Notes Carbohydrates

Introduction:Carbohydrates are normally the first classification of compounds to be discussed which fall within the realm of Biochemistry. All biological systems are run on chemical reactions: some are easy to see, such as the iron ion needed to hold oxygen in red blood cells. The positively charged iron attracts the lone pairs of electrons on the oxygen molecule. HCl in the stomach breaks down many large molecules allowing the small intestines digest more manageable pieces.

The three main classes of molecules metabolized by our bodies:1. Carbohydrates (sugars)2. Lipids (fats)3. Proteins (amino acids)

Carbohydrates are defined as sugars and their derivatives. Animals (such as humans) break down carbohydrates during the process of metabolism to release energy. For example, the chemical metabolism of the sugar glucose is shown below:

glucose + oxygen carbon dioxide + water + energyC6H12O6 + 6 O2 6 CO2 + 6 H2O + Energy

Animals obtain carbohydrates by eating foods that contain them, for example potatoes, rice, breads, etc. These carbohydrates are manufactured by plants during the process of photosynthesis. Plants harvest energy from sunlight to run the reaction described above in reverse:

6 CO2 + 6 H2O + energy (from sunlight) C6H12O6 + 6 O2

A potato, for example, is primarily a chemical storage system containing glucose molecules manufactured during photosynthesis. In a potato, however, those glucose molecules are bound together in a long chain. As it turns out, there are two types of carbohydrates, the simple sugars and those carbohydrates that are made of long chains of sugars - the complex carbohydrates.The simplest carbohydrates are the monosaccharide, a single unit simple sugar. The most common monosaccharide is glucose, and this is the most important one for living organisms.

Metabolism:Processes require energy. The term metabolism is associated with energy. This is just one aspect of metabolism.

Metabolism more specifically refers to a sequence of chemical reactions used to produce one or more products or accomplished one or more processes.

Returning to energy, per gram fats provide the most energy, carbohydrates provide the next most and proteins provide the least energy. The energy of carbohydrates is the most quickly utilized. Think about a 4 year old after sneaking into their Halloween candy bag. They are full of energy!

Structure of Carbohydrates:Lets break down the word carbohydrate. Carbo = carbon and hydrate = water leading one to believe carbohydrates are hydrates of carbon. Remember a hydrate is a compound which has water loosely attached. An example would be FeCl3 6 H2O. This is iron(III) chloride hexahydrate. Each FeCl3 salt molecule has absorbed 6 water molecules. These are not chemically bound and can be removed by heating leaving FeCl3 and H2O. Since the chemical formulas are unchanged there has been no chemical reaction, it has undergone a physical process.

From the above carbohydrate ribose, it should be easy to see why the products of heating carbohydrates is water and a black soot, carbon. When heated the OH groups combine with their associated H and form water, leaving elemental carbon, a black soot. But these are clearly chemical bonds and not hydrates of water. By the way, the R in RNA is ribose.

If you look at the structure of the saccharides you will find they are either an aldehyde or a ketone. Carbohydrates are either polyhydroxy aldehydes or polyhydroxy ketones. Remember, poly means many, hydroxy refers to –OH groups and that the carbonyl carbon is either the terminal carbon, therefore an aldehyde, or it is not a terminal carbon, therefore a ketone. The aldehyde saccharides are called aldose and the ketone saccharides are called ketose.

The ribose above is an aldehyde. The carbonyl is the terminal carbon.

examples:

The structure of saccharides shows them to contain stereocenters. Even the most simple saccharides, glyceraldehydes have a stereocenters on its central carbon. The following section is a review of stereo chemistry.

Isomers: StereoisomersThe stereoisomer is a special type geometric isomer. The connectivity is the same between two stereoisomers, the difference is their arrangement in 3-dimensional space. They are mirror images of each other. They are not super imposable. Your hands are mirror images of each other, they are the same, but opposites. Another term used is chiral, this would be used to describe a carbon that is a stereocenter, the chiral carbon, or a molecule with a stereocenter is a chiral molecule.

