Carbohydrates

ID #2375

Okay, so I was listening to my recorded lecture, and found out Professor made a difference between Starch and Glycogen; saying "You can't make beta glocoses fit next to each other without rotation, but you can make alpha fit without rotations because there are no oxygen on the same side that is gonna get into each others way..." I am confused because when I look at starch and glycogen, they are made of alpha linkage,but professor mentions "beta." I don't know where beta is coming from comparing Starch and Glycogen. Also it will be great if somebody can explain the differences between Starch and Glycogen in both general and what professor said in the lecture. I know Starch is found in the plant and Glycogen in animal. So, besides that point please.


Cellulose, starch, and glycogen are all made up of glucose molecules linked into long chains. 
Cellulose is linked by 1,4 beta linkages. It is organized into long fibrils that can stack on top of each other. At first it doesn't look like the linkages are 1,4 beta, but if you number the carbons and remember that every other glucose is flipped you can see that they are 1,4 beta glycosidic linkages. 
Glycogen is found in animals, as you noted. It is more extensively branched than starch. It contains only alpha linkages. The 1,4 alpha linkages are the "straight" ones (though glycogen chains do curve slightly, compared to the very straight cellulose chains). The 1,6 alpha linkages are the branches (which look like "Y"s on the green dot picture shown in class). 
Starch is found in plants and is less branched than glycogen. Chemically, it follows the same pattern as glycogen - all alpha linkages. Again, 1,4 alpha linkages are the straight ones. 1,6 alpha linkages are the branches. 
The confusion is most likely because Professor Mehrtens was talking about starch and glycogen, but then he compared it back to what he was talking about with cellulose, so the terms may have gotten jumbled. The important point in that section is to know that only cellulose has beta linkages (starch and glycogen do not have beta) and you need to know what having a beta linkage means for the overall structure. 
The result of having a beta linkage in cellulose is that the second sugar in that linkage is rotated 180 degrees, allowing the oxygens between the two sugars in the linkage to have their own space apart from one another, causing the cellulose fibril to be straight. The straightness of the cellulose fibril allows the fibrils to hydrogen bond together to form bundles. You don't see the same bundling in glycogen or starch because (1) they have branches and (2) they have alpha linkages meaning they don't have the same need for rotation to make them fit side by side, but the chain ends up curved not straight. 

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