Why do humans circulate monosaccharides instead of disaccharides as in plants?

Why do humans circulate monosaccharides instead of disaccharides as in plants?

We are searching data for your request:

Forums and discussions:
Manuals and reference books:
Data from registers:
Wait the end of the search in all databases.
Upon completion, a link will appear to access the found materials.

Plants transport food mostly in the form of disaccharides like sucrose but humans transport them in the form of a monosaccharide - glucose.

What is the reason behind this ?

One of the reasons could be -

humans and many other animals have active circulatory systems, there is little need for targeting sugars for specific parts of the body. The entire body would rapidly reach equilibrium with the glucose level of the bloodstream, and thus there likely never evolved the need to target specific organs to receive sugars via disaccharides

Source : This

Other reason could be that disaccharides are harder to transfer inside the cells than monosaccharides.

Why is it that during transportation of carbohydrates in plants it is in the form of Sucrose but in animals it is in the form of Glucose?

Sucrose transport is more efficient for plants. Furthermore, plants and animals have different enzymes and transporters.


#color(blue)"The difference between glucose an sucrose"#

  • Glucose = a monosaccharide, a single building block of sugars
  • Sucrose = a disaccharide, build from the monosaccharides glucose and fructose.

#color(blue)"Why plants use sucrose instead of glucose"#
Sucrose is formed in the cytosol of photosynthesizing cells from fructose and glucose and is then transported to other parts of the plant. This process is favorable for two reasons:

Sucrose contains more energy than a monosaccharide, so it is more energy efficient, both in transport as in storage.

Secondly, sucrose is a so called non-reducing sugar. This means that it is not oxidized i.e. no intermediate reactions with other molecules occur. This in contrast to glucose that is reactive and can form other products during transport.

#color(blue)"Why animals use glucose instead of sucrose"#
The question arises why animals don't use sucrose instead of energy considering the above mentioned advantages.

This has to do with the fact that animal cells do not have the same transport mechanisms and enzyme distribution as plants do:

In plants sucrose is converted back to glucose and fructose by an enzyme called sucrase. Animals produce less sucrase, the presence of these enzymes is limited to certain tissues.

Animals have specific mechanisms to transport glucose to the target tissues. However, they have no sucrase transporters

Some cells convert sucrose into glucose and fructose. Glucose can go into glycolysis in almost all tissues. Fructolysis is limited to the liver, so most cels don't have the enzymes to deal with fructose.

Useful Notes on Disaccharides (With Diagram)

Disaccharides consist of two ringed mono-saccha­rides.

The bonds that unite neighboring mono-sac­charides are called glycosidic bonds and are formed by the condensation of a hydroxyl group of carbon atom number 1 of one monosaccharide with the hy­droxyl group of either the number 2, 4, or 6 carbon atom of another.

The formation of the common disac­charide maltose from two molecules of glucose is shown in Figure 5-9.

In maltose, the oxygen bridge is formed between the number 1 carbon atom of one a-d- glucose unit and the number 4 carbon atom of the other. The bond formed is referred to as an α1→4 glycosidic bond.

Another important disaccharide is sucrose (i.e., or­dinary “table” sugar), which is formed by the conden­sation of a-d-glucose and 0-d-fructose (Fig. 5-10). Milk contains the disaccharide lactose, which consists of the hexoses α-D-galactose and β-D-glucose (Fig. 5- 11). In lactose, the glycosidic bond is of the beta vari­ety i.e., β1 → 4 (compare with maltose).

Why do humans circulate monosaccharides instead of disaccharides as in plants? - Biology


The term carbohydrate was originally used to describe compounds that were literally "hydrates of carbon" because they had the empirical formula CH2O. In recent years, carbohydrates have been classified on the basis of their structures, not their formulas. They are now defined as polyhydroxy aldehydes and ketones. Among the compounds that belong to this family are cellulose, starch, glycogen, and most sugars.

There are three classes of carbohydrates: monosaccharides, disaccharides, and polysaccharides. The monosaccharides are white, crystalline solids that contain a single aldehyde or ketone functional group. They are subdivided into two classes aldoses and ketoses on the basis of whether they are aldehydes or ketones. They are also classified as a triose, tetrose, pentose, hexose, or heptose on the basis of whether they contain three, four, five, six, or seven carbon atoms.

With only one exception, the monosaccharides are optically active compounds. Although both D and L isomers are possible, most of the monosaccharides found in nature are in the D configuration. Structures for the D and L isomer of the simplest aldose, glyceraldehyde, are shown below.

The structures of many monosaccharides were first determined by Emil Fischer in the 1880s and 1890s and are still written according to a convention he developed. The Fischer projection represents what the molecule would look like if its three-dimensional structure were projected onto a piece of paper. By convention, Fischer projections are written vertically, with the aldehyde or ketone at the top. The -OH group on the second-to-last carbon atom is written on the right side of the skeleton structure for the D isomer and on the left for the L isomer. Fischer projections for the two isomers of glyceraldehyde are shown below.

These Fischer projections can be obtained from the skeleton structures shown above by imaging what would happen if you placed a model of each isomer on an overhead projector so that the CHO and CH2OH groups rested on the glass and then looked at the images of these models that would be projected on a screen.

Fischer projections for some of the more common monosaccharides are given in the figure below.

Glucose and fructose have the same formula: C6H12O6. Glucose is the sugar with the highest concentration in the bloodstream fructose is found in fruit and honey. Use the Fischer projections in the figure of common monosaccharides to explain the difference between the structures of these compounds. Predict what an enzyme would have to do to convert glucose into fructose, or vice versa.

If the carbon chain is long enough, the alcohol at one end of a monosaccharide can attack the carbonyl group at the other end to form a cyclic compound. When a six-membered ring is formed, the product of this reaction is called a pyranose, shown in the figure below.