This phenomenon only occurs around carbon atoms that have 4 different connections. The example below shows 4 different atoms, but chains of atoms, for instance a methyl group, could be substituted for these individual atoms. See the second image down.

Example:

One of the following three carbons is chiral, which is it and why are the other two not chiral?

Example A is the chiral carbon. Both B and C do not have 4 different connections. Example B has 2 hydrogen atoms attached to the carbon and example C has 2 methyl groups attached to the carbon.

When this occurs the two mirror images are called enantiomers. The two molecules will have the same name except for a prefix(s). These prefixes originate in one of the properties of compounds with stereocenters, rotation of plane polarized light or in the convention of drawing the structures.

This may not seem like an important point, but as it turns out, even though these two molecules’ structures are almost exactly the same, they do react differently. Sometimes with terrible consequences.

To determine the number of isomers a compound will form:1. Count the number of stereocenters in the molecule2. Take 2 to that power

The ribose molecule to the right has 3 stereocenters. 23 = 8 Ribose will have 8 different isomers.

Carbohydrate Polymer:Carbohydrates are also referred to as saccharides. Saccharides can be found in several forms.

single monosaccharidepair disaccharide

many polysaccharide

monosaccharide basic unit of metabolism normally 3, 4, 5, 6 or 7 carbons in length classified as aldose or ketose classified as D or L isomers based on the stereochemistry

disaccharide use to transport monosaccharides water soluble – as they are short hydrocarbon chains are sweet to taste sucrose, galactose and lactose

polysaccharides structure of plants – cellulose storage of monosaccharides

The D and L prefixes are generated by the convention of drawing the structures. If the -OH group on the carbon before the terminal carbon is on the left it is designated with an L; if the -OH group on the carbon before the terminal carbon is on the right it is designated with an D. You may ask why the -OH group on the carbon before the terminal carbon, why not use the terminal carbon? Well the terminal carbon will not be a stereo center, there will normally be 2 hydrogens on that terminal carbon.

So, if you draw these monosaccharides vertical with the carbonyl carbon on the top or as close to the top as possible it will make identifying them easier.

These structures are normally drawn in a more simplified manner, because as we know chemists are lazy. Below you will find a drawing of a Fischer projection along side its equivalent structures. The Fischer projection is named after Emil Hermann Fischer, winner of the 1902 Nobel Prize in Chemistry.

This structure is an L enantiomer. The –OH on the carbon before the terminal carbon is on the left.

What is the name of this structure? We already have the first piece, L. The carbon chain is 5 carbons long, therefore it is a pent. Lastly, the carbonyl carbon is an aldehyde.

L-aldopentose

Draw the Fischer Projections for lactic acid given the following structure:

Example:

Importance of Carbohydrates: Very effective energy yield

o contains carbono has a reactive bond – carbonyl carbon and is a polar areao does not have 4 bonds to oxygen – which means the carbon is organic carbon,

remember that organic carbon is carbon with an low oxidation number, once the oxidation number becomes + 4 it can no longer be oxidized

Effective building materialo strong not brittle – will bend and not break

H2O solubleo easily transported thru the blood streamo easily passes thru cell walls

Sugars are carbohydrates. Sucrose was used as the standard, all other sugars sweetness is based on sucrose.

carbohydrate

relativesweetness class common

namesucrose 1.00 disaccharide table sugarlactose 0.16 disaccharide milk sugarmaltose 0.32 disaccharide malt sugar

glucose 0.74 monosaccharide blood sugar

galactose 0.22 monosaccharide -

fructose 1.74 monosaccharide fruit sugar

Saccharide Monomers: (important)Glucose classified as an aldohexose – as it is an aldehyde and a 6-carbon compound Most carbohydrates are converted to glucose to be metabolized for energy. dextrose and blood sugar are both common names for glucose one of the monomers found in the disaccharide found in sucrose, maltose and lactose a monomer of starch, cellulose and glycogen 25% less sweet than table sugar, sucrose no digestion needed can be given intravenously found in the urine of diabetics 70-150mg per dl of blood

