Figure 1: Glucose molecule
Carbohydrates are made of hydrogen, oxygen, and carbon that combine to form monosaccharides. These simple sugars are made of five (pentose) or six (hexose) carbon rings. Additional hydrogen and oxygen atoms allow these rings to form disaccharides ("two sugars") such as sucrose and lactose or polysaccharides ("many sugars") such as starch.
Sugars are essential to living things. They provide a way to store chemical energy to operate living things. They are also the structural backbone to the information storage molecules DNA and RNA, as well as many of the other essential molecules in living things. Sugars are transported through the blood stream in the form of the simple sugar glucose. They have 4 calories of energy per gram.
Figure 1 shows glucose, a 6 carbon sugar. Glucose is manufactured from carbon dioxide and water by plants during photosynthesis. The element carbon plays a central role in the features of living things, primarily because it can make four chemical bonds. The chemical formula for the glucose molecule is C
. The formula indicates the presence of 6 carbon atoms, 12 hydrogen atoms and 6 oxygen atoms. In figure 1, carbon atoms are located at each of the corners of loci 1 through 6 (they are implied by the ring structure). At each site you will note that it is bonding with other carbons, oxygen, and/or hydrogen atoms.
These glucose molecules bond with each other in a condensation reaction. A condensation chemical reaction involves the formation of a water molecule when one glucose molecule releases a hydrogen and the other releases an oxygen and a hydrogen. These extra hydrogens and the oxygen combine to make water (H
To help students understand this process in the classroom, physical modeling activities that can be employed to make this less abstract. If molecular modeling kits are available, individual glucose molecules can be constructed and brought next to each other. With the removal of one hydrogen and one oxygen and hydrogen from the adjacent molecule, a condensation reaction can be modeled. Then, the two molecules can be joined to represent the growing carbohydrate polymer chain. If modeling kits are not available, toothpicks (to represent bonds) and different colored jelly beans (to represent different atoms) can be substituted in model making. (Save these models for a later activity that will involve their disassembly.)
Plants use starch to store chemical energy in the form of a polymer of glucose molecules. These chains can be up to 600 units long.
Animals use glycogen to store glucose. We will later see that it is stored in muscles and the liver. This molecule is more compact and stable than glucose.
The energy contained within the bonds of glucose is converted within the mitochondria of cells in to a more accessible form of chemical energy, adenosine triphosphate (ATP). The cells can readily access the chemical energy contained in the ATP and use it to do the work required by living things.
Cellular respiration is illustrated in this simplified chemical reaction.
glucose + 6 O
+ 6 H
This reaction occurs in both the cytoplasm and in the mitochondria. Glucose is transformed into a three carbon sugar, and then in the mitochondria it loses hydrogens. The electrons on these hydrogens are used to create ATP, the energy molecule cells need to do their work. "Waste" products from this reaction are carbon dioxide and water.