by D. DeWitt, PhD
v1.5 11/10/12 Introduction
| Plant Synthesis
Although it might seem straight forward, the synthesis of sucrose, either as a simple condensation reaction (a.k.a. dehydration synthesis), or what actually happens in plants is complicated.
Before we explore sucrose's creation, let's take a look at its structure. In Figure 1, the space-filling model is pretty but rather useless at this point in our journey. We need to see the atoms!
| Figure 1: Space-filling Model of Sucrose (Click on image to visit originating website.)
In Figure 2, you will find the structural formula for sucrose. It is a disaccharide made from the simple sugars glucose (on the left) and fructose (on the right).
I will caution you right now that there are many different view of sucrose on the internet, so if this one does not agree with your impression, there are reasons: 1) errors (e.g., Fructose has 7 oxygens) and 2) synthesis from different forms (isomers) of glucose and fructose.
Figure 2: Structural Model of Sucrose
Note: Carbon atoms are assumed at all angles of each geometric shape (hexagons and pentagons) unless an atom is shown (e.g. oxygen) (Click on image to visit originating website.)
The first issue is the process by which two sugars can be chemically joined.
| The answer is found in the fact that these sugars are literally covered with hydroxyl (-OH) groups. As shown in Figure 3 on the right, a hydroxyl group on one sugar (-OH) can be connected to another hydroxyl group (HO) on a second sugar by removing an -OH from the first sugar and a H- from the second leaving a -O- bond connecting the two sugars in a disaccharide.
In addition, the released -OH and H- join to form H2O (HOH). The name of the -O- bond is called a glycosidic bond. The bond gets its name from the fact that the disaccharide is also called a glycoside.
| sugar1-OH + HO-sugar2
sugar1- + -O-sugar2 + -OH + H-
sugar1-O-sugar2 + HOH
Figure 3: Glycosidic Bond Formation
The overall reaction is called either a a) condensation reaction because water appears, or b) dehydration synthesis because water is removed (dehydration). Therefore the formation of the disaccharide sucrose occurs via a condensation reaction. Because it occurs within cells, the reaction occurs only with the aide of an enzyme.
ACTUALLY... in plant cells, it occurs differently, but I will address the real world later in this discussion.
Let's take a closer look at the specifics (and therefore, the sources of confusion) about condensation reactions.
Two molecules are isomers if they have the same molecular formula, but different structural formulas. Therefore glucose and fructose are structural isomers because they both have the same molecular formula of C6H12O6 but they are arranged differently. (E.g., Glucose is an aldose and fructose is a ketose.) In Figure 4 below, you can see how glucose and fructose carbons are numbered. Reference to carbons 1, 2 and 4 will be forthcoming.
| Figure 4: Glucose (left) and Fructose (right)
(Click on image to visit originating website.)
Note: This website's fructose has 7 oxygens! Look at Carbon 5.)
| With each named monosaccharide, alternative forms exist as well. These are also isomers, based on more subtle differences. In other words, each aldose or ketose has different forms possible which are also isomers. Examples are D and L isomers and more useful to this discussion, alpha and beta isomers (a.k.a., anomers).
THE major source of confusion and the source of the abundance of alternate structures for sucrose, is the lack of attention to alternate forms (a.k.a. isomers) of the building block monosaccharides glucose and fructose.
| Alpha and Beta Isomers (a.k.a., alpha and beta anomers)
If you examine Figure 4 above, and find the Oxygen atom in each ring, you will find a carbon to...
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