Galactose classified as an aldohexose – as it is an aldehyde and a 6-carbon compound found in pectin and gum combined with glucose to form the disaccharide lactose 80% less sweet than table sugar, sucrose Galactosemia

o genetic disease – inability of body to metabolize galactoseo elevated levels of galactose in blood and urineo vomiting, diarrhea, liver enlargemento can cause death in dayso lactose must be removed from their dieto http://www.galactosemia.org/galactosemia.htm

isomer of glucose the #5 carbon has the hydroxyl and hydrogen switched

Fructose classified as a ketohexose as this molecule is a ketone and is a 6-carbon chain found in fruit juice and honey combined with glucose to form the disaccharide sucrose 175% sweeter than table sugar, sucrose this country’s most common sweetener

o high fructose corn syrupo can be metabolized to glucose in the liver

Cyclic Saccharides:The straight form of saccharides is very reactive. For the saccharide to be stable enough to transport, it forms a cyclic structure. Below are drawings for the formation of straight-chained glucose to become cyclic. The reaction breaks the double bond of the carbonyl group and shifts hydrogen of the hydroxyl group on the number 5 carbon to the carbonyl group’s oxygen.

Glucose is classified as an aldose, this ring structure will also form with the ketose saccharides like fructose.

The stereochemistry of these molecules can become overwhelming. As such other prefixes must be introduced to describe the stereochemistry, alpha, and beta, . Below is the cyclic structure of glucose. The carbon to the far right on each ring shows the hydroxyl group in different location. The alpha structure has the hydroxyl group down and the beta group has the hydroxyl group up.

Rings of different numbers of sides are given different names. A five-sided ring is called a furanose and a six-sided ring is called a puranose.

-D-fructose

-D-glucose -D-glucose

These are the straight and cyclic structures for fructose, a five carbon ketose.

Below are the two furanose rings of fructose. The alpha is on the left, hydroxyl on the down. The beta on the right, hydroxyl on the top.

These are the straight and cyclic structures for ribose, a six carbon aldose.

Below are the two furanose rings of ribose. The alpha is on the left, hydroxyl on the down. The beta on the right, hydroxyl on the top.

-D-fructose -D-fructose

D-fructoseD-fructose

-D-ribose -D-ribose

As stated in the beginning of these notes, the R in RNA is from ribonucleic acid. The D in DNA is a molecule whose structure is very close to that of ribose; the molecule is deoxyribonucleic acid. Lets break down that word. The prefix “de“ means loss, “oxy” means oxygen and “ribo” refers to ribose. So, what you have is a ribose that has lost an oxygen.

This may not seem like much of a change, but this demonstrates the specificity of chemistry. One oxygen can change the function of a molecule from making proteins, RNA, and storing the organism genetic information, DNA. Both of the above molecules are furanose, 5-member rings and are in the beta form, the hydroxyl is up.

Disaccharides: The three most important disaccharides are sucrose, lactose and maltose. The monomers are very specific. Meaning you must have stereochemistry exact, the bond

will require an alpha or beta and always the D form. Disaccharides are formed thru a dehydration reaction. This reaction releases a water molecule. To break this bond, named a glycosidic bond, you add water, this reaction is named

hydrolysis. Where in your body will this digestion occur?

Sucrose: most common disaccharide, table sugar 20% of sugar cane is sucrose based on the total consumption in this country, it is estimated that a person will consume 100

pounds of sucrose each year it is composed of one -D-glucose and one -D-fructose monomer not a reducing sugar therefore no reaction occurs with the Benedict’s reagent this reaction will not occur because there is no way to open ring and form an aldehyde in an acidic solution sucrose undergoes hydrolysis, the resulting solution containing glucose

and fructose is sweeter than the original sucrose this can be observed in jams and jellies, the acid in the fruit’s juice, normally citric acid,

causes the sucrose to undergo hydrolysis linked -1 -2 this linkage information tells you that the -D-glucose molecule uses its #1 carbon, whose

