Understanding Carbohydrates: Starch, Disaccharides, and Cellulose, Study notes of Chemistry

Download Understanding Carbohydrates: Starch, Disaccharides, and Cellulose and more Study notes Chemistry in PDF only on Docsity! 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 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