Notes Carbohydrates

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Notes Carbohydrates

Introduction:Carbohydrates are normally the first classification of compounds to be discussed which fallwithin the realm of Biochemistry. All biological systems are run on chemical reactions: some areeasy 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. Cl in the stomachbrea!s down many large molecules allowing the small intestines digest more manageablepieces.

The three main classes of molecules metaboli"ed by our bodies:#. Carbohydrates $sugars%&. 'ipids $fats%(. )roteins $amino acids%

Carbohydrates are defined as sugars and their derivatives. Animals $such as humans% brea!down carbohydrates during the process of metabolism to release energy. *or example, the

chemical metabolism of the sugar glucose is shown below:

glucose + oxygen carbon dioxide + water + energy

C#&-+ -& C-&+ &- + nergy

Animals obtain carbohydrates by eating foods that contain them, for example potatoes, rice,breads, etc. These carbohydrates are manufactured by plants during the process ofphotosynthesis. )lants harvest energy from sunlight to run the reaction described above inreverse:

C-&+ &- + energy $from sunlight% C#&-+ -&

A potato, for example, is primarily a chemical storage system containing glucose moleculesmanufactured during photosynthesis. /n a potato, however, those glucose molecules are boundtogether in a long chain. As it turns out, there are two types of carbohydrates, the simple sugarsand those carbohydrates that are made of long chains of sugars 0 the complex carbohydrates.The simplest carbohydrates are the monosaccharide, a singleunit simple sugar.The most common monosaccharide isglucose, and this is the most important one for livingorganisms.

Metabolism:)rocesses re1uire energy. The term metabolism is associated with energy. This is 2ust oneaspect of metabolism.

3etabolism more specifically refers to a se1uence of chemical reactions used to produce one ormore products or accomplished one or more processes.

4eturning to energy, per gram fats provide the most energy, carbohydrates provide the nextmost and proteins provide the least energy. The energy of carbohydrates is the most 1uic!lyutili"ed. Thin! about a 5 year old after snea!ing into their alloween candy bag. They are fullof energy6


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Structure of Carbohydrates:'ets brea! down the word carbohydrate. Carbo 7 carbon and hydrate 7 water leading one tobelieve carbohydrates are hydrates of carbon. 4emember a hydrate is a compound which has

water loosely attached. An example would be *eCl( &-. This is iron$///% chloride

hexahydrate. ach *eCl(salt molecule has absorbed water molecules. These are notchemically bound and can be removed by heating leaving *eCl (and &-. 8ince the chemicalformulas are unchanged there has been no chemical reaction, it has undergone a physical

process.

C -

C

C

-

C

-

C

-

-

4ibose

*rom the above carbohydrate ribose, it should be easy to see why the products of heatingcarbohydrates is water and a blac! soot, carbon. 9hen heated the - groups combine withtheir associated and form water, leaving elemental carbon, a blac! soot. But these are clearlychemical bonds and not hydrates of water. By the way, the 4 in 4NA is ribose.

/f you loo! at the structure of the saccharides you will find they are either an aldehyde or a!etone. Carbohydrates are either polyhydroxy aldehydes or polyhydroxy !etones. 4emember,poly means many, hydroxy refers to - groups and that the carbonyl carbon is either the

terminal carbon, therefore an aldehyde, or it is not a terminal carbon, therefore a !etone. Thealdehyde saccharides are called aldose and the !etone saccharides are called !etose.

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

examples:

C -

C

C

-

-

glycer aldehyde

&,(0dihydroxypropanal #,(0dihydroxy0&0propanon

dihydroxy acetone

C

C

C

-

-

-

The structure of saccharides shows them to contain stereocenters. ven the most simplesaccharides, glyceraldehydes have a stereocenters on its central carbon. The following sectionis a review of stereo chemistry.


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Isomers: Stereoisomers

The stereoisomer is a special type geometric isomer. The connectivity is the same between twostereoisomers, the difference is their arrangement in (0dimensional space. They are mirrorimages of each other. They are not super imposable. ;our hands are mirror images of eachother, they are the same, but opposites. Another term used is chiral, this would be used todescribe a carbon that is a stereocenter, the chiral carbon, or a molecule with a stereocenter is achiral molecule.

This phenomenon onlyoccurs around carbon atoms that have 4 differentconnections. Theexample below shows 5 different atoms, but chains of atoms, for instance a methyl group, couldbe substituted for these individual atoms. 8ee the second image down.

xample:

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

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Notes Carbohydrates