OH group is down, to bond to the #2 carbon on the -D-fructose molecule, whose OH group is up

the reaction and structures are drawn below

-D-glucose

-D-fructose

Lactose: milk sugar by mass composes 7% of human milk, 5% of bovine milk being lactose intolerant is a fairly common condition, especially in adults who drink less milk,

the body forgets how to make the enzyme, lactase, needed to break lactose’s glycosidic bond

it is composed of one -D-glucose or -D-glucose and one -D-galactose monomer Lactose is odd in the fact that the second monomer can have either or stereochemistry is a reducing sugar therefore when tested with Benedict’s reagent the solution turns from a

clear light blue color to a cloudy rust/brown color this reaction will occur because when the ring opens an aldehyde can form in an acidic solution lactose undergoes hydrolysis, the resulting solution containing glucose

and galactose linked -1 4 this linkage information tells you that the -D-glucose molecule uses its #4 carbon to bond to

the #1 carbon on the -D-galactose molecule, both of which have a OH group in the up position

the dehydration of one -D-glucose and one -D-galactose monomer to form water and lactose

-D-glucose

-D-galactose

lactose

Maltose: malt sugar it is composed of -D-glucose monomers is a reducing sugar therefore when tested with Benedict’s reagent the solution turns from a

clear light blue color to a cloudy rust/brown color this reaction will occur because when the ring opens an aldehyde can form in an acidic solution maltose undergoes hydrolysis, the resulting solution containing glucose linked -1 4 this linkage information tells you that one -D-glucose molecule uses its #1 carbon to bond

to the #4 carbon on the other -D-glucose molecule, both of which have a OH group in the down position

-D-glucose-D-glucose

Polysaccharides:Starch:There are 2 classes of starch, animal and plant. Glycogen in animal starch; amylose and amylopectin are plant starch.

Plant starch is the major storage carbohydrate (polysaccharide) in higher plants, being the end product of photosynthesis. Starch is composed of a mixture of two polymers, an essentially linear polysaccharide -amylose, and a highly branched polysaccharide - amylopectin. Starch is unique among carbohydrates because it occurs naturally as discrete granules (or grains). Starch granules are relatively dense, insoluble and hydrate only slightly in cold water.

Amylose - The constituent of starch in which anhydroglucose units are linked by -D-1 4 glucosidic bonds to form linear chains. The level of amylose and its molecular weight vary between different starch types. Amylose molecules are typically made from 200-2000 anhydroglucose units. Aqueous solutions of amylose are very unstable due to intermolecular attraction and association of neighboring amylose molecules. This leads to viscosity increase, retrogradation and, under specific conditions, precipitation of amylose particles. Amylose forms a helical complex with iodine giving a characteristic blue color. This is what you think of when you think of starch, the meat of a potatoes.

Animal Starch, glycogen, is a polymer of glucose. This is stored in the liver and for quick energy. Glycogen can be quickly hydrolyzed to form glucose and deposited into the blood stream for transport to cells. Glycogen is highly branched. The chain glycosidic linkage is -1 4, the branching occurs with a glycosidic linkage of -1 6.

Cellulose:Cellulose is yet a third polymer of the monosaccharide glucose.  The below diagram is of a portion of a cellulose chain. The glucose monomers are connected by a -1 4 linkage, this prevents most organisms, including humans from breaking the glucose monomers apart. Cellulose gives plants their structure, in their cell walls, for example wood fiber. It is also insoluble.Cellulose differs from starch and glycogen because the glucose units form a two-dimensional structure, with hydrogen bonds holding together nearby polymers, thus giving the molecule added stability (shown in the smaller diagram in the upper right hand corner of the diagram below.Cellulose, also known as plant fiber, cannot be digested by human beings and so cellulose passes through the digestive tract without being absorbed into the body. Some animals, such as cows and termites, contain bacteria in their digestive tract that help them to digest cellulose.  Cellulose is a relatively stiff material, and in plants cellulose is used as a structural molecule to add support to the leaves, stem and other plant parts. Despite the fact that it cannot be used as an energy source in most animals, cellulose fiber is essential in the diet because it helps exercise the digestive track and keep it clean and healthy.