When a five-membered ring is formed, it is called a furanose, shown in the figure below.

There are two possible structures for the pyranose and furanose forms of a monosaccharide, which are called the a - and b -anomers.

The reactions that lead to the formation of a pyranose or a furanose are reversible. Thus, it doesn't matter whether we start with a pure sample of a -D-glucopyranose or b -D-glucopyranose. Within minutes, these anomers are interconverted to give an equilibrium mixture that is 63.6% of the b -anomer and 36.4% of the a -anomer. The 2:1 preference for the b -anomer can be understood by comparing the structures of these molecules shown previously. In the b -anomer, all of the bulky -OH or -CH2OH substituents lie more or less within the plane of the six-membered ring. In the a -anomer, one of the -OH groups is perpendicular to the plane of the six-membered ring, in a region where it feels strong repulsive forces from the hydrogen atoms that lie in similar positions around the ring. As a result, the b -anomer is slightly more stable than the a -anomer.

Disaccharides are formed by condensing a pair of monosaccharides. The structures of three important disaccharides with the formula C12H22O11 are shown in the figure below.

Maltose, or malt sugar, which forms when starch breaks down, is an important component of the barley malt used to brew beer. Lactose, or milk sugar, is a disaccharide found in milk. Very young children have a special enzyme known as lactase that helps digest lactose. As they grow older, many people lose the ability to digest lactose and cannot tolerate milk or milk products. Because human milk has twice as much lactose as milk from cows, young children who develop lactose intolerance while they are being breast-fed are switched to cows' milk or a synthetic formula based on sucrose.

The substance most people refer to as "sugar" is the disaccharide sucrose, which is extracted from either sugar cane or beets. Sucrose is the sweetest of the disaccharides. It is roughly three times as sweet as maltose and six times as sweet as lactose. In recent years, sucrose has been replaced in many commercial products by corn syrup, which is obtained when the polysaccharides in cornstarch are broken down. Corn syrup is primarily glucose, which is only about 70% as sweet as sucrose. Fructose, however, is about two and a half times as sweet as glucose. A commercial process has therefore been developed that uses an isomerase enzyme to convert about half of the glucose in corn syrup into fructose (see Practice Problem 4). This high-fructose corn sweetener is just as sweet as sucrose and has found extensive use in soft drinks.

The monosaccharides and disaccharides represent only a small fraction of the total amount of carbohydrates in the natural world. The great bulk of the carbohydrates in nature are present as polysaccharides, which have relatively large molecular weights. The polysaccharides serve two principal functions. They are used by both plants and animals to store glucose as a source of future food energy and they provide some of the mechanical structure of cells.

Very few forms of life receive a constant supply of energy from their environment. In order to survive, plant and animal cells have had to develop a way of storing energy during times of plenty in order to survive the times of shortage that follow. Plants store food energy as polysaccharides known as starch. There are two basic kinds of starch: amylose and amylopectin. Amylose is found in algae and other lower forms of plants. It is a linear polymer of approximately 600 glucose residues whose structure can be predicted by adding a -D-glucopyranose rings to the structure of maltose. Amylopectin is the dominant form of starch in the higher plants. It is a branched polymer of about 6000 glucose residues with branches on 1 in every 24 glucose rings. A small portion of the structure of amylopectin is shown in the figure below.

The polysaccharide that animals use for the short-term storage of food energy is known as glycogen. Glycogen has almost the same structure as amylopectin, with two minor differences. The glycogen molecule is roughly twice as large as amylopectin, and it has roughly twice as many branches.

There is an advantage to branched polysaccharides such as amylopectin and glycogen. During times of shortage, enzymes attack one end of the polymer chain and cut off glucose molecules, one at a time. The more branches, the more points at which the enzyme attacks the polysaccharide. Thus, a highly branched polysaccharide is better suited for the rapid release of glucose than a linear polymer.

Polysaccharides are also used to form the walls of plant and bacterial cells. Cells that do not have a cell wall often break open in solutions whose salt concentrations are either too low (hypotonic) or too high (hypertonic). If the ionic strength of the solution is much smaller than the cell, osmotic pressure forces water into the cell to bring the system into balance, which causes the cell to burst. If the ionic strength of the solution is too high, osmotic pressure forces water out of the cell, and the cell breaks open as it shrinks. The cell wall provides the mechanical strength that helps protect plant cells that live in fresh-water ponds (too little salt) or seawater (too much salt) from osmotic shock. The cell wall also provides the mechanical strength that allows plant cells to support the weight of other cells.

The most abundant structural polysaccharide is cellulose. There is so much cellulose in the cell walls of plants that it is the most abundant of all biological molecules. Cellulose is a linear polymer of glucose residues, with a structure that resembles amylose more closely than amylopectin, as shown in the figure below. The difference between cellulose and amylose can be seen by comparing the figures of amylose and cellulose. Cellulose is formed by linking b -glucopyranose rings, instead of the a -glucopyranose rings in starch and glycogen.

The -OH substituent that serves as the primary link between -glucopyranose rings in starch and glycogen is perpendicular to the plane of the six-membered ring. As a result, the glucopyranose rings in these carbohydrates form a structure that resembles the stairs of a staircase. The -OH substituent that links the b -glucopyranose rings in cellulose lies in the plane of the six-membered ring. This molecule therefore stretches out in a linear fashion. This makes it easier for strong hydrogen bonds to form between the -OH groups of adjacent molecules. This, in turn gives cellulose the rigidity required for it to serve as a source of the mechanical structure of plant cells.

