CHAPTER 15Carbohydrates

Where in the world do we find carbohydrates?• Most abundant organic compound in nature• Photosynthesis: plants make glucose using the sun’s

energy• They are tasty and yummy (bread, pasta, sugar), broken

down in our cells to provide our bodies with energy• Paper, wood

A. Carbohydrates

• All are made of carbon, hydrogen, and oxygen• Formula: Cn(H2O)n

• The carbohydrate glucose is made by plants during photosynthesis, and it is oxidized by us through respiration:

photosynthesis

6 CO2 + 6 H2O + energy C6H12O6 + 6 O2

respiration glucose

Types of Carbohydrates

• Simplest: monosaccharide, which cannot be split or hydrolyzed into a smaller carbohydrate (example: glucose C6H12O6)

• Disaccharide: two monosaccharide units joined together (example: sucrose C12H22O11)

• A disaccharide can be hydrolyzed in the presence of an acid or enzyme to give the two monosaccharide units

acid or enzyme

sucrose + H2O glucose + fructose

Types of Carbohydrates cont.

• Polysaccharides: naturally occurring polymers containing many monosaccharide units. Can be completely hydrolyzed into many monosaccharides.

Monosaccharides

• Three to eight carbons in length• One of the carbons is in a carbonyl group, and

the rest are attached to hydroxyl groups• Two types of monosaccharides:

▫ Aldose: carbonyl group is on the first carbon▫ Ketose: carbonyl group is on the second carbon as

a ketone

Monosaccharides

• General naming:▫ A three carbon monosaccharide: triose▫ A four carbon monosaccharide: tetrose▫ Five carbons = pentose, and six carbons = hexose

• Put the number of carbons together with the type of sugar for a general naming scheme:▫ Ribose is an aldopentose▫ Fructose is a ketohexose

B. Structures of Monosaccharides

• Monosaccharides contain many chiral carbons. (Chapter 14: review of chirality. Chiral compounds exist as mirror images.)

• Fischer Projections▫ Looking at the Fischer projection for the simplest

aldose: which carbons are chiral?

Structures of Monosaccharides cont.

• Most carbohydrates we deal with will be bigger, so they will have several chiral carbons.

• The designation “L” is given to the stereoisomer having the hydroxyl on the left on the chiral carbon farthest from the carbonyl group.

• The designation “D” is given to the stereoisomer having the hydroxyl on the right on the chiral carbon farthest from the carbonyl group.

C. Cyclic Structures of Monosaccharides

• Chapter 14: aldehyde + one alcohol = hemiacetal

• The same thing happens when the aldehyde and alcohol are in the same molecule, forming a cyclic hemiacetal.

• This happens with monosaccharides – in fact, the ring structure is the most stable form of aldopentoses and aldohexoses.

Drawing Haworth Structures for Cyclic Forms

• For aldohexoses, formation of a six-membered ring is favored. Let’s start from the open chain form and draw the ring.

1.Take an open chain of glucose and think of it falling on its side, to the right. The –OH groups that had been on the right are now pointing down.

Drawing Haworth Structures for Cyclic Forms cont.

2. Fold the chain into a hexagon – I recommend bringing carbon 6 around the back.

3. The –OH on carbon 5 forms a bond with the carbonyl carbon, forming the cyclic hemiacetal.

4. Note that the –OH on carbon 1 can either be pointing “up” or “down”. Either way is a possibility, but they are two isomers called anomers.

Haworth Structures cont.

• Such small differences are important – humans are capable of digesting -glucose, but not -glucose

• In solution, the two forms of the monosaccharide will interconvert. This process is called mutarotation. The ring closes in the form, opens again, closes in the form, opens again, back and forth, back and forth…

Cyclic Structures of Galactose and Fructose

• More examples. Galactose is another aldose, and fructose is a ketose.

D. Chemical Properties of Monosaccharides

• These molecules contain functional groups that can undergo chemical reactions.

• For instance…▫ Aldehyde carboxylic acid (oxidation)▫ Carbonyl group alcohol (reduction)▫ Hydroxyl groups can react with a variety of other

things

Oxidation of Monosaccharides

• In the open-chain form, the aldehyde group is available for oxidation. When it oxidizes, a carboxyl group will be formed.

• When the sugar is oxidized, it gives away electrons and is in turn reducing another compound. Sugars that are able to reduce other compounds are called reducing sugars.

• Reducing sugars form a brick-red precipitate with Benedict’s reagent.

How to tell if something is a reducing sugar?• The sugar has to be able to convert into the open

chain form (that is, reverse from hemiacetal into aldehyde/ketone and alcohol). **If the sugar happens to be in a full acetal form, water alone will not reverse the reaction back to open chain (needs acid catalyst).

• Therefore, sugars that are hemiacetals (or open chain) are reducing; acetals are not.

• A few web links▫ http://www.chem.ucalgary.ca/courses/351/Carey5th

/Ch25/ch25-2-5.html▫ https://www.rpi.edu/dept/chem-eng/Biotech-Enviro

n/FUNDAMNT/reducing.htm

Reduction of Monosaccharides

• Reduction of carbonyl group in sugar gives alcohols.

• Produces “sugar alcohols” – called alditols• Commonly known alditols: sorbitol, xylitol

(artificial sweeteners with side effects!)

E. Disaccharides

• Two monosaccharides linked together• Disaccharide hydrolysis with acid or enzyme

yields two monosaccharidesH+

maltose + H2O glucose + glucose

H+

lactose + H2O glucose + galactose

H+

sucrose + H2O glucose + fructose

Disaccharides cont

• A glycosidic bond links the two monosaccharides – the hydroxyl group in one monosaccharide reacts with a hydroxyl group in another monosaccharide, and the two are joined

• We use numbers to designate where and how the monosaccharides are joined

• Some disaccharides are reducing sugars, and some are not

Why are some disaccharides nonreducing?•It depends on the position of glycosidic

linkage.

F. Polysaccharides

• A polymer of many monosaccharides joined together• There are several polysaccharides of the

monosaccharide glucose that vary only in the type of glycosidic bond and in the type of branching present; different properties

• Cellulose: in wood/plants. Long, unbranched chains of glucose with linkages (we cannot digest!)

• Amylose/amylopectin: the starch we eat (linked glucoses)

• Glycogen: storage of glucose in our muscles. Highly branched glucose, linkages.

• Iodine test: presence of starch turns iodine blue-black