Starch vs. Cellulose:Starch is a form of so-called polysaccharides - macromolecules composed of thousands of small sugar molecules. Starch consists of glucose molecules - and so does cellulose. Starch is used by plants to store energy and is digested when the need for energy arises, e.g., when a seed germinates. Starch may also be digested by humans and most animals. In contrast, cellulose is a structural polysaccharide. It forms cellular walls in plants and is thus present in all plant tissues. In may be found concentrated in wood and straw. Cellulose cannot be digested by humans and most animals but has an important function as dietary fiber in our diet. In ruminants, cellulose is broken down in the rumen by resident microorganisms into smaller fragments which are subsequently utilized by the microorganisms and the ruminant. Decayed microorganisms are also digested and utilized by the ruminant.

There are differences in the molecular structure between the two polysaccharides: Starch consists of alpha-D-glucose and cellulose consists of beta-D-glucose. In the diagram, a starch segment of 3 glucose units (S) is shown at the top and a 3-glucose unit segment of cellulose (C) is shown below it. The first oxygen atom in each segment is below the plane of the sugar ring and the hydrogen atom on the first carbon atom is above that plane. Thus, all — CH2OH groups are above the plane in the starch molecule but their positions alternate in cellulose.

Reactions of Carbohydrates:

The enzymes that break down polysaccharides are specific to the type of linkage in the polysaccharide. The enzyme, cellulase, that hydrolyze the beta linkages in cellulose are different from the enzyme, amylase, that hydrolyze alpha linkages. The beta linkages are not broken down by the enzymes that people have and consequently, cellulose does not provide glucose in our diets. This is the case because most organisms simply do not have the cellulase enzyme in their bodies, but they do have the amylase enzyme.

Benedict’s test, used with disaccharides to distinguish between sucrose, maltose and lactose. Lactose and maltose react to change the Benedict Reagent from a clear blue solution to a cloudy rust/brown solution, but sucrose does not react.

Iodine test, used to check for the presents of starch, specifically amylose. An amylose compound will turn the iodine solution black.

Fermentation will occur when an enzyme found in yeast reacts with sucrose or maltose to create ethanol and carbon dioxide gas. No fermentation will occur with lactose. Yeast does not have that enzyme.

Artificial Sweeteners:These sweeteners are normally used to reduce a person’s the caloric intake. They work two ways. First, your body will not be able to metabolize the molecule, even though it can taste it. So it passes right thru you. Second, the artificial sweeteners are much sweeter, so you need much less than you would for conventional sugars.

For example Sucralose, the newest artificial sweetener is approximately 6000 times sweeter than sucrose. Meaning you would need 6000 times less Sucralose than sucrose.

Sucralose taste a lot like sucrose, because it is made from sucrose. Below is the structure of each.

 Carbohydrates - IntroductionIntroduction:General names for carbohydrates include sugars, starches, saccharides, and polysaccharides. The term saccharides is derived from the Latin word " sacchararum" from the sweet taste of sugars.The name "carbohydrate" means a "hydrate of carbon." The name derives from the general formula of carbohydrate is Cx(H2O)y - x and y may or may not be equal and range in value from 3 to 12 or more. For example glucose is: C6(H2O)6 or is more commonly written, C6H12O6.The chemistry of carbohydrates most closely resembles that of alcohol, aldehyde, and ketone functional groups. As a result, the modern definition of a CARBOHYDRATE is that the compounds are polyhydroxy aldehydes or ketones. The chemistry of carbohydrates is complicated by the fact that there is a functional group (alcohol) on almost every carbon. In addition, the carbohydrate may exist in either a straight chain or a ring structure. Ring structures incorporate two additional functional groups: the hemiacetal and acetal.A major part of the carbon cycle occurs as carbon dioxide is converted to carbohydrates through photosynthesis. Carbohydrates are utilized by animals and humans in metabolism to produce energy and other compounds.Carbohydrate Functions:Carbohydrates are initially synthesized in plants from a complex series of reactions involving photosynthesis.-Store energy in the form of starch (photosynthesis in plants) or glycogen (in animals and humans).-Provide energy through metabolism pathways and cycles.-Supply carbon for synthesis of other compounds.-Form structural components in cells and tissues.

