Assignment 3 February 2, 1999



Provide answers to the following questions/problems based on the assigned readings, lectures and web notes (including direct links). Due by the start of class, February 9, 1999. (10 points)




  1. Schematically illustrate the structures of the three principal starch polymers in plants and compare and contrast these structures with cellulose. (3 points)

    The general structures are given on the web sites for lectures 7 & 8. Amylose is a polymer of glucose with a -1,4 glycosidic bonds. Amylopectin and phytoglycogen are similar, but with a -1,6 branches every 20 to 25 glucose residues in amylopectin and 10-15 in phytoglycogen. Amylose is ~ 103, amylopectin ~ 104 105 and phytoglycogen ~ 105 glucose residues. Because of the bend caused by the a -1,4 glycosidic linkages, amylopectin polymers assume a helical or corkscrew conformation. The b -1,4 glycosidic linkages of cellulose cause 180° flips of the glucose monomers resulting in flat linear structures. These are packed in parallel alignments into microfibrils in which intra- and inter-chain as well as inter-sheet hydrogen bonding occurs.



  3. Outline the main differences in structures of cellulose, hemicellulose, pectin and agar. (2 points)

    Cellulose is described in the answer to question 1. Like cellulose, hemicelluloses are chains of sugar monomers mostly joined by ß-1,4 glycosidic linkages. However unlike cellulose, the chain lengths are shorter (e.g. several hundred residues), they are composed of various monomer units including glucose, mannose, xylose, arabinose, galactose and 4-O-methyl glucuronic acid and the ß-1,4 linked backbone contains numerous short side chains that might be linked a -1,2, a -1,3 or a -1,6. Also some primary cell wall hemicellulose backbones contain ß-1,3 in addition to ß-1,4 linkages. The linear ß-1,4 linked glucose residues are thought to hydrogen bond with cellulose strands like cellulose polymers with each other. Branch chains might bond with cellulose chains of other cellulose fibrils.


    Pectins are another complex group of polysaccharides that are abundant in primary cell walls and in the middle lamella between all plant cells. Like hemicellulose, pectin polymers are chemically diverse molecules. Unlike cellulose or hemicelluloses, pectins are usually acidic and contain a high proportion of D-galacturonic acid residues joined by a-1,4 glycosidic linkages. Some of the carboxylic acids of the galacturonates are esterified to methanol. L-rhamnose (a 6-deoxyhexose) residues are usually interspersed throughout the chain. The linkage of D-galacturonic acid to L-rhamnose is a -1,2 and the linkage from D-galacturonic acid to the next galacturonic acid in the chain is a -1,4. Side chains are often attached to these rhamnose residues. The side chains appear to be neutral sugars polymers containing monomers such as L-arabinose or D-galactose.

    Agar consists of 2 components, agarose and agaropectin. Agarose is composed of alternating D-galactose and 3,6-anhydro-L-galactose with side chains of 6-methyl-D-galactose residues. Agaropectin is like agarose but additionally contains sulfate ester side chains and D-glucuronic acid. The tertiary structure of agarose is a double helix with a so-called threefold screw axis. The central cavity of this double helix can accommodate H2O molecules. Agarose and agaropectin readily form gels that contain high amounts of H2O (up to 99.5%). The structure of agarose is more like amylose than cellulose or hemicellulose. Pectins can also form gels like agar and amylose.



  5. Give the subcellular location of synthesis and deposition of storage and structural carbohydrates. (2 points)

    The main structural carbohydrates, cellulose is synthesized in the plasma membrane and deposited in the cell wall (extracellular).

    Starch is synthesized and stored in plastids.

    Fructans are synthesized and stored in vacuoles.



  7. Outline the regulation of the synthesis and degradation of starch. Give all intermediates between glucose and glucose-1-P and sucrose (3 points).


The synthesis and degradation of starch is regulated by the 3-P-glycerate (3PG)/phosphate (Pi) ratio in chloroplasts at the ADP-glucose pyrophosphorylase step. The 3PG and Pi concentrations of the chloroplast stroma are held constant by triose P-Pi counter-exchange. When sucrose synthesis in the cytoplasm decreases, less Pi is released in the cytosol leading to Pi deficiency (> 3PG/Pi) in chloroplasts which limits photosynthesis. This leads to increased starch synthesis in which Pi is released allowing photosynthesis to resume. High sucrose synthesis in the cytosol depletes 3PG by its conversion into triose phosphates and increases the Pi concentration of plastids reducing the 3PG/Pi ratio and promoting starch catabolism.


See Figures 9.12 and 9.14 of the class (Heldt) text.



Give all intermediates between glucose and glucose-1-P and sucrose.


Glucose from the Calvin-Benson Cycle translocated to the cytosol.

There it can react with UTP forming UDP-glucose.

UDP-glucoseß à glucose-1-P

Glucose-1-P ß à glucose-6-P

Glucose-6-P ß à fructose-6-P

Fructose-6-P + UDP-glucose ß à sucrose-P + UDP

Sucrose-P à sucrose + Pi