INTRODUCTION TO BIOCHEMISTRY


The study of the chemical substances and vital processes occurring in living organisms.
The chemical composition of a particular living system or biological substance.
Biochemistry is the application of chemistry to the study of biological processes at the cellular and molecular level. It emerged as a distinct discipline around the beginning of the 20th century when scientists combined chemistry, physiology and biology to investigate the chemistry of living systems.
Biochemistry has become the foundation for understanding all biological processes. It has provided explanations for the causes of many diseases in humans, animals and plants. It can frequently suggest ways by which such diseases may be treated or cured.
Biochemists study how living organisms extract food and energy from their environment and how they use the extracted molecules to make more of themselves. Buchner, by taking apart yeast cells, had opened the way to ask biochemical questions like: What kinds of molecules cause fermentation? How many different molecules are necessary? Why does the yeast cell do it? Why does it only happen if you keep oxygen out? These are questions that can be answered by separating the "dissolved substances" in the "juice" and asking what they are, how they interact with each other, and how their properties are related to their chemical nature.By using this approach, biochemists have succeeded in...Discovering that although too much cholesterol can cause heart disease, our bodies make cholesterol because it is an essential component of the membranes of our cells.Finding that cells distinctively mark themselves by putting specific groups of sugars, linked together in recognizable patterns, on their surfaces. Your body will reject transplanted tissue if the cells of that tissue have the wrong pattern of sugar groups on their surfaces.Learning that one of the reasons plants require the mineral nutrient magnesium is because it forms part of the structure of chlorophyll, the molecule plants use to trap solar energy.Exploring the way penicillin kills bacteria to discover that it prevents them from putting together the chemical structure of their cell walls.
Biochemistry focuses on the study of life at molecular level - how genes and proteins regulate cells, tissues, organs and ultimately whole organisms like you. As you go about your daily life biochemistry is sure to be involved. It has a role in:
understanding the causes of diseases;
use of engineered therapeutic proteins in medicine;
food production;
understanding how cells function.
This means you can see the effects of biochemistry all around you!
It is central to all areas of the Biological or Life Sciences. The aim is to provide an understanding of every aspect of the structure and function of living things at the molecular level. It is a practical laboratory science that applies the molecular approaches of chemistry to the vast variety of biological systems.Biochemists work at all levels and with all types of biological organisms, ranging from biomolecules to man. There are close links with other specialist life sciences, such as Cell Biology, Genetics, Microbiology, Molecular Biology, Physiology and Pharmacology. In fact, in many cases the distinctions between these disciplines are becoming increasingly blurred. They use biochemical techniques and biochemists work in all these areas. Biochemistry offers the tremendous challenge of seeking to understand the most fundamental of life's processes at the molecular level, and to utilise this knowledge for the benefit of mankind. You will have read, for example, how biochemists, working with colleagues in other disciplines, have developed the new technologies of Molecular Biology and Genetic Engineering. These have enabled the production of therapeutically important human proteins such as insulin and blood clotting factors by cloning procedures, thus avoiding costly, time-consuming and inefficient isolation of these molecules from biological sources; the identification and possible remedying of genetic problems; and the use of DNA fingerprinting in forensic science.
Biochemistry is the study of chemical processes associated with living organisms. Biochemists use concepts of biology, chemistry, physics, mathematics, microbiology, and genetics to unravel the complex puzzles of life. Biochemical techniques are used in clinical diagnosis of infectious diseases, genetic disorders, and cancer; as well as in many forms of research to improve the quality of our lives.