Photosynthesis is a complex series of reactions carried out by algae, phytoplankton, and the leaves in plants, which utilize the energy from the sun. The simplified version of this chemical reaction is to utilize carbon dioxide molecules from the air and water molecules and the energy from the sun to produce a simple sugar such as glucose and oxygen molecules as a by product. The simple sugars are then converted into other molecules such as starch, fats, proteins, enzymes, and DNA/RNA i.e. all of the other molecules in living plants. All of the "matter/stuff" of a plant ultimately is produced as a result of this photosynthesis reaction.

Metabolism: Metabolism occurs in animals and humans after the ingestion of organic plant or animal foods. In the cells a series of complex reactions occurs with oxygen to convert for example glucose sugar into the products of carbon dioxide and water and ENERGY. This reaction is also carried out by bacteria in the decomposition/decay of waste materials on land and in the water. Combustion occurs when any organic material is reacted (burned) in the presence of oxygen to give off the products of carbon dioxide and water and ENERGY. The organic

material can be any fossil fuel such as natural gas (methane), oil, or coal. Other organic materials that combust are wood, paper, plastics, and cloth. The whole purpose of both processes is to convert chemical energy into other forms of energy such as heat.

Carbohydrates – Classification

Classifications:

There are a variety of interrelated classification schemes. The most useful classification scheme divides the carbohydrates into groups according to the number of individual simple sugar units. Monosaccharides contain a single unit; disaccharides contain two sugar units; and polysaccharides contain many sugar units as in polymers - most contain glucose as the monosaccharide unit.Carbohydrates Monosaccharides  Disaccharides  Polysaccharides Glucose  Sucrose  Starch Galactose  Maltose  Glycogen Fructose  Lactose  Cellulose Ribose     Glyceraldehyde    

Number of Carbons:

Monosaccharides can be further classified by the number of carbons present.Hexoses (6-carbons) are by far the most prevalent.Number of Carbons Six = Hexose  Five = Pentose  Three = Triose Glucose Ribose  Glyceraldehyde Galactose FructoseFunctional Groups:

Aldoses contain the aldehyde group - Monosaccharides in this group are glucose,

galactose, ribose, and glyceraldehyde.Ketoses contain the ketone group - The major sugar in this group is fructose.Reducing: Contain a hemiacetal or hemiketal group. Sugars include, glucose, galactose, fructose, maltose, lactoseNon-reducing: Contain no hemiacetal groups. Sucrose and all polysaccharides are in this group.

Common Carbohydrates Name  Derivation of name and SourceMonosaccharides

 Glucose From Greek word for sweet wine; grape sugar, blood sugar, dextrose.

 Galactose Greek word for milk--"galact", found as a component of lactose in milk.

 Fructose Latin word for fruit--"fructus", also known as levulose,found in fruits and honey; sweetest sugar.

 Ribose Ribose and Deoxyribose are found in the backbone structure of RNA and DNA, respectively.

Disaccharides - contain two monosaccharides

  Sucrose French word for sugar--"sucre", a disaccharide containing glucose and fructose; table sugar, cane sugar, beet sugar.

 Lactose Latin word for milk--"lact"; a disaccharide found in milk containing glucose and galactose.

 Maltose French word for "malt"; a disaccharide containing two units of glucose; found in germinating grains, used to make beer.

Common Polysaccharides Name  Source

 StarchPlants store glucose as the polysaccharide starch. The cereal grains (wheat, rice, corn, oats, barley) as well as tubers such as potatoes are rich in starch.

CelluloseThe major component in the rigid cell walls in plants is cellulose and is a linear polysaccharide polymer with many glucose monosaccharide units.

 GlycogenThis is the storage form of glucose in animals and humans which is analogous to the starch in plants. Glycogen is synthesized and stored mainly in the liver and the muscles.