into its alternate properties and uses it has potential to make a large positive impact in the food and pharmaceutical industries. This natural product has many uses and health benefits and, in the future, could be found in more foods.






Daoud, Z., Sura, M., & Abdel-Massih, R, M. (2013). Pectin shows antibacterial activity against Heliobacter phylori. Advances in Bioscience and Biotechnology,04(02), 273-277. doi:10.4236/abb.2013.42a037

Flutto, L. (2003). Encyclopedia of Food Sciences and Nutrition (Second Edition). New Century, KS: Danisco.

Kent, M. (2016). Food and Fitness: A dictionary of Diet and Exercise. Oxford: Oxford University Press.

King, R., Mulligan, P., & Stansfield, W. (2013). pectin. In  (Ed.), A Dictionary of Genetics. : Oxford University Press,. Retrieved 30 Oct. 2018, from

May, C. D. (1990). Industrial pectins: Sources, production and applications. Carbohydrate Polymers. 12(1), 79-99. doi:10.1016/0144-8617(90)90105-2

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Thibault, J.F., Renard, C.M.G.C., Axelos, M.A.V., Roger, P. and Crepeau, M.J. 1993. Studies of the length of homogalacturonic regions in pectins by acid-hydrolysis. Carbohydrate Res. 238: 271–286.

The Structure and Function of Carbohydrates

When one looks at a person moving, what comes to your mind that is fundamental in giving the person energy to breathe, move and function? It is carbohydrates. Carbohydrates are known as saccharides and molecular compounds made from three elements: carbon, hydrogen and oxygen. Carbohydrates are the most abundant macromolecules on earth and they can be accessed easily in our daily intake of food and diet. According to Campbell (2006), the simplest carbohydrates are known as monosaccharides (from the Greek word monos-, single and sacchar), which means single sugars. When two single sugars or two monosaccharides are joined together by the process of condensation, they are known as disaccharides. When two or more single sugars are joined together, they form more complex sugars known as polysaccharides. As these sugars and polysaccharides also known as carbohydrates, which are fundamental in providing energy to human, plants, animals and cells, they give us the opportunity to look at some interesting facts on the structure and function of carbohydrates.

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Firstly, monosaccharides are made up of the structure (CH2O)n1 where n=3, 5 or 6. They are known as Trioses (C3H6O3), Pentoses (C5H10O5) and Hexoses (C6H12O6). An example of it is glucose, which is known as a hexose sugar or six-carbon sugar with the formula of C6H12O6. The other two hexoses, galactose, which is found in milk, and fructose, a plant sugar, both play an important role in mammalian and plant biology respectively. Though the three hexoses have the same formula, their atoms attached to each other is slightly different and they are also known as isomers (Williams, 1999). These sugar isomers’ structures in Figure 1(a) are called straight chain structures. However, when they dissolve in water, they rarely exist as straight-chain structure, they will form ring structures.

Figure 1(a) Two isometric hexoses. Each of these six-carbon sugars has the same molecular formula C6H12O6, hence each is an isomer of the other.
This is because all monosaccharides contain a carbonyl group (C=O) and a number of hydroxyl (-OH) groups. For the hydroxyl groups (-OH) which have the ability to form hydrogen bonds with water molecules, which make these sugars soluble in water. (Grant, 2006) The fact that glucose is readily soluble is important for its absorption during digestion, transport in the blood to cells for respiration, and metabolism by cells.
Thus, as most monosaccharides are soluble in water, they form rings in aqueous solutions, as shown for glucose in Figure 1(b). To form the glucose ring, carbon 1 bonds to the oxygen attached to carbon 5. (Campbell. 2006) As shown in the middle diagram, the ring of glucose and other sugars may not always show the carbon atoms at the corners of the ring. The bonds in the ring are drawn to indicate the atoms and functional groups, such as -OH, extending above and below it.