Biochemists identify biological problems then develop and apply appropriate techniques to solve them at the molecular levelBiochemists study the most basic of life processes; for example, identifying the way in which DNA, which carries the genetic information, is transferred between cells and can be manipulated. This has led to the development of new technologies such as Molecular Biology and Genetic Engineering. The resulting recombinant DNA technology has formed the basis of modern biotechnology (e.g. production of human insulin), medical developments (e.g. prenatal diagnosis and genetic counselling) and forensic science (e.g. DNA fingerprinting).DNA directs the production of proteins. These have diverse functions, such as catalysing biological reactions (enzymes), carrying oxygen round the body (haemoglobin), protecting us from infection (antibodies) and holding us together (collagen). Using both simple and high-technology methods, biochemists work out how these proteins function. Biochemists also develop methods for making use of proteins, such as enzymes in biotechnology and antibodies in hormone analysis.With knowledge of the basic molecular mechanisms, biochemists study how life processes are integrated to allow individual cells to function and interact to form complex organisms. They work with all sorts of organisms, from viruses and bacteria to plants and man.These are just a few of the areas. It would take a whole book, in fact many books, to do justice to the multitude of roles of biochemists.
Biochemists work in many walks of life - in industry, hospitals, agriculture, research institutes, education and associated areas. There are many areas of everyday life as diverse as medical products and diagnostics, new food and its safety, crop improvement, cosmetics and forensic science that owe their development or even existence to biochemists
IndustryPharmaceutical, food, brewing, biotechnology and agrochemical companies all need and employ biochemists to develop new products and to monitor the production, quality control and safety of existing ones.
MedicineHospitals, public health laboratories and medical research institutes, as well as the pharmaceutical industry, all require biochemists. Here they provide a diagnostic service, carrying out tests on blood, urine and other body fluids, alongside researching the underlying causes of disease and the methods of treatment.
Agriculture and the EnvironmentBiochemists and biotechnologists, who often have a biochemistry degree, working in agriculture have been responsible for many developments, such as pest-resistant crops, improvements in crop yields and tomatoes that keep better. They also monitor the environment. Employers include seed companies, local government, the Civil Service and water authorities.
EducationAll levels of education offer prospects for biochemists. The combination of biology and chemistry, along with the training in numerical and analytical skills that is given in any area of science, makes biochemistry ideal for teaching throughout the school age range. There are also opportunities for more advanced teaching, usually associated with research, in universities and colleges, and medical, dental and veterinary schools.
Away from ScienceA science background can be an excellent starting point for many other careers. Biochemistry is a numerate subject that develops analytical thinking, creativity in problem solving, and the ability to handle large amounts of complex information - skills required in jobs in all walks of life including, for example, sales and marketing, accountancy and finance, journalism, and patent work. Biochemists have become successful popular authors and even a national president!


Originally, it was generally believed that life was not subject to the laws of science the way non-life was. It was thought that only living beings could produce the molecules of life (from other, previously existing biomolecules). Then, in 1828, Friedrich Wöhler published a paper about the synthesis of urea, proving that organic compounds can be created artificially. The dawn of biochemistry may have been the discovery of the first enzyme, diastase (today called amylase), in 1833 by Anselme Payen. Eduard Buchner contributed the first demonstration of a complex biochemical process outside of a cell in 1896: alcoholic fermentation in cell extracts of yeast. Although the term “biochemistry” seems to have been first used in 1881, it is generally accepted that the formal coinage of biochemistry occurred in 1903 by Carl Neuber, a German chemist. Since then, biochemistry has advanced, especially since the mid-20th century, with the development of new techniques such as chromatography, X-ray diffraction, NMR spectroscopy, radioisotopic labelling, electron microscopy and molecular dynamics simulations. These techniques allowed for the discovery and detailed analysis of many molecules and metabolic pathways of the cell, such as glycolysis and the Krebs cycle (citric acid cycle).
Today, the findings of biochemistry are used in many areas, from genetics to molecular biology and from agriculture to medicine
macromolecule is a large molecule with a large molecular mass, but generally the use of the term is restricted to polymers and molecules which structurally include polymers. [

