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 • Disaccharides • Starch • Cellulose and Lignocellulose • 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?

Chemistry 104 Prof. Shapley page 1 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.

Chemistry 104 Prof. Shapley page 2 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!

Chemistry 104 Prof. Shapley page 3 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.

Chemistry 104 Prof. Shapley page 4 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,

Chemistry 104 Prof. Shapley page 5 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.

Chemistry 104 Prof. Shapley page 6 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.

Chemistry 104 Prof. Shapley page 7 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.

Chemistry 104 Prof. Shapley page 8 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.

Chemistry 104 Prof. Shapley page 9