Photosynthesis and Carbon Dioxyde Essay

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IB421 Final Take Home Exam. You may consult reference material but you are to work alone in answering these questions. Exams are due by 5 pm on Thursday, Dec 15th in IGB 1406 or e-mail pdf file to

Short Answer: Please answer all 10 questions. Each is worth 5 points and your answers should not exceed 200 words. Please use 1.5 line spacing; calculations and drawings may be done by hand. 1. Describe the importance of genome reduction in plastid evolution.

Genome reduction is usually observed in endosymbiotic species when we have the association of eukaryotes and a cyanobacteria ( ancestor) is established. The inability of some endosymbiotic species to live outside of can sometimes explain the reasons why genome reduction occurs. In most of the cases, the cyanobacteria gets reduced and there is conservation of some of the genetic information via transfer of a large number of genes to the nuclear genome of the host.
The lysis of bacteria occurs during division and enables the transfer of genes to the nucleus.
2. Why do C4 plants and CAM plants typically have higher water use efficiency than C3

plants? (Define water use efficiency in your answer.)
Water use efficiency is the inverse of the transpiration ratio ( water use efficiency =moles
CO2 assimilated/moles H2O transpired). C4 plants and CAM plants unlike C3 plants both undergo little photorespiration. This is the reason why there is a spatial seperation between C3 and C4 photosyntetic pathway and thus C4 plants have a different leaf anatomy where mesophyll cells are close to the leaf surface and exposed to high levels of O2 which will prevent photorespiration and favor photosynthesis and thus water use efficiency by producing more moles of CO2 assimilated . CAM plants present a C3 and C4 seperate pathway in time and they have their stomata opening at night for CO2 uptake whereas their stomata close during the morning and this enables them to retain the water.
3. Describe the steps of starch synthesis in plants. What biochemical and signaling

mechanisms ensure that the rate of starch synthesis is balanced by the rate of photosynthesis and sucrose synthesis?
Starch synthesis starts in the plastids with the formation of ADP gluscose and the release of pyrophosphate. The formation of ADP glucose comes from the reaction of glucose -1-phosphate and ATP and catalyzed by AGPase (ADP-glucose phosphorylase) which is activated by 3phosphoglycerate and inhibited by phosphate (pi). Then starch synthesis ends with the addition of glucose elements at the end of a polymer chain with release of ADP. At the end of this process

starch branches are formed with SBEs, which will form 1, 6 linkages between glucose units. For regeneration of RUBP, the rates of starch synthesis, photosynthesis and sucrose synthesis must be balanced, there is then movement of triose phosphate into the cytosol at the expense of one phosphate (Pi) through the triose phosphate translocator (TPT) pathway which is placed in the chloroplast inner membrane. the mechanims are allosteric regulated: low rate of sucrose synthesis leads to a 3PG:Pi increase in chloroplasts and to high rate of starch synthesis (less TP left the chloroplast). Also high rate of sucrose synthesis leads to a decrease of 3PG:Pi ration in chloroplasts leading to low rates of starch synthesis (more triose phosphate left chloroplast).

4. Explain why activation of Rubisco is necessary and the mechanisms by which Rubisco

activation is achieved.
In order for carboxylation and oxygenation to occur during photorespiration under RUBISCO catalysis, Rubisco needs to be converted from its inactivate form into its active state as the active site of Rubisco will not distinguish those two substrate (CO2and O2). The activation of Rubisco requires the process of carbamylation of a lysine element, this will then result in providing a binding site for magnesium (mg2+). Then, once ribulose 1,5