Illustration of a polypeptide macromolecule
Many examples come from biology and in particular biochemistry. In case of "biomacromolecules" or biopolymers, there are proteins, carbohydrates and nucleic acids (such as DNA). Lipids (fat) are not considered true macromolecules by most biologists as they are not covalently bonded, and so are not true polymers. Synthetic examples include plastics. The integral domains of crystals and metals, while composed of very large numbers of atoms joined by molecule-like bonds, are rarely referred to as "macromolecules."
The term macromolecule is also sometimes used to refer to aggregates of two or more macromolecules held together by intermolecular forces rather than by chemical "bonds". This usage is common in particular when the individual macromolecules involved aggregate or "assemble" spontaneously and rarely exist in isolation. Such an aggregate is more properly called a macromolecular complex. In such a context, individual macromolecules are often referred to as subunits (see e.g. protein subunit).
Substances that are composed of macromolecules often have unusual physical properties. The properties of liquid crystals and such elastomers as rubber are examples. Although too small to see, individual pieces of DNA in solution can be broken in two simply by suctioning the solution through an ordinary straw. This is not true of smaller molecules. The 1964 edition of Linus Pauling's College Chemistry asserted that DNA in nature is never longer than about 5000 base pairs. This is because biochemists were inadvertently and with perfect consistency breaking their samples into pieces. In fact, the DNA of chromosomes can be tens of millions of base pairs long.
Another common macromolecular property that does not characterize smaller molecules is the need for assistance in dissolving into solution. Many require salts or particular ions to dissolve in water. Proteins will denature if the solute concentration of their solution is too high or too low.
According to IUPAC recommendations the term macromolecule is reserved for an individual molecule, and the term polymer is used as to denote a substance composed of macromolecules. Polymer may also be employed unambiguously as an adjective, according to accepted usage, e.g. polymer blend, polymer molecule. [2]
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Major categories of bio-compounds:
Carbohydrates : sugar -- disaccharide -- polysaccharide -- cholesterol -- starch -- glycogen
Lipids : fatty acid -- fats -- essential oils -- oils -- waxes
Nucleic acids : DNA -- RNA -- mRNA -- tRNA -- rRNA -- codon -- adenosine -- cytosine -- guanine -- thymine -- uracil
Proteins :
amino acid -- glycine -- arginine -- lysine
peptide -- primary structure -- secondary structure -- tertiary structure -- conformation -- protein folding
Chemical properties:
molecular bond -- covalent bond -- ionic bond -- hydrogen bond -- ester -- ethyl
molecular charge -- hydrophilic -- hydrophobic -- polar
pH -- acid -- alkaline -- base
oxidation -- reduction -- hydrolysis
Structural compounds:
In cells: flagellin -- peptidoglycan -- myelin -- actin -- myosin
In animals: chitin -- keratin -- collagen -- silk
In plants: cellulose -- lignin -- cell wall
Enzymes and enzyme activity:
enzyme kinetics -- enzyme inhibition
proteolysis -- ubiquitin -- proteasome
kinase -- dehydrogenase
Membranes : fluid mosaic model -- diffusion -- osmosis
phospholipids -- glycolipid -- glycocalyx -- antigen -- isoprene