Cellulose and starch provide an excellent example of the link between the structure and function of biomolecules. At the turn of the century, Emil Fischer suggested that the structure of an enzyme is matched to the substance on which it acts, in much the same way that a lock and key are matched. Thus, the amylase enzymes in saliva that break down the a -linkages between glucose molecules in starch cannot act on the b -linkages in cellulose.

Most animals cannot digest cellulose because they don't have an enzyme that can cleave b -linkages between glucose molecules. Cellulose in their diet therefore serves only as fiber, or roughage. The digestive tracts of some animals, such as cows, horses, sheep, and goats contain bacteria that have enzymes that cleave these b -linkages, so these animals can digest cellulose.

Termites provide an example of the symbiotic relationship between bacteria and higher organisms. Termites cannot digest the cellulose in the wood they eat, but their digestive tracts are infested with bacteria that can. Propose a simple way of ridding a house from termites, without killing other insects that might be beneficial.

For many years, biochemists considered carbohydrates to be dull, inert compounds that filled the space between the exciting molecules in the cell the proteins. Carbohydrates were impurities to be removed when "purifying" a protein. Biochemists now recognize that most proteins are actually glycoproteins, in which carbohydrates are covalently linked to the protein chain. Glycoproteins play a particularly important role in the formation of the rigid cell walls that surround bacterial cells.

Complex Carbohydrates

Some carbohydrates consist of hundreds — or even thousands! — of monosaccharides bonded together in long chains. These carbohydrates are called polysaccharides (“many saccharides”). Polysaccharides are also referred to as complex carbohydrates . Complex carbohydrates that are found in living things include starch, glycogen, cellulose, and chitin. Each type of complex carbohydrate has different functions in living organisms, but they generally either store energy or make up certain structures in living things.

Why do humans circulate monosaccharides instead of disaccharides as in plants? - Biology

What are carbohydrates?

When most people refer to carbohydrates they are talking about foods that are starchy (like bread, pasta, and rice) or are sugary (like candy, cookies, and cake). In science, when we talk about carbohydrates we are talking about specific types of molecules.

Carbohydrates are one of the four major groups of organic molecules the other three being proteins, nucleic acids (DNA), and lipids (fats). Carbohydrates are made up of three elements: carbon, hydrogen, and oxygen.

Carbohydrates are important to the daily lives of living organisms. They store energy (starches), provide energy for cells (glucose), and provide structure to plants and some animals.

Types of Carbohydrates

  • Monosaccharides - Monosaccharides are the simplest form of carbohydrates. They include sugars such as glucose and fructose. Monosaccharides often taste sweet and dissolve in water. Glucose is a common carbohydrate found in plants and is the main product of photosynthesis.
  • Disaccharides - Disaccharides are formed from two Monosaccharides. They are also known as sugars such as sucrose and lactose. Lactose is the carbohydrate found in milk.
  • Oligosaccharides - Oligosaccharides are formed from a small number (usually three to six) of monosaccharides.
  • Polysaccharides - Polysaccharides are long carbohydrate molecules. They are often called complex carbohydrates.

More about Complex Carbohydrates (Polysaccharides)

  • Starches - Starches are a way that many plants store energy. We can then eat starches and our bodies will use the energy.
  • Glycogen - Animals use glycogen to store energy. It is stored in the liver and the muscles to be used when needed.
  • Cellulose - Cellulose is used in plants as a structural molecule. It can't be digested by animals.
  • Chitin - Chitin is used as a structural molecule in fungi and arthropods.

When you eat carbohydrates your body uses them for energy. However, if you eat more than your body needs, it will convert them into fat. Fat is the way that the body stores energy for later use. The body is trying to save up energy for a later time when you don't have any carbohydrates to eat.

Examples of disaccharides


Sucrose, commonly known as table sugar in its refined form, is a disaccharide found in many plants. It consists of the monosaccharides glucose and fructose.

In the form of sugar, sucrose is a very important component of the human diet as a sweetener. Sugar was first extracted and purified from sugarcane in India in the 8th century BC.

In fact, the word candy takes its name in part from the word"khanda", which was a name for sugar crystals in Sanskrit.

H O and in day, about 175 metric tons of sugar are produced every year. People with congenital sucrase-isomaltase deficiency are intolerant to sucrose and can not digest it well because they lack the enzyme sucrose-isomaltase.

A person who is intolerant to sucrose should limit sucrose as much as possible, and should take supplements or medications.


Also known as malt sugar, it is formed from two molecules of glucose. Malt is formed when the grains soften and grow in water, and is a component of beer, starchy foods such as cereals, pasta and potatoes, and many processed foods.

In plants, maltose is formed when starch is broken down for food. It is used to germinate seeds.


Lactose, or milk sugar, is composed of galactose and glucose. Mammalian milk is high in lactose and provides nutrients for babies.

Most mammals can only digest lactose as infants, and they lose this ability as they mature. In fact, humans who are able to digest dairy products in adulthood actually have a mutation that allows them to do so.

That's why so many people are lactose intolerant Humans, like other mammals, did not have the ability to digest lactose in infancy until this mutation was present in certain populations about 10,000 years ago.

Today, the number of people who are lactose intolerant varies widely among populations, ranging from 10% in northern Europe to 95% in parts of Africa and Asia. Traditional diets from different cultures reflect this in the amount of dairy products consumed.


Trehalose is also made up of two glucose molecules like maltose, but the molecules are linked differently. It is found in certain plants, fungi and animals such as shrimp and insects.

The blood sugar of many insects, such as bees, grasshoppers and butterflies, is composed of trehalose. They use it as an efficient storage molecule that provides fast energy for flight when it breaks down.


It consists of two linked glucosamine molecules. Structurally, it is very similar to cellobiose, except that it has an N-acetylamino group where cellobiose has a hydroxyl group.

It is found in some bacteria, and is used in biochemical research in order to study enzymatic activity.

