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: 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 a-glucose, but not b-glucose
• In solution, the two forms of the monosaccharide will
interconvert. This process is called mutarotation. The
ring closes in the a form, opens again, closes in the b
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/BiotechEnviron/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 + 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 blinkages (we cannot digest!)
• Amylose/amylopectin: the starch we eat (alinked
glucoses)
• Glycogen: storage of glucose in our muscles. Highly
branched glucose, alinkages.
• Iodine test: presence of starch turns iodine blue-black