ion channel -- proton pump -- electron transport -- ion gradient -- antiporter -- symporter -- quinone -- riboflavin
Energy pathways :
pigments : chlorophyll -- carotenoids -- xanthophyll -- cytochrome -- phycobilin -- bacteriorhodopsin -- hemoglobin -- myoglobin -- absorption spectrum -- action spectrum -- fluorescence
Photosynthesis : light reaction -- dark reaction
Fermentation : Acetyl-CoA -- lactic acid
Cellular respiration : Adenosine triphosphate (ATP) -- NADH -- pyruvate -- oxalate -- citrate
Chemosynthesis
Regulation
hormones : auxin
signal transduction -- growth factor -- transcription factor -- protein kinase -- SH3 domain
Malfunctions : tumor -- oncogene -- tumor suppressor gene
Receptors : Integrin -- transmembrane receptor -- ion channel
Techniques : electrophoresis -- chromatography -- mass spectrometry -- x-ray diffraction -- Southern blot -- fractionation -- Gram stain
Objective
To describe functional groups of organic compounds of biological interest and provide some examples of chemical reactions and interconversions among these. Reference: Stryer, 4th edition, 1994, Chapter 1, pp. 3-16
I. Some Examples of Chemical Reactions
I. Functional Groups a. Alcohols R-CH2-OH(primary); R2-CH-OH (secondary); R3-C-OH (tertiary) Sustitution on the Carbon atom defines whether the alcohol is primary, secondary or tertiary. Carbon atom in these cases is in sp3 hybrid state.
b. Aldehydes and Ketones
R-CHO; R2-C=O
Carbon atom is double bonded to oxygen in both cases. The difference is in substitutions. Aldehyde has an R group and a H atom whereas in a ketone both substituents are R groups. Carbon atom in these cases is in sp2 hybrid state.
c. Acids R-C=O Acids are able to dissociate into H+ and anions ! OH d. Acid Anhydrides
(R-CO)2O Acid anhydrides are formed from molecules of same or different acids with the elimination of a molecule of water. An example would the formation of pyrophosphate from two molecules of phosphoric acid.
e. Esters
RCH2-O-COCH3
An ester is formed from an alcohol and an acid with the elimination of a molecule of water.Physiological examples include formation of triacyl glycerols from fatty acids and glycerol.
f. Unsaturated Compounds
Unsaturated compouds like R-CH=CH---- are formed either by the elimination of water fron a hydroxy compund like R-CHOH-CH2---- or by dehydrogenation of compunds like R-CH2-CH2---. The double bond may be cis or trans depending on the positions of hydrogen atoms in space. In cis configuration, H atoms would lie in a plane perpendicular to the plane of the double bond whereas iin trans configuration H atoms lie in the same plane as the double bond.
g. Amines and Amides II. Reaction Types A. Oxidation/Reduction (removal of electron or reaction with O2) B. Esterification (carboxylic acid plus alcohol) C. Hydrolysis (cleavage of a bond by water) D. Phosphorolysis (cleavage of a bond by inorganic phosphate) E. Decarboxylation F. Deamination G. Transamination (amino group transfer) H. Phosphorylation (ester bond on sugars, some amino acids, bases) I. Dehydration J. Phosphorylation (transfer of a phosphate group) K. Transmethylation L. Condensation III. Coupled Reactions Decarboxylation and oxidation/reduction
Study Assignment I Draw the structures of molecules that have the functional groups listed above and identify the functional groups. Some molecules will have several functional groups. Study Assignment II For reaction types listed in IIA-L, above, write out the entire reaction, including molecular structure. Use material presented to you in Biochemistry lectures to complete the assignment.