It is also found in chitin, which forms walls of fungi, exoskeletons of insects, arthropods and crustaceans, and is also found in fish and cephalopods such as octopus and squid.

Celobiosa (Glucose + glucose)

Cellobiose is a product of hydrolysis of cellulose or materials rich in cellulose, such as paper or cotton. It is formed by binding two molecules of beta-glucose by a &beta-bond (1 &rarr 4)

Lactulose (Galactose + fructose)

Lactulose is a synthetic (artificial) sugar that is not absorbed by the body but breaks down in the colon into products that absorb water in the colon, which softens the stool. Its primary use is to treat constipation.

It is also used to reduce blood ammonia levels in people with liver disease, as lactulose absorbs ammonia in the colon (removing it from the body).

Isomaltosa (Glucose + Glucose Isomaltase)

Produced during the digestion of starch (bread, potatoes, rice), or artificially produced.

Isomaltulose (Glucose + Fructose Isomaltase)

Sugar cane syrup, and honey is also produced artificially.


Trehalulose is an artificial sugar, a disaccharide composed of glucose and fructose linked by an alpha (1-1) glycosidic bond.

It occurs during the production of isomaltulose from sucrose. In the lining of the small intestine, the enzyme isomalase breaks down into trehalulose to glucose and fructose, which are then absorbed into the small intestine. Trehalulose has a low potency to cause tooth decay.


It is the disaccharide repeat unit in chitin, which differs from cellobiose only in the presence of an N-acetylamino group on carbon-2 instead of the hydroxyl group. However, the non-acetylated form is often also called chytobiose.


It is a crystalline alcohol C 12 H 24 O 11 obtained by hydrogenation of lactose. It is an analog disaccharide of lactulose, used as sweetener. It is also laxative and is used to treat constipation.


An organic disaccharide reducing compound that can be used as a carbon source by bacteria and fungi.


A disaccharide sugar (C12H22O11) formed by partial hydrolysis of raffinose.


A disaccharide consisting of two xylose residues.


A disaccharide present in a sophorolipid.


Gentiobiose is a disaccharide consisting of two D-glucose units linked by a &beta-type glycosidic bond (1 &rarr 6). Gentiobiose has many isomers which differ by the nature of the glycosidic bond connecting the two glucose units.


It is a glycosylfructose consisting of an &alpha-D-glucopyranosyl residue bound to D-fructopyranose via a bond (1 &rarr 5). An isomer of sucrose.


It is a disaccharide present in glycosides.

Caroliniaside A

Oligosaccharides containing two units of monosaccharides linked by a glycosidic bond.

  1. Essay on the Introduction to Carbohydrates
  2. Essay on the Functional Importance of Carbohydrates
  3. Essay on the Synthesis of Carbohydrates
  4. Essay on the Forms of Carbohydrates
  5. Essay on the Absorption of Carbohydrates
  6. Essay on the Role of Vitamins on Carbohydrate
  7. Essay on the Metabolism of Carbohydrates

Essay # 1. Introduction to Carbohydrates:

A carbohydrate is generally defined as a neutral compound made up of carbon, hydrogen and oxygen, the last two elements remaining in the same proportion as in water.

The general formula is Cn(H2O)n. But there may be exceptions. For instance, rhamnose (C6H12O5) is a carbo­hydrate in which H and O remain in a different proportion. Also there are certain other compounds, such as formaldehyde (HCHO), acetic acid (CH3COOH), lactic acid (CH3CHOHCOOH), etc., which have got the same empirical formula but are not carbohydrates. Thus chemically, carbohydrates can be defined as the aldehyde and ketone derivative of higher polyhydric alcohol (having more than one ‘OH’ group).

Essay # 2. Functional Importance of Carbohydrates:

i. It is the readily available fuel of the body.

ii. It also constitutes the structural material of the organism.

iii. It also acts as important storage of food material of the organism.

iv. Carbohydrate also plays a key role in the metabolism of amino acids and fatty acids.

Essay # 3. Synthesis of Carbohydrates:

It is certain that the glycerol, portion of fat, which makes up about 10% of the fat molecule, is converted into glucose in the body but the conversion of fatty acid portion of fat molecules to glucose is a matter of dispute specially in animal body as contrast to plants.

Some of the evidences are given below:

(a) During hibernation the marmot shows a very low respiratory quotient-about 0.3-0.4. The excess oxygen intake is explained on the assumption that oxygen-poor substance (fat) is being converted into an oxygen—rich substance (carbohydrate). But these findings and conclusions are not beyond question.

(b) The same type of conversion is believed to take place in diabetic subjects where a low respiratory quotient is found, and

(c) When fatty acids containing odd number of carbon atoms, such as, propionic, valeric and heptoic acids, etc., are administered to starving rats they are converted into glycogen in the liver. But these fatty acids are not found in the natural fats.

Natural fats, which contain fatty acids with even number of carbon atoms only, do not produce this effect. From this it can be concluded that the synthesis of carbohydrates from fats is not beyond possibility but it takes place indirectly.

2. From Proteins:

There is sample evidence to show that formation of glucose and glycogen may take place from proteins. The administration of certain amino acids in a depancreatised dog raises the urinary glucose. These amino acids are called antiketogenic amino acids. On the whole it is generally accepted that about 60% of the food proteins can form sugar. As to the chemical process of synthesis of carbohydrates from proteins, nature of the mechanism is different with different amino acids.

Some examples are given below:

Essay # 4. Forms of Carbohydrates:

The different forms of carbohydrates which are generally included in diet are as follows:

i. Polysaccharides – Starch, dextrin, glycogen and cellulose.

ii. Oligosaccharides (Disaccharides) – Lactose, maltose, sucrose.

iii. Monosaccharides – Glucose and fructose.

