Sugar Polymers
Plants and animals use polysaccharides, such as starch and glycogen, for the longer-term storage of
energy. These can be readily hydrolyzed by enzymes to simple sugars. Other polysaccharides are
structural components of plant cells.
Outline
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Disaccharides
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Starch
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Cellulose and Lignocellulose
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Homework
Bread, pasta, potatoes, rice and other foods from grains are rich in
starch.
Disaccharides
Condensation of Sugars
Disaccharides are formed by the condensation reactions of two simple sugar molecules. Condensation is
the loss of water in a chemical reaction. Two OH groups, one from each sugar molecule, come together to
release water and form an oxygen bridge between. One of the OH groups is attached to the anomeric
carbon (the carbon that has 2 oxygens bonded to it).
Here you see the formation of sucrose from the 6-membered form of glucose and the 5-membered form of
fructose. Note that linear fructose has a ketone rather than an aldehyde group. Which carbon in glucose
and in fructose would be the carbonyl carbon in the linear form?
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Another example is the condensation of 2 molecules of glucose.
Examples of Disaccharides
Sucrose is the disaccharide of glucose and
fructose. This is common table sugar and
it comes from sugar cane and sugar beets.
Maple syrup also contains sucrose.
Maltose is derived from the coupling of two
molecules of glucose. It is produced when the
enzyme amylase breaks down starch. Maltose is formed in
germinating cereal grains and is important in the production
of alcohol by fermentation.
This is a
disaccharide of galactose and glucose. Lactose is also
called milk sugar and it makes up between 2 and 8 % of
milk.
Step 1: Protonation of Sugars
Most reactions involve the combination of an electrophile and a nucleophile. Remember that a strongly
electrophilic carbon is formed by the protonation of a simple sugar.
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The cation on carbon is stabilized by the adjacent oxygen atom. The empty p orbital on carbon can
overlap with the filled p orbital on oxygen. The carbon is still electron-poor though and will react rapidly
with nucleophiles.
Step 2: Addition of a Nucleophile
Most reactions can be viewed as the addition of a nucleophile to an electrophile. In acid-base reactions,
the base is also a nucleophile and combines with the proton, an electrophile. When carbonyl compounds
are reduced by borohydride reagents, a nucleophilic hydride is added to the nucleophilic carbonyl carbon.
In each of the reactions below, a curved arrow indicates the movement of an electron pair from the
nucleophile to the electrophile. This is a very useful way to show the mechanism of the reaction. Never
show an arrow from an electrophile to a nucleophile!
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An available hydroxy oxygen atom on a simple sugar acts as a nucleophile and adds to the electrophilic
carbon of the sugar atom. Following the addition, the bridging oxygen loses a proton.
Starch
Glucose Polysaccharides
Green plants transform solar energy to
chemical energy in glucose. Glucose isn't
very useful for long-term storage of the
energy, though. It is too soluble in
water. Plant cells (and animal cells)
are mostly aqueous salt solutions.
Glucose dissolves readily in water
and it can pass through the cell
membranes in response to concentration
gradients. In order to store glucose in one location, plants
must condense it to a water-insoluble form.
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Starches are glucose polymers that contain 300 - 1000
glucose units. There are a number of forms of the
condensed polymer of glucose. A linear
polysaccharide found in plants is is amylose (~20 %)
and a related branched polysaccharide is amylopectin
(~80 %).
Glycogen is a closely related molecule that we
use to store glucose in our muscle tissue.
Food Energy
Potatoes, rice, pasta, bread and other grain products are rich in starch and an important part of the human
diet.
The amylase enzymes are catalysts that convert starch and water back into easily digestible glucose.
Amylase is in saliva and in our digestive tract. However, pure crystalline starches are difficult to break
down, even with amylase. Cooking starches causes a partial hydrolysis and makes the starch molecules
more susceptible to further hydrolysis with amylase.
Foods rich in starches and sugars have about the same energy content. In food, energy content is
measured in calories and every gram of starchy food contains about 4 calories of energy. A food calorie is
the same as a kcal and equal to 4.18 kJ, so the energy content should be about 16 kJ/g. How does that
relate to the enthalpy of combustion?
Combustion and respiration are the same thing. For glucose,
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Why is the number we calculate greater (more energy released) than is observed in the metabolism of
starches and sugars?
Cellulose and Lignocellulose
Cell Walls
Cells walls are a tough support that is outside the cell membrane.
They are somewhat flexible but prevent the cell from bursting due
to pressure from water on the inside of the cell. Higher plants,
bacteria, fungi, and algae have cell walls but animals do not.
The cell wall is composed of the glucose polysaccharide cellulose.
The woody parts of trees and certain other plants have a secondary
cell wall that contains another polymeric material called lignin.
Cellulose
Cellulose is the major part of plant cell walls. This polysaccharide
is much like linear (amylose) starch but it can contain up to 10,000
glucose units The big difference between amylose starch and
cellulose is orientation of the bond connecting the glucose units. In both structures, there is an oxygen
bridging the anomeric carbon (C1) and C4.
When the hydroxyl groups on the anomeric carbon and the highest numbered carbon with a hydroxy
group (C1 and C4 in glucose) are on the same side of the ring (alpha configuration), the formation of of
an oxygen bridge between the two carbons results in a structure that curves in on itself. You can see this
with only 4 glucose units in a chain below. With many glucose units the structure forms helices. This is
amylose starch.
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When the oxygen groups on the anomeric carbon and the highest numbered carbon with a hydroxy group
(C1 and C4 in glucose) are on the opposite sides of the ring (beta configuration), the polysaccharide that
forms has a linear structure. This is the structure in cellulose.
Although they have the same empirical formula and are made up of the same monomer units, the
differences in shape and size make the properties of starch and cellulose very different.
Starch is food but neither humans nor any other mammal are able to digest cellulose.
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Lignin
Lignin is a highly non-regular polymer of phenol sub-units. Phenol is a derivative of the very stable
organic molecule benzene with an alcohol functional group. When phenol loses a proton, the benzene unit
stabilizes the negative charge on the oxygen atom. This makes phenol a stronger acid than other alcohols.
A small piece of the lignin polymer is below. Can you locate the phenol units?
Humic Materials, Lignite, and Coal
When woody plants decompose, bacteria are able to break down the cellulose in the cell walls and use
that for respiration. The lignin is much more difficult to decompose.
Pieces of lignin become humic material in soil. The humic material has basic groups and acidic groups. It
serves to balance the acidity of soil and also to bind minerals that can be used for new plant growth. The
humic materials are slowly oxidized to CO2 and water by oxygen.
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When lignin is buried so that it can't react with
oxygen, heat and pressure from within the
Earth can condense the structure and form
coal.
Lignite is a type of coal that closely resembles
lignin. Further condensation leads to
bituminous, then anthracite coal.
The coal that underlies most of the state of
Illinois was derived from tree-ferns and other
plants that date from 300 million years ago. At
that time, the forest was abruptly covered by
the sea (yes, sea-front property in Illinois) and
buried in sediment.
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