CHAPTER 15 Carbohydrates

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: C n (H 2 O) n The carbohydrate glucose is made by plants during photosynthesis, and it is oxidized by us through respiration: photosynthesis 6 CO H 2 O + energy C 6 H 12 O O 2 respiration glucose

Types of Carbohydrates Simplest: monosaccharide, which cannot be split or hydrolyzed into a smaller carbohydrate (example: glucose C 6 H 12 O 6 ) Disaccharide: two monosaccharide units joined together (example: sucrose C 12 H 22 O 11 ) A disaccharide can be hydrolyzed in the presence of an acid or enzyme to give the two monosaccharide units acid or enzyme sucrose + H 2 O 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 ▫ /ch htmlhttp:// /ch html ▫ Environ/FUNDAMNT/reducing.htmhttps:// Environ/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 monosaccharides H+ maltose + H 2 O glucose + glucose H+ lactose + H 2 Oglucose + galactose H+ sucrose + H 2 O 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