Of these types, cellulose containing β-1, 4 linkages cannot be appreciably digested in the human alimentary canal. Monosaccharides need no further digestion, because all carbohydrates are absorbed in the form of monosaccharides. Hence, digestion of carbohydrates includes the digestion of polysaccharides and oligosaccharides.

Digestion of polysaccharides and oligosaccharides starts in the saliva and is completed in the succus entericus. Digestion of oligosaccharides (disaccharides) chiefly takes place in the succus entericus, but may occur to a slight extent by other digestive juices.

The brief details of the digestion of starch and disaccharides are as follows:

I. Digestion in the Saliva:

Saliva contains – (a) chiefly salivary amylase or ptyalin, and (b) traces of maltase (its presence in saliva is doubtful). Salivary amylase (α-type) whose origin in the saliva, acts on starch (which is mostly amylopectin type) and contains straight chains held by 1, 4′-α glucosidic linkages and branch chains whose branch points are 1, 6′- α glucosidic linkages. Maltase acts on maltose.

1. Conditions and Peculiarities of Ptyalin Action:

a. Ptyalin acts on boiled starch only. It cannot penetrate the intact cellulose covering of the un-boiled starch particle.

b. Optimum reaction is slightly acid (pH 6.5), but it can also act in neutral or slightly alkaline medium.

c. Strong acid (such as HCl of gastric juice) destroys ptyalin.

d. Optimum temperature is about 45°C., at 60°C., it is destroyed.

e. Effects of salts (such as chlorides) are necessary for ptyalin action.

f. Ptyalin digests starch up to the maltose stage only.

2. Site of Ptyalin Action:

Although digestion of starch starts in the mouth, yet ptyalin action chiefly takes place in the stomach (before the HCl concentration becomes high). On an average it continues for 30-40 minutes, upper favourable conditions, starch is converted into maltose, isomaltose and maltotriose.

3. Stages of Ptyalin Digestion:

Ptyalin which hydrolyses only α-1, 4′ linkages but not the α-1,6′ linkages and splits the more central linkages, α- and β amylases supplement each other’s action upon amylopectin as β-amylase splits maltose from the end groups and a-amylase splits central linkages to form more end groups.

By its action isomaltose (containing 3 glucose molecules in which there is one α-1, 6′ linkages), maltose (glucose-glucose), maltotriose (glucose-glucose-glucose) and a mixture of dextrins (containing the α-1, 6′ branches and averaging six glucose residues per molecule) are produced.

The stages are briefly as follows (Fig. 9.57):

II. Digestion in the Gastric Juice:

Gastric juice does not possess any carbohydrate splitting enzyme, but gastric HCl can carry on some hydro­lysis of sucrose.

III. Digestion in the Pancreatic Juice:

Pancreatic juice contains two enzymes acting on carbohydrates:

Pancreatic amylase acting on starch and dextrin. Hopkins has divided the amylases into two groups – α-am­ylase or endoamylase is of animal origin and β-amylase or exoamylase is of plant origin, α-amylase acts on the polysaccharide on the interior of the chain and β-amylase from the non-reducing end.

Maltose (in traces) acting on maltose.

Pancreatic Amylase:

1. Conditions of action of pancreatic amylase are as follows:

i. It can act on both boiled and un-boiled starch.

ii. Its action is much more rapid than ptyalin. Most of the starch is converted into maltose within a few minutes.

iii. Optimum reaction ranges from pH 6.7 – 7.0, i.e., slightly acid or neutral. It can also act in slightly alkaline medium.

iv. Optimum temperature is about 45°C.

v. Salts and CI ions are essential for its action.

vi. Amylase is not present in the pancreatic juice of infants up to the age of about 6 months. Hence, during this period the baby should not be given any starchy food.

2. Stages of Action of Pancreatic Amylase:

Stages of action of pancreatic amylase are same as that of ptyalin. All starch and dextrins when exhaus­tively acted both by salivary and pancreatic amylase successively are converted into maltose, dextrin and isomaltose. The latter two are not hydrolysed to maltose and ultimately glucose due to the absence of oligo-α-1, 6′ glucosidase and appreciable amount of maltase in either saliva or pancreatic juice. After this, digestion by intestinal juice begins.

IV. Digestion in the Succus Entericus:

Succus entricus contains oligo-α-1, 6′ glucosidase which splits α-1, 6′ linkages of dextrin formed by the action of a-amylase thus providing scope of further activity of α-amylase and of isomaltose converting it to glucose. It also contains maltase which hydrolyses maltase to glucose.

So the starch is completely hydrolysed to glucose by joint action of α-1, 4′ amylase present in saliva and pancreatic juice and α-1, 6′ glucosidase and maltase in the succus entericus. The other disaccharides taken in food are hydrolysed by lactase and sucrase (invertase) present in this juice.

The enzymes, and their substrates upon which they act, and the respective end products are given below:

Acts on sucrose producing one, molecule of fructose and one molecule of glucose.

Acts on lactose giving one molecule of glucose and one molecule of galactose.

Converts isomaltose into glucose and splits α-1, 6′ linkages of dextrin.

Acts on maltose giving two glucose molecules.

(It has been suggested that there are more than one kind of sucrase, lactase or maltase).

Traces of this enzyme may be present in the succus entericus. The presence of this enzyme at a low concentration has been established. It is supposed to act on that little quantity of starch and dextrin which might have escaped pancreatic digestion.

Evidences indicate that many or all intestinal enzymes are intracellular. Their presence in the juice is due to cell desquamation.

Thus, the digestible carbohydrates are all converted into monosaccharides in which form they are absorbed. The process of starch digestion is slow and prolonged. So monosaccharides are slowly produced as observed by restricting disaccharides in the diet. Thus their absorption becomes slow and a high rise of blood sugar is prevented. Consequently, this can be considered as a process for maintaining blood sugar level within a constant range.