Figure 1(b) Structural formula of glucose ring with carbon atoms numbered.
On the other hand, the carbonyl group, depending on its location, makes the sugar either an aldose (an aldehyde sugar) or a ketose (a ketone sugar). Glucose is an aldose, also known as reducing sugar and fructose is a ketose (non-reducing sugar).
Monosaccharides, especially glucose, are the main fuel molecules for cells to work. “Cells use the carbon skeletons of monosaccharides as raw material for manufacturing other kinds of organic molecules, such as amino acids.” (Campbell, 2006)
However, when cells that do not use the monosaccharides immediately, they are usually incorporated into disaccharides and polysaccharides. Two monosaccharides linked together into a double sugar to form a disaccharide. The reaction is known as condensation where a molecule of water is removed as shown in the equation below. (Grant, 2006)
2C6H12O6  C12H22O11 + H2O
Chemical reaction for condensation process where a molecule of water is removed when two monosaccharides are linked together.
One of the major sugars in milk is lactose, and sucrose is found in most plants such as sugarcane and beet. Sucrose is extracted to use as table sugar. Both lactose and sucrose are disaccharides and contain a molecule of glucose bonded to a different monosaccharide. The bond formed between them is called glycosidic bond. What happens during the condensation process is when one monomer gives up a hydroxyl group and the other gives up a hydrogen atom from a hydroxyl group. As a result, H2O forms, an oxygen atom is left, linking the two monomers through a glycosidic bond as shown in Figure 2(a) between glucose and fructose to form sucrose. Maltose, another disaccharide, is formed from two glucose monomers. It is commonly used in making beer and malted milk candy (Denise R. Ferrier. and Richard A Harvey., 2014).

Figure 2(a) Disaccharide formation by a condensation reaction between glucose and fructose to form sucrose. (
Furthermore, disaccharides cannot cross the intestinal wall and enter the bloodstream. For absorption to occur, they have to be hydrolysed to their monosaccharides’ monomers, which are then transported across the gut epithelial cells to enter the blood for distribution to all cells. (Grant 2006, p. 14) Lactose, which is a key sugar in mammalian milk is a disaccharide is made up of monosaccharides glucose and galactose. The enzyme lactase is only produced in mammals and most mammals’ secretion will stop when the young have been weaned. This is interesting because humans in many cultures, will continue to consume milk throughout adult life. Most of the people in these milk-drinking societies will continue to produce lactase long after childhood and be able to digest milk with ease. However, in cultures where milk consumption is discontinued, that is when lactase production will stop and people can develop lactose intolerance symptoms such as diarrhoea because the lactose in milk is not hydrolysed and it cannot be absorbed in the gastrointestinal tract.
Sucrose is digested easily to produce the monosaccharides that are used to provide energy for the body. Sugar is metabolised in the human bodies to produce to produce glucose, which can be used for immediate energy or if not metabolised, it can be stored by the conversion to a polysaccharide called glycogen. According to Ennis (2005, p. 16), the design of our bodies only allows us to store about a day’s worth of energy in the form of glycogen. If we eat more sugar than we can use or store as glycogen, the excess sugar is converted to fat. Therefore, it is not advisable to eat too much food that is high in sugar as it can result in obesity.
As mentioned previously that sugar which is not metabolised can be stored by the conversion to a polysaccharide known as glycogen. Polysaccharides are essentially polymers of monosaccharides linked together by condensation. Polysaccharides can function as energy storage in animals and plants or as a structural material for plants. For animals, energy storage for polysaccharides is known as glycogen, for plants’ energy storage it is known as starch and for structural material in plants, it is known as cellulose. The bonds joining the glucose units together in a polysaccharide are also known as glycosidic bonds. In addition, there are different types of bonds. Most of the glucose units in glycogen are linked by
(1-4) glycosidic bonds (Denise R. Ferrier. and Richard A Harvey., 2014). The numbers refer to the positions in the sugar where the links are formed (refer to Figure 3a). The α(1-4) bonds produce straight chains of glucose units. The branches in glycogen are formed by α(1-6) glycosidic bonds.