CARBOHYDRATE METABOLISM

THE CARBOHYDRATE MThe function of carbohydrates includes energy storage and providing structure. Sugars are carbohydrates, although there are carbohydrates that are not sugars. There are more carbohydrates on Earth than any other type of biomolecule. The simplest type of carbohydrate is a monosaccharide, which among other properties contains carbon, hydrogen, and oxygen, mostly in a ratio of 1:2:1 (generalized formula CnH2nOnETA

CARBOHYDRATE CHEMISTRY

The function of carbohydrates includes energy storage and providing structure. Sugars are carbohydrates, although there are carbohydrates that are not sugars. There are more carbohydrates on Earth than any other type of biomolecule. The simplest type of carbohydrate is a monosaccharide, which among other properties contains carbon, hydrogen, and oxygen, mostly in a ratio of 1:2:1 (generalized formula CnH2nOn, where n is at least 3). Glucose, one of the most important carbohydrates, is an example of a monosaccharide. So is fructose, the sugar that gives fruits their sweet taste. Some carbohydrates (especially after condensation to oligo- and polysaccharides) contain less carbon relative to H and O, which still are present in 2:1 (H:O) ratio. Monosaccharides can be grouped into aldoses (having an aldehyde group at the end of the chain, e. g. glucose) and ketoses (having a keto group in their chain; e. g. fructose). Both aldoses and ketoses occur in an equilibrium between the open-chain forms and (starting with chain lengths of C4) cyclic forms. These are generated by bond formation between one of the hydroxy groups of the sugar chain with the carbon of the aldehyde or keto group in a semiacetal bond. This leads to saturated five-membered (in furanoses) or six-membered (in pyranoses) heterocyclic rings containing one O as heteroatom.
Two monosaccharides can be joined together using dehydration synthesis, in which a hydrogen atom is removed from the end of one molecule and a hydroxyl group (—OH) is removed from the other; the remaining residues are then attached at the sites from which the atoms were removed. The H—OH or H2O is then released as a molecule of water, hence the term dehydration. The new molecule, consisting of two monosaccharides, is called a disaccharide and is conjoined together by a glycosidic or ether bond. The reverse reaction can also occur, using a molecule of water to split up a disaccharide and break the glycosidic bond; this is termed hydrolysis. The most well-known disaccharide is sucrose, ordinary sugar (in scientific contexts, called table sugar or cane sugar to differentiate it from other sugars). Sucrose consists of a glucose molecule and a fructose molecule joined together. Another important disaccharide is lactose, consisting of a glucose molecule and a galactose molecule. As most humans age, the production of lactase, the enzyme that hydrolyzes lactose back into glucose and galactose, typically decreases. This results in lactase deficiency, also called lactose intolerance.
Sugar polymers are characterised by having reducing or non-reducing ends. A reducing end of a carbohydrate is a carbon atom which can be in equilibrium with the open-chain aldehyde or keto form. If the joining of monomers takes place at such a carbon atom, the free hydroxy group of the pyranose or furanose form is exchanged with an OH-side chain of another sugar, yielding a full acetal. This prevents opening of the chain to the aldehyde or keto form and renders the modified residue non-reducing. Lactose contains a reducing end at its glucose moiety, whereas the galactose moiety form a full acetal with the C4-OH group of glucose. Saccharose does not have a reducing end because of full acetal formation between the aldehyde carbon of glucose (C1) and the keto carbon of fructose (C2).
When a few (around three to six) monosaccharides are joined together, it is called an oligosaccharide (oligo- meaning "few"). These molecules tend to be used as markers and signals, as well as having some other uses.
Many monosaccharides joined together make a polysaccharide. They can be joined together in one long linear chain, or they may be branched. Two of the most common polysaccharides are cellulose and glycogen, both consisting of repeating glucose monomers. Cellulose is made by plants and is an important structural component of their cell walls. Humans can neither manufacture nor digest it. Glycogen, on the other hand, is an animal carbohydrate; humans use it as a form of energy storage.
