Daily Newsletter January 18, 2012

 

 Daily Newsletter                                        January 18, 2012

---

Today's Topic: Carbohydrates and Lipids

While carbohydrates are mainly used as chemical energy storage, carbohydrates are also used as modifiers of proteins. The goal today is to get a good understanding of the structure of carbohydrates, and a little about their naming.

A topic that will come up throughout the semester is how carbons are numbered in carbohydrates.  This is important as we will find carbohydrates being components of monomers and when we move through the carbohydrate catabolism.  The following image from Rensselaer Polytechnic Institute shows the linear form of glucose, and the two possible cyclic (pyranose ring) isomers.

 The formation is based on aldehyde chemistry, so we will leave some of this discussion to organic chemistry and biochemistry.  For our purpose this semester, what is important is that we number carbons from the aldehyde.  Notice in the above diagram that carbon 1 is to the left of the oxygen, we go around to carbon 5, and then carbon 6 is outside of the ring.  If you see the expression 3', it is referring to the third carbon.  5' the fifth carbon.  6' the sixth carbon, and so forth. 

Notice also, that when the ring was formed, there were differences in the groups coming off of carbon 1.  These differences are important, and can influence how the sugar is metabolized. 

One critical difference here comes when linking two monosaccharides together to form disaccharides and polysaccharides.  For instance (again from rpi.edu), here is maltose:

 This is an α 1-4 glycosidic linkage.  We have an α Maltose (look at carbon 1) bound from carbon 1 to carbon 4.  Since the maltose on the left hand side is α at the 1 carbon, we form an α linkage.  In comparison, look at cellobiose:

Cellobiose has a β 1-4 glycosidic linkage.  The designation of β comes from the sugar unit that donates carbon 1 to the bond.

So, what is the big deal?  Maltose is digestible by humans, cellobiose is not.  Just this slight isomeric difference changes the metabolism.

---

Today's Notes:  In our face-to-face time, we will talk about triglycerides and phospholipids.  Please take the time to read about these two lipids, and come prepared to discuss.

---

Today's Challenge:
Beyond dietary sugar, what can sugars do in biological organisms?  HINT:  Look at things as diverse as cellular receptors and markers (blood type), all the way to mucous.