Essay # 5. Absorption of Carbohydrates:

End products of carbohydrate digestion are all monosaccharides, such as glucose, laevulose, galactose, xylose, mannose, arabinose, etc. It is in this form that carbohydrates are absorbed. It is believed that little quantities of disaccharides may also be absorbed, but the still higher forms are not absorbed at all.

Mechanism of Absorption:

The monosaccharides being highly soluble in water, the physical forces like osmosis, diffusion, filtration, etc., certainly play a considerable part in their absorption. But if these were the only forces concerned in the process, the rate of absorption should have been directly proportional to their concentration in the gut.

But this linear relation is found to hold only in the cases of xylose, arabinose, and mannose. With other sugars this principle does not apply for instance, the amount of glucose and galactose absorbed per hour is entirely independent of their concentrations in the gut. Laevulose occupies an intermediate position.

Moreover, the rate of absorption varies with the nature of sugars. If the rate of glucose absorption is taken as the standard (i.e., 100%), then galactose is found to be absorbed much more rapidly (225%) whereas the rate of absorption of laevulose is about half that of glucose, (i.e., 44%) of mannose (33%) and xylose (30%) is about one-third that of glucose.

Other sugars are still more slowly absorbed. Hydroxyl group at position 2 of the sugar molecule seems to be essential for their active transport. It is found that when the temperature of the mucous membrane is brought down between 0°C., to 20°C., the difference in the rates of absorption between glucose, galactose, laevulose and other sugars disappears. Under such conditions, they are all absorbed slowly, but at the same rate.

These observations indicate that the preferential treatment done towards glucose, galactose and laevulose is due to some chemical activity going on inside the cells. Phloridzin (phlorrhizin) or iodo-acetic acid retards the high rate of glucose and galactose absorption. Since, these substances paralyse the activity of phosphatase it is probable that this enzyme is intimately concerned with the process.

Verzer has suggested that during absorption, phosphorylation of sugars takes place and the corresponding hexose phosphates are formed. The formation of these compounds keeps down the concentration of free glucose, etc., in the cells and thereby ensures their rapid absorption.

In this way the high rate of absorption of these sugars can be explained. It is almost certain that on the other side of the epithelial cells a reversible reaction takes place (dephosphorylation) in which the hexose phosphate is broken down, free glucose enters the blood stream and the phosphoric acid is retained in the cells for further phosphorylation. It is interesting to note that the same mechanism also takes place in the reabsorption of glucose by the renal tubules.

Thus from the above it can be concluded that in addition to the physical forces the following factors are necessary for rapid absorption of sugars:

i. Complete digestion into monosaccharides.

ii. Presence of phosphoric acid and phosphokinase enzyme in the epithelial cells.

iii. Adrenal cortex and insulin which probably control the process of phosphorylation.

iv. Anterior pituitary whose exact role is not known, but which may act indirectly through adrenal cortex.

v. Sodium salts also exert some effects. Because in adrenalectomised animals the rate of absorption of glucose can be restored to normal by giving sodium salts.

vi. Vitamins Thiamine, pantothenic acid and pyridoxine may help in the process of absorption.

vii. Blood glucose level has got no effect on absorption of glucose.

Pathway of Absorption:

Carbohydrates are almost completely absorbed through the portal system, because portal blood always shows a higher concentration of sugar during absorption. An inconspicuous amount may pass through the lymphatics.

Site of Absorption:

Glucose is maximally absorbed in jejunum.

Essay # 6. Role of Vitamins on Carbohydrate:

1. Thiamine (Vitamin B1):

Thiamine acts as a coenzyme of the carboxylase which helps in oxidative decarboxylation of pyruvic acid. It has a potential role in the oxidation of sugar in tissues including brain. In its absence, pyruvic and lactic acids fail to be metabolised with a result of accumulation of these substances in blood and tissues. This metabolic disorders produced by this vitamin deficiency, results beriberi. In the tissues thiamine exists as thiamine pyrophosphate ester and helps in decarboxylation of α-ketonic acid as a coenzyme.

Since riboflavin is related with tissue oxidation, so it takes part in carbohydrate metabolism. In the tissues this vitamin exists as FMN and FAD. These two coenzymes in combination with apoenzyme play a great role in a number of enzyme systems. Deficiency of this vitamin results disorder of intermediary metabolism leading to condition known as cheilosis.

3. Nicotinic Acid (Niacin):

It remains as a prosthetic group of at least two enzyme systems- NAD and NADP, and takes part in tissue oxidation. Niacin helps in the formation of fats from carbohydrates. The deficiency of this vitamin results disorder of intermediary metabolism leading to condition known as pallegra.

4. Pantothenic Acid (Vitamin B3):

Since pantothenic acid is a component of coenzyme A, so it takes part in carbohydrate metabolism. The condi­tion of alopecia and certain gastro-intestinal disorders are produced by the deficiency of this vitamin.

5. Cyanocobalamin (Vitamin B12):

This vitamin acts as a cofactor (cobamide) for the enzyme methyl malonyl CoA isomerise which is concerned for the conversion of methyl malonyl CoA to succinyl CoA or succinyl CoA to methyl malonyl CoA. Thus this vitamin is essential in the biochemical conversion of carbohydrate to fat or fat to carbohydrate. After administration of B12 hyperglycaemia may be corrected.

6. Ascorbic Acid (Vitamin C):

Ascorbic acid takes part in the tissue oxidation probably by acting as hydrogen-carrier. Deficiency of this vitamin results disorders of metabolism leading to condition known as scurvy (scorbusis).