Glucose is the major energy source in most life forms; a number of catabolic pathways converge on glucose. For instance, polysaccharides are broken down into their monomers (glycogen phosphorylase removes glucose residues from glycogen). Disaccharides like lactose or sucrose are cleaved into their two component monosaccharides. Glucose is mainly metabolized by a very important and ancient ten-step pathway called glycolysis, the net result of which is to break down one molecule of glucose into two molecules of pyruvate; this also produces a net two molecules of ATP, the energy currency of cells, along with two reducing equivalents in the form of converting NAD+ to NADH. This does not require oxygen; if no oxygen is available (or the cell cannot use oxygen), the NAD is restored by converting the pyruvate to lactate (e. g. in humans) or to ethanol plus carbon dioxide (e. g. in yeast). Other monosaccharides like galactose and fructose can be converted into intermediates of the glycolytic pathway. In aerobic cells with sufficient oxygen, like most human cells, the pyruvate is further metabolized. It is irreversibly converted to acetyl-CoA, giving off one carbon atom as the waste product carbon dioxide, generating another reducing equivalent as NADH. The two molecules acetyl-CoA (from one molecule of glucose) then enter the citric acid cycle, producing two more molecules of ATP, six more NADH molecules and two reduced (ubi)quinones (via FADH2 as enzyme-bound cofactor), and releasing the remaining carbon atoms as carbon dioxide. The produced NADH and quinol molecules then feed into the enzyme complexes of the respiratory chain, an electron transport system transferring the electrons ultimately to oxygen and conserving the released energy in the form of a proton gradient over a membrane (inner mitochondrial membrane in eukaryotes). Thereby, oxygen is reduced to water and the original electron acceptors NAD+ and quinone are regenerated. This is why humans breathe in oxygen and breathe out carbon dioxide. The energy released from transferring the electrons from high-energy states in NADH and quinol is conserved first as proton gradient and converted to ATP via ATP synthase. This generates an additional 28 molecules of ATP (24 from the 8 NADH + 4 from the 2 quinols), totaling to 32 molecules of ATP conserved per degraded glucose (two from glycolysis + two from the citrate cycle). It is clear that using oxygen to completely oxidize glucose provides an organism with far more energy than any oxygen-independent metabolic feature, and this is thought to be the reason why complex life appeared only after Earth's atmosphere accumulated large amounts of oxygen.
In vertebrates, vigorously contracting skeletal muscles (during weightlifting or sprinting, for example) do not receive enough oxygen to meet the energy demand, and so they shift to anaerobic metabolism, converting glucose to lactate (lactic acid). The liver regenerates the glucose, using a process called gluconeogenesis. This process is not quite the opposite of glycolysis, and actually requires three times the amount of energy gained from glycolysis (six molecules of ATP are used, compared to the two gained in glycolysis). Analogous to the above reactions, the glucose produced can then undergo glycolysis in tissues that need energy, be stored as glycogen (or starch in plants), or be converted to other monosaccharides or joined into di- or oligosaccharides.
Carbohydrates are molecules that contain oxygen, hydrogen, and carbon atoms. They may also contain other elements such as sulfur or nitrogen, but these are usually minor components. They consist of monosaccharide sugars, of varying chain lengths, that have the general chemical formula Cn(H2O)n or are derivatives of such.The smallest value for n is 3. A 3-carbon sugar is referred to as a triose, whereas a 6-carbon sugar is called a hexose (see monosaccharides below). Certain carbohydrates are important for storing and transporting energy in most organisms, including plants and animals, and are major structural elements in many organisms (eg cellulose in plants). In addition they play major roles in cell to cell communication, the immune system, fertilization, pathogenesis, blood clotting, and development. Carbohydrates can be classified by the number of constituent sugar units: monosaccharides (such as glucose and fructose), disaccharides (such as sucrose and lactose), oligosaccharides, and polysaccharides (such as starch, glycogen, and cellulose).