Figure 3(a) shows the structure of the storage polysaccharides glycogen and starch. The main chain is α(1-4) bond. Side chains are connected to the main chain by α(1-6) bonds.
Most of the glycogen in animals is stored in the skeletal muscles and the liver. According to Grant (2006, p.14), it is as much as 10% of the weight of the liver that can be glycogen. The glycogen is an important store of glucose that is used to maintain blood sugar concentrations between meals and during night time. Most of the organ and tissues in the body, including the brain, will depend on the liver’s glycogen. When it comes to exercises and heavy activities, skeletal muscle stores energy as glycogen too. Its synthesis from glucose is carried out by the enzyme glycogen synthase. The function of glycogen is to supply glucose on demand to skeletal muscle as well.
For plants, the energy storage for them via polymers of glucose is starch, and there are two forms to it. Amylose is a linear chain of glucose units all in α(1-4) bonds. Amylopectin is a branched form of starch, it is similar to glycogen but it has fewer branches, and only one α(1-6) bond per 30 α(1-6) bond (Grant, 2006). When it comes the human diets, over half of the carbohydrates is actually made up of starch from the plants. Starch is a valuable source of energy in our diet and is contained in large quantities in staple foods such as potato, bread, rice and pasta. If one spends a long time chewing a piece of bread, it will start to taste sweet. This happens because an enzyme in saliva, known as amylase, begins to break down the starch in the bread, releasing some of the glucose units. In addition, starch also has been used as a polymer to replace plastics in packaging (Ennis, 2005). Starch has the advantages of being biodegradable and as a renewable resource.
For cellulose, another major polysaccharide of plants, is also a polymer of glucose. However, unlike starch, it cannot be digested by humans. It provides structural support for plant cell walls. Like amylose, cellulose is an unbranched but linear chain of glucose units made up of β(1-4) glycosidic bonds. This allows cellulose to form long, straight chains. These long, straight chain cellulose polymers form fibrils of cellulose, which have high strength, providing the structural scaffold that supports plant cell walls (see Figure 3b).

Figure 3(b) shows the long straight chains which form fibrils of cellulose, that supports plant cell walls.
However, when compared to α(1-4) glycosidic bonds of starch and glycogen, the molecular shape of starch and glycogen is seen as a hollow coil or helix (Cummings, 1997). 
On the other hand, as the nutritional value of plants and fruits is recognised as mammals do not have cellulases except for herbivores such as cows, sheep and horses, thus cannot digest cellulose fibres. Hence, that makes cellulose a main component of dietary fibre, which forms an important part of a balanced diet.
According to John Cummings (1997), dietary fibre – also known as non-starch polysaccharides, which is made up of carbohydrates such as cellulose and pectin or plant cell walls. It includes the polysaccharides apart from starch that is found in fruits, vegetables, cereals and nuts. There had been many health benefits from these dietary fibre. Two important effects are firstly, non-starch polysaccharides, in the form of plant cell walls delay the absorption of nutrients such as starch and sugars enclosed within the plant cell, which can influence the blood glucose response to foods. Secondly, some soluble, non-starch polysaccharides, which are obtained from plant cell walls, form a gelatinous physical barrier which slows absorption from the gut and lowers cholesterol in blood. Furthermore, Cummings (1997) stated that a high-fibre diet rich in fruit and vegetables may lower the risk of coronary heart disease.
Besides being a source of dietary fibre for humans and plant structure for cell walls, cellulose is also a key raw material of cotton, linen and paper (Grant, 2006). The chemical treatments of cellulose had allowed it to become part of the production of fabrics such as rayon. There had been much exploration to find sustainable energy sources and carbohydrate polymers had been seen as major components of biofuels, as a renewable and carbon-rich source of energy by Grant (2006).
In conclusion, carbohydrates are extremely important macromolecules, from its interesting and meticulous structures to major functions by storing the energy of plants captured from sunlight and providing animals with nutrition. The vital structural roles played by carbohydrates in biology have also shown that they are not only an important energy source for plants and animals, but they have been also a sustainable source for the future industries of energy sources.
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Denise R. Ferrier. And Richard A Harvey. (2014). Lippincott’s Illustrated Reviews: Biochemistry. 6th ed. Baltimore, MD: Lippincott, Williams & Wilkins, pp.83-90
Ennis, C. (2005). Carbohydrates. Chemistry Review, 14,(3), pp. 14-16 (2010-2019). Biochemistry and cell organisation. [diagram] Available at: [Accessed 20 Oct. 2019].
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