Mod+5+Reading+Guide

__**Reading Guide for Module 5: Carbohydrates**__ The main point of Module 5 and Chapter 15: Carbohydrates is to understand the composition, structure, and uses for common monosaccharides, disaccharides, and polysaccharides.

__15.1: Carbohydrates__ ->Summary: Carbohydrates contain carbon, hydrogen, and oxygen atoms. **Monosaccharides**, such as glucose, are carbohydrates that cannot be split into smaller carbohydrates. **Disaccharides**, such as sucrose, are carbohydrates that are made by joining two monosaccharides. **Polysaccharides** are carbohydrates that contain many monosaccharide units. Most O atoms in monosaccharides are found in OH groups, but one is in a carbonyl group. An **aldose** is a monosaccharide with the carbonyl group as an aldehyde, and a **ketose** is a monosaccharide with the carbonyl group as a ketone. A monosaccharide can be named using the names triose, tetrose, pentose, or hexose for 3, 4, 5, or 6 carbon atoms, respectfully. Then aldo- or keto- can be used as a prefix to name the monosaccharide such as ketopentose or aldohexose.

->Struggling Topic 1: HW question 15.4 says that polysaccharides are made of thousands of monosaccharides. This sounds a little outrageous, but I assume the HW program is correct here. Does this mean that the polysaccharides we get in starches are essentially a storage bin for extra glucose made during photosynthesis of the plants that produce these starches?

->Struggling Topic 2: Each monosaccharide that the book and HW have asked us to name using aldo- or keto- and tri-, tetra-, etc has also given a secondary name. Is there no definitive naming method for carbohydrates like there is in IUPAC names for other organic compounds, or is it that these IUPAC names aren't used as commonly? Like the picture it has for glucose could be named something like 2,3,4,5,6-penthydroxyl-hexanal for example.

__15.2: Structures of Monosaccharides__ -> Summary: Monosaccharides contain chiral carbons. Many monosaccharides contain more than 1 chiral carbons. When a Fischer Projection for a monosaccharide has more than 1 chiral carbon, the L or D for the molecule is determined by the location of OH group on the chiral carbon that is located farthest from the carbonyl group. Because Fisher Projections are written with the most oxidized carbon on the top, the chiral carbon that is farthest from the carbonyl group will be the chiral carbon that is closest to the bottom of the Fisher Projection. Then, the molecule with the OH on the left is L, and the one with the OH on the right is D. **D-glucose** is the most common hexose and is also known as dextrose and blood sugar. **D-galactose** does not exist in nature, is formed when lactose is broken down, is almost identical to D-glucose, and requires a specific enzyme to convert it to glucose so that the body can use it. **D-fructose** is identical to D-glucose except that the carbonyl group is a ketone for fructose and an aldehyde for glucose.

->Struggling Topic 1: The Mod 5 Objectives state that we should be able to "recall the important monosaccharides." Do these "important" monosaccharides include more than glucose, fructose, and galactose? Which other monosaccharides should we be familiar with?

->Struggling Topic 2: Is galactosemia one of the main contributors to lactose intolerance? The treatment for galactosemia seems similar to some people with more severe lactose intolerance, but I have also heard of many mild cases of lactose intolerance. Galactosemia doesn't sound like it would cause mild lactose intolerance. There must be several possible causes of lactose intolerance.

__15.3: Cyclic Structures of Monosaccharides__ -> Summary: **Cyclic hemiacetal** is the most stable form for aldopentoses and aldohexoses. In this form, the O from the final OH group bonds with the C from the aldehyde group to close the open chained aldose into a ringed **heterocyclic hemiacetal**. These forms are better known as a **Haworth structure**. This structure can be written as **anomers** because the **OH group** on carbon 1 may be drawn as **up (in the beta anomer)** or **down (in the alpha anomer)**. A similar structural change happens for fructose and other ketoses where the ketone bonds with the alcohol from carbon 5 and forms a heterocyclic compound with 5 atoms in the ring.

-> Struggling Topic 1: The book mentioned how a six atom cyclic compound was favored in equilibrium for aldose compounds. What causes fructose to form a five atom cyclic compound with an extra carbon atom (carbon 6) left outside of the ring? I would have thought that if six atom rings were preferred then the carbon atom would be included in the ring for fructose too.

-> Struggling Topic 2: As far as I can tell there are no homework problems about writing Haworth structures for saccharides. The objectives for Module 5 discuss being able to draw the difference between a-D-glucose and B-D glucose to illustrate the difference between our ability to digest glucose and our inability to digest cellulose. This is due to the revered OH group on carbon 1, and the fact that we don't have the needed enzyme, B-amylase, to digest the cellulose although we do have the a-amylase to digest the glucose, correct?

__15.4: Chemical Properties of Monosaccharides__ -> Summary: An aldose's **aldehyde group can be oxidized to a carboxylic acid** group, and a **carbonyl group** from either an aldose or a ketone **can be reduced to an OH group**. **Reducing sugars** are monosaccharides that reduce another substance (such as Benedict's reagent). **Alditols** are sugar alcohols that are formed when the carbonyl group of a monosaccharide is reduced.