Essay # 7. Metabolism of Carbohydrate:

Although the products of carbohydrate digestion, absorbed into the blood, are mainly hexose monosaccharides, i.e., glucose, fructose, galactose as well as partly pentose sugars but animal cells utilize mostly glucose so monosaccharides are converted into glucose by the cells mainly for their oxidation and partly for their storage as glycogen (through UDPG cycle).

As fructose and galactose are utilized to a lesser extent in the body so they are reconverted from glucose as evident from the following:

i. Large quantities of galactose are found in the brain tissue.

ii. The brain probably uses carbohydrates as galactose only.

iii. During the normal process of glucose utilization, fructose diphosphate is formed as an intermediary step.

iv. When liver function is deficient, fructose and galactose fail to be converted into glucose and are turned out into the blood stream as such. Due to their low renal threshold they are excreted in the urine. The fructose and galactose tolerance tests of liver function depend upon this principle.

Since carbohydrate is utilized by the cells of all animals including man mainly as glucose so carbohydrate metabolism is meant essentially as metabolism of glucose and other substances, convertible to glucose or vice versa.

Pentose sugars, viz., xylose, arabinose and ribose present in the diet may be absorbed but their fate is not known whereas α-ribulose and α-2-deoxyribose after absorption are utilized in the body for the synthesis of nucleoproteins.

Normally glucose metabolism not only supplies major amount of energy for the body but also- (1) other forms of monosaccharides (viz., fructose, galactose, α-ribulose and α-2-deoxyribose, etc.), disaccharides (lac­tose) and polysaccharides (glycogen) (2) reserve depot fats, (3) tissue glycolipids, mucopolysaccharides, and (4) amino acids are formed from glucose and reversely protein and lipids are metabolized through glucose pathway.

Thus it appears likely that glucose metabolism holds a central key position in carbohydrate metabolism which is closely associated with the metabolism of protein and fat.

The metabolism of carbohydrate, i.e., mainly of glucose, in the mammalian organism is discussed under the following points (fate and functions):

i. Storage (by the Process of Glycogenesis):

When glucose is not immediately required by the tissues, it remains stored. The chief form of storage is glycogen. Total about 500-700 gm of glucose may remain stored in this form. The main storehouses are liver and muscles in which glycogen is almost equally distributed. Some glucose may remain stored as such temporarily in the skin and subcutaneous tissue.

ii. Sources of Energy (By the Process of Glycolysis and Oxidation of Pyruvic Acid through TCA Cycle):

Carbohydrates are oxidized in the tissues and supply energy. One gram yields 4 C of energy. The major part of daily energy requirement under normal conditions is derived from carbohydrate oxidation. It is the most readily available source of energy and its combustion involves the least oxygen requirement.

iii. Maintenance of Blood Sugar (Homeostasis):

Glucose remains in blood and its level is always kept within a narrow range. Normal blood sugar varies from 80-120 mgm per 100 ml whenever it tends to fall, glucose is mobilized from glycogen and blood sugar is maintained.

iv. Synthesis of Hexose Phosphate:

This is an intermediary step formed during oxidation of glucose, absorption of glucose from intestine and reabsorption from kidney tubules. It is a very important form in which hexose exists in the body. It is present in large quantities in the muscles, intestines, liver, etc.

v. Synthesis of Lactose:

In the lactating mother lactose is synthesized from blood glucose. The mammary glands convert glucose into galactose and then unite the latter with another molecule of glucose to form lactose.

vi. Synthesis of Glycoproteins.

vii. Synthesis of Complex Fats Containing Sugar:

For instance, cerebrosides are synthesized by the nerve cells from galactose and fats. Galactose is locally synthesized from blood glucose.

viii. Synthesis of Fat (Lipogenesis):

It is an established fact that the body can convert carbohydrate into fats.

ix. Synthesis of Proteins:

Simple amino acids may be formed by uniting ammonia with pyruvic acid, etc., which may easily be derived from carbohydrates.

Excretion of Glucose:

Glucose is not excreted from the body in normal health and Fehling’s test is negative. It is a high threshold substance (170 mgm per 100 ml of blood). In metabolic disorders (e.g., diabetes mellitus), glucose appears in the urine and Fehling’s Test is positive. When blood sugar level gives higher value than renal threshold value, the glucose appears in the urine, the condition is known as glycosuria.

In lactating mother, lactose may be found in the mother’s urine. Pentose is found in a condition known as pentosuria. Glucose may be found in urine in higher quantities in some pathological conditions.

Biology Essay on Carbohydrates | Organic Molecules | Biology

Are you looking for an essay on the ‘Carbohydrates and Its Types’? Find Paragraphs, long and short essays on ‘Carbohydrates and Its Types’ especially written for school and college students.

Essay # 1. Introduction to Carbohydrates:

Chemically, carbohydrates are organic molecules in which carbon, hydrogen, and oxy­gen bond together in the ratio Cx (H2O)y, where x and y are whole numbers that differ de­pending on the specific carbohydrate. They are reduced compounds having large quantities of hydroxyl groups. The presence of the hydroxyl groups allows carbohydrates to interact with the aqueous environment and to participate in hydrogen bond formation, both within and between chains.

The simplest carbohydrates also contain either an aldehyde moiety (termed polyhydroxyaldehydes) or a ketone moiety (polyhydroxyketones). Derivatives of the carbohydrates can contain nitrogen/s, phosphates and sulfur compounds. Carbohydrates can also combine with lipid to form glycolipids or with protein to form glycoproteins.

The aldehyde and ketone moieties of the carbohydrates with five and six carbons will spontaneously react with alcohol groups present in neighboring carbons to produce in­tramolecular hemiacetals or hemiketals, respectively. This results in the formation of five- or six-membered rings.