Glucose as a straight-chain carbohydrate (Fischer projection)


Fructose (Fischer projection)
Pure carbohydrates contain carbon, hydrogen, and oxygen atoms, in a 1:2:1 molar ratio, giving the general formula Cn(H2O)n. (This applies only to monosaccharides, see below, although all carbohydrates have the more general formula Cn(H2O)m.) However, many important carbohydrates deviate from this, such as deoxyribose and glycerol. Sometimes compounds containing other elements are also counted as carbohydrates (e.g. glucosamine and chitin, which contain nitrogen).
The simplest carbohydrates are monosaccharides, which are small straight-chain aldehydes and ketones with many hydroxyl groups added, usually one on each carbon except the functional group. Other carbohydrates are composed of monosaccharide units and break down under hydrolysis. These may be classified as disaccharides, oligosaccharides, or polysaccharides, depending on whether they have two, several, or many monosaccharide units.
Monosaccharides
Monosaccharides may be divided into aldoses, which have an aldehyde group on the first carbon atom, and ketoses, which typically have a ketone group on the second. They may also be divided into trioses, tetroses, pentoses, hexoses,and many more. This all depends on how many carbon atoms they contain. For instance, glucose is an aldohexose, fructose a ketohexose, and ribose an aldopentose.
Further, each carbon atom that supports a hydroxyl group (except for the first and last) is a stereogenic centre, allowing a number of different enantiomers and stereoisomers for carbohydrates with the same basic structure. For instance, galactose is an aldohexose but has different properties from glucose because the atoms are arranged differently.
A heterocyclic form of ribose (Haworth projection)
The straight-chain structure described here is only one of the forms a monosaccharide may take. The aldehyde or ketone group may react with a hydroxyl group on a different carbon atom to form a hemiacetal or hemiketal, in which case there is an oxygen bridge between the two carbon atoms, forming a heterocyclic ring. Rings with five and six atoms are called furanose and pyranose forms and exist in equilibrium with the straight-chain form.
It should be noted that the ring form has one more stereogenic centre than the straight-chain form, and so has both an alpha and a beta form, which interconvert in equilibrium. However, the carbohydrate may further react with an alcohol to form an acetal or ketal, in which case the two forms become distinct. This is the basic type of link between the monosaccharide units of larger carbohydrates. Excessive consumption may cause obesity.
An aldose is a monosaccharide (a certain type of sugar) containing one aldehyde group per molecule and having a chemical formula of the form CnH2nOn (n>=3).
Triose: glyceraldehyde
Tetroses: erythrose, threose
Pentoses: ribose, arabinose, xylose, lyxose
Hexoses: allose, altrose, glucose, mannose, gulose, idose, galactose, talose

With only 3 carbon atoms, glyceraldehyde is the simplest of all aldoses.
Aldoses isomerize to ketoses in the Lobry-de Bruyn-van Ekenstein transformation
An aldehyde is an organic compound containing a terminal carbonyl group. This functional group, which consists of a carbon atom which is bonded to a hydrogen atom and double-bonded to an oxygen atom (chemical formula -CHO), is called the aldehyde group. The aldehyde group is also called the formyl or methanoyl group.
The word aldehyde seems to have arisen from alcohol dehydrogenated. In the past, aldehydes were sometimes named after the corresponding alcohols, for example vinous aldehyde for acetaldehyde. (Vinous is from Latin vinum = wine, the traditional source of ethanol; compare vinyl.)
The aldehyde group is polar. Oxygen, being more electronegative, pulls the electrons in the carbon-oxygen bond towards itself, thus creating an electron deficiency at the carbon atom.


Disaccharides

Disaccharides are composed of two monosaccharide units bound together by a covalent glycosidic bond. The binding between the two sugars results in the loss of a hydrogen atom (H) from one molecule and a hydroxyl group (OH) from the other.
The most common disaccharides are sucrose (cane or beet sugar - made from one glucose and one fructose), lactose (milk sugar - made from one glucose and one galactose), maltose (made of two glucoses linked alpha-1,4) and cellobiose (made of two glucoses linked beta-1,4). The formula of these disaccharides is C12H22O11.
Other examples of disaccharides include trehalose, chitobiose, laminaribiose, kojibiose and xylobiose.











Oligosaccharides and polysaccharides
Main articles: Oligosaccharide and Polysaccharide
Oligosaccharides and polysaccharides are composed of longer chains of monosaccharide units bound together by glycosidic bonds. The distinction between the two is based upon the number of monosaccharide units present in the chain. Oligosaccharides typically contain between two and nine monosaccharide units, and polysaccharides contain greater than ten monosaccharide units. Definitions of how large a carbohydrate must be to fall into each category vary according to personal opinion. Examples of oligosaccharides include the disaccharides mentioned above, the trisaccharide raffinose and the tetrasaccharide stachyose.
Oligosaccharides are found as a common form of protein posttranslational modification. Such posttranslational modifications include the Lewis oligosaccharides responsible for blood group incompatibilities, the alpha-Gal epitope responsible for hyperacute rejection in xenotransplanation, and O-GlcNAc modifications.
Polysaccharides represent an important class of biological polymer. Examples include starch, cellulose, chitin, glycogen, callose, laminarin, xylan, and galactomannan.