->Struggling Topic 1: In the HW problems, when I was drawing the reduced product of d-Xylose, I am unsure why one orientation of the answer was counted wrong and the other was accepted. It must matter, but can you explain why for me? See pictures below:

->Struggling Topic 2: How do you name the product of a reduction reaction for a sugar where the answer is a sugar alcohol? For example, d-glucose is reduced to d-glucitol, and xylitol is reduced from xylose. Does this mean that the reduced sugar is named for the original sugar but the -ose ending is replaced with -itol?

__15.5: Disaccharides__ -> Summary: **Maltose** is a common disaccharide from starches that yields two glucose molecules after hydrolysis. A **glycosidic bond** is a bond between two monosaccharide hydroxyl groups that forms water and a disaccharide. **Lactose**, milk sugar, is a common disaccharide that yields a glucose molecule and a galactose molecule after hydrolysis. **Sucrose**, table sugar, is a common disaccharde that yields a glucose and a fructose molecule after hydrolysis. Sucrose is unusual for a disaccharide because the glycosidic bond is between carbon 1 of glucose and carbon 2 of fructose, so it cannot open to form an aldehyde group. This makes sucrose unusual because it is NOT a reducing sugar.

-> Struggling Topic 1: Do the bonds that connect Maltose, Sucrose, and Lactose always stay the same, or can the a/b part of them change? Each example of maltose used an a-1,4-glycosidic bond, each example of sucrose used an a,b-1,2-glycosidic bond, and each example of lactose used a b-1,4-glycosidic bond. I would assume that this means that these bonds are always the same.

-> Struggling Topic 2: Is it a trend that all 1,4-glycosidic bonded disaccharides are reducing sugars while 1,2-glycosidic bonded disaccharides are not reducing sugars? This seemed to be the reason why sucrose wasn't a reducing sugar although maltose and lactose were reducing sugars. Also, the 1,6 glycosidic bond in isomaltose is a reducing sugar. How do we know this?

__15.6: Polysaccharides__ -> Summary: Four biologically important polysaccharides are amylose, amylopectin, cellulose, and glycogen. All of these polysaccharides are constructed of d-glucose and differ only in the types of glycosidic bonds that connect these d-glucose molecules. Amylose and amylopectin make up starches in plants. **Amylose** is formed of d-glucose that is bonded only with a-1,4-glycosidic bonds in a continuous chain. **Amylopectin** is almost the same as amylose except that about every 25 glucose units there is a branch of glucose molecules attached by an a-1,6-glycosidic bond. Amylose makes up about 20% of plant starch, and amylopectin makes up the other 80%. These are easily broken down (hydrolyzed) by our bodies using amylase and maltase enzymes. **Glycogen**, animal starch, is very similar to amylopectin except that it is branched more often (about every 10-15 glucose units). Glycogen is used in the liver and muscle of animals to maintain the proper blood glucose level and maintain energy between meals. **Cellulose**, the major structural material in plants, is made from glucose molecules in long unbranched chains similar to amylose, but cellulose chains are made exclusively from b-1,4-glycosidic bonds that do not form coils like amylose and are not water soluble. Animals don't produce the enzyme needed to hydrolyze the b-1,4-gylocosidic bonds, so animals cannot digest cellulose (although some have help from bacteria). An **iodine test** using I2 causes reactions with any of these four polysaccharides that form bold colors, but these colors don't from in the present of mono- or disaccharides.

-> Struggling Topic 1: Why does iodine react with these polysaccharides when it will not react with mono or disaccharides? It would seem more likely to be the other way around since the mono and disaccharides are smaller molecules.

-> Struggling Topic 2: Is there a difference in the rate of metabolism for glycogen and for amylose or amylopectin in our digestive systems because of the relative length of the starch molecules? It would seem like glycogen should be digested more quickly than the plant starches because it is shorter.

__Critiques of this chapter__ --A. How clearly the author communicated individual topics --->15.5 - I liked the simple breakdown of the 3 main disaccharides that the author wanted us to know. It was clear to see the difference between maltose, lactose, and sucrose.

--B. Specifics about the amount of content (did you need more examples?) --->15.2-15.4 - I would have liked to see a table or chart with **the** important monosaccharides compared. There were a few pictures of examples of glucose, fructose, etc in fischer projections, but I think it would be helpful to have all of the ones that we should recognize in the same place.

--C. Chapter's place in the overall text --->15.3 - It is good that we have covered cyclical alkanes, alkenes, and aromatic compounds before getting to cyclical monosaccharides because the folding of the monosaccharide into a hexagon or pentagon shape is considerably more difficult than these previous examples. This is mostly due to the extra substituent OH groups that must be kept in the correct orientation as the molecule is bent into a Haworth Structure.