As the five-membered ring structure resembles the organic molecule furan, the derivatives with this structure are termed as furanoses. Those with six-membered rings, resemble the organic molecule pyran are termed pyranoses and are depicted by either Fischer or Haworth style diagrams. The numbering of the carbons in carbohydrates proceeds from the carbonyl carbon, for aldoses, or the carbon nearest the carbonyl, for ketoses.

The rings can open and re-close, allowing rotation to occur about the carbon bearing the reactive carbonyl, yielding two distinct configurations (α and β) of the hemiacetals and hemiketals. The carbon about which this rotation occurs is the anomeric carbon and these two forms are termed anomers. Carbohydrates can change spontaneously between α and β configurations- a process known as mutarotation. In the Fischer projection, α configuration places the hydroxyl attached to the anomeric carbon to the right, towards the ring, while in the Haworth projection, α configuration places the hydroxyl downward.

Carbohydrates can exist in either of two conformations, as determined by the orientation of the hydroxyl group about the asymmetric carbon farthest from the carbonyl. With a few exceptions, those carbohydrates that are of physiological significance exist in the D- conformation. Carbohydrates are the main energy source for the human body. Animals (including humans) break down carbohydrates during the process of metabolism to release energy.

For example, the chemical metabolism of the sugar (glucose) is shown below:

Carbohydrates are manufactured by plants during the process of photosynthesis. Plants har­vest energy from sunlight and stores in carbohydrate moieties.

All carbohydrates can be classified as monosaccharides, oligosaccharides or polysaccha­rides. Two to ten monosaccharide units, linked by glycosidic bonds, make up an oligosac­charide. Polysaccharides are much larger and contain hundreds of monosaccharide units.

Essay # 2. Classification of Carbohydrates:

Monosaccharides are simple sugars, having 3 to 7 carbon atoms. They can be bonded together to form polysaccharides. Cells also use simple sugars to store energy and construct other kinds of organic molecules. The names of most sugars end with the letters ‘ose’. Glucose and other kinds of sugars (fructose, and galactose) may be linear molecules (C6H12O6) but in aqueous solution they take ring form.

There are two isomers of the ring form of glucose. They differ in the location of the OH group on the number 1 carbon atom. The number 1 carbon atom of the linear form of glu­cose is attached to the oxygen on the number 5 carbon atom.

Disaccharides are composed of 2 monosaccharides joined together by a condensation reaction.

There are three common disaccharides:

i. Maltose (or malt sugar) consists of glucose monomers. Amylase enzyme digests starch molecules to produce maltose.

ii. Sucrose (or cane sugar) composed of glucose and fructose. Plants synthesize sucrose to transport to non-photosynthetic parts of the plant, because it is less reactive than glucose.

iii. Lactose (or milk sugar) is made up of galactose and glucose. It is found only in mam­malian milk.

(c) Polysaccharides:

Monosaccharides may be bonded together to form long chain compounds called polysaccha­rides. The monomeric building blocks used to generate polysaccharides can be varied in all cases, however, the predominant monosaccharide found in polysaccharides is D-glucose. Polysaccharides that are composed of a single monosaccharide building block are termed as homopolysaccharides, while polysaccharides composed of more than one type of monosac­charide, they are termed as heteropolysaccharides.

For examples, starch and glycogen are composed of glucose monomers bonded together, producing long chains. They serve the function as stored food, starch in plants and glycogen in animal, in the liver and muscles. Glycogen is poly (1-4) glucose with 9% (1-6) branches (Fig. 3.5).

Starch is a long (100s) polymer of glucose molecules, where all the sugars are oriented in the same direction. Unbranched starch is called amylose, while branched starch is known as amylopectin. Amylose is simply poly-(1-4) glucose units in a straight chain. In fact the chain is floppy, and it tends to coil up into a helix. Amylopectin is poly (1-4) glucose with about 4% (1-6) branches. This gives it a more open molecular structure than amylose.

As it has more ends, it can be broken more quickly than amylose by amylase enzymes. Amy­lopectin is a form of starch that is very similar to glycogen except for a much lower degree of branching (about every 20-30 residues). Another example of polysaccharide is cellulose. Cellulose is a long (100’s) polymer of glucose molecules. However, the orientation of the sugars is little different. In Cellulose, every other sugar molecule is “upside-down”. Glyco­gen is different from both, starch and cellulose in that the glucose chain is branched or “forked” (Fig. 3.6).

Section Summary

Living things are carbon-based because carbon plays such a prominent role in the chemistry of living things. The four covalent bonding positions of the carbon atom can give rise to a wide diversity of compounds with many functions, accounting for the importance of carbon in living things. Carbohydrates are a group of macromolecules that are a vital energy source for the cell, provide structural support to many organisms, and can be found on the surface of the cell as receptors or for cell recognition. Carbohydrates are classified as monosaccharides, disaccharides, and polysaccharides, depending on the number of monomers in the molecule.

Lipids are a class of macromolecules that are nonpolar and hydrophobic in nature. Major types include fats and oils, waxes, phospholipids, and steroids. Fats and oils are a stored form of energy and can include triglycerides. Fats and oils are usually made up of fatty acids and glycerol.

Proteins are a class of macromolecules that can perform a diverse range of functions for the cell. They help in metabolism by providing structural support and by acting as enzymes, carriers or as hormones. The building blocks of proteins are amino acids. Proteins are organized at four levels: primary, secondary, tertiary, and quaternary. Protein shape and function are intricately linked any change in shape caused by changes in temperature, pH, or chemical exposure may lead to protein denaturation and a loss of function.

Nucleic acids are molecules made up of repeating units of nucleotides that direct cellular activities such as cell division and protein synthesis. Each nucleotide is made up of a pentose sugar, a nitrogenous base, and a phosphate group. There are two types of nucleic acids: DNA and RNA.